Volume 80, Issue 4

Martensitic stainless steels are an important group of steels for different applications. Their typical hardening consists of austenitizing, quenching and (multiple) tempering (Q&T). As many of these steels contain at least smaller amounts of retained austenite (RA) after quenching, partitioning of carbon and nitrogen from martensite into RA can take place during tempering, changing it from pure Q&T to combined Q&T and quenching & partitioning (Q&P). This contribution provides a systematic overview of such partitioning effects on micro-structure and subsequently on material properties and were investigated with several steel grades and a wide variation in heat treatment parameters. The results demonstrate that partitioning effects dominate over tempering at temperatures up to 500 °C. Higher quenching temperatures can increase the RA-content similar to higher austenitizing temperatures. Lower quench rates can reduce RA due to carbide (nitride) precipitation. Rising tempering temperatures enhance austenite stabilization. Larger amounts of RA promote transformation induced plasticity (TRIP), leading to high ductility and tensile strength but reduced yield strength. Increased amounts of RA with sufficient stability raise impact toughness at slightly reduced hardness. Tempering above 500 °C in contrast promotes carbide and nitride precipitation, resulting in the elimination of the RA and therefore a typical tempering condition.

Laser Power Bed Fusion (LPBF) fabricated parts often require a modified heat treatment to accommodate for slight differences in composition due to the additive manufacturing process. Powders used for LPBF fabrication of the precipitation hardening 17-4PH stainless steel are often spray-atomized using N 2 cover gas, resulting in a significant nitrogen uptake which lowers the martensite start (M s ) temperature of the steel. Sub-zero treatments can promote martensite transformation in such alloys. In this work, wrought 17-4PH samples were high temperature solution nitrided (HTSN) to achieve nitrogen contents comparable to LPBF 17-4PH. Sub-zero dilatometry was used to track martensite formation behavior at sub-zero temperatures in the HTSN wrought and LPBF 17-4PH. Samples were further characterized through XRD, hardness, and optical microscopy. It was found that martensite morphology changes from lath to predominantly twinned with decreasing M s (higher nitrogen), and thermally activated martensite formation was recorded on heating. The HTSN wrought samples also exceeded the standard peak hardness for 17-4PH, demonstrating that sub-zero treatments are effective at producing a hard martensitic microstructure for both HTSN and AM parts produced with N 2 -atomized powders.

The present paper examines additively manufactured tool inserts in the series production of a die casting tool for a clutch housing. The inserts are manufactured by laser powder bed fusion using the well-established maraging steel 1.2709. Various post-treatment approaches are examined as part of the investigations. In addition to different downstream heat treatments, a component variant with a specific surface structure in the direction of demolding is also tested. A comprehensive damage analysis is carried out to determine the causes of failure and to derive recommendations for action to improve the service life of the mold insert and, thus, ultimately the quality of the casting. The mold inserts selected for the test series demonstrate that additive manufacturing can increase the service life of this specific tool insert by more than three times, while the acquisition costs for the insert are reduced by more than 50 %. In tools with areas that are subjected to high thermal stress, which benefit from conformal cooling and therefore cannot be produced using conventional manufacturing processes, the implementation of additively manufactured tool inserts is therefore recommended for both economic and technological reasons.