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Study on the softening in overlapping zone by laser-overlapping scanning surface hardening for carbon and alloyed steel
ARTICLE IN PRESS Optics and Lasers in Engineering 48 (2010) 20–26
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Study on the softening in overlapping zone by laser-overlapping scanning surface hardening for carbon and alloyed steel Chengwu Yao a,b,, Binshi Xu c, Jian Huang a,b, Peilei Zhang a,b, Yixiong Wu a,b a
School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, China Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai 200030, China c National Key Laboratory for Remanufacturing, Beijing 100072, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 January 2009 Received in revised form 19 April 2009 Accepted 5 May 2009 Available online 22 August 2009
Softening in overlapping passes by laser-overlapped scanning surface hardening is a difficult problem of laser surface modification. Despite the advantage of laser quenching, softening in overlapping zone limits its practical application. In this paper, 45, 9Cr2Mo and W18Cr4V steel were hardened by laseroverlapping quenching. Softening occurred in all overlapping zones. The results of hardness testing indicated the softening width of 45 steel was the broadest and that of W18Cr4V steel was the narrowest. Different mixed microstructures composed the overlapping zone for three steels, i.e., tempered sorbite and a little tempered martensite in 45 steel, tempered martensite, tempered sorbite and a small amount of carbides in 9Cr2Mo steel, tempered martensite and a little carbide in W18Cr4V steel. The effect of activation energy of diffusion for carbon in steel and cooling rate on the decomposition of martensite have been investigated by developing a diffusion model based on the principle of carbon diffusion in martensite. The results indicated that action energy for diffusion of carbon in steel plays a main role in hindering decomposition of carbide and cooling rate has a limited action in reducing temper softening during laser-overlapping scanning. & 2009 Published by Elsevier Ltd.
Keywords: Laser quenching Overlapping zone Tempering softening Activation energy of diffusion Cooling rate
1. Introduction Laser quenching is an important surface-treatment technology for carbon steel and alloyed steel components. During laser quenching, the surface of the components is heated to form austenite, and then quenched to form martensite. Compared to the standard hardening process, the most significant features of laser quenching are high heating and self-quenching rates [1–4]. Moreover, it does not need any liquid or gaseous quench media for the quenching of austenitized surface of the large components during laser quenching. Laser surface transformation hardening not only improves the wear resistance, but also increases the fatigue strength under certain conditions, due to the compressive stresses induced on the surface of the components [5–7]. However, laser quenching is seldom applied to the components needing large areas hardening, such as cold roll, in industrial production. This is attributed to temper softening phenomena occurring at the contact parts subjected to laser beam repeating scan. While the laser beam passes a pervious pass a small distance away, the overlapping zone is exposed to a thermal cycle and is
tempered in a short time [8]. In previous quenching pass, a tempering zone with lower hardness was formed. For the practical application of large areas laser quenching, softening in overlapping zone is a problem that has to be solved. Recently, Tani et al. [9,10] presents an original tempering model for the prediction of the hardness reduction in multiple laser paths. The reaustenitization of the martensite and the non-constant tempering temperature are considered during the modeling of tempering. In this paper, the softening phenomena in overlapping passes by laser-overlapping scanning surface hardening for three types of steel was investigated. After laser treatment, the steel shows different features in overlapping zone. An analytical model is developed based on the principle of martensite decomposition and carbon diffusion migration to reveal the reasons why the tempering effect is quite strong for 45 steel and somewhat weak for W18Cr4V steel despite the short duration time of the laser passage. The effects of the action energy for diffusion of carbon in steel and cooling rate on tempering softening are analyzed.
2. Experimental procedures Corresponding author at: School of Materials Science and Engineering,
Shanghai Jiaotong University, Shanghai 200030, China. E-mail addresses: [email protected] (C. Yao), [email protected] (J. Huang). 0143-8166/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.optlaseng.2009.05.001
The 45, 9Cr2Mo and W18Cr4V steel were processed by laser quenching. The chemical compositions are listed in Table 1. The specimen is 150 mm long, 100 mm wide and 10 mm thick. Before
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Table 1 Chemical compositions of the materials used (wt%). Name
C
Mn
Si
P
S
Cr
W
V
Mo
45 9Cr2Mo W18Cr4V
0.42–0.50 0.85–0.95 0.70–0.80
0.50–0.80 0.20–0.35 0.10–0.40
0.17–0.37 0.25–0.45 0.2–0.4
r0.035 r0.025 r0.030
r0.035 r0.025 r0.030
r0.25 1.70–2.10 0.80–4.40
– – 17.50–19.50
– – 1.0–1.40
–
laser treatment, the specimens were sand blasted to obtain a rough surface. Then the specimens were cleaned with acetone and sprayed with a kind of absorbing coating to enhance the surface absorptivity. The coating is made up of 80 wt% talcum powder, 10 wt% titanium dioxide powder and 10 wt% Hydroxyethyl cellulose. A continuous-wave CO2 laser system (Trumpf TCF15000) with a maximum output power of 15 kW and laser beam diameter of 0.7 mm was used, and the wave length of the laser beam was 10.6 mm. The power of the laser as measured on the specimen was 4.0 kW, and the distance of the focus point above the specimen was 80 mm resulting in a spot size of 5.5 mm. The overlap of the laser quenching tracks was about 20% of the width. The scanning velocity was 4 m/min and the delay time between two passes was 5.0 min to cool the samples down to room temperature again. After the laser treatment, cross-sectional specimens were prepared for hardness measurement, optical microscopy (OM), and scanning electron microscopy (SEM).
0.20–0.40 r0.30
Overlapping zone
1.0mm Fig. 1. Cross section image of overlapping passes (9Cr2Mo steel).
3. Results 3.1. Hardness test The region between the pass and the second pass is the overlapping zone. The structure of the overlapping zone is shown in Fig. 1. In the overlapping zone, if the peak temperature of the second pass is below the austenitized temperature of the steel during laser quenching, martensite formed in the first pass might be decomposed which leads to tempering softening. Fig. 2 shows the hardness distribution of the surface treated by laser quenching. It can be seen from Fig. 1 that tempering softening happens in overlapping zone. In the hardening zone, the average hardness of 45, 9Cr2Mo and W18Cr4V steel are HV760, HV955 and HV908, respectively. It defines that tempering softening happens when the hardness below 85 pct of hardening zone. The widths of softened zones of 45, 9Cr2Mo and W18Cr4V steel are about 1.0, 0.75 and less 0.5 mm, respectively. In the overlapping zone, the hardness of 45, 9Cr2Mo and W18Cr4V steel decreases in sequence, as shown in Fig. 1.
