1091

Sensors and Materials, Vol. 31, No. 3 (2019) 1091–1103
MYU Tokyo
S & M 1837

Tempering Effects of Multitrack Laser Surface Heat Treatmentof AISI 1045 Steel
Chi-Liang Kung,1 Hao-En Shih,2 Chao-Ming Hsu,1* and Cheng-Yi Chen3,4**
1

Department of Mechanical Engineering, National Kaohsiung University of Science and Technology,
Kaohsiung 80778, Taiwan
2
Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-sen University,
Kaohsiung 804, Taiwan
3
Department of Electrical Engineering, Cheng Shiu University, Kaohsiung 83347, Taiwan
4Super Micro Mass Research and Technology Center, Cheng Shiu University, Kaohsiung 83347, Taiwan(Received November 30, 2018; accepted January 11, 2019)

Keywords: laser surface heat treatment, hardening width, hardening depth, tempering effect

A thermo-elastic-plastic finite element model was employed to simulate and analyze theeffects of high-power parameters on the laser heat treatment of AISI 1045 steel. Tempering
effects of multitrack laser heat treatment using various process parameters—laser power, laser
feed rate, laser overlap rate, and laser spot size—were analyzed to estimate the distribution of ahardening layer on the steel surface. Numerical simulation results indicated that the proposed
finite element model is effective in analyzing the laser heat treatment of the steel surface andcalculating the possibility of decreases in steel hardness due to tempering effects, enabling it toundergo rapid temperature increases and decreases.

1.

Introduction

Laser surface processing technology has undergone more than 40 years of development
and has been applied by numerous well-known international enterprises in the automotive,
aerospace, military, shipbuilding, and material-related industries because of its abundantadvantages for manufacturing the products of these industries. Unlike the conventional
heat treatment method, laser surface heat treatment involves transmitting a laser beamthrough optics to irradiate a high-energy, high-intensity laser beam onto a metal surface toimmediately facilitate austenite transformation. At room temperature and without any fixed
thermal convection conditions, the austenite form of the metal surface undergoes martensitic
transformation through self-quenching.(1)In 1960, Maiman(2,3) installed a ruby cylinder (diameter: 1 cm; length: 2 cm) on the axisof a helical xenon flashlamp, which was then inserted into a polished aluminum cylinder.
When the ruby cylinder was illuminated by the flashlamp, the device emitted red light; hence,
the first working laser was invented. According to DeMichelis,(4) a German scientist used alaser beam to perform heat treatment on a metal, which was the first experiment in history*

Corresponding author: e-mail: jammy@kuas.edu.tw
Corresponding author: e-mail: k0464@gcloud.csu.edu.twhttps://doi.org/10.18494/SAM.2019.2229**

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

1092

Sensors and Materials, Vol. 31, No. 3 (2019)

