The influence of microstructure on hydrogen embrittlement of high strength steels for fastener applications is explored in this study. Space limiting applications in areas such as the automotive or agricultural industries provide a need for higher strength fasteners. Albeit, hydrogen embrittlement susceptibility typically increases with strength. Using a 9260 steel alloy, the influence of retained austenite volume fraction in a martensitic matrix was evaluated with microstructures generated via quenching and partitioning. X-ray diffraction and scanning electron microscopy were used to assess the influence of retained austenite in the matrix with different quenching parameters. The quench temperatures varied from 160 °C up to 220 °C, and a constant partitioning temperature of 290 °C was employed for all quench and partitioned conditions. The target hardness for all testing conditions was 52-54 HRC. Slow strain rate tensile testing was conducted with cathodic hydrogen pre-charging that introduced a hydrogen concentration of 1.0-1.5 ppm to evaluate hydrogen embrittlement susceptibility of these various microstructures. The retained austenite volume fraction and carbon content varied with the initial quench temperature. Additionally, the lowest initial quench temperature employed, which had the highest austenite carbon content, had the greatest hydrogen embrittlement resistance for a hydrogen concentration level of 1.0-1.5 ppm.

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.

Retained austenite may be helpful or detrimental to the life of heat-treated components, but it can be difficult to accurately measure in manufactured steels. Commonly used visual sample investigations are subjective and often incorrect, magnetic measurements require part-specific calibration, and electron backscattering involves expensive equipment, intensive sample preparation, and long measurement times. Recent developments in X-ray diffractometry, however, provide measurements in minutes and can compensate for the influence of carbides in high-carbon steels as well as texture orientations in rolled sheet metals. This paper discusses the use of X-ray diffraction for measuring retained austenite and compares and contrasts it with other methods. It also provides a brief review of the formation of austenite and its effect on carburized gears, TRIP steels, and bearings.

The notion that compressive residual stresses can extend the service life of components subject to rolling contact fatigue is well documented. However, the exact nature of the relationship between effective case depth and the residual stress state is not well understood for components with case depths greater than 0.050 in. (1.27 mm). It is expected that compressive residual stresses gradually transition to tensile stresses as case depth increases beyond a threshold value. This study will measure the residual stress state of components with different case depths before and after simulated service in order to determine where the compressive to tensile transition occurs. It will also investigate the role of retained austenite and the effect of strain-induced transformation caused by rolling contact. Residual stress and retained austenite measurements will be conducted using X-ray diffraction.

Microstructure refinement strategies for carburized steel were evaluated to assess their effect on the fatigue performance of case carburized components. Commercial 52100 steel samples were subjected to various treatments and analyzed to determine the micro-geometry of plate martensite and the size distribution of retained-austenite regions. Decreasing reheat temperature produced finer austenite grain size, while multiple reheating cycles helped narrow grain size distribution. The refinement of austenite grain size also led to a reduction in martensite plate size and finer distribution of retained austenite.