Laser Hardening of XC42 Steel: Numerical Analysis of Tempered Area E. Saavedra, A. Ramil, A.J. López, J.C. Álvarez

Escola Politécnica Superior, Universidade da Coruña, 15403 Ferrol, Spainesotero@cdf.udc.es

Fig.º2. Width of tempered area as a function Fig. 3. The apparent depth as a function of the of maximum temperature. real depth (only measured results are used).

Fig. 4. Real depth as a function of maximum Fig. 5. Jominy’s curves both calculatedtemperature. and measured (969W, 10 mm s-1, 3 mm).

Fig. 6. Maximum hardness as a function of Fig. 7. Depth, where the maximum hardnessthe maximum temperature. is placed, as a function of the maximum

temperature (only experimental measurements).

References[1] R. Patwa, Y.C. Shin. Predictive modeling of laser hardening of AISI5150H steels. Int. J. of Machine Tools & Manufacture. Vol. 47, No. 2 (2007), pp. 307-320.[2] R.S. Lakhkar, Y.C. Shin, M.J.M. Krane. Predictive modeling of multi-track laser hardening of AISI4140 steel. Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing. Vol. 480, No. 1-2 (2008), pp.

209-217.[3] G. Tani, L. Orazi, A. Fortunato. Prediction of hypo eutectoid steel softening due to tempering phenomena in laser surface hardening. CIRP Annals-Manufacturing Technology. Vol. 57, No. 1 (2008), pp. 209-212.[4] E.A. Brandes, G.B. Brooks. Smithells metals reference book, 7th edition, Butterworth-Heinemann, Oxford (1992).[5] E. Saavedra, A. Ramil, J.C. Álvarez, J.M. Amado, A.J. López, J. Sanesteban, M.J. Tobar, A.J. Yáñez. Laser hardening simulation with finite element method: an analysis of the errors introduced by the discretization. Proceedings of the 4th

European Congress on Computational Methods in Applied Sciences and Engineering, Jyväskylä (2004).[6] Max-Planck Institut für Einsenforschung. Atlas zur wärmebahandlung der stähle, Band 1-3. Verlag, Stahleisen M.B.H., Düsseldorf (1961).[7] M. Melander, J. Nicolov. Heating and cooling transformation diagrams for the rapid heat treatment of two alloy steels. J. Heat Treating, Vol. 4, No. 1, (1985), pp. 32-38.[8] S. Denis, D. Farias, A. Simon. Mathematical model coupling phase transformation and temperature evolutions in steels. ISIJJ International, Vol. 32 (1992), No. 3, pp. 316-325.[9] J. Xie, A. Kar, J.A. Rothenflue, W.P. Latham, Temperature-dependent aborptivity and cutting capability of CO2, Nd:YAG and chemical oxygen-iodine lasers, Journal of Laser Applications (1997), 9, 77-85.

Abstract. In contrast with conventional tools, laser processing allows hardening of a restricted zone while keeping the structural properties of the steel bulk. This last quality indicates that, in order to verify a laser hardening, only some specific information of the tempered area is required. The aim of this article is to analyse the reliability of finite element numerical simulation by comparing numerical and experimental outcomes. To do that, we define some magnitudes: the maximum width, the real depth of the laser penetration, the maximum hardness and the hardness versus depth profiles (Jominy’s curves). The tests show the good behaviour of the model and how this contributes important information to the choice of the laser parameters.

Table 2 Summary of experimental errors

Error [%] Error

Net power 2 2 % Tmax

Speed 3 1 % Tmax

Width 5.4 7.4 % Tmax

Global error 8 % Tmax

Table 1 Summary of laser parameters

Net power [W] Speed [mm s-1] Width [mm]

969.0 10 3.00

940.5 10 3.51

883.5 10 3.51

741.0 10 3.51

883.5 13 3.51

883.5 8 3.51

940.5 14 3.51

t

Tem

pera

ture

Fig. 1. Dicretization of constant phase diagram in steps.

t

T

it 1it

iT

1iT

1iy

iy

Model. Finite element model + discretization of phase diagrams as a staircase (time steps linked to others of constant volume fraction).

Parameters. It is common to require hardening treatments affecting an area as large as possible and a tempered depth suitable to specific industrial use. As a result keeping this in mind, we will control the process only with the laser power and speed, while the beam width is kept constant with the required value.

Model and measurement dispersions. To estimate the model dispersions, the thermal cycles are enlarged or compressed in proportion to the maximum temperature variations, 8 %, and, later, the phases and hardness measurements are calculated. Then, the arithmetic mean and the standard deviation are used to obtain the desired values. As the average hardness in the untreated steel is around 200 Vickers, we take 300 Hv as the border delimiting the tempering area; moreover, we will use the linear interpolation with consecutive data when the 300 Hv has not accurately been reached.

Jominy’s curves. To obtain these curves, the centre of the tempered area is estimated from the borders of 300 Hv. Then, from the heated surface and tracing a straight line, the hardness is measured.

Maximum hardness. From an experimental point of view, finding the maximum hardness would take a lot of time and, furthermore, it will always remain uncertain whether the measurement is really the maximum. Anyway, it is necessary to have a useful definition. Therefore, we use Jominy’s profiles to extract the maximum from the curves and, hence, we consider this value as the maximum hardness.

Real depthAparente depth

y

z

Width

Area with Hd < 300 Hv

AcknowledgmentThe authors gratefully acknowledge the computing support provided by

the Centro de Supercomputación de Galicia (CESGA), both the hardware and the ANSYSTM license.