Spatially resolved temporal stress evolution during laser surface spot hardening of steel

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• 作者：V. Kostov ; J. Gibmeier ; ^{jens.gibmeier@kit.edu" class="auth_mail" title="E-mail the corresponding author} ; A. Wanner
• 关键词：Time resolved synchrotron X-ray diffraction ; In-situ stress analysis ; Residual stress ; Laser surface hardening
• 刊名：Journal of Materials Processing Technology
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：239
• 期：Complete
• 页码：326-335
• 全文大小：3438 K

文摘

The time-dependent stress evolution, the resulting residual stresses as well as the microstructure of the heat treatable low alloyed steel AISI 4140 induced by laser surface spot hardening was investigated systematically by means of synchrotron X-ray diffraction. In‐situ stress analyses with a time resolution up to 100 ms were carried out at the synchrotron beamlines P05@PETRAIII(*), DESY, Hamburg and PDIFF@ANKA, Karlsruhe, by the application of the measurement and evaluation approach for very fast X-ray diffraction stress analyses. During the laser surface spot hardening using a homogenization working optic with a spot size of approx. 8 × 8 mm² at a maximum temperature T_A,max of 1150 °C and heating/cooling rates v_heat/cool of 1000 K/s time-resolved diffraction data were collected for various measurement positions inside and outside of the processed zone aiming to analyze the different origins for residual stress build-up. The in-situ tests were supplemented by X-ray residual stress analyses and microscopical investigations of the microstructure subsequent to the laser hardening process (ex-situ analyses).

The results show that inside of the martensitic transformed region (process zone) in radial and in tangential direction homogeneous compressive residual stresses are generated. The data of the in-situ diffraction experiments reveal that these compressive residual stresses develop due to (i) local compressive elasto-plastic deformations and (ii) local phase specific transformation strains. Outside the process zone, the compressive residual stresses are balanced by rather high inhomogeneous tensile residual stresses. By means of the in-situ determined diffraction data it is proven that these tensile residual stresses have their origin in the superposition of (i) quenching effects outside the process zone, (ii) local elasto-plastic deformations and (iii) the effect of phase transformations in the nearby process zone.