A Fundamental Study of Laser Transformation Hardening (original) (raw)
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Applied Mathematical Sciences, 2010
A two-dimension numerical model for pulsed Laser Transformation Hardening (LTH) is developed using the Finite Differences Method (FDM). In this model, the starting temperature of martensite formation is taken into account as a crucial factor in martensite formation. The numerical results are compared to analytical solution and numerical solution of the previous study [2], showing a good agreement. Two experiments are carried out varying two input variables namely, heating duration and laser beam intensity, to investigate their impact on the temperature distribution and martensitic transformation. The results of these experiments will be explained more detail in this paper.
Status of laser transformation hardening of steel and its alloys: a review
Emerging Materials Research, 2019
The state of laser transformation hardening in surface modification of steel and its alloys is reported in this review paper. The laser hardening process involves the use of high-intensity laser radiation to heat the surface of steel into the austenitic region rapidly. Due to high rates of heat transfer, steep temperature gradients result in rapid cooling by conduction. This causes the transformation from an austenite to a martensite microstructure without the need for external quenching and generates a hard wear-resistant surface. The introduction of high-power lasers, such as neodymium-doped yttrium aluminum garnet (Nd:YAG), diode and fiber lasers, has made laser-assisted hardening an attractive candidate for surface hardening processes. Recently, fiber lasers appeared as a competitor to Nd:YAG and diode lasers for material processing. This review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with those of conventional hardenin...
International Journal of Computational Materials Science and Surface Engineering, 2013
This study develops a thermodynamic model to investigate the quasi-steady thermal process of a wide thin steel workpiece irradiated with a moving Gaussian laser beam. Equations are established for temperature distribution, transformation boundaries, homogenisation time of austenite and cooling rate. The equations are numerically solved with an error of less than 10-8. The temperature distributions for various thicknesses are compared with that for infinite thickness at different laser traverse speed. The lag of the peak temperature relative to the centre of laser beam is found to be limited. The conditions to produce full and partial martensite are investigated. The model is verified by comparing the calculated Ac 1 and Ac 3 depths and temperature cycles with the experimental results. For AISI 4340 steel, correction coefficients are applied to the model to produce an empirical equation for temperature cycles above 488.4°C.
An Analysis of the Effect of Laser Beam Geometry on Laser Transformation Hardening
Journal of Manufacturing Science and Engineering, 2006
The effect of transformation hardening depends upon both heating and cooling rates. It is desirable to have a slow heating rate and a rapid cooling rate to achieve full transformation. To date laser transformation hardening has been carried out using circular or rectangular beams which result in rapid heating and cooling. Although the use of different beam intensity distributions within the circular or rectangular laser beams has been studied to improve the process, no other beam geometries have been investigated so far for transformation hardening. This paper presents an investigation into the effects of different laser beam geometries in transformation hardening. Finite element modeling technique has been used to simulate the steady state and transient effects of moving beams in transformation hardening of EN 43A steel. The results are compared with experimental data. The work shows that neither of the two commonly used beams, circular and rectangular, are optimum beam shapes for ...
Laser transformation hardening of tempered 4340 steel
Metallurgical Transactions A, 1992
A CO2 laser with a fixed laser power of 1.8 KW was employed to harden the surface of some AISI 4340 steel specimens, with a scan rate from 5 to 10 mm/s. The influence of scan rates and tempering treatments of the alloy on the hardness profile and microstructure of the laserhardened zone was analyzed. Microstructures in the hardened zone consisted of mainly lath and twinned martensites. However, depending on the scan rate, autotempered martensite has also been found. In the transition zone of laser-treated specimens, partially dissolved carbides with austenite envelopes and/or austenite islands in a matrix of martensite were observed. The time required for complete carbide dissolution into austenite during laser treatment depended on the tempering conditions. A lower tempering temperature of the alloy produced a deeper hardened zone and a narrower transition zone in the hardness profile. A simple mathematical estimation of the hardness profile, based on the carbon diffusion distance in austenite, was performed. The calculated results are in reasonably good agreement with the measured hardness profiles and the microstructural observations in the laser transformation hardening process.
Materials Research, 2004
The aim of this work is to develop a mathematical model to predict the depth of laser treated zone in the LTH process. The Fourier equation of heat conduction is solved by using the Finite Difference Method in cylindrical coordinates in order to study the temperature distribution produced in a workpiece and hence to obtain the depth to which hardening occurs. The theoretical simulations are compared with results produced experimentally by a CO 2 laser operating in continuous wave, showing good agreement.
