BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI [600534]

BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI
Tomul LIII (LVII), Fasc. 1, 2007
Secția
ȘTIINȚA ȘI INGINERIA MATERIALELOR

NEW HEAT TREATMENTS APPLIED TO ALUMINIUM ALLOYS
AlCu2.5Mg

BY

R. M. POPESCU, V.N. CÂNDEA and D. SENCHETRU

Abstract: This work describes a model of thermal treatment fo r the aluminium alloy AlCu-2.5Mg with the
advisable use of the equipment and the thermal trea tment appliances.

Keywords : materials, thermal treatment

1. Introduction

A recent tendency in the industrial hot processing development is the application
of computer aided heat treatment processes and tool ing in combination with the
reduction of the energetic consumptions. The work m ethodology suggested for
studying the improvement of the heat treatment tech nologies to be applied to the
AlCu2.5Mg aluminium alloy comprise the following ph ases:
– adapting the heat treatment process to the said alu minium alloy;
– choosing the tooling and the installations required for carrying out the heat
treatment cycles for the said aluminium alloy;
– choosing the installations and the devices to be us ed in the study of the
mechanical characteristics and in the structural an alysis of the said aluminium
alloy;
– carrying out the experiment and analytical interpre tation of the results.
Mention must be done that this methodology is an or iginal one and has been
conceived for enabling the establishment of the fin al heat treatment opportunities at
high accuracy rates.
Thus, in order to improve heat treatment technologi es for this aluminium alloy,
there have been taken into account as follows:
– application of methods and techniques to eliminate the physical human labour;
– elimination of manual control manoeuvres by applyin g automated control
systems, capable to maintain the proper operation o f the tooling without any
intervention on behalf of the process operator, no matter the external or internal
actions;
– reduction of heat losses due to the lack of air-tig htness, by using a proper
insulation;
To this respect, components and semi-manufactured p arts made of the said
aluminium alloy shall be heated in electric ovens f eaturing a forced air circulation. The
temperature of the heating mechanisms shall be set and controlled using automatic
self-registration devices with a precision class as high as possible, so that the

R. M. POPESCU et al. 128
measuring accuracy be within ±3°C. Control and adju stment devices shall be checked
as soon as the operational area reaches the maximum temperature.
In order to perform treatment cycles as set forth, there has been chosen a
SUPERTHERM oven manufactured by the Nabertherm comp any in Germany. This
equipment is available in the endowment of the S.C. Compania Apa S.A. Joint stock
trading company in Sibiu. The utilized heating inst allation is in accordance with the
ISO 9001 standard requirements.
The heating oven meets all requirements of a final heat treatment applicable to
such aluminium alloys. By means of this equipment, some heat treatment diagrams
have been successfully accomplished and also, some heating – holding curves have
been successfully maintained. [1]
By programming the desired temperature or a treatme nt cycle can be performed
by the instrumentality of the 903 P controller. Usi ng the same controller, several
programs can be interconnected or the same cycle re peated under identical
circumstances, for several times.
Typical µR1000-Yokogawa registration devices are fi tted in addition in the oven,
thus enabling, in parallel, the measurement of the heating / cooling temperature and
speed, as well as the registration of the temperatu re – time diagrams and printing the
main treatment cycle parameters onto the same.
With a view to modelling the final heat treatment p rocess for the AlCu2.5Mg
aluminium alloy [2], a minimum number of 9 experime nts are necessary according to
the 3 k factorial experiment theory; in this specific case k = 2, i.e. there are two
variation factors: the quenching heat treatment tem perature for putting into solution
(T c) and the heating temperature for ageing (T r)

2. Performing the experiment and providing the anal ytical interpretation of the
results

In order to optimize the heat treatment process for the chosen aluminium alloy, it
is necessary to settle some heat treatment versions within the limits set forth in the
standards as well as to research the mechanical, te chnological and structural properties
of the test pieces to be obtained under differently directed heating, holding and cooling
conditions.
In order to be able to perform mathematical modelli ng as desired, this experiment
has to be programmed. This involves as follows:
• establishing the necessary and sufficient number of experiences and the
accomplishment conditions thereof;
• establishing the regression equation representing t he model of the process;
• establishing the conditions to attain the optimum v alue in terms of the
accomplished process performance
Therefore, for each variable basic levels as well a s variation intervals shall be
determined. By adding the variation level to the ba sic level, the superior level of the
variable, and by subtracting it, the inferior varia ble level is obtained. Variation
intervals shall be chosen so that possibly most acc urate values be involved in terms of
a functional viewpoint. As a first step, basic leve ls and variation intervals shall be
obtained.

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 129
For modelling the AlCu2.5Mg aluminum alloy final he at treatment process, the
quenching and aging temperatures available as varia bles over the different physical or
technological parameters obtained by way of experie nce.
Table 1 shows the variation interval and the basic level for programming the
experiment in order to establish the optimum final heat treatment technology.
Table 2 shows the experiments that are necessary in order to obtain the mathematical
model. Mention must be done that each test piece ha s been pressed and labelled using
a code number so that it can be properly pursued du ring the experiment.

Table 1

Factor Quenching temperature, T c [°C] Aging temperature, T r [°C]
Basic level 515 160
Variation interval 10°C 10°C
Superior level (+) 525 150
Inferior level (-) 505 170

Table 2

Experiment Test piece
code number Quenching temperature,
Tc [°C] Aging temperature,
Tr [°C]
1 1.1.1 505 150
2 1.2.1 505 160
3 1.1.2 505 170
4 2.1.1 515 150
5 2.2.1 515 160
6 2.1.2 515 170
7 3.1.1 525 150
8 3.2.1 525 160
9 3.1.2 525 170

Using the obtained experimental results, it will be determined which are the ones
yielding the most advantageous mechanical and techn ological properties among the
proposed versions. Mention must be done that the ho lding times and the cooling
conditions are the same for each experiment, and th at the variables are only the heating
temperatures used for the purpose of the heat treat ment.

4. Conclusions

We deem that the chosen original research methodolo gy enables carrying out
experiments for the purpose of the improvement of h eat treatment technologies
applicable to aluminium alloys due to the following considerations:
– proper adoption of the heat treatment technology fo r the AlCu2.5Mg aluminium
alloy taken into consideration;

R. M. POPESCU et al. 130
– choosing a high-performance heating equipment by me ans of which the heating
and holding process can be monitored for the perfor med heat treatment operations;
– choosing an up-to-date equipment for establishing t he exact chemical composition;
– using the MTS 810 tooling for establishing the tens ile strength values, the elasticity
module, as well as other mechanical parameters. Th is is tooling of last generation
equipment and provides failure diagrams as well as the registered parameters using
a computer software.
– employing a more accurate apparatus for establishin g hardness, at a 1 mm spacing
over the parts diameter;
– using a proper and up-to-date method for analyzing material structures;
– experiment management and possibility to analytical ly analyze the results

Received April 26, 2007

REFERENCES

1. Brăgaru, A., and others: Optimizarea produselor și echipamentelor tehnologic e (Optimization of
technological processes and equipment) , București, Editura Didactică și Pedagogică, 1996
2. Giacomelli, I., et al. : Tratament termice (Heat treatments) , Ed. Universitatea Transilvania, 1980
3. Zgură, G., and others: Tehnologia sudării prin topire (Welding Engineering by Melting) , București,
Editura Didactică și Pedagogică, 1983

NOI TRATAMENTE TERMICE APLICATE ALIAJELOR DE ALUMIN U AlCu2,5Mg

Rezumat : Această lucrare descrie un model de trament termi c aplicat aliajului de aluminiu AlCu2,5 Mg,
împreună cu recomandări de utilizare a echipamentul ui și aparatelor de tratament termic.

BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI
Tomul LIII (LVII), Fasc. 1, 2007
Secția
ȘTIINȚA ȘI INGINERIA MATERIALELOR

RESEARCHES ON MECHANICAL PROPERTIES FOR STEELS SINT ERED
WITH NICKEL AND MOLYBDENUM

BY

N.V. CÂNDEA, R. M. POPESCU and D. SENCHETRU

Abstract: This work describes the metallurgy of poor alloyed steel obtained from powders containing Ni and
Mo, for producing sintered elements of high mechani cal resistance.

Keywords : sintered materials

Low alloy steels obtained using Ni and Mo content p owders are used for
producing sintered components featuring high streng th values.
Nickel influences iron powder compressibility which decreases as the nickel
content decreases due to its ferrite hardening effe ct (Fig. 1). [1]
Nowadays, the tendency in obtaining low Ni, Cu, Mo alloy steels is to use
partially alloyed powder obtained by diffusion. Thi s is especially intended to avoid
direct handling of nickel powders by the users, sin ce it is well known that nickel has a
noxious effect as to the human organism.
Fig. 1 The effect of the alloying elements onto the iron powder compressibility

The advantages of using partially alloyed powders include, among others, the
avoidance of the component segregation during handl ing (for instance copper in Fe-
Cu-C powder mixtures), increasing the homogeneity o f the sintered material alloyed
with a relatively low diffusion coefficient in Fe s uch as Cu, Ni, or Mo, providing some
inter-diffusion as early as the treatment phase, th e increase of the dimensional stability
due to the sinter process.
0 0,4 0,8 1,2 1,6 2,0
Alloying elements concentration [%]
C Mn
Ni
Mo
Cr 0,6
0,5
0,4
0,3
0,2
0,1
0 Compressibility reduction [g/cm 2]

N.V. CÂNDEA et al. 132
Steels obtained from partially alloyed powders thro ugh diffusion feature a higher
strength than the ones obtained using mixtures havi ng the same composition. Strength
in this latter case is subject to the carbon conten t (Fig. 2).
Fig. 2 Tensile strength variation depending on the Fe-4Ni-0.5Mo-1.5Cu steel carbon content obtained
from 1) powder mixture; 2) partially alloyed powder through diffusion with additions of alloying
elements

Table 1 and Table 2 show, by way of example, the mo st important chemical
compositions and engineering properties featured by partially alloyed powders through
diffusion. Such typical copper, nickel and molybden um powder alloys can be regarded
as being representative for partially allied powder s through diffusion.
Typically, Fe-1.75Ni-1.5Cu-0.5Mo alloys (Distaloy S A) are the most
recommended ones.

Table 1

Concentration [%] of the alloying elements Powder type Iron powder
used as a base
Cu Ni Mo
Distaloy SA SC 100.26 1.50 1,75 0.50
Distaloy AB ASC 100.29 1.50 1,75 0.50
Distaloy SE SC 100.29 1.50 4.00 0.50
Distaloy AE ASC 100.29 1.50 4.00 0.50
Distaloy DC-1* Astaloy Mo – 2.00 1.50
Distaloy DH-1* Astaloy Mo 2.00 – 1.50
Distaloy HP-1** Astaloy Mo 2.00 4.00 1.50
* DC – dimensional control
** HP – high performance

The same simple compacting – sinter process, tensil e strength can be increased to
values as high as 650-700 MPa in typical Fe-4Ni-1.5 Cu-0.5Mo steels obtained using
atomized ASC 100.29 powder as a base and increasing the Ni concentration from 1.75
to 4.0% (Distaloy AE) in a mixture with 0.5% C. Suc h powders can be used for
manufacturing large parts. In order to obtain Ni, C u and Mo alloy steels, pre-alloyed
powders can be used, just in the same way.

0,2 0,4 0,6 0.8
Carbon concentration 700
600
500
400 Tensile strength [MPa]

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 133
Table 2

Typical powder Maximum
particle size
[µm] Apparent
density
[g/cm 3] Fluidity
[s/50g] Density
at 600
MPa
[g/cm 3] O2
(loss in
hydrogen)
[%] C
[%]

Distaloy SA 20-150 2.8 27 7.15 0.10 <0.01
Distaloy AB 20-180 3.05 24 7.15 0.10 <0.01
Distaloy SE 20-150 2.8 27 6.90 0.10 <0.01
Distaloy AE 20-180 3.05 24 7.14 0.10 <0.01
Distaloy DH-1* 20-250 3.15 25 7.09 0.10 <0.01
Distaloy DC-1* 20-250 3.10 25 7.10 0.10 <0.01
Distaloy HP-1** 20-250 3.15 25 7.09 0.10 <0.01
* DC – dimensional control
** HP – high performance

In the pre-alloyed powder particles the alloy eleme nts are evenly distributed in
the solid solution or available under the shape of other phases. This is due to the fact
that they are obtained by spraying the already melt ed alloys. Compressibility in such
alloys is, however, somewhat lower. Such typical po wders are preferred no matter
their high costs, in cases where higher structural homogeneity is required for the
material in order to obtain higher properties. Tabl e 3 shows the chemical compositions
of some typically representative, slightly pre-allo yed powders to be used in sintered
steels for constructions. It has been demonstrated that oil sprayed powders feature the
best quality for a low oxygen content. The apparent oxygen content of these powders
lies within 3.0 – 3.1 g/cm 3 while the particle size in this case is lower than 0.15 mm.

Table 3

Concentration of the alloy
elements [%] Typical
powder O2
(loss in
hydrogen) C Ni Mo Cr Mn Spraying
environment Producer
Astaloy A 0.1 0.05 1.90 0.50 0.08 0.25 water Höganäs
Astaloy D 0.15 0.10 0.25 0.50 0.18 0.35 water Höganäs
Sumiron
4600 0.1 0.05 1.90 0.50 – 0.15 oil Sumitomo
HF 4 0.2 0.02 1.90 0.55 – 0.25 water Mannesmann

The highest raw strength is attained in compacts ma de of oil sprayed powders
(Sumiron 4600), due to the optimum morphology of th e particle. The oxygen content
of water sprayed Ni-Mo powders subsequently submitt ed to H 2 reduction treatments
(Astaloy A, Astaloy D, HF 4) does not exceed 0.2% a nd that of the oil sprayed and
treated powder does not exceed 0.1%.

N.V. CÂNDEA et al. 134
The compressibility of such pre-alloyed powders is subject to the chemical
composition and to the purity. The relevant values for the qualities given in Table 3,
are shown in Fig. 3.
Fig. 3 Raw density variation in compacts obtained u sing different pre-alloyed iron powders according
to the compacting pressure

The highest compressibility is featured by the Sumi ron 4600 powder as a result
of the oil spraying operation and the subsequent hy drogen reduction treatment. The
lowest compressibility is shown by water sprayed HF 4 which, therefore, has a high
oxygen content. Table 4 compares the mechanical pro perties of the sintered steel and,
respectively, of the sintered and forged steel of t he Fe2Ni-0.5Mo type including 0.3
and 0,8% graphite obtained using a mixture of iron powders with a powder made up of
the alloy elements and, respectively, of pre-alloye d powder. Mention must be done
that the considered mechanical properties in steels obtained using powder mixtures are
higher in both conditions as compared to those obta ined using pre-alloyed powder .

