A New Systemic Study Regarding [602265]

10
A New Systemic Study Regarding
the Behaviour of Some Alloy Steels
During Low Cycles Fatigue Process
Macuta Silviu
Dunarea de Jos University of Galati
Romania
1. Introduction
The research of the metallic materials used in machine manufacturing to which high stress
and a small number of cy cles is applied have been increasing ly gained interest in the last 40
years; this is because during cyclical stress at critical points in terms of resistance repeated
strain occurs in many major constructions.
Fatigue breaking to a small number of cycles and high strains is encountered in the
operation of various types of machines: power ma chine, elements of heating boilers or heat
exchangers in electro nuclear industry, pressure vessels, st eam and hydro turbines, turbo
compressors, the plane landing trains an d other transport means and mechanisms.
The process of cracking and breaking by fatigu e is characterized by different mechanisms at
different levels of strain. Thus Wöhler's curve can be studied in 3 areas:
The first area called the quasi-static field, where N=0 104 cycles, when ther e are large plastic
strains;
The second area called the area of limited durability, where N=10410106 cycles, breaking in
this situation implies elastoplastic strain and calculations will be carried out to limited
durability.
The third area – the area of unlimited durability, or the fatigue resistance range, when
N>10106 cycles. Breaking is characte rized by elastic strain only.
Regarding the first area, a more careful resear ch led to the conclusion that there are two
distinct zones:
The quasi-static area itself, for N <103 cycles – area where high st rains close to the material
yielding point occur, where breaking is characterized by large plastic strain, similar to static
fracture.
The low-cycle fatigue zone also called the low cycle fatigue where N=5 102 104 cycles, where
this time breaking is characterized by elastoplas tic strain. A deeper insi ght into this area has
revealed the existence of anomalies (discontin uities) of the fatigue curve. The problem was
first studied by R. Moore in 1923, then by Sabolin, Finney and Mann .
www.intechopen.com

Alloy Steel – Properties and Use
238
The phenomenon of fatigue to a small number of cycles ha s three specific features.
1. High level of strains
2. Low testing time (103-104, max. 105 cycles)
3. Reduced testing frequency( up to50 cycles/min)
Investigating the low cycle fatigue damage, it can be distinguished between a rigid and a
soft loading regime.
The studies carried out have a general charac ter, the fatigue behavi our being determined
from the fatigue curves, the curv es of mechanical hysteresis and the cyclic cold-hardening
curves. Based on the investigations, a set of rules and criteria to predict the material
behaviour to low cycle strains has been established.
2. A new systemic approach of the fatigued surface layer behavior
The concept of a structural cybernetic pattern has been introduced in order to obtain an as
complete as possible approach of the surface layer behavior and also with a view to
analyzing and emphasizing the main factors and parameters which determine the fatigue
process. This concept, introduced by professo r I. Crudu from Dunarea de Jos University of
Galati-Romania in order to characterize a trib osystem, was extended to the characterization
of the fatigued surface layer and it represents a totally new approach of the fatigue process.
In this chapter a new research methodology has been developed along with a way to
approach the fatigue behaviour of the surface layer to high strains and small number of
cycles by extending the concept of structural cybernetic tribo-system.
For the research purpose, special equipment was needed consisting of a patented Universal
Machine for fatigue testings and related facilities. The development of certain surface layer
parameters was monitored (1st and 2nd order strains, dislocation density, crystalline lattice
parameter, texture, microstructure, and micr o-hardness of the surface layer) during the
fatigue process according to the control paramete r (number cycles, strain, frequency, type of
material required).
In order to have an as extensively as possible approach to the surface layer behavior and to
track and highlight the main factors and paramete rs determining the process of destruction by
fatigue, the concept of structural cybernetic model was introduced. This concept was introduced
to characterize a tribosystem and allowed its ex tension to the characterization of the surface
layer of the material subject to fatigue, which is an entirely new approach to fatigue processes.
Fig. 1 shows a structural cybernetic model by means of which the changes of the input
parameters can be systematically monitored by measuring the output parameters of the
surface layer undergoing the action of destruction by fatigue.
The input-output parameters of the cybernetic model include the parameters of the surface
layer ((S s-S’s) and the control parameters (U). Surfac e layer parameters can be grouped into:
geometric parameters (macro-geometry and micro-geometry- X 1- X’1)
mechanical parameters (hardness and micro hardness – X 2 -X’ 2 and the strain state – X 3- X’ 3);
physical and metallurgical parameters (chemical composition – X 4- X’ 4, structure – X 5- X’ 5,
purity – X 6- X’ 6)..
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
239
Some of these parameters, such as micro hardness – X 2, strain – and structure X 3 – X 5, may be
modified from outside so that the life time of a material under fatigue process can be
modified within certain limits, as desired.
The control parameters (U) also called external factors are those parameters which by their
action on the functioning of the fatigue testing machine may modify some parameters of the
surface layer of the material the test-piece is made of. The control parameters can be
grouped into:
Constructive parameters (nature of the material – U 1, test-piece shape – U 2, the test-piece
dimensions – U 3);
Operating parameters (working environment – U4, cinematic – U5 and energy parameters –
U6).
Under the experimental program, out of all surface layer parameters, the evolution with
respect to the initial state (input parameters) of the following parameters (output
parameters) has been investigated: mechanical parameters (micro ha rdness – X2; state of
strain – X3 namely: 1st order strain (I) – 1
3X,) -2nd order strain ( II) – 2
3X; -,3rd order strain
(() – 3
3X, – and from the physical metallurgical parameters, the structure changes were
monitored – X5 (network parameters, the texture).