3.2. Microstructures of the laser-hardening zone Fig. 3 shows the cross-sectional microstructures in laserhardening zone. It is found that there is lath martensite and a little retained austenite phases for 45 steel, which the white phase is retained austenite that did not transform during the rapid quenching, as shown in Fig. 3a. Needle martensite, retained austenite and a little of carbide phase are found in laser-hardening zone of 9Cr2Mo steel, as shown in Fig. 3b. Fig. 3c shows the microstructures of W18Cr4V steel. The microstructures of hardening zone are composed of needle martensite and retained austenite. Comparing with 9Cr2Mo steel, a large amount of residual austenite is produced due to the existence of
Fig. 2. Hardness distribution of the surface treated by laser-overlapping scanning.
alloy elements in W18Cr4V steel during laser transformation hardening, which decreases surface hardness, as shown in Fig. 2.
3.3. Microstructures of the overlapping zone The overlapping zone microstructures of 45, 9Cr2Mo and W18Cr4V steels show different features, as shown in Figs. 4 and 5. Fig. 4a shows the cross-sectional overlapping zone microstructures of 45 steel, which consists of tempered sorbite and a little tempered martensite. It can be seen that martensite formed in the first pass are decomposed into sorbite after heating cycle of the second track. Comparing with 45 steel, 9Cr2Mo steel has a large amount of tempered martensite and carbide and relatively less quantities of sorbite, as shown in Fig. 4b.
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C. Yao et al. / Optics and Lasers in Engineering 48 (2010) 20–26
20µm
20µm
20µm Fig. 3. Optical micrograph in laser-hardening zone: (a) 45 steel; (b) 9Cr2Mo steel; and (c) W18Cr4V steel.
20µm
20µm
Fig. 4. Cross-sectional views of multi-track LTH samples in the overlapping zone: (a) 45 steel; and (b) 9Cr2Mo steel.
Overlapping zone
0.4mm Fig. 5. Cross-sectional views of the overlapping zone of W18Cr4V steel: (a) profile of the overlapping zone; and (b) SEM micrograph showing the microstructure of the overlapping zone.
ARTICLE IN PRESS C. Yao et al. / Optics and Lasers in Engineering 48 (2010) 20–26
Fig. 5a shows the cross-sectional microstructures of W18Cr4V steel in overlapping zone. The width of the overlapping zone is about 0.25–0.35 mm. Fig. 5b shows the SEM microstructure of the overlapping zone. Unlike 45 and 9Cr2Mo steel, sorbite was not observed in W18Cr4V steel. Instead, overlapping zone microstructures of W18Cr4V steel are mainly composed of tempered martensite, retained austenite and a small amount of carbide. In general, the hardness of sorbite is significantly lower than that of martensite. The hardness profiles in Fig. 2 show that the hardness in overlapping zone of 45, 9Cr2Mo and W18Cr4V steel increases in sequence, corresponding to the reduction of sorbite. During a heat treatment after tempering cycle, metastable phases are usually transformed to be more stable phases. In this study, although the duration of tempering cycle during second pass laser treatment is very short, the martensite of hardening zone in the first pass is still decomposed into sorbite and/or tempering martensite. However, the degree of decomposition of three kinds of steel is different, which is related to material properties.
4. Discussion For 45, 9Cr2Mo and W18Cr4V steel processed by laseroverlapping quenching, the softening of overlapping zone seems to be caused mainly by the tempering of the martensite. The results demonstrate that laser heat treatment can produce significantly different microstructures in three types of steel under the same conditions. From the metallographic point of view, the decomposition of martensite results from the diffusion of the solid solution elements. In general, carbon atom diffusion in carbon and alloyed steel is easier than that in other alloying elements, due to its lower diffusion activation energy of carbon atom [11–13]. From this perspective, the tempering is a process controlled by the diffusion of carbon atom. Tempering heat treatment is heating martensitic steel to a temperature below the eutectoid point and holding for a specified time period. Such heat treatment leads to the formation of tempered martensite according to the reaction [11], i.e., martensite ðBCT; single phaseÞ ! tempered martensite ða þ Fe3C phasesÞ where the single-phase BCT martensite supersaturated with carbon transforms to the tempered martensite composed of the stable ferrite and cementite phases. As mentioned in the above discussion, a diffusion model was developed to describe carbon diffusion in the martensite phase after laser treatment and to check whether the differences can be
C2 (martensite)
C1 (dislocation)
C2
t2> t1> t0
t0
t1
0
C2
M
explained by diffusion mechanisms, as shown in Fig. 6. Fig. 6 is the distribution profile of carbon concentration in martensite during tempering. The carbon atoms in martensite phase enters intergranular dislocation cluster by one dimension diffusion couple, as shown in Fig. 6a. Laser tempering is a process with large cooling rate. The carbon diffusion in this case obeys the Fick’s second law @C @2 C ¼ DðtÞ 2 @t @x
(1)
where C is concentration of carbon atom, t diffusion time determined by cooling rate, D(t) the diffusivity of carbon atom in martensite which is a function of time t, and x is space coordinate measured from dislocation to martensite matrix (the plane at x ¼ 0 is an interface between dislocation cluster and martensite matrix). The carbon concentration profile of Eq. (1) is plotted by the real line in Fig. 6b. Usually, the value of C(x) on the plane at x ¼ 0 is equal to (C1+C2)/2. However, for martensite laser tempering, there is no such case in overlapping zone. Carbides forms during carbon atoms in the martensite matrix entering into the dislocation cluster, resulting in the trapping of carbon atom on the plane at x ¼ 0. The corresponding carbon concentration profile is plotted by the dash line in Fig. 6b. The boundary condition of this equation can be expressed as ( 0; ðx ¼ 0Þ (2) Cðx; t40Þ ¼ C 0 ðx ¼ 1Þ In this study, the temperature T of laser tempering in overlapping zone is the function of time t. Normally, tempering is carried out at temperatures between 200 and 700 1C [14]. However, considering the thermodynamic non-equilibrium phenomenon during laser-overlapping scanning, tempering peak temperature in overlapping zone should be higher than that of conventional tempering. T can be expressed as TðtÞ ¼ T p
Z
t p þ Dt tp
Ai dt
(3)
where Tp is peak temperature (K), tp the time of peak temperature (s) and Ai cooling rate in tptp+Dt (K/s). Since the heating and selfquenching rates are typically on the order of 104 K/s or greater during laser surface hardening [3], the relationship of T and t is assumed to be linear in all continuous cooling process. Eq. (3) can be simplified as TðtÞ ¼ T p At
(4)
