Short time tempering on 39MnCrB6-2 steel to find best compromise between mechanical properties and use of copper-rich scraps

On the one hand, copper (Cu) segregates forming solute clusters/precipitates at the austenite grain boundary. It delays in austenite to ferrite phase transformation which impaisr hot-ductility property. copper can lead to metallurgical problems during downstream thermo-mechanical processing, such as resistance to hot rolling and surface hot shortness during casting. Cu-oxysulphides or Cu-sulfides at the grain boundaries favor intergranular fracture. Thus, embrittlening elements, especially segregating at grain boundaries, strongly influence the susceptibility of a material to temper embrittlement. Temper embrittlement results in the loss of ductility (or the increase of the ductile-to brittle transition temperature) after tempering.

On the other hand, Copper is the most pervasive contaminant for steel scrap, present as wiring in vehicles, appliances and equipment, and alloyed with steel in engine blocks and powder metallurgy products. During hammer shredding, copper wiring entangles with the fragmented steel scrap. Subsequent magnetic separation is not completely effective, and steel-encased motors often remain with the steel scrap. The amount of steel discarded yearly will triple from the present day to 2050, as predicted by Pauliuk et al. Moreover, much more steel must be produced from scrap to meet emissions targets, and utilizing this growing resource is a sound economic strategy. However, the presence of contaminating elements restricts the applications in which end-of-life scrap can replace primary steel.

Finaly the use of low alloyed grade to reach high mechanical characteristics obliges often to apply low tempering temperatures by maintaining long tempering time. In this paper is proposed to study the possibility to use short tempering times for a 39MnCrB6-2 steel grade to reduce on one side the TE but also to support the sustainable trend of increasing Cu-content in ferrous scraps.