INFLUENCE OF LASER HEAT TREATMENT ON METAL COATING LAYERS [601410]

INFLUENCE OF LASER HEAT TREATMENT ON METAL COATING LAYERS

OLÁH Arthur 1, Mircea Horia TIEREAN 1, VER Ő Balázs 2,
Corneliu MUNTEANU 3, Andreea BARBINTA3
1Transilvania University of Brasov, Romania, 2Bay Zoltán Institute for Materials Science and
Technology Budapest, Hungary, 3Gheorghe Asachi Technical University Iasi, Romania
([anonimizat], [anonimizat])

Abstract: The paper presents a research concerning the influ ence of laser heat treatment on welded coating laye rs.
The research was made with four types of electrodes for welding coating. Evaluation of results was mad e by
researching the microhardness, wear resistance, SEM and EDS. Results reveal a good effect of laser hea t treatment in
case of coating with electrodes having low carbon a nd moderate chrome content.
Usually, the hardness obtained after coating depend s only on the filler material characteristics. The goal of this
research is to demonstrate the increasing of hardne ss after the laser heat treatment of the welded coa ting layer. The
welding coating process enables the recovery by mea ns of reworking of some parts or machine components that
reached the wear limit. In this situation, some exa mples are: dies, crankshafts, different machinery a xles, camshaft
which can be brought back to the nominal quota.
After the analysis of the laser-hardened and remelt ed shaft under different power conditions, the conc lusion was
that the laser beam heating produces two kinds of r egions inside the laser tracks. One region is compo sed
predominately of martensite; the other region consi sts of unchanged proeutectoid ferrite, martensite a nd some
pearlite.

Keywords: metal coating, laser beam, hardness, microstructur e

1. INTRODUCTION

Laser surface hardening is a method of producing ma rtensite on selected regions of steel components. A
continuous wavelength laser is scanned over the ite m to heat up the surface to the austenite range (ap proximately
1000 °C in most steels). Since the substrate acts a s an efficient heat sink, the material quickly cool s to a temperature
below Ms (martensite start temperature). The result ing microstructure is composed of fine martensite, which
improves mechanical and chemical surface properties , but maintains unchanged material bulk properties, including
ductility and toughness. Because of wide availabili ty, both CO 2 and Nd:YAG lasers have been used to produce
hardened surfaces on steels [1].
The current technological problem to be solved is h ow to propose an alternative route for surface hard ening of a
specific automotive shaft that has usually been ind uction-hardened. The laser technology is considered a good
candidate because the variety of part shapes to be hardened could pose problems for the induction coil manufacturing
and for induction coupling, and also because one wa nts to choose specific treatment regions. In case o f metal coating
layers, due to differences between the deposited la yer and core, specific problems appear, that have n ot been
adequately studied.
The energy absorbed during laser beam heating is ma inly from heat conduction dissipated to the solid v olume;
thus, the temperature field could be calculated fro m the heat diffusion equation. Hunziker [2] has pro posed a solution
for a Gaussian heat source at constant velocity, Vb , over a semi-infinite solid. This solution is base d on integration of
the uniform source solution originally proposed by Rosenthal [3] with axis origin at the intersection of the laser beam
axis with the materials surface. The model assumes constant and isotropic thermal properties, conducti vity and
specific heat, and negligible latent heat. The stea dy-state temperature distribution of a Gaussian hea t source is then
given by:
()
( )22
22
20230
2 1 21exp 1
2) , , (
ξσ
ξσξσξ
ξσκπβ



+
+

+

+
=+−× += ∫∞
zyPx
HdH PTz y x T
t (1)