where A is average cooling rate in tempering process; according to the Arrhenius equation [15], the diffusivity D(t) at any given value of Q after time t will be written empirically as Q DðtÞ ¼ D0 exp (5) RTðtÞ where D0 is diffusion constant (m2/s), Q activation energy (J/mol) and R the gas constant. R Defining Z ¼ D(t) dt, this integrates to Z ¼ t0D0 exp (Q/RT(t)) dt. Thus, Eq. (1) becomes
t2
(C1+C2)/2 C1
23
C1 x=0 x Fig. 6. Distribution of carbon concentration in martensite
@C @2 C ¼ @Z @x2 The general solution of Eq. (6) is given by ! x Cðx; ZÞ ¼ A þ Berf pffiffiffiffiffiffi 4Z
(6)
(7)
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where A and B are constant. Combining with the boundary condition Cðx; Z40Þ ¼ ( 0; ðx ¼ 0Þ , a particular solution of Eq. (7) can be given by C 0 ðx ¼ 1Þ ! x Cðx; ZÞ ¼ C 0 erf pffiffiffiffiffiffi 4Z
(8)
It is well known that martensite is a non-equilibrium singlephase structure which results from diffusionless transformation of austenite and all the carbon atoms remain as interstitial impurities in martensite [16]. Therefore, the carbon concentration C0 in Eq. (8) can be regarded as the carbon content in the steels studied in this paper. The diffusion coefficient D0 for carbon in a-iron is 0.2 105 m2/s and the gas constant R is 8.3145. To solve the Eq. (8), the diffusion activation energy Q should be confirmed first. Q may be thought of as the energy required driving the diffusive motion of 1 mol carbon atoms. D is controlled by Q. The presence of alloying elements other than carbon (e.g., Cr, Ni, Mo, and W) will slowdown the mitigation diffusion rate of carbon atom in martensite by increasing its activation energy Q. Some works on the calculating of Q of steels containing Cr, Mn, and Mo have been carried out [17]. The hardness of the steel was tested after tempering at different temperature and the activation energy was measured by indirect method such as statistics, equivalence or equivalent weight. According to these studies, the empirical equation of diffusion activation energy of carbon in carbon or low-alloyed steel is obtained Q ¼ 114:69ðMoeq Þ þ 46:64 ðkcal=molÞ
Fig. 7. Carbon distributions in martensite with different diffusion energy (peak temperature Tp ¼ 1173 K).
(9)
where Moeq is equivalent weight of Mo element in steel, given by 1 Moeq ¼ Mo þ 15Mn þ 10 Cr
(10)
Calculating by Eqs. (9) and (10), the ratio of Q in 45 steel and 9Cr2Mo steel is 1:1.7. The result indicates that the diffusion activation energy of 45 steel is significant lower than that of 9Cr2Mo steel containing Mo and Cr element. For 45 steel, the activation energy Q is 263.9 kJ/mol (63 kcal/mol). As for 9Cr2Mo or W18Cr4V steel, the diffusion activation energy Q for diffusion of carbon is higher than those of 45 steel, due to larger amount of Cr, Mo and W (see Table 1). However, the values of diffusion activation energy for diffusion of carbon in 45, 9Cr2Mo and W18Cr4V steels have not been reported. Therefore, different activation energy Q has been chosen to evaluate their influence on the laser tempering. The carbon content in martensite w(C) of 0.85% and cooling rate A of 200 1C/s are used in the calculation. Peak temperature Tp is selected as 900 1C. The results are shown in Fig. 7. It can be seen from Fig. 7 that the decreased degree of carbon concentration in a certain part of martensite has obvious slowdown tendency as the activation energy Q increases. When Q are 3.0 105, 2.5 105, 2.0 105, and 1.5 105 J/mol, carbon concentration in martensite drop within the range of 6.5, 96, 1325 and 45000 A˚, respectively. The literature [17] revealed that a 10 A˚ segregation of carbon in martensite after low temperature (20–150 1C) tempering was detected by analyzing the peak intensity of X-rays diffuse scattering, which was regarded as the sign of martensite entering into decomposition stage. According to this inference and principle of martensite tempering, conclusions on Fig. 7 may be given: (1) on the condition of Q of 1.5 105 and 2.0 105 J/mol, full carbon segregation occurs in 10 A˚ and martensite have entered into decomposition stage; (2) on the condition of Q of 2.5 105 J/mol, imperfect carbon segregation occurs in 10 A˚ and martensite have partially entered into decomposition stage; and (3) on the condition of Q of 3.0 105 J/mol,
Fig. 8. Carbon distributions in martensite with different diffusion energy (peak temperature Tp ¼ 900 1C).
the distance of carbon migration was only 6.5 A˚ and martensite have not entered into the decomposition stage. Peak temperature Tp has great influence on martensite decomposition. Fig. 8 shows the results in which peak temperature Tp is selected as 1100 1C. Under the same activation energy Q, the degree of martensite decomposition at peak temperature Tp of 1100 1C is significantly higher than that of Tp of 900 1C, as shown in Figs. 7 and 8. On the condition of Q of 3.0 105 J/mol, martensite has not entered into decomposition stage in Fig. 7 (Tp ¼ 900 1C), but the result in Fig. 8 (Tp ¼ 1100 1C) is that martensite have partially entered into the decomposition stage. It can be seen from Fig. 8 that when activation energy Q is 3.5 105 J/mol, the largest diffusion distance of carbon is 7.7 A˚ and martensite have not entered into the decomposition stage. According to Eqs. (9) and (10), the activation energy Q of 9Cr2Mo
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The overlapping zone The first pass
The second pass
0.2mm Fig. 9. Carbon distributions in martensite at different cooling rate (Q ¼ 2.5 105 J/ mol).
Fig. 10. Hardness distribution of surface after laser-overlapping scanning and tempering.
steel is about 398 KJ/mol (95 kcal/mol). Based on Fig. 8, martensite in 9Cr2Mo steel cannot be decomposed. However, in this study, martensite in laser-overlapping area of 9Cr2Mo steel has partially decomposed into sorbite during laser tempering, as shown in Fig. 4b. Therefore, for laser tempering, actual peak tempering temperature Tp may be higher than 1100 1C, due to its nonequilibrium thermodynamics process. If peak temperature Tp is selected as 1300 1C, computering by model, martensite with activation energy Q of 398 KJ/mol will partially enter into the decomposed stage. Fig. 9 shows the carbon distribution in martensite at different tempering cooling rate on the condition of Q of 2.5 105 J/mol. It can be seen from Fig. 10 that on the condition of cooling rate of 200, 1000 1C/s, full carbon segregation occurs in 10 A˚ and martensite have entered into the decomposition stage. Moreover, even cooling rate is up to 104 1C/s, carbon concentration drop within the range of 94 A˚ and martensite have partially entered into decomposition stage, as shown in Fig. 9. Through analyzing the results shown in Figs. 7–9, it can be concluded that the activation energy Q for diffusion of carbon in steel is a main reason that causes the difference in microstructures and temper softening in overlapping zone of 45, 9Cr2Mo and W18Cr4V steel during laser-repeating scanning.