to employ lasers as a heat treatment method. Scientists in the United States then started to
study the absorption and hardening of a graphite-coated metal surface irradiated with a rubylaser beam.(5) Researchers in the United States and Japan(6,7) also began to examine the heat
treatment ability of lasers on metal surfaces by observing their austenitization and to study theeffects of laser heat treatment on alloys. In 1965, the Nd:YAG laser was the first industrial useof this technology to repair connectors inside assembled television tubes.(8) Lasers were applied
to industrial production in 1974, when the Saginaw Steering Gear Division, part of General
Motors, began to use laser beams to harden steering gear housings.(9,10) Loeffer(11) proposed thefirst computer numerical control (CNC) laser soldering machine, which used a CO2 laser andkeyhole welding technology. However, no mature dynamic equations were developed in related
research until 1995, when Fuerschbach and MacCallum(12) irradiated a CO2 laser beam onto 304
stainless steel at 6 MW/cm2 and derived the relationship among laser absorption rate, weld pool
width, and weld pool depth using a weld pool geometry model.
Laser surface heat treatment has the following advantages:(1) First, it causes the metal
to self-quench; thus, the metal surface remains clean and does not require further cleaning
after treatment. Second, changing the laser power and the position of the optics enables its
application in heat treatment processes for different materials with different requirements.
Third, laser sources installed on CNC machines can be used to perform heat treatment on
workpieces with complex shapes. Fourth, workpieces that have received laser heat treatment
undergo minimal deformation. Fifth, workpieces that have received laser heat treatment do not
require grinding. Sixth, production can be automated, effectively reducing human resource
costs.
Bojinović et al.(13) proposed a 3D model of the laser hardening process using a highpower diode laser for 50CrV4 steel, based on the finite element method. Nguyen and Yang(14)presented a sequential method to estimate surface absorptivity in the laser surface hardening
process using a 1D transient conduction heat transfer model. Sarkar et al.(15) proposed amodified 1D heat conduction model to analyze the effects of temperature and surface hardening
on low-carbon thin steel (thickness: 1 mm) using an Yb-fiber laser. Martínez et al.(16) proposeda scanner-based laser hardening process for AI1045 steel and discussed the effects of scanningspeed on the hardened layer thickness. Lakhkar et al.(17) proposed a numerical model topredict the back tempering in multitrack laser hardening for AISO4140 steel. Recently,
Cordovilla et al.(18) have studied laser surface hardening analysis to propose the design of asuitable overlapping distance for Cr-Mo steel. Overall, the main difficulties are how to choose
adequate laser technological parameters and a suitable laser scanning (tracking) method forsurface hardening to obtain the optimum hardening quality with an excellent temperature field
distribution.
In this study, the temperature distribution of SAE-AISI 1045, a medium carbon steel, wasanalyzed by adopting the finite element method as the numerical simulation tool and employing
a neodymium-doped yttrium aluminum garnet (Nd:YAG) solid-state laser as the laser source.(19)The material properties of SAE-AISI 1045 were obtained from Ding and Yung.(20) In addition,on the basis of studies by Kusuhara et al.,(21) a thermo-elastic-plastic finite element model wasconstructed to examine and discuss the effects of laser feed rate, laser power, and laser spotSensors and Materials, Vol. 31, No. 3 (2019)1093

size on the temperature distribution, the depth of the hardened layer (hardening depth), andthe width of the laser-hardened area (hardening width). Moreover, the effects of the overlap
and power level of a diode laser beam on the hardened layer depth of SAE-AISI 1045 were
examined.

2.

Finite Element Modeling and Analysis

By simulating and analyzing the application of a moving Gaussian laser beam as the source
for the heat treatment of a tool steel surface, a set of research procedures to identify the adequate
ranges of various process parameters was established in this study. We used a multitrack laser
beam with a Gaussian profile to perform heat treatment under the effects of circular laser
spots and examined the possible tempering effects of the laser beam during the heat treatment
process. The details are described below.
2.1. Setup of moving heat source
The employed Nd:YAG laser was adjusted for heat source distribution in transverse
electromagnetic mode 00 (TEM00), as described in Ref. 22. This commonly used mode differsfrom that of a conventional welding heat source, which exhibits a bi-elliptic distribution. In
the TEM00 mode, the power density is highest at the center of the beam and diminishes withincreasing distance from the center, resulting in a Gaussian distribution of light intensity acrossthe beam diameter. The power density of the laser (Ee) in the TEM00 mode can be expressed as
Ee (re )= Pi ⋅ Φ (re ) ,(1)

where re is the distance from the center of a laser beam, Pi is the laser power size, and F is theGaussian distribution function. The center of the 2D Gaussian distribution was rotated 360° toobtain a 3D Gaussian distribution, which is expressed as

Φ
=
(re )

 re 2 exp − 2  ,2πs 2
 2s 
1(2)

where s is the standard deviation of the Gaussian distribution. In accordance with the research
in Refs. 22 and 24, the radius of the spot size, Re, was used to calculate the area in which 95% of
the laser beam power passes through a circle of its focused area, which can be written as
2 π Re1∫0 ∫0 2πsr2
− e2e 2 s re dre dθ = 0.95 ,2

where q is the angular variable. From Eq. (3), the following can be obtained:

(3)

1094

Sensors and Materials, Vol. 31, No. 3 (2019)

=
s

Re

2ln(20)

≈

Re

6

.