Table 4

Typical
powder C
[%] Density
[g/cm 3] Tensile
strength
[MPa] HB hardness Conditions
A1 0.15 6.55 330 72
A2 0.57 6.55 400 137
B1 0.52 6.58 250 85
B2 0.60 6.59 330 128 sinter at
1200°C for
30 minutes in
dissociated
ammonia
A1 0.17 7.70 750 141
A2 0.54 7.68 1120 211
B1 0.14 7.70 675 156
B2 0.47 7.72 925 159 forging
300 400 500 600 700
Compacting pressure [MPa]
Raw density [g/cm 3 7,2
7,0
6,8
6,6
6,4
6,2
6,0
5,8 4
1
2 3 1- Astaloy A
2- Astaloy D
3- HF 4
4- Sumiron 4110

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 135
In Table 4 the mechanical properties in typical Fe- 2Ni-0.5Mo sintered steels
were obtained using A – powder mixture; B – Astaloy A pre-alloyed powder with
additions of: 0.3% graphite – 1 and 0.8% graphite – 2.
Another method to obtain high strength alloy steels is to add to the pre-alloyed
Ni and Mo powder another alloy element (Cu) in an e lementary form. The effect of the
copper addition to two types of pre-alloyed powders as to the mechanical properties
under sinter and forging conditions are shown in Ta ble 5. Copper enables that the
system be sintered in the presence of the liquid ph ase. The effect of the copper
addition as to the mechanical properties featured b y sintered steels (6.5% density).
Graphite addition: 0.3%. sintered at 1,200°C, 30 mi nutes. Atmosphere: dissociated
ammonia; forging at 1,060°C in table 5.

Table 5

Typical
powder C
[%] Tensile
strength
[MPa] Elongation
[%] HV 10
hardness
Conditions
1.00 314 2.6 104
1.50 325 3.3 118 A*
2.00 312 2.3 121
1.50 256 3.5 87
2.00 294 3.0 87 D**
2.50 308 2.8 95 sinter at
1200°C for
30 minutes in
dissociated
ammonia
1.00 765 16.5 204
1.50 882 11.9 215 A*
2.00 903 12.5 222
1.50 650 20.5 221
2.00 685 17.3 227 D**
2.50 708 16.7 247 forging

Conclusions

The compressibility of such pre-alloyed powders is subject to the chemical
composition and to the purity.
The considered mechanical properties in steels obta ined using powder mixtures
are higher in both conditions as compared to those obtained using pre-alloyed powder.
By adding Cu in an elementary form, to pre-alloyed Ni and Mo powder, high
strength alloy steels were obtained. The effect of the copper addition to two types of
pre-alloyed powders as to the mechanical properties under sinter and forging
conditions were shown. Copper enables that the syst em be sintered in the presence of
the liquid phase.

Received April 26, 2007

N.V. CÂNDEA et al. 136
REFERENCES

1. Popescu, R.: Tehnologia materialelor (Technology of materials), Bucharest, Editura Lux Libris,
2000
2. Brăgaru, A., and others: Optimizarea proceselor și echipamentelor tehnologic e (Optimization of
engineering processes and equipment, Bucharest, Edi tura Didactică și Pedagogică, 1996

CERCETĂRI ASUPRA PROPRIETĂȚILOR MECANICE ALE OȚELUR ILOR SINTERIZATE CU
NICHEL ȘI MOLIBDEN

Rezumat : Această lucrare descrie metalurgia oțelurilor sla b aliate, obținute din pulberi care conțin Ni și Mo ,
pentru a produce elemente sinterizate de înaltă rez istență mecanică.

BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI
Tomul LIII (LVII), Fasc. 1, 2007
Secția
ȘTIINȚA ȘI INGINERIA MATERIALELOR

THE FISSURING MECHANISMS IN WELDED JOINTS

BY

N.V. CÂNDEA, R. M. POPESCU and D. SENCHETRU

Abstract: This work describes the looseness gearing of the we lded conjunction due to thermal and structural
stresses, by expansion, contraction, and phase tran sformations, that influences the technological and exploitation
characteristics of metallic materials.

Keywords : materials, welding

When joining parts through welding, internal stress es might occur in the resulting
products to be balanced within their volume, fully or partially remaining there as
residual stresses. Stresses incurred by the welding process may be residual stresses and
reaction stresses. The presence of the residual str esses sensibly influences product
behaviour during the subsequent processing operatio ns and during exploitation.
According to the cause which gives rise to them, in ternal stresses are of two
kinds: thermal, i.e. due to the non-uniformity of the dilatations and the contractions
further to the non-simultaneity of heating and cool ing in different micro-volumes of
the metallic body, and structural , i.e. due to the unevenness of the dilatations and
contractions accompanying the phase transformations subject to the non-simultaneous
heating and cooling in different micro-volumes of t he metallic body.
The presence of the internal stresses sensibly infl uences the technological and
operational characteristics of metallic materials, that is the metal products behaviour
during the processing operations in service or even during storage.
From among the consequences that result in damages due to the presence of
internal stresses in metal products, the most impor tant ones are as follows:
1. alteration in shape and size further to subseque nt processing and operation or
during storage, in certain parts of the product, ch anges in the stress values which result
in a disturbance of the forces and moments equilibr ium and thus, in shape and size
changes.
2. failure at much lower loads that the ones which would normally be withstood
by the product in the absence of internal stresses. As these add up to the resulting
stresses with the external loads, they may be incre ased or decreased. It is well known
that if external tensile stresses overlap the reman ent tensile stresses, cracks or failure
can occur.
Internal stresses sensibly affect product behaviour under the chemical action of
the environment combined or not with a mechanical a ction.
As to the extent and the distribution of remanent s tresses, beyond uneven
deformations due to the changes in the material vol ume while cooling, a certain

N.V. CÂNDEA et al. 138
influence is also due to the volume changes occurri ng under the austenite
decomposition temperature.
With different steel qualities, changes are subject to the carbon concentration and
to the alloying elements. [1].
On grounds of the data shown in Table 1, assessment s can be made as to the role
of the composition on the occurrence of remanent st resses in welded joints.

Table 1

Steel quality Austenite
transfor-
mation start
temperature
[°C] Internal
deformation
before
transfor-
mation starts
[%] Stress before
transfor-
mation starts
[daN/mm 2] Temperature
interval of
compression
stress
availability
[°C] Internal
deformation
after cooling
down to
200°C
[%] Stresses after
cooling down
to 200°C
[daN/mm 2]
OL 370 680 +0.78 +1.7 670-550 0.33 +29
OLT 45K
OLC 20 670 +0.72 +1.5 630-480 +0.30 +22
40Cr10 430 +1.4 +4.0 430-220 -0.22 +10
10TiNiCr180 – + + none +1.65 +24

In cross-section, heating is strong on the welding side and less strong on the
opposite side. Over the entire length of the weld, heating is maximum next to the
electric arc; in front of the arc components are co ld and behind the arc they are under
cooling process. Another source of stress is repres ented by possible expansions or
contractions in welded structures.
The total amount of stress in welded parts is zero. The extent of the remanent
stress further to this and to the deformations is s ubject to the heat quantity delivered
during the unit time. This can vary much according to the applied welding procedure.
Thus, at electric arc welding 50% lower heat quanti ty is transmitted during the unit
time than in gas welding. The greater the melted me tal volume the greater the
contraction during the solidification period. The r espective stress will occur in the
same direction as the contractions, changing the st eel structure and resulting in
additional stress. Intense cooling of the welded ar ea (or if using addition materials that
can be welded in the air), welding structures such as bainite or martensite can build up.
Alongside with these transformations a volume incre ase takes place accompanied by
an internal stress generation.
Hydrogen penetration is intense both in the melted metal and in the basic
material heated above 994°K which diffuses from the wet of the electrodes sheath. On
cooling, as soon as structural transformations are completed, hydrogen, which is less
soluble in ferrite and pearlite, agglomerates in th e imperfection areas of the crystal
lattice, thus giving rise to high local stresses. T herefore, it is necessary to observe the
electrodes drying recommendations as well as to cle an the joints, removing any trace
of grease, rust, etc.
Welding under a 5°C temperature shall be avoided or performed by protecting
the welding spot since steel contraction takes place more rapidly t hus shortening t 8/5
and there is the risk to obtain welds with welding constituents. The great thickness
featured by the parts to be welded results in high remanent stresses since inn addition