where T 0 is the ambient temperature; β is the laser-matter absorptivity; P is the laser po wer; k is the thermal
conductivity; σ is the variance of the Gaussian; ξ is the integration variable calculated as ( α.t) 1/2 , with α as the
thermal diffusivity and t as the elapsed time; P t is the Péclet number for thermal diffusion defined as P t = σVb/(2 α).
High-energy density beam processing is a special te chnology that uses a high-energy density beam as a heat
source for such applications as welding, incision, punching, spray painting, surface treatment, etchin g, and fine
machining [4]. Surface treatments that are effected by laser beam irradiation include laser hardening, laser alloying,
and laser cladding [5, 6]. The common feature of al l of these processes is the production of certain t hermal cycles in
small, highly localized regions on the surface of t he work-piece, which then takes on new properties t hat allow it to
cope better with wear, fatigue, and corrosion while maintaining most of its other original properties [7].
Recent reviews of the principles and applications o f laser treatments describe the use of lasers as a controlled
heat source for transformation hardening [8]. The c lassical approach to modeling the heat flow induced by a
distributed heat source moving over the surface of a semi-infinite solid starts with the solution for a point source and
integrates it over the area of the beam. This widel y used method requires numerical procedures for its evaluation, as
do the finite difference solutions of Shuja and co- workers [9]. They developed a 3-D heat flow model a nd varied the
beam power and traverse speed to determine the dime nsional analysis of heat flow during heat treating and melting.
But the results were not easy discretion of complex shapes and needed complicated calculation. Another approach of
heat flow modeling applied to moving heat sources b y Rosenthal and several authors have used finite-el ement
method for numerical evaluation, as do the FEM anal yses of W. Dai and co-workers [10].
High power diode lasers have reached a high mean po wer level, a high energy efficiency conversion and a
small size of the laser system that permits to moun t the whole laser head directly onto the machine, s aving a beam
guiding system [11]. In short, this kind of laser h as become an adequate tool to carry out the ‘‘harde ning by
transformation’’ [12], especially for the heat trea tment of the surface of steel due to the high absor ption coefficient
for diode laser radiation on shiny or oxidized meta l surface. Usually, a cinematic stage to move the s ample has to be
included in the experimental arrangement to harden a desired surface. Far from the simple experimental set-up, new
components for the hardening process have to be emp loyed in order to assure the control of the thickne ss of the
hardened layer without the remelting of the sample surface. The reason to involve more complexity in t he
experimental system can be attributed to the requir ements related to the hardness homogeneity, that ca n be disturbed
not only by the ‘‘noise’’ that affects the processi ng parameters during the process, but the geometry of the sample to
be hardened like complex shaped parts with difficul t heat flow conditions (near edges, boreholes or at 3D-curved
surfaces) [13]. Furthermore, the reproducibility of the mechanical properties obtained cannot be guara nteed due the
in homogeneity of the surface quality that changes the absorption coefficient and as a result the ther mal cycles that
affect the phase transformation [14].
The welding coating process enables the recovery by means of reworking of some parts or machine compon ents
that reached the wear limit. In this situation, som e examples are: dies, crankshafts, different machin ery axles,
camshaft which can be brought back at the nominal q uota. Usually, the hardness obtained after coating depends only
on the filler material characteristics. The goal of this research is to demonstrate that the increasin g of hardness and
wear resistance, if a laser heat treatment will be applied on the welded layer.

2. SAMPLE PREPARATION AND TESTING EQUIPMENT

The metal coating condition was made with arc weldi ng with Luftarc 150 Ductil equipment, using four ty pes of
electrodes. The welding current intensity was 700 A and the welding voltage was 40 V. The base metal was S275JR
SR EN 10025-2:2004, 20 mm thick. The coating layer was 5 mm thick. After the coating, the samples were tempered
at 600șC. The results were evaluated with PMT – 3 m icro-hardness tester, electronically microscope Nova Nano
SEM and chemical analyzer EDAX Orbis Micro-XRF.

3. EXPERIMENTAL RESEARCH

Laser heat treatment was applied on four types of w elding coating layers, presented in Table 1. The sa mple was
subjected to laser heat treatment in nine variants, with laser source Nd:YAG – Rofin DY 570 Germany, d irected with
ABB – Sweden robots.

