Improved process prediction of laser surface hardening on basis of experimental in-situ data

在工业上，激光表面硬化是一种成熟的钢构件局部表面硬化技术。该工艺的特点是通过聚焦激光束使零件局部奥氏体化，并通过局部短时间热处理后发生的自淬而使马氏体硬化。这伴随着耐磨性、抗氧化性和疲劳强度的提高，通过显著的硬度增加和通过形成有益的压残余应力。大量的工艺参数和各个工艺变量之间复杂的相互作用影响所得硬度，阻碍了通过工艺模拟进行预测。本课题的目的是通过有限元模拟，使激光表面线硬化得到显著改善。该方法的基础仍然是通过原位同步加速器x射线衍射实验实时了解快速局部热处理过程，使用一种完善且不断改进的测量和评估策略，用于局部，时间分辨分析相变动力学和应力演化。如中期报告所述，在第一个资助期的范围内，激光表面线硬化的过程预测已经取得了相当大的进步。然而，结果表明，在激光路径的纵向上确定应力分量是必须的，并且在这个方向上可以更好地预测最终的残余应力分布。这需要对原位实验的仪器进行修改，这将在项目延续的范围内实现。最后，在进一步的同步加速器光束时间内确定激光轨迹纵向应力的时间演化。这些数据将与已有的实验数据一起应用，进一步完善仿真模型。此外，使用折射x射线透镜将大大减少模拟的吞吐时间，并进一步提高原位测量系列的可达测量频率。根据实验确定的残余应力深度分布，对连续的局部电化学亚层去除引起的残余应力状态的再分布进行数值计算。利用这些结果，可以有效地修正x射线衍射测定的一定深度内的残余应力分布。通过这种方法，最后可以在深度分解实验结果的基础上对激光表面线硬化模拟进行验证。

In industry laser surface hardening is a well-established technique for local surface hardening of steel components. The process is characterized by a local austenitization of a component by means of a focused laser beam and the martensitic hardening through self-quenching that occurs subsequent to the local short time heat treatment. This is accompanied by the improvement of the wear resistance, the oxidation resistance and the fatigue strength through a significant hardness increase and through the formation of beneficial compressive residual stresses. A large number of process parameters and a complex interaction between individual process variables affect the resulting hardness and hamper the prediction via process simulations. The aim of the continuation of the project is unchanged the significant improvement of the laser surface line hardening by means of FEM simulations. Basis for the approach are still real time insights into the rapid local heat treatment process through in-situ synchrotron X-ray diffraction experiments using a well-established and continuously improved measuring and evaluation strategy for local, temporal resolved analyses of the phase transformation kinetics and the stress evolution. As presented in the mid-term report, a considerable improvement regarding the process prediction of laser surface line hardening was already achieved within the scope of the first funding period. However, the results have indicated that the determination of the stress component in longitudinal direction of the laser path is mandatory that also in this direction a better agreement for the prediction of the final residual stress distributions can be achieved. This requires a modification of the instrumentation of the in-situ experiment, which will be realized within the scope of the project continuation. Finally, the temporal evolution of the stresses in longitudinal direction of the laser track will be determined in a further synchrotron beamtime. These data will be applied together with the already existing experimental data for the further improvement of the simulation model. Furthermore the throughput time of the simulation will be significantly reduced and the reachable measuring frequencies of the in-situ measuring series will be further increased by using refractive X-ray lenses. Regarding the experimentally determined residual stress depth distribution the redistributions of the residual stress states that result from the successive local, electrochemical sub-layer removal will be calculated numerically. Using these results the residual stress distribution determined in defined depths by X-ray diffraction will be effectively corrected. By this means, finally the validation of the laser surface line hardening simulations can be additionally done on basis of the depth resolved experimental results.