The effect of tempering and auto-tempering on the microstructure–property relationship of two ultra-high strength press hardening steels (PHS1500 and PHS2000) was studied. Both steels were austenitized, oil quenched, and subsequently tempered at four different temperatures ranging from 180 °C to 300 °C. For auto-tempering, the steels underwent austenitization and quenching using a press equipped with planar tools and were subsequently ejected at varying cooling durations. The tensile properties, hardness, microstructure, and dislocation densities after heat treatment were characterized. The results showed that the effect of tempering temperature on tensile properties and microstructure features was more pronounced than the effect of tempering time for both steels. Tensile strength and hardness decreased slightly with increasing tempering temperature up to 200 °C. Above that temperature, there was a further decrease in tensile strength and hardness, which is suggested to be due to the formation and coarsening of carbides in the highly dislocated martensitic matrix. In contrast to the tensile strength and hardness, the yield strength increased with increasing tempering temperatures, which is most probably due to internal stress relaxation. Total elongation was increased with increasing tempering temperatures, except for the samples tempered at 250 °C and 300 °C. These samples experienced a reduction in elongation at fracture, which was more pronounced after tempering at 300 °C than at 250 °C. This was most likely attributed to the so-called tempered martensite embrittlement effect. Calculation of dislocation densities before and after tempering treatments confirmed dislocation annihilation and recovery of martensitic microstructure. 

This study explores the use of Fe-Mn-Cr-Si-C master alloy as a sustainable alternative to nickel in press and sinter powder metallurgy, with the goal to improve hardenability and mechanical performance. Two master alloy compositions were optimized for liquid phase sintering using thermodynamic software and then produced through gas atomization. The melting behavior of the master alloys was characterized and compared with thermodynamic predictions. Sintering experiments were performed on compacts from mixes with and without master alloy additions. Continuous cooling transformation (CCT) diagrams were generated to assess hardenability. The results demonstrated that the master alloys improve the hardenability, even at lower cooling rates compared to alloying with Ni. Mechanical testing showed notable improvements in yield strength and apparent hardness comparable to those achieved with Ni additions. These findings support the use of Fe-Mn-Cr-Si-C master alloy as a viable, more sustainable alternative to nickel in powder metallurgy steels.

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

The alloy 21-6-9 is a nitrogen-strengthened austenitic stainless steel often used in aerospace applications due to its high strength, good fabrication properties, and toughness at cryogenic temperatures. However, minimal research has been conducted on alloy 21-6-9 using the additive manufacturing process laser powder-bed fusion (L-PBF). The L-PBF technique has been seen as a key to reducing production time and avoiding costly machining. Therefore, there is an interest in investigating L-PBF-processed 21-6-9 to determine the effects of L-PBF on properties at elevated and cryogenic temperatures. In this study, prior to tensile testing the alloy 21-6-9 underwent heat treatments that simulated aerospace applications and the alloy was analyzed and characterized to evaluate phase stability. The effects of elevated and cryogenic temperatures (77K) on the tensile behavior and microstructure were investigated using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The tensile tests showed that the yield strength and ultimate tensile strength improved, while ductility varied depending on the conditions and test environment. The ultimate tensile strength was approximately 80% higher at 77K than at room temperature, although the elongation decreased by around 90%, possibly due to the formation of strain-induced martensite.

The effect of ultrasonic treatment (UST) and thermal annealing (THA) post-processes on the mechanical properties and the related microstructural mechanisms of the tensile pre-strained 316 stainless steel was investigated. It was shown that both processes reduce the microhardness and the yield point as well as increasing the elongation of the pre-deformed alloy. A 10% reduction of the yield point and 28% increase in the elongation was observed after the higher power UST (500 W), while an enhanced ductility of 56% and 41% reduction of the yield point was measured for the high-temperature THA (800°C) treated steel. The increased ductility was related to de-twinning and dislocation annihilation mechanisms, which increase the mean free path distance of dislocations. The de-twinning mechanism was proposed as the boundary migration mechanism and reverse gliding of the partial dislocations by cyclic shear stress for the THA and UST processes, respectively. Unlike the UST process, the high-temperature thermal annealing was associated with the formation of M₂₃C₆ precipitates, which causes depletion of alloying elements from the vicinity of grain boundaries and makes the alloy more prone to intergranular corrosion. Compared with THA, the advantages of the UST process are as follows: a rapid and straightforward process, low energy consumption, enhanced ductility without significant reduction in strength, and inhibition of grain boundary precipitation.

Carbon partitioning from martensite to austenite is essential for austenite stabilization during quenching and partitioning (Q&P), while a few competitive phenomena, such as bainitic transformation and carbide precipitation, alter the microstructural evolution. So, there is a need of using in-situ in combination with ex-situ characterisation techniques to understand the C partitioning at high temperature in relation to simultaneous competitive phenomena that might occur during the partitioning stage.

In this study, microstructural evolutions of a medium carbon steel ( 0.6C–1.6Si–1.25Mn–1.75Cr wt%) during Q&P treatment were investigated by using an in-situ High-Temperature X-Ray Diffraction (HTXRD) equipment at three partitioning temperatures. Results confirmed that carbon enrichment of austenite at 280 and 400 ℃ originates from partial carbon depletion from martensite and bainitic transformation, while partitioning at 500 ℃ results in the complete depletion of carbon from initial martensite and ferrite formation. Short diffusion distance (~0.13 µm) of carbon at 280 ℃ caused a poor carbon homogenization of austenite and formation of 8 vol% fresh martensite after final quenching. High Si content of the steel stabilized transitional carbides and, concurrently, suppressed Fe₃C formation during Q&P. The outcome of this study could contribute to the design of suitable chemistry and process parameters for producing quenched and partitioned steels.