(4)

Substituting Eq. (4) to Eq. (2), Eq. (1) can be rewritten as=
Ee (re ) 3re 2 exp − 2  .
πRe 2
 Re 
3Pi(5)

Because most materials are unable to fully absorb the energy of a laser beam, we considered
the effects of the steel thermal absorptivity (ηe). Referring to the data presented by DeKock,(25)we set the thermal absorption rate he at 40%; thus, the final amount of energy absorbed by thesteel (Pe) can be expressed as 3r 2 
3PPe =ηe ⋅ i2 exp  − e2  .
 R 
πRe
e 


(6)

The diameter of the laser spot set between 2 and 4 mm is chosen in this study.
2.2. Construction of finite element model
In this study, MSC.MARC, a commercial finite element analysis software program, wasused as a numerical equation solving tool, and the pre- and post-Mentat processor provided by
MSC.MARC was employed to construct a finite element model mesh and boundary conditions,and to obtain the temperature and stress data for the analysis node. AISI 1045 medium carbon
steel was selected in bar-shaped pieces with dimensions of 100 × 12 × 6 mm3 (l × w × h). The
use of a 3D finite element model mesh for multitrack laser heat treatment was proposed, as
shown in Fig. 1. Note that the mesh in the x- and y-directions is evenly distributed, but the meshin the z-direction is smaller when close to the material surface. The moving heat source of thelaser with heat distribution in the TEM00 Gaussian mode was simulated using MSC.MARC, as
shown in Fig. 2.
2.3. Multitrack laser heat treatment model
Steel materials generally cool rapidly after laser treatment. To reduce the calculation timerequired for numerical simulation, the laser beam was fed through an S-shaped path (hereafterreferred to as SSP), as illustrated in Fig. 3, which was employed in the heat treatment simulation
model. Data were collected from nine nodes in the targeted treatment area (length: lh; width:3 mm) on the surfaces of the steel materials, as shown in Fig. 4. In this article, the length of the
proposed S-shaped laser track for the heat treatment was set at 3 mm to achieve a hardening
width of 6 mm on the surface of the treatment area when the light spot diameter (Ri) was 3 mm.

Sensors and Materials, Vol. 31, No. 3 (2019)

Fig. 1. (Color online) 3D finite element model meshconstruction.

Fig. 3.

1095

Fig. 2.

(Color online) Moving heat source of laser.

SSP traced by multitrack laser heat treatment.

Fig. 4. Schematic of the nine nodes designating the data acquisition points during the multitrack laser heat
treatment.

Figure 5 depicts the nine nodes (labeled O1–O9) selected from the area between the first andsecond laser tracks for temperature data acquisition and further analysis. In terms of laser
power output, laser power and energy were continuously fed from the beginning to the end of
the SSP.

1096

Sensors and Materials, Vol. 31, No. 3 (2019)

(a)

(b)

(c)
Fig. 5.

3.

(Color online) Temperature profiles of O8 on AISI 1045 at laser overlap rates of (a) 25, (b) 55, and (c) 75%.

Results and Discussion

In this study, we examined the tempering effects of multitrack laser surface heat treatment
by using the finite element method to simulate and analyze the effects of all process parameterson the heat treatment process. The appropriate ranges and optimal values of the processparameters were identified. The multitrack laser heat treatment process was analyzed to
examine the tempering effects of these process parameters on multiple nodes along the surfacesof the materials. In the following subsection, the effects of laser overlap rate, laser power, laser
feed rate, and laser spot size on the temperature distribution of AISI 1045 steel materials will
be discussed. Because of the considerable amount of data collected from the nine nodes, not
all of the nodes are presented in the figures; only nodes that reflected significant effects of thefour parameters on temperature distribution, namely, O8 and O9, are discussed. Tp denotes thehighest peak temperature during the laser heat treatment.
3.1