Table 1 – Electrode used in research

Electrodes C Si Mn Cr W Nb
ElCrW8Co 0.3 0.8 0.8 2.0 7.0 1.2
El CrW2 0.4 1.3 – 1.2 2.3 –
El 62 H 0.7 0.8 0.3 3.5 3

Table 2 – Laser power and notation of sample
Laser power
[W]
Electrode
type Non heat
treatment 1400 1500 1600 1700 1875 2150 2425 2600 2700
El CrW2 [1-0] [1-1] [1-2] [1-3] [1-4] [1-5] [1-6] [ 1-7] [1-8] [1-9]
ElCrW8Co [2-0] [2-1] [2-2] [2-3] [2-4] [2-5] [2-6] [2-7] [2-8] [2-9]
El 62 H [K-0] [K-1] [K-2] [K-3] [K-4] [K-5] [K-6] [ K-7] [K-8] [K-9]

4. EVALUATION OF RESULTS
4.1. Micro-hardness

a) Metal coating with electrode El CrW2

b) Metal coating with electrode ElCrW8Co

c) Metal coating with electrode El 62 H
Figure 1. Variation of micro-hardness

In the case of coating with El CrW2 electrode (figu re 1a), which has 0.4% Carbon, and the alloys eleme nts are
very low, maximum is Vanadium (3%), the influence o f laser heat treatment is very good, the micro-hard ness
increasing from 151 HV 0.1, in the case of classical heat treatment after wel ding coating, to 274 HV 0.1 , 206 HV 0.1 and
236 HV 0.1 , in the case of application the laser heat treatme nt after welding coating.
In the case of coating with electrode ElCrW8Co (fig ure 1b), which has a medium carbon content of 0.45% , and
alloys elements are relatively high, Cr – 2%, W – 7 %, the effect of laser heat treatment is very good for
microstructures and micro hardness. So the micro ha rdness in case of classical treatment (600șC) after welding

coating the micro hardness is 87,6 HV 0.1 , and in the case of application of laser heat trea tment after welding coating
the micro hardness is 128 HV 0.1 , 151 HV 0.1 , 160 HV 0.1 .
In case of coating with El 62 H (figure 1c), which have big carbon content 0.7%, the effect of laser h eat
treatment has significantly increased the micro-har dness starting with 1400 W laser power.

4.2. Wear resistance

Determination of wear resistance was made with dete rmination of weight loss after 15, 30, 60 and 120 m inutes
of wear the samples were weighed with ”Oertling – E ngland” balance, accuracy 10 -2 grams. Results are presented in
the wear diagrams (figures 2…4).

00,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2
15 30 60 120
Time [min.] Weight loss [gr.] 1-0
1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
1-9

Figure 2. Wear resistance of metal coating layers w ith ElCrW2
00,02 0,04 0,06 0,08 0,1 0,12 0,14
15 30 60 120
Time [min.] Weight loss [gr.] 2-0
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9

Figure 3. Wear resistance of metal coating layers w ith ElCrW8Co

00,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16
15 30 60 120
Time [min.] Weight loss [gr/] K-0
K-1
K-2
K-3
K-4
K-5
K-6
K-7
k-8
k-9

Figure 4. Wear resistance of metal coating layers w ith El62H

In wear diagrams, in figure 2 to figure 4, variatio ns of wearing resistance are presented. The tests w ere made
measuring of mass loss after 15 minute, 30 minute, 60 minute and 120 minute. The diagrams present a co mparison
between the case of just surface coated (non TT) an d different variants of laser surface heat treatmen t after surface
coating with different powers. In all cases one can observe the wearing resistance is much better when applied
surface hardening with laser after surface coating, rather than tempering at 600șC.

4.3. Electronically microscopy – SEM

Figure 5. SEM metal coating layers with ElCrW2+ las er surface hardening 1700 W

Figure 6. SEM metal coating layers with ElCrW2+ las er surface hardening 2700 W

Figure 7. SEM metal coating layers with ElCrW8Co+ l aser surface hardening 2425 W

Figure 8. SEM metal coating layers with El 62 H+ la ser surface hardening 1400 W

Figure 9. SEM metal coating layers with El62H+ lase r surface hardening 2150 W

Figure 10. SEM metal coating layers with El62H+ las er surface hardening 2425 W

Figure 11. SEM metal coating layers with El62H+ las er surface hardening 2700 W

Analyzing the SEM structures one can observe that t he interface between these two kinds of martensite is not
visible and the martensite itself has similar shape and dimensions at the centre or the surface of the track. The
martensite laths have a typical width of 0.2 µm in all analyzed parts of the specimen. From the f igure, the melted
layer only surpasses 50 µm in depth when the laser power is 1800 W. During h eating, the eutectoid structure of
pearlite quickly changes to austenite when the temp erature rises above A c1 . The rapid diffusion of C between
cementite and ferrite is aided by the small spacing of these phases, only about 0.3 µm. This means that the interface
between the base material and the heat treat zone i s quite sharp, as observed, and the reaction does n ot need
overheating. Therefore, the phase transformation fr om pearlite to austenite begins just above A c1 . On the other hand,
the reaction between the austenite and the proeutec toid ferrite at A c3 requires long-range diffusion of C and other
elements, as well as a BCC-to-FCC phase change.