Fig. 11. Cross-sectional image and microstructure of W18Cr4V through furnace tempering at 560 1C for 2.0 h after laser multi-pass quenching.
In the practical application, quenching is an important heat treatment process for steel. The aim of this treatment is to obtain martensitic microstructure with excellent hardness and strength properties. For carbon and low-alloy steel such as 45 and 9Cr2Mo steel, the diffusion activation energy of carbon and the decomposition temperature of martensite are low due to their small alloying element content. Therefore, low temper temperature under 200 1C is usually chosen to treat these steels. The structures of these tempered steels are tempered martensite. However, for 45 steel, there is mainly sorbite in overlapping zone by multi-passes laser quenching. From this perspective, multi-passes laser quenching of steels similar to 45 steel cannot meet the requirement of large areas laser hardening. It is essential to point out that increasing cooling rate will limit the carbon diffusion in martensite, i.e., reducing decomposition of martensite, but this action is limited, as shown in Fig. 9. Tempering temperature of about 560 1C is usually chosen in W18Cr4V steel, which is attributed to the alloying elements of Cr, W and V. Tempering microstructures of W18Cr4V steel are mainly tempered martensite and carbide. In this study, the microstructures in the overlapping zone of W18Cr4V steel are also tempered martensite and carbide, which is a structural fundament for subsequent hardness homogenizing treatment. Fig. 10 shows the hardness distribution of surface of W18Cr4V steel through furnace tempering at 560 1C for 2.0 h after multi-pass laser quenching. It can be seen from Fig. 10 that the hardness in overlapping zone is somewhat increased and the whole surface hardness becomes uniform. Fig. 11 shows the OM cross-sectional morphology of multi-pass laser quenching of W18Cr4V steel after furnace tempering at 560 1C for 2.0 h, where the dash lines denote the location of laser-hardening zone. Macromorphology properties between the laser hardness zone and the overlapping zone tend to be uniform, as shown in Fig. 11.
5. Conclusions In this study, softening in overlapping passes by laser-overlapped scanning surface hardening was formed in 45, 9Cr2Mo and W18Cr4V steel. The width of softened zone in turn is 45, 9Cr2Mo and W18Cr4V steel and the lowest hardness in softened zone are increased successively. There is big difference in microstructure of softened zone for three types of steels. The 45 steel mainly contains sorbite and 9Cr2Mo mainly contains tempered martensite, sorbite and carbide. It is noticeable that there is
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mainly tempered martensite, retained austensite and a small amount of carbide for W18Cr4V steel. Sorbite was not observed in softened zone of W18Cr4V steel. A model for analysizing tempering softening in overlapping zone was proposed. Computering by the model, it can be concluded that cooling rate has a limited action in reducing tempering softening and activation energy of diffusion for carbon in steel plays a main role in hindering decomposition of carbide in overlapping zone during laser-overlapping scanning.
Acknowledgement Funding support from Shanghai Key Laboratory of Materials Laser Processing and Modification No. 06DZ22102, it is gratefully acknowledged. References [1] Steen William M. Laser materials processing [M]. Berlin: Springer; 1991. [2] Duley WW. Laser processing analysis of materials [M]. New York, NY: Plenum Press; 1983. [3] Bradley John R, Kim Sooho. Laser transformation hardening of iron-carbon iron- carbon-chromium steels [J]. Metallurgical and Materials Transactions A 1988;19(8):1015–2013. [4] Kou Sindo, Sun DK, Le YP. A fundamental study of laser transformation hardening [J]. Metallurgical and Materials Transactions A 1983;14(3):643–53.
[5] Kaul Rakesh, Ganesh P, Tiwari Pragya, Nandedkar RV, Nath AK. Characterization of dry sliding wear resistance of laser surface hardened En 8 steel [J]. Journal of Materials Processing Technology 2005;167(1):83–90. [6] Shao Tianmin, Hua Meng, Tam Hon Yuen. Impact wear behavior of laser hardened hypoeutectoid 2Cr13 martensite stainless steel [J]. Wear 2003; 255(1–6):444–55. [7] Senthil Selvan J, Subramanian K, Nath AK. Effect of laser surface hardening on En18 (AISI 5135) steel [J]. Journal of Materials Processing Technology 1999;91(1–3):29–36. [8] Tani G, Orazi L, Fortunato A. Prediction of hypoeutectoid steel softening due to tempering phenomena in laser surface hardening. CIRP Annals—Manufacturing Technology 2008;57(1):209–12. [9] Tani G, Orazi L, Fortunato A, Campana G, Ascari A. 3D modelling of LASER hardening and tempering of hypo-eutectoid steels. JLMN—Journal of Laser Micro/Nanoengineering (online) 2008;3:124–8. [10] Hegge HJ, De Beurs H, Noordhuis J, De Hosson JThM. Tempering of steel during laser treatment [J]. Metallurgical Transactions A 1990;21:987–95. [11] Bhadeshia HKDH, Honeycombe RWK. Steels-microstructure properties [M]. 3rd ed. London: Elsevier Ltd; 2006. [12] Cardona M, Fulde P, von Klitzing K, Merlin R, Queisser H-J. Diffusion in solids – fundamentals, methods, materials, diffusion – controlled processes [M]. New York: Springer Berlin Heidelberg; 2007. [13] Porter DA, Easterling KE. Phase transformation in metals and alloys [M]. 2nd ed. 2–6 Bormdary Row, London SEl 8HN, UK: Chapan & Hall; 1993. [14] Smallman RE, Bishop RJ. Modern physical metallurgy materials engineering [M]. 6th ed. Oxford: Butterworth-Heinemann; 1999. [15] Kostorz G, Zu¨rich ETH, Phase Transformationsin Materials [M]. New York: Chichester Brisbane Singapore Toronto, 2001. [16] Callister Jr. William D. Materials science engineering: an introduction [M]. 7th ed. Australia: Wiley; 2005. p. 344. [17] Zhong SH. The temper technology temper equation of steels [M]. Beijing: China machine press; 1993.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Study on the softening in overlapping zone by laser-overlapping scanning surface hardening for carbon and alloyed steel Chengwu Yao a,b,, Binshi Xu c, Jian Huang a,b, Peilei Zhang a,b, Yixiong Wu a,b a
School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, China Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai 200030, China c National Key Laboratory for Remanufacturing, Beijing 100072, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 January 2009 Received in revised form 19 April 2009 Accepted 5 May 2009 Available online 22 August 2009
Softening in overlapping passes by laser-overlapped scanning surface hardening is a difficult problem of laser surface modification. Despite the advantage of laser quenching, softening in overlapping zone limits its practical application. In this paper, 45, 9Cr2Mo and W18Cr4V steel were hardened by laseroverlapping quenching. Softening occurred in all overlapping zones. The results of hardness testing indicated the softening width of 45 steel was the broadest and that of W18Cr4V steel was the narrowest. Different mixed microstructures composed the overlapping zone for three steels, i.e., tempered sorbite and a little tempered martensite in 45 steel, tempered martensite, tempered sorbite and a small amount of carbides in 9Cr2Mo steel, tempered martensite and a little carbide in W18Cr4V steel. The effect of activation energy of diffusion for carbon in steel and cooling rate on the decomposition of martensite have been investigated by developing a diffusion model based on the principle of carbon diffusion in martensite. The results indicated that action energy for diffusion of carbon in steel plays a main role in hindering decomposition of carbide and cooling rate has a limited action in reducing temper softening during laser-overlapping scanning. & 2009 Published by Elsevier Ltd.