Effects of laser overlap rate on tempering effects

This subsection focuses on analyzing the effects of laser overlap rate (g) on the temperature
profiles of nodes O8 and O9. The laser power Pi was set at 350 W, the laser feed rate Vi at 8 mm/s,

Sensors and Materials, Vol. 31, No. 3 (2019)1097

the laser spot size diameter Ri at 3 mm, and the laser feed path as the SSP. Figure 5 shows thetemperature profiles of node O8 (see Fig. 4) on the steel material for laser overlap rates of g = 25,
55, and 75%. It can be seen that these rates all generated temperatures above the quenchingtemperature Th, which may have induced hardening. In addition, all of the O8 temperatures
reached the tempering temperature, which might have caused tempering effects. Note that thetempering effects of O8 are relatively more prominent at g = 75%. When the laser overlap rateis increased from 25 to 55%, the first peak temperature decreases by 7%, but the second peaktemperature increases by 160%. When the overlap rate is increased from 55 to 75%, the firstpeak temperature increases by 252%, the second by 10%, and the third by 70%.
Figure 6 illustrates the temperature profiles of node O9 on AISI 1045. Similarly to O8, thethree laser overlap rates generated temperatures over the quenching temperature Th and induced
hardening. At g = 55 and 75%, tempering effects might have occurred on node O9, and theseeffects were more prominent. When the rate was increased from 25 to 55%, the first peaktemperature remained unchanged, but the second peak temperature increased by 178%. Whenthe rate increased from 55 to 75%, the first peak temperature increased by 14% and the second
by 73%. From the information in Figs. 5 and 6, we selected the temperature profile of the 55%
laser overlap rate to discuss the effects of laser power, laser feed rate, and laser spot size on thetempering effects of O8 and O9.

(a)

(b)

(c)
Fig. 6.

(Color online) Temperature profiles of O9 on AISI 1045 at laser overlap rates of (a) 25, (b) 55, and (c) 75%.

1098

3.2

Sensors and Materials, Vol. 31, No. 3 (2019)

Effect of laser power on tempering effects

In this subsection, the effects of laser power on the temperature profiles of O8 and O9 are
examined. The laser feed rate Vi was set at 8 mm/s, the laser spot size diameter Ri at 3 mm, thelaser overlap rate g at 55%, and the laser feed path as the SSP. Figure 7 shows the temperature
profiles of O8 for laser powers of Pi = 300, 350, and 400 W. All these powers generated heatover the quenching temperature Th and may have induced hardening. When Pi = 300 W, thethird peak temperature reached the tempering temperature Tt. However, when Pi = 350 and 400 W,
the third peak temperature was higher than the tempering temperature Tt and might have caused
tempering effects on the steel material. In particular, when Pi = 400 W, the temperature at nodeO8 exceeded the melting temperature (Tm) of AISI 1045, which could cause the steel to melt.
Figure 8 presents the temperature profiles of O9 for laser powers of Pi = 300, 350, and
400 W. Similarly to O8, the generated temperatures for these laser powers were higher than thequenching temperature Th and hence might have caused hardening. However, the second peaktemperature (tempering temperature) at node O9 increased by around 15–17% when its laser
power was increased by 50 W. We concluded that laser power control is not adaptive for SAEAISI 1045 and that 350 W is the suitable laser power.

(a)

(b)

(c)
Fig. 7.

(Color online) Temperature profiles of O8 at laser powers of (a) 300, (b) 350, and (c) 400 W.

Sensors and Materials, Vol. 31, No. 3 (2019)

1099

(a)

(b)

(c)
Fig. 8.

3.3(Color online) Temperature profiles of O9 at laser powers of (a) 300, (b) 350, and (c) 400 W.