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 139
to the longitudinal and transversal stresses, therm al stresses occur over the weld
thickness, as well. In this way spatial stresses oc cur, which are highly dangerous in
terms of the welded construction security.
With steels showing no phase transformations within the welding temperature
range, the arising issues are the grain size increa se, carbide precipitations, and, in
steels including much chromium, the occurrence of t he σF phase [2] in the area
influences by the heat. At high temperatures, some alloying elements such as
chromium, build up carbides and get deposited in in ter-grain layers making the steel to
become fragile. In order to avoid this phenomenon, a rapid cooling is performed
within the 500°C – 700°C range, by alloying with ta ntalum, niobium or titanium as
well as by reducing the carbon content of steel.
The σ phase is an iron carbide at a 50% ratio each. It is hard and brittle. At the
beginning it precipitates into inter-grain layers b ut as soon as it covers the layers, it
occurs inside the austenite grains. Consequently, s teel hardness is increased while its
plasticity decreases.
The explanation of the structural fragility mechani sm of the heat affected zone
(HAZ), in weldable carbon and low alloy steels, sha ll also take into account the
specific undulations imposed by the heat cycle on w elding. Thus in HAZ favourable
conditions are created for the occurrence of hard a cicular welding structures, of high
internal stresses and, finally, of micro-fissures a nd cracks.
Specific to such fissures is the fact that in the m echanism of their formation the
main role is played by the concentration of the int ernal stresses, due to the
countersinking phenomenon. This phenomenon is characterized by th e crowding of the
force lines in the vicinity of some discontinuities in the material or is due to other
causes of geometrical nature. Thus, the countersink ing phenomenon results in the
transition of the material from the tenacious into a fragile condition.
As far as the countersinking phenomenon is concerne d, this is due to the tensile
stress level, required both for the development of the fissuring germs, and for the
fissure propagation up to breaking (Griffith – Orow oan).
The fissures built up in the HAZ can be explained b y the micro-model of
dislocation agglomerations at the intersection of t wo sliding planes (Fig. 1) provoked
by the deviation of the force lines next to the int ernal stress concentrators. [3].
The essence of the dislocation agglomeration mechan ism resides in the idea that
the sliding provoked by the movement of the disloca tions from one source (Frank –
Read source) is blocked by certain obstacles, such as, for example, grain boundaries.
Internal stresses around the group head might reach the value of the steel cohesion
resistance which, under certain load conditions, ca n result in local creeping
phenomena and, implicitly, to the occurrence of mic ro-fissures. Then this micro-
fissure propagates provided that Griffith's conditi on is met. Have not produced any
prior sliding process whatsoever, since due to such processes stresses get relaxed. Any
micro-fissure can propagate if the energy released thereby is at least equal to the one
delivered from outside and if due to such increase and to the excess of released energy,
the fissure propagation process is continued.
There are several methods for reducing the level of residual stresses in welded
joints, which might result in fragility – fissures, but the most widely known is the
application of a stress relieving heat treatment (T TD) after welding.

N.V. CÂNDEA et al. 140
Fig. 1 Fissure formation due to the dislocations ag glomeration mechanism according to Cottrell's
theory

Such treatments feature multiple metallurgical impl ications, such as:
/square4 restoring deformed crystal lattices;
/square4 rearranging dislocations, carbide coalescence;
/square4 providing some finely dispersed precipitates in the basic mass, as established
during the TTD;
/square4 blocking the sliding phenomena and the grain bounda ries by a fine dispersion of
stable particles (this dispersion is necessary for avoiding some uncontrolled
structural transformations in cases where discontin uous precipitates occur, as
well.
The above implications influence favourably or not certain mechanical
characteristics (R m, R p02, A, Z, R cor , KV, etc.) and structural features (shape and
distribution of carbides, typical structures, etc.) , onto which the welded structures
behaviour in operation depends.
However, under these sophisticated circumstances, s tress relieving heat
treatments shall provide for the optimum reduction of the residual stress level, the
restoration of the ductility in areas that got frag ile, and the minimum degradation of
the mechanical characteristics of the basic aim, un affected by the thermal cycles on
welding.
Beyond the effects that are favourable from the vie wpoint of the reduction or the
avoidance of fragile or inter-grain breaking, of di mensional stability or of corrosion
under tension, the application of the stress reliev ing heat treatment after welding might
result under certain circumstances in the degradati on of the thermally influenced area,
further to the fragility – fissuring phenomena.
Research carried out for the elucidation of damages in welded structures made of
non-alloy (carbon) steels and alloy (low Cr-Mo allo y) steels submitted to stress
relieving heat treatments highlighted the presence of some fine fissures, featuring a
reduced (as low as 225µm) size, which are not notic eable by macroscopic analysis nor
by X-rays.

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 141
Conclusions

Thus, in HAZ favourable conditions are created for the occurrence of hard
acicular welding structures, of high internal stres s and finally of micro-fissures and
cracks.
The above implications influence favourably or not certain mechanical
characteristics (R m, R p02 , A, Z, R cor , KV, etc.) and structural features (shape and
distribution of carbides, typical structures, etc.) , onto which the welded structures
behaviour in operation depends.
For reducing the level of residual tensions in weld ed joints, which might result in
fragility – fissures, there are several methods, bu t the most widely known is the
application of a stress relieving heat treatment (T TD) after welding.

Received April 26, 2007

REFERENCES

1. Zgură, G., and others, Tehnologia sudării prin topire (Welding engineering by melting) , Bucharest,
Ed. Facla, 1983
2. Voicu, S., Controlul îmbinărilor și produselor sudate (Control of welded joints and products) ,
Timișoara, Ed. Facla, 1984
3. Pascu, D. R., Influența factorilor structurali asupra caracteristi cilor tehnologice și proprietăților
mecanice ale oțelurilor slab aliate pentru construc ții sudate supuse tratamentelor termice post-sudare
(Structural factor influences onto the engineering characteristics and mechanical properties in low
alloy steels for welde dconstructions submitted to heat treatments after welding) , Doctorate thesis,
Bucharest, Institutul Politehnice, 1982

MECANISME DE FISURARE ALE ÎMBINĂRILOR SUDATE

Rezumat : Această lucrare descrie mecanismul de creare a jo cului în îmbinările sudate din caza tensiunilor
termice și structurale, prin dilatare, contracție ș i transformări de fază, care influențează caracteri sticile
tehnologice și de exploatare ale materialelor metal ice

BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI
Tomul LIII (LVII), Fasc. 1, 2007
Secția
ȘTIINȚA ȘI INGINERIA MATERIALELOR

METHODS FOR DETERMINING THE RELIABILITY OF SERVICE PIPES
FOR TRANSPORTING CINDER IN POWER PLANTS

BY

N.V. CÂNDEA, R. M. POPESCU and D. SENCHETRU

Abstract: This work describes researches concerning the relia bility of steel pipes, used for transporting cinder
derived from thermo-electrical power stations and t he problems that intervene of their consequent acti on.