Keywords: Laser quenching Overlapping zone Tempering softening Activation energy of diffusion Cooling rate
1. Introduction Laser quenching is an important surface-treatment technology for carbon steel and alloyed steel components. During laser quenching, the surface of the components is heated to form austenite, and then quenched to form martensite. Compared to the standard hardening process, the most significant features of laser quenching are high heating and self-quenching rates [1–4]. Moreover, it does not need any liquid or gaseous quench media for the quenching of austenitized surface of the large components during laser quenching. Laser surface transformation hardening not only improves the wear resistance, but also increases the fatigue strength under certain conditions, due to the compressive stresses induced on the surface of the components [5–7]. However, laser quenching is seldom applied to the components needing large areas hardening, such as cold roll, in industrial production. This is attributed to temper softening phenomena occurring at the contact parts subjected to laser beam repeating scan. While the laser beam passes a pervious pass a small distance away, the overlapping zone is exposed to a thermal cycle and is
tempered in a short time [8]. In previous quenching pass, a tempering zone with lower hardness was formed. For the practical application of large areas laser quenching, softening in overlapping zone is a problem that has to be solved. Recently, Tani et al. [9,10] presents an original tempering model for the prediction of the hardness reduction in multiple laser paths. The reaustenitization of the martensite and the non-constant tempering temperature are considered during the modeling of tempering. In this paper, the softening phenomena in overlapping passes by laser-overlapping scanning surface hardening for three types of steel was investigated. After laser treatment, the steel shows different features in overlapping zone. An analytical model is developed based on the principle of martensite decomposition and carbon diffusion migration to reveal the reasons why the tempering effect is quite strong for 45 steel and somewhat weak for W18Cr4V steel despite the short duration time of the laser passage. The effects of the action energy for diffusion of carbon in steel and cooling rate on tempering softening are analyzed.
2. Experimental procedures Corresponding author at: School of Materials Science and Engineering,
Shanghai Jiaotong University, Shanghai 200030, China. E-mail addresses: [email protected] (C. Yao), [email protected] (J. Huang). 0143-8166/$ - see front matter & 2009 Published by Elsevier Ltd. doi:10.1016/j.optlaseng.2009.05.001
The 45, 9Cr2Mo and W18Cr4V steel were processed by laser quenching. The chemical compositions are listed in Table 1. The specimen is 150 mm long, 100 mm wide and 10 mm thick. Before
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Table 1 Chemical compositions of the materials used (wt%). Name
C
Mn
Si
P
S
Cr
W
V
Mo
45 9Cr2Mo W18Cr4V
0.42–0.50 0.85–0.95 0.70–0.80
0.50–0.80 0.20–0.35 0.10–0.40
0.17–0.37 0.25–0.45 0.2–0.4
r0.035 r0.025 r0.030
r0.035 r0.025 r0.030
r0.25 1.70–2.10 0.80–4.40
– – 17.50–19.50
– – 1.0–1.40
–
laser treatment, the specimens were sand blasted to obtain a rough surface. Then the specimens were cleaned with acetone and sprayed with a kind of absorbing coating to enhance the surface absorptivity. The coating is made up of 80 wt% talcum powder, 10 wt% titanium dioxide powder and 10 wt% Hydroxyethyl cellulose. A continuous-wave CO2 laser system (Trumpf TCF15000) with a maximum output power of 15 kW and laser beam diameter of 0.7 mm was used, and the wave length of the laser beam was 10.6 mm. The power of the laser as measured on the specimen was 4.0 kW, and the distance of the focus point above the specimen was 80 mm resulting in a spot size of 5.5 mm. The overlap of the laser quenching tracks was about 20% of the width. The scanning velocity was 4 m/min and the delay time between two passes was 5.0 min to cool the samples down to room temperature again. After the laser treatment, cross-sectional specimens were prepared for hardness measurement, optical microscopy (OM), and scanning electron microscopy (SEM).
0.20–0.40 r0.30
Overlapping zone
1.0mm Fig. 1. Cross section image of overlapping passes (9Cr2Mo steel).
3. Results 3.1. Hardness test The region between the pass and the second pass is the overlapping zone. The structure of the overlapping zone is shown in Fig. 1. In the overlapping zone, if the peak temperature of the second pass is below the austenitized temperature of the steel during laser quenching, martensite formed in the first pass might be decomposed which leads to tempering softening. Fig. 2 shows the hardness distribution of the surface treated by laser quenching. It can be seen from Fig. 1 that tempering softening happens in overlapping zone. In the hardening zone, the average hardness of 45, 9Cr2Mo and W18Cr4V steel are HV760, HV955 and HV908, respectively. It defines that tempering softening happens when the hardness below 85 pct of hardening zone. The widths of softened zones of 45, 9Cr2Mo and W18Cr4V steel are about 1.0, 0.75 and less 0.5 mm, respectively. In the overlapping zone, the hardness of 45, 9Cr2Mo and W18Cr4V steel decreases in sequence, as shown in Fig. 1.