Effect of laser feed rate on tempering effects

In this subsection, the effects of laser feed rate on the temperature profiles of O8 and O9are explored. The laser power Pi was set at 350 W, the laser spot size diameter Ri at 3 mm, thelaser overlap rate g at 55%, and the laser feed path as the SSP. The temperature profiles of nodeO8 are given in Fig. 9 for three laser feed rates of Vi = 5, 10, and 15 mm/s. The temperatures
on node O8 are higher than Th and could result in hardening. In these cases, the second peaktemperature exceeded the tempering temperature Tt, causing tempering effects. Figure 10
presents the temperature profiles of node O9 for AISI 1045 at three different laser feed rates.
These rates also contributed to heated temperatures higher than Th on O9 that might cause
hardening. For laser feed rates of 5 and 10 mm/s, the second peak temperature reached or
exceeded the tempering temperature Tt and therefore would also cause tempering effects.
A laser feed rate of 15 mm/s would not induce tempering effects because the second peaktemperature did not exceed Tt. Note that the second peak temperature (tempering temperature)of node O9 for AISI 1045 was reduced by approximately 13–18% when the laser feed rate wasincreased by 5 mm/s. From the simulation results shown in Figs. 9 and 10, we can conclude thattempering effects are better at node O9 than at node O8.

1100

Sensors and Materials, Vol. 31, No. 3 (2019)

(a)

(b)

(c)
Fig. 9.

(Color online) Temperature profiles of O8 at laser feed rates of (a) 5, (b) 10, and (c) 15 mm/s.

(a)

(b)

(c)
Fig. 10. (Color online) Temperature profiles of O9 at laser feed rates of (a) 5, (b) 10, and (c) 15 mm/s.

Sensors and Materials, Vol. 31, No. 3 (2019)

3.4

1101

Effect of laser spot size on tempering effects

In this subsection, the effects of laser spot size on the temperature profiles of O8 and O9
are analyzed. The laser power Pi was set at 350 W, the laser feed rate Vi at 8 mm/s, the laser
overlap rate g at 55%, and the laser feed path as the SSP. Figures 11 and 12 respectively depict
the temperature profiles of O8 and O9 for laser spot diameters of Ri = 2.8, 3.0, and 3.2 mm. It
can be observed from Figs. 11 and 12 that these spot diameters produced a temperature higher
than Th at both O8 and O9, inducizng hardening. Note that AISI 1045 could also melt when Ri =2.8 mm because the heated temperature exceeded its melting temperature Tm. Also, note thatthe tempering effects of O8 and O9 would only occur when Ri = 3 mm.

(a)

(b)

(c)
Fig. 11. (Color online) Temperature profiles of O8 at spot sizes of Ri = (a) 2.8, (b) 3, and (c) 3.2 mm.

1102

Sensors and Materials, Vol. 31, No. 3 (2019)

(a)

(b)

(c)
Fig. 12. (Color online) Temperature profiles of O9 at spot sizes of Ri = (a) 2.8, (b) 3, and (c) 3.2 mm.

4.

Conclusions

In this study, a thermal–mechanical coupling model was utilized to simulate the peaktemperature at the center of a laser spot as well as the hardening width and depth for various
laser process parameters during the multitrack heat treatment of AISI 1045 medium carbon steel
using a Nd:YAG laser. A finite element analysis model was proposed to effectively analyze thetemperature field distribution on the surface of AISI 1045 for various process parameters duringlaser heat treatment. The obtained temperature field distributions on the material surface wereused to observe and estimate the uniformity of the hardened surface layer and the relationshipbetween tempering effects and process parameters. The appropriate ranges of the four processparameters—laser power, laser spot size, laser feed rate, and laser overlap rate—were identified
for multitrack laser heat treatment. Numerical results reveal that the proposed finite element
models are feasible for simulating the laser surface heat treatment process and tempering effects
on AISI 1045 steel. Hence, the results may serve as a useful reference for researchers regarding
the process parameters of laser heat treatment when conducting machining experiments.

Acknowledgments
The authors acknowledge partial financial support under grant no. MOST 106-2221-E-151017.