Keywords : materials, reliability

Reliability represents the totality of features pro viding for the proper operation of
a product in keeping with the prescribed norms, eve n beyond the warranty period.
Reliability is defined as:
a) the entirety of the qualitative characteristics featured by a technical system
which establishes the capacity thereof to be used u nder given conditions for the
longest possible while according to its intended us e;
b) a measure characterizing the safe operation of a technical system;
c) a measure of the probability of the safe operati on of a technical system in
keeping with the prescribed norms.
Reliability means “quality in time”, the concept of reliability being closely
related to the one referring to breakdown.
Breakdowns (failures) represent ceasing of the apti tude of a device to meet its
intended use.
Also, reliability may be defined as being the aptit ude of a device to meet its
intended use under given circumstances and over a p re-set period of time.
Another component part of the concept of reliabilit y is the ambient environment,
the actual environmental conditions under which the product is operated. Environment
strongly influences product performances as well as the actual life time / period of
utilization thereof.
Should the product be utilized under environmental conditions other than those
taken into account during its design and manufactur ing phase, its operation might be
seriously disturbed.
Reliability is a basic characteristic of any techni cal system or element, strictly
depending on the quality of the given product or as sembly, on the sale-purchase costs
thereof, and, since it defines the behaviour during the entire lifetime of such technical
systems or elements, also on the actual operating c osts of the technological
engineering. The general expression of correlation is given in Fig. 1.
In order to assess the reliability parameters the p rocedure to be used is pursuing
the piping behaviour throughout its operation. Let's suppose that the piping is used

N.V. CÂNDEA et al. 144
after a prior run-in (removal of the initial breakd owns/ failures) and before wear
occurs (defects due to ageing), i.e. that the produ ct is in its useful lifetime.
Fig. 1 Depreciation of reliability from its design phase to its operation

Statistical methods of reliability calculus on grou nds of failure intensities (λ)
feature a widespread, requiring, however, a longer period of time for the collection of
statistic data. Probability methods are realistic a nd result in rational manufacturing
output since they remove the shortcomings of the de terminist methods introducing the
value dispersion of input and output measures about the rated values, thus each and
every input measure can be weighed against the outp ut value.
Table 1 shows the lifetime of 10 ducts included in this study.

Table 1

Pipe no. Operating time [h] Pipe no. Operating time [h]
1 228694 6 270605
2 242302 7 258605
3 242408 8 261308
4 258605 9 235986
5 264332 10 239794

The average operating time is:
2 ,2735764412037353
Nti
tn
1i= =∑
== (1)
The main square deviation of the operating time is calculated using:
Ideal reliability lev el (social control)
Reliability level revealed by the market
Reliability level of the concept (potential)
Acceptable implemented reliability level
Marketin gCreation Manufacturing Utilization Utilization A
B
C
D
EF
GHM N
Depreciation of reliability
during the operation due to a
slight wear under proper
maintenance conditions
Depreciation of
reliability due to
wear (under
improper
maintenance
conditions)
Repair

Repair

Level of
reliability in
operation
(without
preventive Level of reliability
in operation (with
periodical revisions
and maintenance)

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 145
7 ,32776Nti
ti1N1
1Ntti
sN
1i2N
1i2N
1i= ∑∑
=∑
=
== = (2)
The calculation algorithm is initiated at a t= 250, 500 hours and a c = 275,400
hours value. The average operating time, in another calculation version, results in:
6 ,296863cNkiti
atN
1i1
=+∑
== (3)
The main square deviation of the operating time is determined using:
( ) ( )( )
32802,5Nkiti
1N1kiti1N1asN
1iN
1i21
12
= ∑ ∑ =
= = (4)
Piping in steam power plants shall operate without failures whatsoever during a
certain time interval, usually for a long while, un der given operating conditions. In the
power supply industry, sizing relies on codes and c alculation norms. Establishing
permissible theoretical strength values is an impor tant issue. Some norms confine
themselves only to showing the way how permissible strength values are calculated,
while others provide actual values for each accepta ble material. Both ways, when
establishing permissible strength values, the start ing point is to find values that are
guaranteed by norms for the strength characteristic s of materials. All these
characteristics are static measures. Piping element s in steam power plants feature an
acceptable breaking probability even in their desig n phase. The breaking probability is
lower in the design phase and increases in time, wh ile the safety coefficient decreases
down to a minimum value established for the calcula ted operating time.
Metallurgical degradation is mainly due to the stru ctural deteriorations and
especially to the fluctuations of the precipitates as well as of the alloy elements in the
metal matrix. Structural degradations are difficult to be detected in operation from a
quantitative point of view. Such degradations resul t in inter-grain erosions by
cavitation, which determine the fragile rupture at high temperatures in steels.
Mechanical degradation takes place due to cavity an d micro-fissure forming processes,
which are more easily detectable parameters in maki ng estimates as far as the
operating time is concerned.
Micro-structural research works related to the stee ls covered by this study
highlight that further to a prolonged operation at normal parameters, irreversible
changes occur therein, ferrite grains increase in s ize, pearlite areas get dispersed,
carbides get precipitated and coalescence thereof t akes place. The incurred structural
modifications are the reason for changes in mechani cal characteristics. Thus, the
precipitation of some very fine carbides results at the beginning in increased
mechanical strengths, and the quantity increase in precipitated carbides on the limit in
between the grains as well as the coalescence there of result in a decrease of the
strength properties and ultimately, of the strength reserve. The mechanical
characteristics of the piping materials, as establi shed by static tests (R m, R p02 , δ 5, Z),

N.V. CÂNDEA et al. 146
show values that are greater than the ones prescrib ed for the material in delivery
condition. The same holds true for resilience, too, i.e. shows values greater than the
ones prescribed by the norms.
Studying the reliability curve is based upon the su rvival curve of the piping. An
operation to be carried out prior to establishing t he theoretical law is to remove from
among the multitude of collected data, the non-homo geneous ones, which, being gross
errors, are not included in the given repair.
Reliability, as a modern concept with technical, ec onomical and social
reverberations imposed by the industrial developmen t, succeeds to define and
characterize from a qualitative viewpoint the const ructive concept adopted by the
designer, the manufacturing method, the technical s ecurity in operation, the level of
acceptance and maintenance expenditures, as well as production losses due to the
frequency and the duration of breakdowns (failures) .
There is no 100% reliability nor technical security since practically one must take
into account the occurrence of the so called force major events. It is necessary to be
aware of the optimum reliability or, respectively, technical security levels, and also, of
the fact that about 95% of the piping failures are due to technological engineering
activities and only about 5% are due to the working personnel.
On grounds of the professional knowledge concerning the manifestation logics of
the piping behaviour in steam power plants, the hyp othesis concerning the theoretical
law of reliability is formulated to be followed by the experimental data. Non-
homogeneous data (gross errors) are removed on grou nds of the known tests. If
reliability is high, failure rates are low. The rel iability, the strength reserve as
compared to the yielding limit and the strength res erve as compared to the breaking
limit provide full information as far as steam powe r plant piping behaviour is
concerned.

Conclusions

The mechanical characteristics of the piping materi als as established by static
tests (R m, R p02 , δ 5, Z) show values that are greater than those prescr ibed for the
material in delivery condition. The same holds true for resilience, too, i.e. shows
values greater than the ones prescribed by the norm s.

Received April 26, 2007

REFERENCES

1. Dima, A., et al , Cuptoare și instalații de încălzire – elemente de î ncălzire și proiectare constructiv-
funcționale (Heating ovens and installations – heating element s and constructive – functional designs),
Iași, Ed. Gh. Asachi, 1993
2. Mitelea, I. et al , Selectarea și utilizarea materialelor metalice (Selecting and using metal materials),
Timișoara, Ed. Politehnica, 1998

METODE DE DETERMINARE A FIABILITĂȚII CONDUCTELOR DE TRANSPORT AL ZGUREI ÎN
CENTRALELE TERMICE

Rezumat : Această lucrare descrie cercetările referitoare l a fiabilitatea conductelor de oțel, utilizate pentr u
transportul zgurei provenite din centralele termice și a problemelor ce intervin din acțiunea lor.

BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI
Tomul LIII (LVII), Fasc. 1, 2007
Secția
ȘTIINȚA ȘI INGINERIA MATERIALELOR

THE INFLUENCE OF ALLOYING ELEMENTS ON STEELS USED I N
STEAM POWER PLANTS

BY

R. M. POPESCU, V.N. CÂNDEA and D. SENCHETRU

Abstract: This work describes the influence of alloying eleme nts Ni, Mn, Cr, Si, P, S and Mo on the structure
and mechanical properties of steel used in thermo p ower stations.

Keywords : metallurgy, materials

Steels used in steam power plants shall feature goo d mechanical properties,
stability and withstanding to corrosion, as well as heat conductivity, dilatation and
elasticity module of acceptable values.
Subject to the working pressure and the operating t emperature, either carbon
steels or especially Cr, Mo or Cr-Ni alloy steels a re used.
In cases where the working pressure does not exceed P max = 6 MPa and the
operating temperature T max = 400°C, OLT 35K and OLT 45K (STAS 8148-87) carbon
steels and 16Mo3, 14CrMo10, K410, K460, K510 (STAS 2883/3-88) alloy steels are
used. [1]
For a p min = 15 MPa working pressure and a T = 550°C operatin g temperature
ferrite and martensite steels are used. Such steels feature the following characteristics:
• a good creeping / yielding limit and fatigue streng th at the operating temperature;
• tenacity both at ambient temperatures and at high t emperatures;
• phase transformation spots occur at temperatures hi gher than the ones at which
they are used;
• withstanding corrosions due to chemical substances, and wears due to erosion
Austenite steels are used for piping operated at T = 800°C in cases of short
operating times and at a T = 700°C in cases of long operating times. There are alloy
steels including 14÷ 30% Cr and 8÷35% Ni and eleme nts such as Mo, W, Ti, Nb, V,
Cu, Si, Mn, N in a smaller quantity and a C content under 0.1%. Austenite structures
in these steels are hardened due t the precipitatio n of certain secondary phases
(carbides, carbonates, inter-metallic compounds).
The mechanical properties featured by steels used i n steam power plants at high
temperatures are influenced by the chemical composi tion, by micro-structures and
by the size of the grain. Breaking strength, creepi ng / yielding limits in these steels can
be improved by Mo, Ti and Nb additions.
In Cr-Ni Austenite steels carbides are instable in the 427÷870°C temperature
range, precipitated at the grain limits and decreas e the withstanding to corrosion,
however do not decrease the breaking strength or te nacity at ambient temperatures.

R. M. POPESCU et al. 148
Cr-Ni austenitic steels can undergo an ageing durin g operation, thus resulting in
fragility. This is decreased in cases where the ste el includes Ti or Al additions.

Ni influences .

Alloy steels including Ni are more expensive, since the Ni is obtainable at a price
of conjuncture. Ni alloy steels (Fig. 1) having a p earlitic structure, feature an
elongation and an entirely special resiliency as we ll as a high breaking strength.
Fig. 1 Structural diagram of Ni alloy steels

Martensitic steels (Fig. 2) feature a very high bre aking strength and limit of
elasticity. However, the maxim strain to failure is low and therefore these steels have
not much practical use, since they are fragile and difficult to be processed.
Fig. 2 Variation of mechanical properties according to the Ni content

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 149
Austenitic steels feature a breaking strength and e lastic limit of low values, but
their elongation and specific constriction are high . Since nickel does not form carbides,
it has no influence on the steel hardness in anneal ed condition. Ni is associated with
elements which form carbides (Cr, W, Mo), thus obta ining steels with obviously
superior mechanical properties

Mn influences

.Manganese does not build special carbides, but a M n xC, which is totally
miscible in cementite. Mn alloy steels (Fig. 3) fea ture the same patterns as the Ni alloy
steels, but a smaller quantity is needed for changi ng the structures. A 0.2% C and 12%
Mn steel is austenitic, while the same steel in a N i alloy features a pearlitic structure.
Mn incurs a breaking strength increase in annealed condition.
Fig. 3 Variation of mechanical properties according to the Mn content

For a 055% carbon content and a proper heat treatme nt, Mn produces a R m, R p02
and HB increase and a decrease in elongation. Auste nitic Mn alloy steels feature a
pronounced tendency towards cold-hardening. Therefo re their tensioning curve is a
constantly ascending one. This high tendency toward s cold-hardening featured by
austenitic Mn alloy steels is the very reason why a re they so difficult to be processed.
Mn additions in small quantities result in signific ant hardening capacity increases.
Manganese does not confer good magnetic properties to steels

Chromium influences

. Low carbon and 18% Cr content steels are ferritic at any temperature. As the
carbon content increases, the steel can be austenit ized and therefore, also hardened.
Depending on the C and Cr content, steels can be pe arlitic (1.67% C and 0 ÷ 8% Cr),
martensitic (0 ÷ 1.66% C and 8 ÷ 18% Cr) which is c haracterized by its high hardness
values due to the stable carbides that are built up to confer to the steel a high
withstanding capacity to wear and qualities that ma ke it suitable for undergoing chip
removing processes. Hardening steels using Cr is a ccompanied by a decrease in
resiliency and elongation. In order to cancel this effect, Ni quantities are added.
The steel hardening capacity increases as Cr conten t increases:
– a Cr content of at most 1% favours steel hardening capacity;
– a Cr content of 1 ÷ 3% increases withstanding to hy drogen under pressure and
favours steel nitriding;
0,55%C
1 2 3 4 5 6 7 8 Mn % HB
Rp02
A5 Mechanical properties

R. M. POPESCU et al. 150
– as the Cr content increases, steels turn to be more withstanding to oxidization and
corrosion;
– Cr contents over 30% make steels refractory;
– Chromium increases hardness, withstanding capacity to wear, decreasing however
resiliency

Si influences

When smelting some steel qualities, Si is added to bind oxygen and reduce
ferrous oxide (FeO) formed during the refining proc ess. Should steels be rich in C, the
Si can form carbides, too, and when Si is present i n great quantities (over 3.5%), then
it can graphitize the steel. Since Si features a gr eat affinity to O 2, it forms silicate
oxydes (FeO) 2SiO 2, (MnO)SiO 2, which persist as inclusions and might result in fib rous
patterns during plastic deformations.
Si increases R r and R e with about 100 MPa for each 1% Si. Specific elonga tion is
slightly decreased when the content does not exceed 2.2% Si; also, Si increases the
strength to wear of the steel. When steels include Cr and Al, then the additions result
in an increase in terms of the withstanding capacit y to oxidation at high temperatures.

P influences

Phosphor can reduce weldability, favouring the occu rrence of fragile breaking.
Weldability of steels featuring a low (max. 0.17%) C content is not affected by the
presence of the phosphor.
Phosphor dissolved in ferrite results in increasing the capacity to withstand corrosion
in slightly corrosive atmosphere (industrial enviro nment in metallurgical plants,
glassware factories, building workshops). Thus, pho sphor is one of the main impurities
and brittles steel, so that its content is limited (to as low as 0.03÷0.06%).

S influences .

At high temperatures, sulphur produces brittleness and also, decreases steel
tenacity, so that the S content thereof is set to 0 .02% at most. Sulphur is an element
which is suitable for chip removing processes due t o the manganese sulphur which
acts as a lubricant. Sulphur does not influence ste el weldability unless participating in
great quantities, and it decreases corrosion withst anding, as well.