Fig. 1. A cybernetic model used in study of friction process adapted in study of the low
cycle fatigue process.
From among the control parameters (U), for th e purpose of the experiment, the constructive
parameter was acted upon by the nature of the material U1, using two grades of steel OL52
and 10TiNiCr180, both shape and dimensions of the test pieces remaining unchanged
throughout the experiment. From among the operating parameters, it was maintained the
same working environment – air at ambient te mperature (U4), acting upon the cinematic
parameters U 5, namely the testing frequency 1= 20 cycles / min or 2= 40 cycles / min
It was also acted upon the energy parameters (U 6),in an attempt to investigate the evolution
of the surface layer parameters by changing the imposed strains ( 1, 2, 3)) and the number
of strain cycles (N1, N2, N3, N4, N5) under an experimental program.
www.intechopen.com

Alloy Steel – Properties and Use
240
Knowing at any time the parameters of the surfac e layer, this is one of the safest procedures
in assessing and forecasting the degree of degradation of metallic materials under fatigue
processes .
Determination of the structural changes in the surface layers may allow for the optimization
of the metal components manufacturing technolo gy. In practice the control of the surface
layer parameters often requires the use of physical methods of investigation which do not
affect its structure and physical-chemical condition.
3. Experimental researches
This chapter presents only the experimental research carried out for the steel OL52. The
experimental research was performed in steps from 2000 to 2000 cycles up to 104 cycles.
At each step of the number of cycles (2000 cycles) investigations were carried out on the
evolution of the crystalline network parameter, trap, density, texture, analysis of the micro
hardness and the evolution of the layer micro-structure. For all these investigations use was
made of a diffractometer of X radiation, DR ON-3, micro hardness meter PMT3, and optical
microscope Olympus BX60M of Japanese construction.
Mention must be made that there has not been made a systematic research of the surface
layer for the following reasons: wide variety of materials, wide range of physical, chemical,
mechanical and metallurgical factors influenc ing on the surface layer, various deficient
physical methods of non-destructive control. The relatively long time taken for some of the
analyzing methods as compared to the stress relaxation period/time of certain structure
modifications, gets in the way of finding a common point of view on and general metho ds
for analyzing and controlling the different pr ocesses occurred in the surface layer.
The experimental program consists of:
Experiment 1 – carbon steel sample OL52 under stress at frequency 1=20 cycles/min;
Experiment 2 – carbon steel sample OL52 under stress at frequency 2=40 cycles/min;
Experiment 3 – alloyed steel sample 10TiNiCr180 under stress at frequency 1=20
cycles/min;
Experiment 4 –alloyed steel sample 10TiNiCr180 under stress at frequency 2=40
cycles/min;
All the 4 experiments were focused on the modifications of the surface layer parameters
depending on the number of cycles N 1, N 2, N 3, N 4, N 5 and the prescribed deformations 1, 2,
3.
Tables 1, 2, 3, 4 illustrates the experimental program, highlighting all the parameters
involved in the experiment.
TEST PROGRAM (Tables 1,2)
OAL ji, 1AL ji – A(OL52) steel samples at two stress frequencies I i,j,k – Ist order stress a i,j,k –
lattice parameter j – induced deformations, (j=1…3). II i,j,k – IInd order stress texture i,j,k
Nk – number of stress cycles, (k=1…5). i,j,k – displacement density, HV i,j,k – micro-hardness
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
241

TEST 1
OL52 steel – frequency 1=20 cycles/min OAL 11 1 [m/m]
OAL 21 2 [m/m]
OAL 31 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k OAL 12 1 [m/m]
OAL 22 2 [m/m]
OAL 32 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
surface layer
properties
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k surface layer properties
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k surface layer properties
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k OAL 13 1 [m/m]
OAL 23 2 [m/m]
OAL 33 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k
Table 1. Test program for OL52 steel – frequency 1=20 cycles/min

