Influence of delay strategies and residual heat on in-situ tempering in the laser metal deposition of 300M high strength steel

Abstract

This study investigates the role of delay times between tracks and layers in the laser metal deposition (LMD) to replicate the required quench and temper process for 300M steel in-situ. By allowing time for cooling, the use of a delay produced different microstructures compared to ones where retained heat could accumulate. Delays between layers produced a sawtooth hardness profile in the vertical orientation ranging from 450 HV to 550 HV between the different layers, with each fluctuation corresponding to a delay. Delays between tracks showed a similar pattern, with the range increasing to 475 HV–675 HV and a sawtooth pattern in evolving in both horizontal and vertical directions. The last track/layer had a hardness above 650 HV as these were not tempered by subsequent deposition, while the softest regions were tempered close to the AC1 temperature. The accumulation of residual heat in continuous deposition also has the capacity for in-situ tempering where the temperature can fall below MS. The hardness profile for this form of in-situ tempering was more uniform than the delay strategies and showed improved hardness in the upper tempered layers. This effect was due to an increase in the proportion of retained austenite in these layers compared to the delay strategies, which would revert into martensite on subsequent reheating and experience tempering at lower temperatures compared to pre-existing martensite.