Mo influences

Molybdenum influences the increase of the mechanica l properties maintaining
the steel tenacity. At high temperatures, it builds up with C and the compounds thereof
Mo carbides (Mo 2C), which are fragile and very hard.
Introducing too much molybdenum induces in steels s ome tendency towards
fragility. The influence of the molybdenum is shown by very small contents, i.e. of as
low as 0.1% and progressively increases as this sma ll content increases. Usually,
molybdenum is added to steels in 0.4÷1% quantities, and only in cases of exception

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 151
this exceeds 2%. Molybdenum rises the re-crystalliz ation temperature and favours the
occurrence of stable carbides and some intermetalli c compounds thus resulting in a
breaking limit increase.
Pipes tubes in steam power plants are made of low a lloy steels according to
STAS -2883-88 and STAS 8184-87, in lamination sizes according to STAS-404-59.
Technical conditions for steam pipes shall meet the requirements given in the
STAS-2478-82 standard. For ducts operated at temper atures above 450°C, the
technical conditions shall be supplemented accordin gly, with the conditions set forth
for high temperatures, i.e. with the hot mechanical characteristics and with the micro-
structure analysis; for temperatures in excess of 5 50°C, these technical conditions shall
the supplemented with the inter-crystalline resista nce to corrosion. Romanian low
alloy steels shall meet the technical conditions se t forth in STAS-8184-87 and STAS
2883/3-88 (see Tables 1, 2). [1].

Table 1

Quality C [%] Si [%] Mn [%] Cr [%] Mo [%] Other elements
[%]
16Mo3 0.12÷0.2 0.15÷0.35 0.5÷0.8 – 0.25÷0.4 Al 0.01 ÷0.03
14MoCr10 0.1÷0.18 0.15÷0.35 0.4÷0.7 0.7÷1.1 0.4÷0.55 Al 0.01 5÷0.045
12MoCr22 0.08÷0.15 0.15÷0.5 0.4÷0.7 2÷2.5 0.9÷1.1 Al 0.015÷0.045

Table 2

Rc [MPa] Quality
200°C 300°C 400°C 450°C 500°C
16Mo3 225 180 160 155 150
14MoCr10 240 215 190 180 175
12MoCr22 245 230 205 195 185

At temperatures over 250°C, steel oxidizes so quick ly, that it gets covered with
magnetite (Fe 3O4) in a very short while under favorable conditions building up a
compact and adherent layer that acts as a protectio n layer for the metal.
The subsequent evolution of the corrosion is mainly defined by diffusion, since
iron and water cannot react in between themselves u nless and to the extent iron
diffuses through the protective layer. However, the diffusion process is very slow so
that there is no metal loss to be mentioned. Should the protection layer be destroyed, a
rapid oxidation takes place. When this phenomenon o ccurs repeatedly in the same
place, then the pipe is destructed. The magnetite layer built up by the overheated
steam is not free from strains, since its volume is twice bigger than the oxidized iron
quantity.
At temperatures over 570°C, Fig. 4, instead of magn etite (Fe 3O4), ferrous oxide
can be built up, which no longer features protectiv e properties. It builds up rapidly and
damages the pipe in a short while. The chemical rea ction that takes place with this type
of corrosion is:

R. M. POPESCU et al. 152
3Fe + 4H 2O → Fe 3O4 4H 2 (1)
Fig. 4: Corrosion variation according to temperatur e

Usually, the reaction starts at a 450°C steam tempe rature. Speed is significant for
the reaction progress: over 15 m/s no corrosion occ urs, at 4 m/s there occurs H 2 and at
0.12 m/s corrosions are severe. Steam decomposition and its reaction with the steel has
been studied by Chaudron. Steam action can be reduc ed almost entirely if a Fe 3O4
protective layer is provided before

4. Conclusions

For ducts operated at temperatures, the technical c onditions shall be
supplemented accordingly, with the conditions set f orth for high temperatures, i.e. with
the hot mechanical characteristics and with the mic ro-structure analysis; for
temperatures in excess, these technical conditions shall the supplemented with the
inter-crystalline resistance to corrosion. The subs equent evolution of the corrosion is
mainly defined by diffusion, since iron and water c annot react in between themselves
unless and to the extent iron diffuses through the protective layer.

Received April 26, 2007

REFERENCES

1. Dima, A., et al. , Cuptoare și instalații de încălzire – elemente de î ncălzire și proiectare constructive
-funcționale (Heating ovens and installations – heating element s and constructive – functional
designs), Iași, Ed. Gh. Asachi, 1993
2. Mitelea, I. and others, Selectarea și utilizarea materialelor metalice (Selecting and using metal
materials), Timișoara, Ed. Politehnica, 1998

INFLUENȚA ELEMENTELOR DE ALIERE ASUPRA OȚELURILOR U TILIZATE LA
COINDUCTELE CENTRALELOR TERMICE

Rezumat : Această lucrare descrie influența elementelor de aliere Ni, Mn, Cr, Si, P, S și Mo asupra structurii și
proprităților mecanice ale oțelurilor utilizate pen tru conductele din centralele termice.

BULETINUL INSTITUTULUI POLITEHNIC DIN IAȘI
Tomul LIII (LVII), Fasc. 1, 2007
Secția
ȘTIINȚA ȘI INGINERIA MATERIALELOR

THE BEHAVIOUR OF PLASTIC DEFORMATION SLIM BANDS
WITH HIGH MECHANICAL PROPERTIES

BY

R. M. POPESCU, V.N. CÂNDEA and D. SENCHETRU

Abstract: This work describes the technology for obtaining me tallic straps with high mechanical properties, and
the heat treatments used for reducing the level of plastic yielding and obtaining some reduced grain s ize.

Keywords: materials, plastic yielding

The behaviour during a plastic deformation of a met allic material as well as the
effect of the deformation in terms of the material structure and properties differ
depending on the ratio between the temperature at w hich the said deformation took
place as compared to a certain characteristic tempe rature known as re-crystallization
temperature. Cold plastic deformations incur in met allic materials three main types of
structural alterations:
• in the shape and the dimensions of the crystal grai n;
• in the spatial orientation of the crystal grains;
• a fine modification in the grain
As the extent of the cold deformation increases, gr ains get more and more
elongated in the direction of the lamination gettin g the aspect of a fiber, the
corresponding structure being named fibrous structu re or cold plastic deformation
fibering.
Along with the alteration of the crystal grain stru cture a change in their spatial
orientation by the rotation of the crystal lattice takes place, too. Thus certain
crystallographic elements are prone to be directed towards the direction of the
deformation. The structure of such materials is cal led texture. The deformation texture
mainly depends on the processing method. Cold plast ic deformations result in
significant alterations of the physical and mechani cal properties. Thus, the tensile
strength, yielding / creeping limit, hardness are i ncreased as the deformation degree
increases, while plasticity properties, just as the elongation and the constriction,
decreases. The material becomes harder, stronger, less plastic – and this phenomenon
is called hardening by deformation or cold-hardenin g. The cold-hardening capacity of
a material is characterized by the cold-hardening c oefficient. During the cold
deformation process alterations of the physical pro perties occur, as well: the electric
resistivity and the coercive field increase as the deformation degree increases. The
condition attained by cold plastic deformation is a condition featuring a higher internal
energy than the one of the non-deformed material. S o, cold-hardened materials feature
a spontaneous tendency to reach a condition of equi librium. Applying a thermal