TEST 2
OL52 steel – frequency 1=40 cycles/min 1AL 11 1 [m/m]
AL 21 2 [m/m]
1AL 31 3 [m/m]
N1 N 2 N3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k 1AL 12 1 [m/m]
1AL 22 2 [m/m]
1AL 32 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k 1AL 13 1 [m/m]
1AL 23 2 [m/m]
1AL 33 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k
Table 2. Test program for OL52 steel – frequency 1=40 cycles/min
www.intechopen.com

Alloy Steel – Properties and Use
242

TEST 3
10TiNiCr180 alloy steel – frequency 1=20 cycles/min OBL 11 1 [m/m]
OBL 21 2 [m/m]
OBL 31 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k OBL 12 1 [m/m]
OBL 22 2 [m/m]
OBL 32 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
surface layer
properties
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k surface layer properties
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k surface layer properties
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k OBL 13 1 [m/m]
OBL 23 2 [m/m]
OBL 33 3 [m/m]
N1 N2 N 3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k
Table 3. Test program for 10TiNiCr180 alloy steel – frequency 1=20 cycles/min

TEST 4
10TiNiCr180 alloy steel – frequency 2=40 cycles/min 1BL 11 1 [m/m]
1BL 21 2 [m/m]
1BL 31 3 [m/m]
N1 N 2 N3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k 1BL 12 1 [m/m]
1BL 22 2 [m/m]
1BL 32 3 [m/m]
N1 N 2 N3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k 1BL 13 1 [m/m]
1BL 23 2 [m/m]
1BL 33 3 [m/m]
N1 N 2 N3 N4 N5 N1 N2 N3 N4 N5 N1 N2 N3 N 4 N 5
I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-
hardness HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k I i,j,k; II i,j,k; i,j,k; a i,j,k;
texture, micro-hardness
HV i,j,k
Table 4. Test program for 10TiNiCr180 alloy steel – frequency 2=40 cycles/min
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
243
TEST PROGRAM
0BL ji, 1BL ji – B (10TiNiCr180) steel samples at two stress frequencies I i,j,k – Ist order stress a
i,j,k – lattice parameter j – induced deformations, (j=1…3). II i,j,k – IInd order stress texture i,j,k
Nk – number of stress cycles, (k=1…5). i,j,k – displacement density HV i,j,k – micro-hardness
3.1 Sample preparation
The samples have been prepared metallographica lly according to the standards in force.
The metallographic analysis has been made on samples from the two steel grades
investigated. Samples have been taken longitudin ally and investigated at a size of (x100).
For purity purpose samples have been prep ared and analyzed acc to [121], and for
microstructure and grain size acc. to STAS 7626-79, STAS 5490-80 and SR ISO 643-93 [120].
The attack for the sample made from OL 52 wa s achieved by means of the natal reactant ,
2% and for the sample made from TiNiCr 180 with nitrogen acid reactant, 50% under
electrolytic attack regime. Results are given in table 5 and 6.
Material Purity
STAS 5949-80 Microstructures
STAS 7626-79 and SR ISO 643-93 Figures
OL52 Silicates +
punctilious
oxides
score>5 Ferrite + perlite
Grain size = 9
Ratio Pe/Fe=15/85 2, 3
10TiNiCr180 Titanium
nitrure
score=2,5 Austenitic structure with macla ți
grains and chrome carbides distributed
in rows; M:G:=4-5 4
5
Table 5. Initial result for samples
In order to closely watch any microstructure modifications with the samples coded OAL ji,
1AL ji, OBL ji and 1BL ji, surface micro-photos were taken at size (x200, x500, x1000) directly on
the samples subject to strain.
Due to the big sample sizes, vs an optimum si ze of a metallographic slif , it was rather
difficult to prepare the su rface being investigated .
The samples not chemically atta cked by reactants were photographed at size (x100), and
those attacked at (x200, x500, x1000).
Since the samples made from alloyed steel ( aust enitic stainless steel strongly anti-corrosive)
are of bigger size, they have been immersed in to the reactant for 30 min. The chemical
composition of the reactant was: nitrogen acid (1,4) 5 ml, fluoride acid 1 ml, distilled water
44 ml, and for the carbon steel , the reactant was inital 2%.
In order to adjust the machine to 3 prescrib ed deformations, acc to another experimental
program, captor-sampl es were prepared.
www.intechopen.com

Alloy Steel – Properties and Use
244
sample Structure Notifications
Șlif 1

Carbon
steel
Fig. 2.
Ferrite-perlitic microstructure
OL52x100
grain size = 9
ratio Pe/Fe=15/85
natal attack 2%
Șlif 2

oțel
carbon
Fig. 3.
OL52x100
purity – nonmetallic inclusions
fragile silicates – punctilios
oxides- score > 5
Șlif 3