Optimal beam spot diameter for the laser surface hardening of a medium carbon steel
1986
The results of experiments and FEM-simulation on laser surface hardening of a medium carbon steel are described. A 2-dimensional computer program, which can be used generally for the determination of transient temperature distributions in welding and surface heat treatment, was used in the first place to investigate the effects of traverse speed and beam spot diameter on the shape and size of hardened zones. For the confirmation of the accuracy of the numerical analysis a medium carbon steel of 5 mm thickness was heat-treated with a 1 kW CO2 laser. A simulation scheme for the cooling time and the corresponding CCT-diagram showed that the cooling rate is high enough to consider the heated zone above the Acl temperature as the martensitic hardening zone. With proper assumption of the absorptivity the numerical and experimental shape and size of hardened zones were in good agreement. The numerical analysis showed that with increasing beam spot diameter the width and depth of the harden...
Laser Surface Hardening of Tool Steels—Experimental and Numerical Analysis
Journal of Surface Engineered Materials and Advanced Technology, 2013
This research work is focused on both experimental and numerical analysis of laser surface hardening of AISI M2 high speed tool steel. Experimental analysis aims at clarifying effect of different laser processing parameters on properties and performance of laser surface treated specimens. Numerical analysis is concerned with analytical approaches that provide efficient tools for estimation of surface temperature, surface hardness and hardened depth as a function of laser surface hardening parameters. Results indicated that optimization of laser processing parameters including laser power, laser spot size and processing speed combination is of considerable importance for achieving maximum surface hardness and deepest hardened zone. In this concern, higher laser power, larger spot size and lower processing speed are more efficient. Hardened zone with 1.25 mm depth and 996 HV surface hardness was obtained using 1800 W laser power, 4 mm laser spot size and 0.5 m/min laser processing speed. The obtained maximum hardness of laser surface treated specimen is 23% higher than that of conventionally heat treated specimen. This in turn has resulted in 30% increase in wear resistance of laser surface treated specimen. Numerical analysis has been carried out for calculation of temperature gradient and cooling rate based on Ashby and Easterling equations. Then, surface hardness and hardened depth have been numerically estimated based on available Design-Expert software. Numerical results indicated that cooling rate of laser surface treated specimen is high enough to be beyond the nose of the CCT diagram of the used steel that in turn resulted in a hard/martensitic structure. Numerically estimated values of surface temperature, surface hardness and hardened depth as a function of laser processing parameters are in a good agreement with experimental results. Laser processing charts indicating expected values of surface temperature, surface hardness and hardened depth as a function of different wider range of laser processing parameters are proposed.
Laser transformation hardening of tool-steel specimens
Journal of Materials Processing Technology, 1990
Experimental work on laser transformation hardening has been performed on Assab DF-2 toolsteel specimens of 5 mm thickness, with a de-focussed beam. The power inputs were 0.4, 0.5 and 0.6 kW, the nozzle gaps were 5, 10 and 15 mm and the calculated beam diameters were 0.129, 1.174 and 2.276 mm, respectively. It has been found that the case depth is not affected significantly by the beam diameter. However, the beam diameter affects the gradient of the plot of P~ (DbV) 1/2 against Dp, which explains why considerable scatter was encountered in the work of Courtney and Steen. It has been found that the case depth is dependent on P~ V 1/2 and not on P~ (DbV)1/2, as had been assumed previously. A new empirical equation for the case depth has been proposed in the light of this knowledge. The beam diameter, on the other hand, is found to affect the width of the hardened zones. An empirical equation for predicting the width from the known geometry of a parabola and the experimentally observed case depth has been formulated.
Laser surface hardening of H13 steel in the melt case
Materials Letters, 2005
Laser heating caused a melting layer to form on the H13 steel, which usually has bad thermal conductivity and diffusivity. Therefore, the modified Ashby-Eastering heat-transfer equation was used to provide the temperature field for laser surface hardening in the melt. When the laser hardened H13 steel through surface melting, the basic microstructure of the dendrites was surrounded by an extremely fine lamellar structure in the melt layer. It is clear that the contours of the melting point isotherms and the critical phase transition temperature of H13 in the quenched and as-received conditions were comparable in the temperature distribution field under different laser energy densities. When the laser moves on, the phase transition temperature of H13 is raised and it becomes higher than the A 1 temperature because the heating rate during laser processing is usually N10 4 8C/s. The larger the grain size or the more heterogeneous the structure, the higher the temperature and the longer the duration required for transforming the steel into austenite. D