R. M. POPESCU et al. 154
activation (heating), cold-hardened metallic materi als can get a condition of
equilibrium.
The heat treatment [1] i.e. heating the cold-harden ed material for getting into the
condition of equilibrium is called re-crystallizati on tempering. Subject to the
temperature and the heating duration, inside the co ld-hardened material there take
place the following phenomena: restoring or re-esta blishing, re-crystallization and the
increase of the grains.
Restoration involves two stages: expansion or detent and polyg onizing.
Expansion is the phase where physical properties ar e restored without any
possibility whatsoever to notice changes in the col d-deformed metal micro-structure
During restoration, electric conductibility increas es rapidly, heading towards a value
corresponding to the annealed material. Restoration takes place at temperatures under
the re-crystallization temperature, since in this p hase there take place the diffusion
process of the punctual defects, the annihilation o f some punctual defects and of some
dislocations featuring a contrary sign, located on the same sliding plan.
In the final restoration phase, in the vicinity of the re-crystallization temperature,
dislocation structures are re-arranged into configu rations of equilibrium.
The actual re-crystallization or primary re-crystallization is the process throu gh
which cold plastic deformation structures are repla ced by a new generation of non-
deformed grains. Actual re-crystallization takes pl aces at a temperature above the re-
crystallization temperature and is achieved through a germination and increase
process. The motive force of the re-crystallization process is the elastic deformation
energy of the crystal lattice.
Grain increase represents the last phase of the re-crystallizatio n process and
takes place as the temperature rises (beyond the re -crystallization temperature) and of
the temperature hold period. Grain increase is dete rmined by the high superficial
energy featured by the small and uneven grains obta ined by re-crystallization.
Mainly, re-crystallization is influenced by the fol lowing factors: the size of the
deformation degree, the deformation temperature, th e initial dimension of the grain,
the chemical composition, and, respectively, the pu rity degree of the material.
The starting re-crystallization temperature decreas es as the deformation degree
increases up to a value known as re-crystallization threshold. Temperature, as a factor,
is more important than the time factor. Doubling th e annealing time is approximately
equivalent to a 10°C increase in terms of the annea ling temperature. During the re-
crystallization process, all properties are changed reversely to the variation of the cold
plastic deformation.
The continuous increase in of the user requirements and exigencies as far as
product quality is concerned, as well as the high t echnical level attained due to the
upgrading works carried out in production units, re sulted in the accomplishment of a
new and efficient system for steel bands lamination process monitoring. Such
upgrading works involve the following:
1.- Utilization of high-performance control and mea suring equipment [2], band
speed measuring device using the laser technology, band profile measuring device
using a measuring roll provided with piezoelectric sensors, numeric systems intended
for the adjustment of the working cylinders speed, high-performance hydraulic
systems for the cylinders displacement, high-accura cy apparatuses for measuring the

Bul. Inst. Polit. Iași, t. LIII (LVII), f. 1, 2007 155
band thickness deviations, etc.
2. -Using high-performance hierarchic calculation s ystems to provide for the
management of the lamination process according to d ifferent priority levels;
level 0 , accomplished by the instrumentality of micro-cont rollers intended
for the management of the electric actuating system s;
level 1 , represented by automatic systems intended for the adjustment of the
band thickness and profile;
level 2 , represented by artificial intelligence (neuro-fuz zy) interfaces for
modelling and general management purposes in the la mination process.
Up-to-date industrial management systems are mainly based on the experience
gained by the human operators. Laminating tensions, and especially the tension
distribution subsequently applied to the band, affe ct both the smoothness thereof and
the control of the band shape according to its widt h in cold lamination processes. As
the band smoothness decreases during lamination, an uneven subsequent tension
distribution builds up in the deformation area of t he band. The presence of the
subsequent tension influences the transversal yield in metals, improving the band
smoothness.
Nickel/iron alloys featuring high nickel contents, show special magnetic, thermal
or elastic properties. Nickel strongly influences t he linear expansion coefficient in
iron; the Fe-Ni alloys expansion coefficient passin g through a value near zero with
36% Ni. Changing the nickel content in iron-carbon alloys, steels featuring the
expansion coefficient of some materials can be achi eved, so that they can be welded
with those very materials. Thus, 40% Ni steels have the same expansion coefficient as
porcelain, 0.2%C and 46% Ni have te same expansion coefficient as platinum and
glass. Nickel exerts an especially strong influence on the magnetic properties of the
iron. The initial magnetic permeability (µ a) highly increases with 78.5% Ni. Cold
plastic deformation results in a reduction in terms of the magnetic permeability.
However, if followed by a crystallization annealing , it results in the formation of a
crystal texture that improves the magnetic permeabi lity.
The applied subsequent tension influences smoothnes s in cold laminated bands
by the intensity of the lateral creeping. The value of the subsequent tension is the one
to [provide for and make even the band smoothness. It is mandatory to take this into
account in calculi accompanying smoothness control band laminations. Entering the
shape altering coefficient (α j), the evenness of the subsequent tension applied t o the
laminated band can be quantitatively assessed.
Roll mills are up-graded according to the following directions: increasing the
cylinder strength (reduction of wear and of the spe cific consumption), reducing the roll
mill rotary parts as well as the weight thereof, in creasing the laminating accuracy and
improving the laminate quality; automation of the e ngineering processes and reducing
the power consumption used in the manufacturing pro cess. Roll mills will further be
developed as lamination units according to the incr ease of the customers' exigencies,
with the possibility to directly increase the refer enced characteristics. Such exigencies
shall especially result in reduced thicknesses, in combination with an increase in terms
of the mechanical characteristics. Nowadays, lamina tion is no longer a simple method
to get the final geometry of the product in the met allurgical industry, since managing
the lamination process in a certain way, it becomes possible to obtain a given structure

R. M. POPESCU et al. 156
to endow laminated products with the desired mechan ical characteristics. When
choosing the technological characteristics for lami nation purposes, there shall be taken
into account of the influence exerted thereby onto the structural transformations
occurring during deformation (re-crystallization, p hase transformation). Reviewing the
measures playing a role of influence and on which m aterial properties depend,
technological measures can be defined for obtaining steel aspect and structure. Up-to-
date roll mills enable now to meet close tolerances and, in addition, up-to-date
lamination engineering enables the exertion of cert ain influences on the material
properties, as well. Modern roll stands feature hig h laminating forces and high-
performance measuring and adjusting installations.
Moreover, in addition to high productivity rates, r ecent band roll mills are
provided with the most up-to-date improvements for accomplishing high quality
products. A special importance is given to providin g close tolerances for thicknesses
over the width and the length of the band. Therefor e, the most recent roll stands
feature a very high rigidity (given by the big diam eter of the supporting cylinders and
the great weight of the frames), an automatic thick ness control (gage-meter AGC)
function to be operated during lamination, cylinder counter-curving devices and
thickness measuring installations for providing the most suitable transversal profiles in
bands.

4. Conclusions

The temperature and the heating duration, inside th e cold-hardened material there
take place the following phenomena: restoring or re -establishing, re-crystallization and
the increase of the grains. Restoration involves tw o stages: expansion or detent and
polygonizing.
Primary re-crystallization is the process through w hich cold plastic deformation
structures are replaced by a new generation of non- deformed grains. Grain increase
represents the last phase of the re-crystallization process and takes place as the
temperature rises (beyond the re-crystallization te mperature) and of the temperature
hold period. The continuous increase in of the user requirements and exigencies as far
as product quality is concerned, as well as the hig h technical level attained due to the
upgrading works carried out in production units, re sulted in the accomplishment of a
new and efficient system for steel bands lamination process monitoring.

Received April 26, 2007

REFERENCES

1. Cazimirovici, E.: Teoria deformării plastice (Theory of plastic defor mations) , Bucharest, Editura
Academiei, 1995
2 Crivineanu, C.: Referat teză de doctorat: Aliaje Fe – Ni și obținerea acestora (Doctorate thesis
reference: Fe-Ni alloys and production thereof

COMPORTAREA BENZILOR SUBȚIRI, CU PROPRIETĂȚI MECANI CE RIDICATE, OBȚINUTE
PRIN DEFORMARE PĂLASTICĂ

Rezumat : Această lucrare descrie tehnologia de obținere a benzilor metalice, cu proprități mecanice ridicate,
tratamentele termice folosite pentru a reduce nivel ul curgerii plastice și a obține o granulație scăzu tă.