3.2. Microstructures of the laser-hardening zone Fig. 3 shows the cross-sectional microstructures in laserhardening zone. It is found that there is lath martensite and a little retained austenite phases for 45 steel, which the white phase is retained austenite that did not transform during the rapid quenching, as shown in Fig. 3a. Needle martensite, retained austenite and a little of carbide phase are found in laser-hardening zone of 9Cr2Mo steel, as shown in Fig. 3b. Fig. 3c shows the microstructures of W18Cr4V steel. The microstructures of hardening zone are composed of needle martensite and retained austenite. Comparing with 9Cr2Mo steel, a large amount of residual austenite is produced due to the existence of
Fig. 2. Hardness distribution of the surface treated by laser-overlapping scanning.
alloy elements in W18Cr4V steel during laser transformation hardening, which decreases surface hardness, as shown in Fig. 2.
3.3. Microstructures of the overlapping zone The overlapping zone microstructures of 45, 9Cr2Mo and W18Cr4V steels show different features, as shown in Figs. 4 and 5. Fig. 4a shows the cross-sectional overlapping zone microstructures of 45 steel, which consists of tempered sorbite and a little tempered martensite. It can be seen that martensite formed in the first pass are decomposed into sorbite after heating cycle of the second track. Comparing with 45 steel, 9Cr2Mo steel has a large amount of tempered martensite and carbide and relatively less quantities of sorbite, as shown in Fig. 4b.
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20µm
20µm
20µm Fig. 3. Optical micrograph in laser-hardening zone: (a) 45 steel; (b) 9Cr2Mo steel; and (c) W18Cr4V steel.
20µm
20µm
Fig. 4. Cross-sectional views of multi-track LTH samples in the overlapping zone: (a) 45 steel; and (b) 9Cr2Mo steel.
Overlapping zone
0.4mm Fig. 5. Cross-sectional views of the overlapping zone of W18Cr4V steel: (a) profile of the overlapping zone; and (b) SEM micrograph showing the microstructure of the overlapping zone.
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Fig. 5a shows the cross-sectional microstructures of W18Cr4V steel in overlapping zone. The width of the overlapping zone is about 0.25–0.35 mm. Fig. 5b shows the SEM microstructure of the overlapping zone. Unlike 45 and 9Cr2Mo steel, sorbite was not observed in W18Cr4V steel. Instead, overlapping zone microstructures of W18Cr4V steel are mainly composed of tempered martensite, retained austenite and a small amount of carbide. In general, the hardness of sorbite is significantly lower than that of martensite. The hardness profiles in Fig. 2 show that the hardness in overlapping zone of 45, 9Cr2Mo and W18Cr4V steel increases in sequence, corresponding to the reduction of sorbite. During a heat treatment after tempering cycle, metastable phases are usually transformed to be more stable phases. In this study, although the duration of tempering cycle during second pass laser treatment is very short, the martensite of hardening zone in the first pass is still decomposed into sorbite and/or tempering martensite. However, the degree of decomposition of three kinds of steel is different, which is related to material properties.
4. Discussion For 45, 9Cr2Mo and W18Cr4V steel processed by laseroverlapping quenching, the softening of overlapping zone seems to be caused mainly by the tempering of the martensite. The results demonstrate that laser heat treatment can produce significantly different microstructures in three types of steel under the same conditions. From the metallographic point of view, the decomposition of martensite results from the diffusion of the solid solution elements. In general, carbon atom diffusion in carbon and alloyed steel is easier than that in other alloying elements, due to its lower diffusion activation energy of carbon atom [11–13]. From this perspective, the tempering is a process controlled by the diffusion of carbon atom. Tempering heat treatment is heating martensitic steel to a temperature below the eutectoid point and holding for a specified time period. Such heat treatment leads to the formation of tempered martensite according to the reaction [11], i.e., martensite ðBCT; single phaseÞ ! tempered martensite ða þ Fe3C phasesÞ where the single-phase BCT martensite supersaturated with carbon transforms to the tempered martensite composed of the stable ferrite and cementite phases. As mentioned in the above discussion, a diffusion model was developed to describe carbon diffusion in the martensite phase after laser treatment and to check whether the differences can be
C2 (martensite)
C1 (dislocation)
C2
t2> t1> t0
t0
t1
0
C2
M
explained by diffusion mechanisms, as shown in Fig. 6. Fig. 6 is the distribution profile of carbon concentration in martensite during tempering. The carbon atoms in martensite phase enters intergranular dislocation cluster by one dimension diffusion couple, as shown in Fig. 6a. Laser tempering is a process with large cooling rate. The carbon diffusion in this case obeys the Fick’s second law @C @2 C ¼ DðtÞ 2 @t @x
(1)
where C is concentration of carbon atom, t diffusion time determined by cooling rate, D(t) the diffusivity of carbon atom in martensite which is a function of time t, and x is space coordinate measured from dislocation to martensite matrix (the plane at x ¼ 0 is an interface between dislocation cluster and martensite matrix). The carbon concentration profile of Eq. (1) is plotted by the real line in Fig. 6b. Usually, the value of C(x) on the plane at x ¼ 0 is equal to (C1+C2)/2. However, for martensite laser tempering, there is no such case in overlapping zone. Carbides forms during carbon atoms in the martensite matrix entering into the dislocation cluster, resulting in the trapping of carbon atom on the plane at x ¼ 0. The corresponding carbon concentration profile is plotted by the dash line in Fig. 6b. The boundary condition of this equation can be expressed as ( 0; ðx ¼ 0Þ (2) Cðx; t40Þ ¼ C 0 ðx ¼ 1Þ In this study, the temperature T of laser tempering in overlapping zone is the function of time t. Normally, tempering is carried out at temperatures between 200 and 700 1C [14]. However, considering the thermodynamic non-equilibrium phenomenon during laser-overlapping scanning, tempering peak temperature in overlapping zone should be higher than that of conventional tempering. T can be expressed as TðtÞ ¼ T p
Z
t p þ Dt tp
Ai dt
(3)
where Tp is peak temperature (K), tp the time of peak temperature (s) and Ai cooling rate in tptp+Dt (K/s). Since the heating and selfquenching rates are typically on the order of 104 K/s or greater during laser surface hardening [3], the relationship of T and t is assumed to be linear in all continuous cooling process. Eq. (3) can be simplified as TðtÞ ¼ T p At
(4)