Alloyed
steel
Fig. 4.
Austenitic microstructure
10TiNiCr180x100
Șlif 4

Alloyed
steel
Fig. 5.
10TiNiCr180x100

www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
245
3.2 Determination of the prescribed deformations
In order to determine the upper limit of the co nventional elastic range, and to assess the
prescribed deformations used for the experi ment purpose an experimental program was
designed consisting of :
1. preparation of the captor-samples
2. analysis of the captor –samples operation
3. partial plotting of the characteristic cu rves for the two materials specifying the
prescribed deformations
4. experimental determination of the longitudinal elastic module and max strains.
Preparing the captor-samples
The samples were marked to facilitate identification:
the samples from material A (OL52) was marked 2.1, 2.2, 2.3 acc to the three prescribed
deformations to be used in the experiment, and those from material B (10TiNiCr180)
marked 1.1, 1.2,1.3. In the cent ral measuring zone, on both sides a mechanical processing of
the surfaces with abrasive paper to achieve the desired roughness : 1.5 … 2 m.
After this processing the cross sections were me asured and the results given in Table 6 (fig 6).
sample
Dimensions
[mm]
1.1
1.2
1.3
2.1
2.2
2.3
b 10.16 10.17 10.17 10.13 9.35 10.05
h 4.78 4.85 4.60 4.25 4.35 4.60
Table 6. Sample dimensions
The processed surfaces were marked (the middle of the surface was plotted on both
directions) and subsequently chemically prep ared .The chemical preparation involves
degreasing with dicloretan and carbon tetraclorur ă, with a number of flushes in isopropilic
alcohol. The adhesive used to stick the ma rks was “Z70” – Hottinger, a fast-hardening
cyanoacrylate. For an even as possible hardness ov er the entire surface of the mark (without
polimerisation poles) a neutralizing soluti on “NZ70” Hottinger was resorted to

Fig. 6. Captor sample
The tensometric marks were of the type: 3P/120LY11 made by Hottinger with constant
k=2.04 1%. The complex mark-adhesive and coatin g was chosen depending on the degree
of compliance with the rule of the dissipated power over tensometric mark. The dissipated
power on the grill surface is calculated
www.intechopen.com

Alloy Steel – Properties and Use
246
Two grades of rolled steels mainly used in pressure boilers and vessels industry, namely,
OL52k and austenite alloyed steel 10TiNiCr180, were considered. Samples made from the
above mentioned steels were tested on the universal machine at 50 tf hydraulically-driven
pull at the department of material strength, the Faculty of Mechanical Engineering Galati.
On the same machine pure bending testings were conducted by making use of a
construction illustrated in figure 7.

Fig. 7. The construction used for pure bending testings, were: 1 – plate, 2 – roller, 3 – sample,
4 – base roller, 5 – watch, 6 – digital tens ometric amplifier, a – tensometric mark, b –
connector.
The tensometric marks used are of the type 3P/120LY11 manufactured by Hottinger,
constant k=2.04±1%. The two tensometric mark s were connected in half-bridge and the
connector was tied to the tensometric am plifier by a 6-thread cable. The arrow f was easily
measured in the section a by means of a comparator (5) at all testing stages.
Using the relation Mohr-Maxwell procedure Ve resceaghin, the arrow expression becomes:
2
00 1
A0 1
yFl l lf( 2ll)2EI 3 2   
    (1)
Relation (1) indicates the prop ortionality between the force A and the applied force F, or
between fA (the arrow) and the bending moment 0FlM2 . Since the beam is subject to pure
bending at the middles zone, the tensions  are proportional to the bending moment
(Navier relation).
The proportionality zone on the characteristics curve is highlighted by plotting an arrow
curve (f) depending on the specific deformation ( ). From the experimental results, the
diagram of the austenite alloyed steel 10TiNiCr180 was plotted in Figure 8.
At the Tensometry Laboratory of ICEPRONAV bending testing were conducted at variable
moment. Each sample was installed on a device to the diagram in Figure 9. The samples
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
247
were built in at one end while forces of known values were attached to the other end,
gradually.
The point for forces application is 100 mm from the central reference of the tensometric
mark according to the diagram. The applied forces were obtained with calibrated weigts
(order 4) of 0.5 and 1 kgf respectively, which allow for calibrations higher than the accuracy
class 0.5. After the experiments and calculat ions performed the curve in Figure 10 was
plotted.