4.4. Chemical Composition – EDS

Figure 12. EDS metal coating layers with ElCrW2+ la ser surface hardening 2700 W

Figure 13. EDS metal coating layers with ElCrW8Co+ laser surface hardening 2425 W

Figure 14. EDS metal coating layers with El62H+ las er surface hardening 2700 W

Studying the EDS chemical analyzing results, one ca n observe in case of metal coating layers with El C rW2
(fig.12) after laser surface hardening with 2700 W power, when formation the vitrified structures, the Cr content
determinate formation of Cr chemical compound. In c ase of electrode El CrW8Co (fig.13) when the Cr con tent is
smaller the percent of Cr chemical compound is smal l. In case of electrodes El 62H (fig.14) when the C r content is
bigger because C content is big, the formation of C r chemical compound is blocked.

5. CONCLUSIONS

In terms of microstructure, the longitudinal cross- section metallography revealed two regions: one com posed of
martensite, and the other composed of proeutectoid ferrite, unchanged pearlite and martensite. Except at 1400 W
where the material remained practically unchanged, the rippled shape of the upper surface indicates th at part of the
martensite came from melting followed by rapid soli dification. Another portion of martensite, after th e resolidified
layer, came from the homogenization of the microstr ucture, austenitization and rapid cooling.
The base material has ferrite – pearlite structure, with lamellar pearlite. This research reveals colu mnar –
dendrites structures in Fe matrix with god delimita tion. One can observe the metal coating layers is influenced by
the laser heat treatment, by disappearing of dendri tes structures, appearing the compound of Carbon an d Chrome
conglobated in the Fe matrix.
Distribution of chemical composition in EDS analyze s reveals the increasing of chemical compound of C and
Cr. This increasing follows the increasing of inten sity of laser surface treatment. Same influence is observed in
dilution of Fe percentage of this element extractin g.
After the present analyses of the laser-hardened an d remelted shaft under different power conditions, the
following conclusions could be drawn:
• The laser beam heating produces two kinds of region s inside the laser tracks. One region is composed
predominately by martensite, and other region prese nt is unchanged proeutectoid ferrite, martensite an d some
pearlite;
• The case depth varies with the laser power. The max imum hardened depth is 0.3 mm for a laser power of 2700
W. Under high power, 1400 and 1800 W, the laser tra cks partially overlap; therefore, some tempering oc curs at
the overlapped zones;

• The current methodology shows a promising alternati ve to induction-hardened shafts and could be easily
implemented within the production process. The meth od is rapid and allows treatment of specific surfac es on the
piece.
• Increasing of C content determines increasing of mi cro-hardness and wear resistance (fig.2…4). Incre asing of Cr
content, determines formation of chemical compound (fig.12) that also effects increase of wear resista nce.
• As one may notice from the experimental outcomes, t he samples with laser heat treatment present a wear ing
resistance that is mostly superior to the classical ly thermal treated samples. In the case of laser he at treatment
layers, the existence in the chemical combination l ayer of isolated dot-shaped pores does not signific antly
influence the hardness of the laser heat treatment area or the cohesion with the diffusion sub-layer. It does allow
though the enhancement of surface lapping capacity as a result of lubricant retention in the superfici al pores,
linked to the surface through channels.
• The optimal thickness of the combination layer with best wearing resistance is 10 – 20 µm.
• The current methodology shows a promising alternati ve to induction-hardened metal coating layers and c ould be
easily implemented within the production process. T he method is rapid and allows treatment of specific surfaces
on the piece.

Acknowledgement: This paper is supported by Sectori al Operational Programme Human Resources Developmen t
(SOP HRD), finance from the European Social Fund an d by Romanian Government under project number
POSDRU/89/1.5/S/59323

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