where A is average cooling rate in tempering process; according to the Arrhenius equation [15], the diffusivity D(t) at any given value of Q after time t will be written empirically as Q DðtÞ ¼ D0 exp (5) RTðtÞ where D0 is diffusion constant (m2/s), Q activation energy (J/mol) and R the gas constant. R Defining Z ¼ D(t) dt, this integrates to Z ¼ t0D0 exp (Q/RT(t)) dt. Thus, Eq. (1) becomes
t2
(C1+C2)/2 C1
23
C1 x=0 x Fig. 6. Distribution of carbon concentration in martensite
@C @2 C ¼ @Z @x2 The general solution of Eq. (6) is given by ! x Cðx; ZÞ ¼ A þ Berf pffiffiffiffiffiffi 4Z
(6)
(7)
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where A and B are constant. Combining with the boundary condition Cðx; Z40Þ ¼ ( 0; ðx ¼ 0Þ , a particular solution of Eq. (7) can be given by C 0 ðx ¼ 1Þ ! x Cðx; ZÞ ¼ C 0 erf pffiffiffiffiffiffi 4Z
(8)
It is well known that martensite is a non-equilibrium singlephase structure which results from diffusionless transformation of austenite and all the carbon atoms remain as interstitial impurities in martensite [16]. Therefore, the carbon concentration C0 in Eq. (8) can be regarded as the carbon content in the steels studied in this paper. The diffusion coefficient D0 for carbon in a-iron is 0.2 105 m2/s and the gas constant R is 8.3145. To solve the Eq. (8), the diffusion activation energy Q should be confirmed first. Q may be thought of as the energy required driving the diffusive motion of 1 mol carbon atoms. D is controlled by Q. The presence of alloying elements other than carbon (e.g., Cr, Ni, Mo, and W) will slowdown the mitigation diffusion rate of carbon atom in martensite by increasing its activation energy Q. Some works on the calculating of Q of steels containing Cr, Mn, and Mo have been carried out [17]. The hardness of the steel was tested after tempering at different temperature and the activation energy was measured by indirect method such as statistics, equivalence or equivalent weight. According to these studies, the empirical equation of diffusion activation energy of carbon in carbon or low-alloyed steel is obtained Q ¼ 114:69ðMoeq Þ þ 46:64 ðkcal=molÞ
Fig. 7. Carbon distributions in martensite with different diffusion energy (peak temperature Tp ¼ 1173 K).
(9)
where Moeq is equivalent weight of Mo element in steel, given by 1 Moeq ¼ Mo þ 15Mn þ 10 Cr
(10)
Calculating by Eqs. (9) and (10), the ratio of Q in 45 steel and 9Cr2Mo steel is 1:1.7. The result indicates that the diffusion activation energy of 45 steel is significant lower than that of 9Cr2Mo steel containing Mo and Cr element. For 45 steel, the activation energy Q is 263.9 kJ/mol (63 kcal/mol). As for 9Cr2Mo or W18Cr4V steel, the diffusion activation energy Q for diffusion of carbon is higher than those of 45 steel, due to larger amount of Cr, Mo and W (see Table 1). However, the values of diffusion activation energy for diffusion of carbon in 45, 9Cr2Mo and W18Cr4V steels have not been reported. Therefore, different activation energy Q has been chosen to evaluate their influence on the laser tempering. The carbon content in martensite w(C) of 0.85% and cooling rate A of 200 1C/s are used in the calculation. Peak temperature Tp is selected as 900 1C. The results are shown in Fig. 7. It can be seen from Fig. 7 that the decreased degree of carbon concentration in a certain part of martensite has obvious slowdown tendency as the activation energy Q increases. When Q are 3.0 105, 2.5 105, 2.0 105, and 1.5 105 J/mol, carbon concentration in martensite drop within the range of 6.5, 96, 1325 and 45000 A˚, respectively. The literature [17] revealed that a 10 A˚ segregation of carbon in martensite after low temperature (20–150 1C) tempering was detected by analyzing the peak intensity of X-rays diffuse scattering, which was regarded as the sign of martensite entering into decomposition stage. According to this inference and principle of martensite tempering, conclusions on Fig. 7 may be given: (1) on the condition of Q of 1.5 105 and 2.0 105 J/mol, full carbon segregation occurs in 10 A˚ and martensite have entered into decomposition stage; (2) on the condition of Q of 2.5 105 J/mol, imperfect carbon segregation occurs in 10 A˚ and martensite have partially entered into decomposition stage; and (3) on the condition of Q of 3.0 105 J/mol,
Fig. 8. Carbon distributions in martensite with different diffusion energy (peak temperature Tp ¼ 900 1C).
the distance of carbon migration was only 6.5 A˚ and martensite have not entered into the decomposition stage. Peak temperature Tp has great influence on martensite decomposition. Fig. 8 shows the results in which peak temperature Tp is selected as 1100 1C. Under the same activation energy Q, the degree of martensite decomposition at peak temperature Tp of 1100 1C is significantly higher than that of Tp of 900 1C, as shown in Figs. 7 and 8. On the condition of Q of 3.0 105 J/mol, martensite has not entered into decomposition stage in Fig. 7 (Tp ¼ 900 1C), but the result in Fig. 8 (Tp ¼ 1100 1C) is that martensite have partially entered into the decomposition stage. It can be seen from Fig. 8 that when activation energy Q is 3.5 105 J/mol, the largest diffusion distance of carbon is 7.7 A˚ and martensite have not entered into the decomposition stage. According to Eqs. (9) and (10), the activation energy Q of 9Cr2Mo
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The overlapping zone The first pass
The second pass
0.2mm Fig. 9. Carbon distributions in martensite at different cooling rate (Q ¼ 2.5 105 J/ mol).
Fig. 10. Hardness distribution of surface after laser-overlapping scanning and tempering.
steel is about 398 KJ/mol (95 kcal/mol). Based on Fig. 8, martensite in 9Cr2Mo steel cannot be decomposed. However, in this study, martensite in laser-overlapping area of 9Cr2Mo steel has partially decomposed into sorbite during laser tempering, as shown in Fig. 4b. Therefore, for laser tempering, actual peak tempering temperature Tp may be higher than 1100 1C, due to its nonequilibrium thermodynamics process. If peak temperature Tp is selected as 1300 1C, computering by model, martensite with activation energy Q of 398 KJ/mol will partially enter into the decomposed stage. Fig. 9 shows the carbon distribution in martensite at different tempering cooling rate on the condition of Q of 2.5 105 J/mol. It can be seen from Fig. 10 that on the condition of cooling rate of 200, 1000 1C/s, full carbon segregation occurs in 10 A˚ and martensite have entered into the decomposition stage. Moreover, even cooling rate is up to 104 1C/s, carbon concentration drop within the range of 94 A˚ and martensite have partially entered into decomposition stage, as shown in Fig. 9. Through analyzing the results shown in Figs. 7–9, it can be concluded that the activation energy Q for diffusion of carbon in steel is a main reason that causes the difference in microstructures and temper softening in overlapping zone of 45, 9Cr2Mo and W18Cr4V steel during laser-repeating scanning.