Fig. 8. Pure bending diagram for 10TiNiCr180

Fig. 9. The sample installed on a device.
www.intechopen.com

Alloy Steel – Properties and Use
248
It should be underlined that the specific defo rmations in Figures 9 and 10 are those read on
the digital tensometric amplifier N2313; the tensometric marks being connected in half-
bridge , real = citit/2.

Fig. 10. Bending at variable moment diagram for 10TiNiCr180
From both the experiments and the characterict ic curves, three deformations were obtained
for the alloyed steel in the transiti on from the elastic-plastic domain.
The same methodology was used for the carbon steel OL52 and other three deformations
were obtained for the above transition domain (according to table 7).
Steel type Imposed deformations
1
[m/m] 2
[m/m] 3
[m/m]
10TiNiCr180 1500 2000 2500
OL52 K 2000 2500 3500
Table 7. Imposed deformations in elasto-plastic area.
The values in the table are those recorded while the real ones are half due to the half-bridge
arrangement
3.3 The evolution of certain parameters in the surface layer during low cycle fatigue
proces
In this chapter the evolution of certain para meters in the surface layer during low cycle
fatigue process are presented: evolution of lattice parameter, evolut ion network parameter,
evolution of texture level, variations of micr ohardness, evolution of microstructures in the
surface layer only for ol52 samples tested to pure bending fatigue.
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
249
3.3.1 The internal 3rd order strain. The trap density
Figure 11 presents the dependence of  max/fII on the number of strain cycles for 3
imposed strains 1, 2, 3 at frequency 1= 20 cycles / min for steel OL52.
In general, it is found out the existence of a proc ess of decrease in the tr ap density in case of
small strains ( 1, 2) relative to the original state (level 1 and level 2) and an increase in the
trap density ( ) in case of higher strains 3. With increased strain ,a lower degree of
deformation of the crystalline lattice around atom s or groups of atoms (dislocation density)
is visible.

2.8652.872.8752.882.8852.892.895
0 2000 4000 6000 8000 10000a [A]
Number of cycles"a" network parameter in OL52, frequency 20 cycles / min
Deformation 1 Deformation 2 Deformation 3

Fig. 11. Evolution of lattice parameter for f = 20 cycles/min
For the average strain 2=2500 m/m it is found a tendency to increase the trap density with
the number of strain cycles, said increase taking place in jump s. This increase in the trap
density can lead to their agglomeration and subs equently to the generation of micro-cracks.
For a strain of 1=2000 m/m the trap density decreases with increasing number of cycles,
which can be accounted for by a greater durability of the sample put to strain, when
compared with the strain 2=2500 m/m.
As regards the development of the trap densities in the case of strains 1= 2000 m/m and 2
= 2500 m/m, two opposite trends of its variation are found. This may be related to a
process of transition from th e elastic to elasto-plastic.
With 3=3500 m/m there is a general tendency to in crease the trap density, which will
hasten the destruction by fatigue as compared with the strain 2 = 2500 m / m.
www.intechopen.com

Alloy Steel – Properties and Use
250
Figure 12 shows the dependence of  max/fII on the number of strains for the 3 strain
imposed at the frequency of 40 cycles / min for carbon steel OL52.
It is found that for the 3 imposed strains ther e is an overall decrease in the trap density
relative to the initial state.
There is a tendency to increase the trap density with the number of cycles, for the strains of
2= 2500 m / m and 3 = 3500 m/m, and a slight downward te ndency of the trap density
for a small strain of 1=2000 m/m.
2.862.8652.872.8752.882.8852.892.8952.92.905
0 2000 4000 6000 8000 10000a [A]
Number of cycles"a" network parameter in OL52, frequency 40 cycles / min
Deformation 1 Deformation 2 Deformation 3

Fig. 12. Evolution of the lattice parameter for f = 40 cycles/min
The downward and upward slopes of the trap density ( 1=2000 m/m and 2=2500 m/m)
are lower as in the case of frequency of 20 cy cles / min above. In addition, the slopes are
reversed when compared with the case of frequency of 20 cycles/min.
The fact that there is a change of sign in the slopes at 1=2000 m/m and 2=2500 m/m,
again justifies the existence, between the two strain s, of a strain of transition from elastic to
elasto-plastic.
The trap density variation with the number of cycles has a minimum value the position of
which appears increasingly later, as the degree of strain decreases.
From the analysis of the two di agrams it is found that, in or der to compensate for the fatigue
damage to large strains, high-frequency strains should be applied.
Therefore, at low frequency and large strain, the occurrence of cracks through the process of
trap agglomeration is much higher.
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
251
3.3.2 Analysis of metallurgical characteristi cs of the surface layer. Structure analysis.
Network parameter
From the analysis of Figures 13 and 14 it is fo und that the network parameter tends to decrease
with increasing number of strain cycles for the 3 strains imposed. The de crease is stronger at
small strain and low frequencie s and at high frequencies and large strains, respectively.
Modification of the network parameter may be associated with the existence of a migration
process of the atoms in the alloying elements from the network to the material surface.

