Advanced characterization techniques and modeling are used to get new insight on the microstructural evolutions occurring during the tempering of low-alloyed steels with initial martensitic microstructure. Tempering temperatures from 150°C to 600°C, are considered to make vary the metallurgical phenomena activated, form carbon segregation to defects to precipitation of different types of carbides (transition, cementite, alloyed). A large range of carbon compositions, from 0.1 to 0.7 wt.% are investigated, with the same main experimental technique: in situ HEXRD at synchrotron beamlines, with complementary post mortem fine-scale characterizations by TEM and 3D-APT. In the middle of this range (~0.3wt.%), the usual sequence is observed: successive precipitation of transition and cementite carbides. New observations concern the carbon concentrations outside this range. For high carbon concentrations (~0.6wt.%), the same sequence occurs but the martensite/ferrite matrix remains highly supersaturated in carbon compared to equilibrium, for a long time and even after the precipitation of cementite. For low carbon concentrations (~0.1wt.%) most of the carbon starts to segregate at defects (dislocations, lath boundaries). This enters in competition with the transition carbides which are almost fully hindered, whereas cementite precipitates afterwards. Two previous models from literature are combined to predict the concomitant kinetics of carbon segregation and precipitation. Segregation puts the transition carbides at a disadvantage with cementite and for this reason, the latter precipitates earlier than usually reported. The effects of nitrogen enrichment (up to ~0.4 wt.%N, context of carbonitriding thermochemical treatments) in austenite domain of stability (before the martensitic quench) are also investigated. In low-alloyed steel considered (23MnCrMo5), nitrides are formed upon enrichment (CrN, MnSiN 2 ). This has a strong impact on the precipitation sequence, compared to model systems previously investigated (Fe-N, Fe-C-N).

Quenched and tempered (Q&T) medium-C steels with various V and Mo additions were studied to understand the relationship between alloy carbide precipitation and hydrogen absorption and trapping behaviours. Heat treatments were selected in the temperature range favourable for V carbide formation, 500-600 °C, leading to higher hardness compared to similar V- and Mo-free alloys due to precipitation hardening. Heat-treated coupons were electrochemically charged to introduce hydrogen, and the bulk hydrogen concentration was measured using melt extraction analysis. Hardness and dislocation density were measured for each tempered condition to relate these properties to the hydrogen absorption and trapping behaviours of each material. Results indicate that dislocation density as well as V and Mo carbide precipitation increase the extent of hydrogen absorbed during charging and the amount of hydrogen remaining trapped after holding at ambient temperature for up to 168 h (1 week).

The 13% Cr 0.2%C steel is extensively used in OCTG application. This steel was examined in the double hardened condition at 1040°C/OQ followed by 980°C/OQ as against conventional hardening treatment at 980°C/OQ. It was observed that the double hardening heat treatment resulted in higher carbon dissolution in the matrix without any residual grain boundary necklace carbides while single hardening at 980°C shows remnant grain boundary carbide. Double hardening heat treatment was found to refine the grain size by recrystallization of the defects introduced in first hardening. During double tempering heat treatment, the carbides in the 980°C treatment were coarse and found to nucleate along grain boundary. The double hardened sample on tempering showed an even distribution of carbide throughout without grain boundary carbide. The double hardened sample show improved strength and toughness compared to the single hardened sample at similar tempering conditions. The microstructural analyses at various stages of processing have been correlated to the mechanical properties obtained.

This paper presents a new approach for predicting nitriding and nitrocarburizing results. The model calculates thermodynamic and kinetic effects based on material composition and pre-nitrided conditions. It can simulate up to three-stage recipes with varying temperatures, nitriding potentials, and carburizing potentials while also taking nucleation time into account. The simulation result gives compound layer thickness, precipitation layer, and total diffusion depth and calculates surface hardness, core hardness, and effective case depth.