Sensors and Materials, Vol. 31, No. 3 (2019)

1103

References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

J. H. Gur and J. Pan: Handbook of Thermal Process Modeling Steels (CRC Press, Boca Raton, 2009) p. 499.
T. Maiman: Phys. Rev. Lett. 4 (1960) 564. https://doi.org/10.1103/PhysRevLett.4.564
T. Maiman: Nature 187 (1960) 493. https://doi.org/10.1038/187493a0
C. DeMichelis: IEEE J. Quantum Electron. QE-6 (1970) 630. https://doi.org/10.1109/JQE.1970.1076329
D. Lichtman and J. F. Ready: Phys. Rev. Lett. 10 (1963) 342. https://doi.org/10.1103/PhysRevLett.10.342
G. R. Speich and A. Szirmae: Transform. Metall. Soc. AIME 245 (1969) 1063.
S. Namba, P. H. Kim, S. Nakayama, and I. Ida: Jpn. J. Appl. Phys. 4 (1965) 153. https://doi.org/10.1143/
JJAP.4.153
D. Belforte and M. Levitt: Industrial Laser Handbook (Springer-Verlag, New York, 1992) p. 13.
C. Wick: Manuf. Eng. 76 (1976) 35.
J. E. Miller and J. A. Wineman: Metal Prog. 111 (1977) 38.
J. R. Loeffer: Assembly Eng. 20 (1977) 32.
P. W. Fuerschbach and D. O. MacCallum: Proc. 14th Int. Congress on Applications of Lasers & Electro-Optics(1995) 493–497.
M. Bojinović, N. Mole, and B. Štok: Surf. Coat. Technol. 273 (2015) 60. https://doi.org/10.1016/
j.surfcoat.2015.01.075
Q. Ng uyen a nd C.-Y. Ya ng: I nt. J. Heat Mass Tra nsfer 95 (2016) 224. ht t ps://doi.org /10.1016/
j.ijheatmasstransfer.2015.11.087
S. Sarkar, M. Gopinath, S. S. Chakraborty, B. Syed, and A. K. Nath: Surf. Coat. Technol. 302 (2016) 344.
https://doi.org/10.1016/j.surfcoat.2016.06.045
S. Martínez, A. Lamikiz, E. Ukar, A. Calleja, J. A. Arrizubieta, and L. N. Lopez de Lacalle: Opt. Lasers Eng.
90 (2017) 72. https://doi.org/10.1016/j.optlaseng.2016.10.005
R. S. Lakhkar, Y. C. Shin, and M. J. M. Krane: Mater. Sci. Eng., A 480 (2008) 209. https://doi.org/10.1016/
j.msea.2007.07.054
F. Cordovilla, Á. García-Beltrán, P. Sancho, J. Domínguez, L. Ruiz-de-Lara, and J. Ocaña: Mater. Design 102(2016) 225. https://doi.org/10.1016/j.matdes.2016.04.038.
K. Wissenbach, A. Gillner, F. Dausinger, E. Beyer, and K. Wissenbach: Oberflächenbehandlung mit
Laserstrahlung. Allgemaine Grundlagen (Springer-Verlag, Berlin, 1998) p. 19 (in German).
H. T. Ding and S. Yung: J. Manuf. Sci. Eng. 10 (2012) 051014. https://doi.org/10.1115/1.4007464
T. Kusuhara, J. Morimoto, N. Abe, and M. Tsukamoto: Proc. 4th Int. Conf. Modeling, Simulation and Applied
Optimization (IEEE, 2011) 1–4.
C. Dawes: Laser Welding (Ablington Publishing and Woodhead Publishing in Association with the Welding
Institute, Cambridge, 1992) p. 1.
T. Zacharia, S. A. David, J. M. Vitek, and T. Debroy: Weld. J. 68 (1989) 499.
T. Zacharia, S. A. David, J. M. Vitek, and T. Debroy: Weld. J. 68 (1989) 510.
J. DeKock: Laser Heat Treating Advances for the Gear Industry. http://www.gearsolutions.com/article/
detail/5632/laser-heat-treating-advances-for-the-gear-industry (accessed 2 April 2018).