Fig. 13. Variation of the  max/fII 220 on number of testing cycles for frequency 1=20
cycles/min
This is also supported by the slight increase in the network parameter for a given number of
cycles. This increase occurs ear lier when the strain is greater. The process of atom migration
in and from the elementary ce ll of the ferritic phase indicates a high atom kinetics in the
surface layer during the fatigue process.
The high kinetics may have adverse effects if th e material would be put to strain in corrosive
environments.
Analyzing the two figures it can be seen that the process of atom migr ation in and from the
elementary cell occurs more slowly for low fr equencies and faster for high frequencies and
large strain; at small strains the process is more pronounced at lower frequencies.
www.intechopen.com

Alloy Steel – Properties and Use
252
It follows that if the strains were applied in aggressive environments, the life time would be
much shorter for low frequencies/low stra in and high frequency/ large strains.

Fig. 14. Variation of the  max/fII 220 on number of testing cycles for frequency 1=40
There is therefore the possibility to "manage" from outside the life time by appropriate
adjustments of the relationship between the strains imposed and the frequency applied .
3.3.3 Evolution of texture level
Diffractometry investigations with X-rays have highlighted the degree of texturing of OL52
steel sample subjected to high fatigue at the limit of the elastic range and to low freq uencies.
As in previous cases, the texture analysis was made in increments of 2,000 cycles to 10,000
cycles and 3 imposed strains. The histogram in Figure 15 shows the dependence of I max/I0
on the number of cy cles N and strain , at a frequency of 20 cycles / min.
Analyzing the resulting graphical representa tion, it is found the predominance of a
retexturing process of the material to crystallographic direction (220).
The highest degree of retexturing becomes apparent at the largest strain value, 3. This
retexturing is associated with the mechanical micro-processes leading to the loss of
preferential orientation of crystal planes accord ing to the direction (220) with respect to the
state reached after rolling (I max/I0=1).
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
253
With the first strain 1, it is found a slight tendency of texturing which increases with
increasing number of cycles. With the intermediate strain it is found that there is a stronger
texturing which can be associated with the forced orientation of the crystalline planes (220).
This material behaviour can be explained by analyzing the first micro processes of
elastoplastic strain although, a ccording to the characteristic cu rve of the material, we found
ourselves in the elastic range. Figure 16 provides the histogram of the dependence of I max/I0
on the number of cycles N and the required strains 1, 2, 3 at the frequency of 40 cycles /
min

Fig. 15. Evolution of texture level for f = 20 cycles/min
Compared with the previous case, the graphical representation analysis shows that th ere is a
general tendency of increased texture of the crystalline network both with increased number
of cycles and the strain required.
With a frequency of 40 cycles / min. no te ndency of retexturing was revealed for any
amount of strain or number of strain cycles, as in the case described above at a frequency of
20 cycles / min. The fact that at low frequency ( 1= 20 cycles / min) there are texturing and
retexturing processes leads us to the conclusion that the material does not present inertia to
changes in structure (hysteresis functions normally), while with high frequency (40 cyc les /
www.intechopen.com

Alloy Steel – Properties and Use
254
min), the material loses part of its elastic proper ties responding to extern al factors – inertia to
structure changes being much lower.

Fig. 16. Evolution of texture level for f = 40 cycles/min
3.3.4 Analysis of the surface layer microhardness
Figure 17 and 18 show the micro hardness HV variation for sample OL52 HV depending
on the number of cycles to the strain 3=3500 m/m for the two frequencies. The analysis
of the experimental data and curves shows th at micro hardness decreases in jumps. The
decrease in the micro hardness occurs through processes of hardening a nd softening.
With low frequencies ( 1=20 cycles/min) decreases in micro hardness is less than the
initial state, while at high frequency, the decrease in micro hardness is higher than the
initial state. The amplitude of the softening and hardening processes is much higher for
low frequencies than for high frequency ( 2= 40 cycles / min). The period of the
hardening and softening processes is lower at low frequencies and higher at h igher
frequencies. It can be said, therefore, that the velocities of the hardening and softening
processes are higher at low fr equencies than at higher freq uencies for the same strain
imposed.
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
255

Fig. 17. Variation of microhardn ess vs testing cycles for ǎ = 20 cycles/min

Fig. 18. Variation of microhardn ess vs testing cycles for ǎ = 40 cycles/min
www.intechopen.com