Fig. 11. Cross-sectional image and microstructure of W18Cr4V through furnace tempering at 560 1C for 2.0 h after laser multi-pass quenching.
In the practical application, quenching is an important heat treatment process for steel. The aim of this treatment is to obtain martensitic microstructure with excellent hardness and strength properties. For carbon and low-alloy steel such as 45 and 9Cr2Mo steel, the diffusion activation energy of carbon and the decomposition temperature of martensite are low due to their small alloying element content. Therefore, low temper temperature under 200 1C is usually chosen to treat these steels. The structures of these tempered steels are tempered martensite. However, for 45 steel, there is mainly sorbite in overlapping zone by multi-passes laser quenching. From this perspective, multi-passes laser quenching of steels similar to 45 steel cannot meet the requirement of large areas laser hardening. It is essential to point out that increasing cooling rate will limit the carbon diffusion in martensite, i.e., reducing decomposition of martensite, but this action is limited, as shown in Fig. 9. Tempering temperature of about 560 1C is usually chosen in W18Cr4V steel, which is attributed to the alloying elements of Cr, W and V. Tempering microstructures of W18Cr4V steel are mainly tempered martensite and carbide. In this study, the microstructures in the overlapping zone of W18Cr4V steel are also tempered martensite and carbide, which is a structural fundament for subsequent hardness homogenizing treatment. Fig. 10 shows the hardness distribution of surface of W18Cr4V steel through furnace tempering at 560 1C for 2.0 h after multi-pass laser quenching. It can be seen from Fig. 10 that the hardness in overlapping zone is somewhat increased and the whole surface hardness becomes uniform. Fig. 11 shows the OM cross-sectional morphology of multi-pass laser quenching of W18Cr4V steel after furnace tempering at 560 1C for 2.0 h, where the dash lines denote the location of laser-hardening zone. Macromorphology properties between the laser hardness zone and the overlapping zone tend to be uniform, as shown in Fig. 11.
5. Conclusions In this study, softening in overlapping passes by laser-overlapped scanning surface hardening was formed in 45, 9Cr2Mo and W18Cr4V steel. The width of softened zone in turn is 45, 9Cr2Mo and W18Cr4V steel and the lowest hardness in softened zone are increased successively. There is big difference in microstructure of softened zone for three types of steels. The 45 steel mainly contains sorbite and 9Cr2Mo mainly contains tempered martensite, sorbite and carbide. It is noticeable that there is
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mainly tempered martensite, retained austensite and a small amount of carbide for W18Cr4V steel. Sorbite was not observed in softened zone of W18Cr4V steel. A model for analysizing tempering softening in overlapping zone was proposed. Computering by the model, it can be concluded that cooling rate has a limited action in reducing tempering softening and activation energy of diffusion for carbon in steel plays a main role in hindering decomposition of carbide in overlapping zone during laser-overlapping scanning.
Acknowledgement Funding support from Shanghai Key Laboratory of Materials Laser Processing and Modification No. 06DZ22102, it is gratefully acknowledged. References [1] Steen William M. Laser materials processing [M]. Berlin: Springer; 1991. [2] Duley WW. Laser processing analysis of materials [M]. New York, NY: Plenum Press; 1983. [3] Bradley John R, Kim Sooho. Laser transformation hardening of iron-carbon iron- carbon-chromium steels [J]. Metallurgical and Materials Transactions A 1988;19(8):1015–2013. [4] Kou Sindo, Sun DK, Le YP. A fundamental study of laser transformation hardening [J]. Metallurgical and Materials Transactions A 1983;14(3):643–53.
[5] Kaul Rakesh, Ganesh P, Tiwari Pragya, Nandedkar RV, Nath AK. Characterization of dry sliding wear resistance of laser surface hardened En 8 steel [J]. Journal of Materials Processing Technology 2005;167(1):83–90. [6] Shao Tianmin, Hua Meng, Tam Hon Yuen. Impact wear behavior of laser hardened hypoeutectoid 2Cr13 martensite stainless steel [J]. Wear 2003; 255(1–6):444–55. [7] Senthil Selvan J, Subramanian K, Nath AK. Effect of laser surface hardening on En18 (AISI 5135) steel [J]. Journal of Materials Processing Technology 1999;91(1–3):29–36. [8] Tani G, Orazi L, Fortunato A. Prediction of hypoeutectoid steel softening due to tempering phenomena in laser surface hardening. CIRP Annals—Manufacturing Technology 2008;57(1):209–12. [9] Tani G, Orazi L, Fortunato A, Campana G, Ascari A. 3D modelling of LASER hardening and tempering of hypo-eutectoid steels. JLMN—Journal of Laser Micro/Nanoengineering (online) 2008;3:124–8. [10] Hegge HJ, De Beurs H, Noordhuis J, De Hosson JThM. Tempering of steel during laser treatment [J]. Metallurgical Transactions A 1990;21:987–95. [11] Bhadeshia HKDH, Honeycombe RWK. Steels-microstructure properties [M]. 3rd ed. London: Elsevier Ltd; 2006. [12] Cardona M, Fulde P, von Klitzing K, Merlin R, Queisser H-J. Diffusion in solids – fundamentals, methods, materials, diffusion – controlled processes [M]. New York: Springer Berlin Heidelberg; 2007. [13] Porter DA, Easterling KE. Phase transformation in metals and alloys [M]. 2nd ed. 2–6 Bormdary Row, London SEl 8HN, UK: Chapan & Hall; 1993. [14] Smallman RE, Bishop RJ. Modern physical metallurgy materials engineering [M]. 6th ed. Oxford: Butterworth-Heinemann; 1999. [15] Kostorz G, Zu¨rich ETH, Phase Transformationsin Materials [M]. New York: Chichester Brisbane Singapore Toronto, 2001. [16] Callister Jr. William D. Materials science engineering: an introduction [M]. 7th ed. Australia: Wiley; 2005. p. 344. [17] Zhong SH. The temper technology temper equation of steels [M]. Beijing: China machine press; 1993.