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. The use of low alloyed quenching and tempering steel grade such as 39MnCrB6-2 to reach high mechanical characteristics (around 1000 MPa) obliges often to apply low tempering temperatures for which tempering embrittlement may be observed. In this paper, it is proposed to reduce the hold time and to increase the temperature during conventional tempering to (1) reduce the embrittlement because of segregation of elements like copper, (2) to change the fracture mechanism with finer martensite sub-grains and (3) to promote θ particles with smaller dimensions but higher density.

Press hardening steel (PHS) applications predominately use 22MnB5 AlSi coated in the automotive industry. This material has a limited supply chain. Increasing the tensile strength and bendability of the PHS material will enable light-weighting while maintaining crash protection. In this paper, a novel PHS is introduced, and properties are compared to 22MnB5. The new Coating Free PHS (CFPHS) steel, 25MnCr, has increased carbon, with chromium and silicon additions for oxidation resistance. Its ultimate tensile strength (UTS) of 1.7 GPa with bending angle above 55° at 1.4 mm thickness improves upon the 22MnB5 grade. This steel is not pre-coated, is oxidation resistant at high temperature, thus eliminating the need for AlSi or shot blasting post processing to maintain surface quality. Microstructural mechanisms used to enhance bendability and energy absorption are discussed for the novel steel. Performance evaluations such as: weldability, component level crush and intrusion testing and e-coat adhesion, are conducted on samples from industrial coils.

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

As a novel manufacturing technology additive manufacturing (AM) has advantages such as energy saving, reduced material waste, faster design-to-build time, design optimization, reduction in manufacturing steps, and product customization compared to conventional manufacturing processes. Heat treatment is widely used to improve the properties of conventional manufactured steel parts. The response of additively manufactured steel parts to heat treatment may be different from conventionally manufactured steel parts due to variations in microstructure. An understanding of heat treatment processes for additively manufactured steel parts is necessary to develop their heat treatment process parameters. In the present work 20MnCr5 steel was selected to investigate the carburization heat treatment of additively manufactured parts. These parts were fabricated by selective laser melting (SLM) for the carburization study. It was found that the AM parts fabricated by the SLM process show the microstructure of tempered martensite while the microstructure of as-received wrought part is ferrite and pearlite. It was also experimentally found that the SLM process decarburizes the entire SLM part. Before carburizing, a normalization process was conducted on both SLM and wrought 20MnCr5 parts to reduce the effect of the pre-carburizing microstructure. The objective of this project is to determine the carburization performance of additively manufactured steel parts. The results for the SLM parts in terms of carbon concentration and microhardness profiles are compared with the results for the wrought steel. It was found that the carburized SLM part in the present work has higher carbon concentration near the surface, deeper case depth, and higher total carbon flux than the carburized wrought part.

Low pressure carbonitriding (LPCN) has the potential to improve the impact and fatigue strength of steel components through the enrichment of nitrogen and the effect of carburizing at higher temperatures. The work described in this paper investigates the influence of boron on the LPCN response of 20MnCr5 steel and the effect of niobium on that of 8620. LPCN treatments were developed to achieve a surface hardness of ~700 HV and case depth of 0.65-0.75 mm in four alloys: 20MnCr5, 20MnCr5 + B, 8620, and 8620 + Nb. The hardness and case microstructure of treated and quenched test samples are correlated with bending fatigue measured in Brugger fatigue specimens, which simulate the root of a gear tooth.

The present work is the first stage of an ongoing research project concerning with integration of quenching and partitioning heat treatment into the traditional hot stamping process, for 22MnB5 alloy, high Si and high Mn steels, last two proposed for the research with a special chemical composition to enhance Quenching and partitioning (Q&P) process. The above, in order to produce a microstructure containing retained austenite and martensite to improve the ductility of resultant components. The investigations of quenching and partitioning process were made by means of dilatometry and hot stamping plus quenching and partitioning (HS - Q&P) process experiments by a special experimental tool. The resulting microstructure was examined using light optical and scanning electron microscope as X-ray diffraction too. Some results obtained shown that compared with conventional hot stamping samples, the HS-Q&P process improves elongation effectively and maintains high-strength at the same time.