Alloy Steel – Properties and Use
256

www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
257
3.3.5 Analysis of microstructure
The microstructure analyses were also carried ou t in increments of 2,000 cycles up to 10,000
cycles for 3 imposed strains and at the two frequencies ( 1 și 2).
The Figures 19 and 20 present the microstructu res in the surface layer of the OL52 samples
tested to pure bending fatigue to the strain 3=3500 m/m, at the two frequencies.
The analysis of the microstructures reveals that with increasing number of cycles there is an
increase in the density of the sliding bands in the ferrite grains for a given frequency. With
the same strain and number of strain cycles it is observed that with higher frequency the
sliding bands density is lower compared with that at lower frequencies.
3.4 Macroscopic aspects of the fractures
Generally, in all cases (Figure 21, 22, 23) the fatigue fracture process is initiated from the
sample surface from spots featuring microscopic surface defects (roughness, more intense
local hardening because of previous processing, surface defects of the material structur e,
such as inclusions, intermetallic phases, intersecting the processing surface ).
In the section damaged by fatigue process, th e samples present a characteristic shiny area
and the sudden breaking zone under the strain applied to them.
In case of the sample shown in Figure 23a, on the polished surface near the fracture zone,
sliding bands are clearly visible due to the relatively high sp eed of the strain propagation
onto the crystalline grains favorably oriented and of relatively low total hardening intensity.
The weight of the plastic strains under elasto-plastic regime is relatively high in a relatively
small period of time ((N 1r = 30,065 cycles until br eaking) to the strain 3=3500 m/m.
The weight of the fatigue fracture surface is relatively small and located near the originator
(concentrator) of the breaking/ fracture process (Figure 23b, c).
In Figure 21a, the polished surface reveals sliding bands specific to a very large number of
cycles (N 3r = 106,488 cycles up to breaking) at a relatively large distance from the
break/fracture zone. This indicates that for strains with small strains (( 1=2000 m/m), the
elasto-plastic strain zone before fracture is more extensive. The explanat ion is that the rate of
hardening of the material is relatively small and therefore we believe that plastic strains will
be taken, at the next cycles, by less hardened neighboring areas which feature lower st rain
resistance. Extension of the plastic strain area in the vicinity of the fracture zone is accounted
for by the propagation of plastic strains, progressively to grains from the neighboring
hardened areas.
In Figure 22a, with the sample put to strain 2=2500 m/m, on the polished surface in the
vicinity of the breaking zone it is highlighted the presence of sliding bands of highly fine
granulation due to the extension of the elasto-pla stic range to cover a larger period of time
and a larger number of cycles until breaking (N 2r = 62,635 cycles). We believe that the
weight of plastic strains under elasto-plastic regime is smaller than the previous case.
In Figure 22 b,c the fatigue fracture surface (shiny area) has a relatively greater expansion in
the vicinity of the fracture or iginator and is developed over the entire width of the sample.
Both on the previous sample and the sample mentioned above, the fatigue fracture surfac e is
unilateral (on one side).
www.intechopen.com

Alloy Steel – Properties and Use
258

(a) (b) (c)
Fig. 21. Macroscopic aspects of crack for ε3 = 3500 Ǎm/m

(a) (b) (c)
Fig. 22. Macroscopic aspects of crack for ε2 =2500 Ǎm/m

(a) (b) (c)
Fig. 23. Macroscopic aspects of crack for ε1=2000 Ǎm/m
www.intechopen.com

