Abstract
Thermochemical treatments like carburizing and carbonitriding allow to improve the properties in low-alloyed steels, which depend mainly on the distributions of residual stresses and microstructures. As the fatigue properties depend mainly on the latter, a fundamental understanding must be established regarding their formation during the cooling after the enrichment treatment.
This study introduces an experimental and simulation analysis of microstructure and internal stresses evolutions and their couplings. Influence of the carbon and nitrogen enrichments is highlighted. An original experimental technique is introduced to follow in situ by High-Energy XRD the phase transformation kinetics and the evolutions of the internal stresses during cooling, inside laboratory scale samples with C/N composition gradients.
The usual trends are confirmed regarding the carburizing: the carbon-enriched case is the last to undergo phase transformations. Due to the phase transformation strains, the surface ends up with compression residual stresses, whereas the center is put in tension. Conversely, for carbonitriding, unusual profiles of microstructures and residual stresses are observed. The presence of nitrogen induces a drastic loss of hardenability in the enriched case. This modifies the chronology of the phase transformations and this leads to tensile residual stresses at the surface for the studied cooling conditions. In the nitrogen-enriched case, a fine microstructure is formed during cooling and retained austenite remains, leading to a lower hardness than in the martensite layer beneath.
A coupled thermal, mechanical and metallurgical model predicting the phase transformation kinetics and the evolutions of internal stresses is set up. It takes account of the local carbon and nitrogen concentrations in the case. For carburizing, predictions are in good agreement with experiment. Simulations for carbonitriding achieve to predict the tensile stresses in the nitrogen-enriched case, which are due to the loss of hardenability. In both cases, residual stresses come mostly from phase transformation plasticity strains.