A New Systemic Study Regarding the
Behaviour of Some Alloy Steels During Low Cycles Fatigue Process
259
Sliding bands have greater width, this being possible by the accumulation of plastic strain in
a relatively large time, i.e. for a larger number of cycles.
In Figure 21b, c, as far as the fracture section is concerned, th e fatigue damaged area is much
larger than the sudden fr acture area, developed over the entire width of the sample, and of
bilateral aspect. This can be explained by the fact that the development speed of the fatigue
fracture surface from a concentrator is small which allows the initiation of the fatigue
fracture from a concentrat or on the opposite side.
In the previous cases, since strains are higher, it is sufficient to initia te the fatigue fracture
from a stress concentrator because the growth rate of the fatigue fracture is much higher
which makes no longer possible the initiation of a fatigue fracture from another
concentrator.
4. Conclusions
1. By extension of the tribolayer and tribosys tem concepts to the study low cycle fatigue
process of the steel the structural changes in the superficial layer are sh own. This
allows to establish a relationship between structural parameters of superfi cial layer
and damage degree during fatigue tests. It was evinced a microfatigue proce ss which
is strong influenced of: frequency testing, strain level, and numb er of the fatigue tests.
2. Our results can be used to account for the damage mechanism of the tested samples
subjected to low frequency fatigue test and high tensions
5. References
Buzdugan, Gh., & Blumenfield, I. (1979), Calculul de rezistenta al organelor de masini ,
Editura Tehnica Bucurest i –Romania, Bucuresti
Constantinescu, I. & Stefanescu, D.M. & Sandu, M., (1989), Masurarea marimilor mecanice cu
ajutorul tensometriei, Editura Tehnica Bucuresti, IS BN 973-31-0127-3, Bucuresti
Crudu, I.& Macuta, S. & Palaghian, L. & Fazekas L.(1991) “ Masina universala de incercat
materiale ”, Patent nr. 102714/1991 ,Bucuresti
Gheorghes, C. (1990), Controlul structurii fine a metalelor cu radiatii X, Editura Tehnica
Bucuresti, ISBN 973-31-0151-6, Bucuresti
Lieurade, H.P. (1982) La Pratique des Essais de Fatigue, PYC Editon , ISBN 2-85330-053-6,
Paris
Macuta, S. – “ Evolution of some structural fine paramet er in the superficial layer during low cycle
fatigue process ”, Tome I of International Conference on Advanced in Materials and
Processing Technologies AMPT’01, vol1 .ISBN 84-95821-06-0, Leganes, Madrid –
Spania. September 2001
Macuta, S. & Rusu, E., (2008)., Experimental researches regarding th e evolution of some
parameters of the superficial layer in low cycle fatigue processes ,In: Maritime
Industry Ocean Engineering and Coastal Resources, Guedes Soares & Kolev, pp. 219
– 223, Taylor and Francis Group , ISBN 978-0-415-45523-7, London
Macuta, S. & Rusu, E., (2009), Experimental researches regarding the evolution of so me
parameters of the superficial laye r in low cycle fatigue processes, Proceedings of
www.intechopen.com

Alloy Steel – Properties and Use
260
13th International Congress International Ma ritime Association of Mediterranean, tom.
1 – ISBN 978-975-561-356-7, Istanbul Turkey, October 2009
Macuta, S. & Rusu, L., (2009), Modelling by finite element method of stress state
establshing and experimental research re garding the elasto-plastic deformations
of some steels alloys , Proceedings of 13th International Congress International
Maritime Association of Mediterranean, tom. 3 – ISBN 978-975-561-358-1, Istanbul
Turkey, October 2009
Macuta, S. (2007),. Oboseala oligociclica a materialelor , Editura Academiei Romane, ISBN
978-973-27-1382-2, Bucuresti
Macuta, S. (2010) – The Evolution of Cert ain Parameters In The Surface Layer During
Low Cycle Fatigue Process- Metalugia International Journal vol.XV Special Issue
no.8 , (augusut 2010), pp .20-25 ,ISSN 1582-2214
Macuta, S., (2004)- “ Establishing the elasto-plastic deformations of some steel alloys ”,
Proceeding of The 29-th Annual Congre ss of the American Romanian Academy
of Arts and Sciences, ISBN 973-632-140-1 Bochum,Germany september 2004.
Mocanu, Ds. (1982), Incercaera materialelor Vol. 1 & 2, Editura Tehnica Bucuresti – Romania
, Bucuresti
www.intechopen.com

Alloy Steel – Properties and Use
Edited by Dr. Eduardo Valencia Morales
ISBN 978-953-307-484-9
Hard cover, 270 pages
Publisher InTech
Published online 22, December, 2011
Published in print edition December, 2011
InTech Europe
University Campus STeP Ri
Slavka Krautzeka 83/A
51000 Rijeka, Croatia
Phone: +385 (51) 770 447
Fax: +385 (51) 686 166
www.intechopen.comInTech China
Unit 405, Office Block, Hotel Equatorial Shanghai
No.65, Yan An Road (West), Shanghai, 200040, China
Phone: +86-21-62489820
Fax: +86-21-62489821The sections in this book are devoted to new approa ches and usages of stainless steels, the influence of the
environments on the behavior of certain classes of steels, new structural concepts to understand some fatigue
processes, new insight on strengthening mechanisms, and toughness in microalloyed steels. The kinetics
during tempering in low-alloy steels is also discus sed through a new set-up that uses a modified Avram i
formalism.
How to reference
In order to correctly reference this scholarly work , feel free to copy and paste the following:
Macuta Silviu (2011). A New Systemic Study Regardin g the Behaviour of Some Alloy Steels During Low Cyc les
Fatigue Process, Alloy Steel – Properties and Use, Dr. Eduardo Valencia Morales (Ed.), ISBN: 978-953-3 07-
484-9, InTech, Available from: http://www.intechope n.com/books/alloy-steel-properties-and-use/a-new-
systemic-study-regarding-the-behaviour-of-some-allo y-steels-during-low-cycles-fatigue-process