Effects of Si on the stability of retained austenite and temper embrittlement of ultrahigh strength steels

Effects of silicon （Si） content on the stability of retained austenite and temper embrittlement of ultrahigh strength steels were investigated using X-ray diffraction （XRD）,transmission electron microscopy （TEM）,and other experimental methods.The results show that Si can suppress temper embrittlement,improve temper resistance,and hinder the decomposition of retained austenite.Reversed austenite appears gradually with the increase of Si content during tempering.Si has a significant effect on enhancing carbon （C） partitioning and improving the stability of retained austenite.Si and C atoms are mutually exclusive in lath bainite,while they attract each other in austenite.ε-carbides are found in 1.8wt% Si steel tempered at 250℃,and they get coarsened obviously when tempered at 400℃,leading to temper embrittlement.Not ε-carbides but acicular or lath carbides lead to temper embrittlement in 0.4wt% Si steel,which can be inferred as cementites and composite compounds.Temper embrittlement is closely related to the decomposition of retained austenite and the formation of reversed austenite.

Overaging Phenomenon of Fe-2% Mn-Sb-Ce Structural Steels during Tempering

The temper embrittlement of Fe-2%Mn-Sb-Ce structure steels was studied by measurements of the ductile-brittle transition temperature and observations by AES and SEM of the fracture surface of isothermally embrittle steel. It shows that Sb and Mn can produce non-equilibrium co-segregation to grain boundaries and bring about the temper embrittlement of Fe-2%Mn-Sb-Ce structure steels. Cerium may reduce the temper embrittlement of the steels and the segregation of cerium to grain boundaries may play an important part in reducing the temper embrittlement of the steels.

KINETICS OF TEMPER EMBRITTLEMENT IN 2.25Cr-1Mo STEEL USED FOR HOT-WALL HYDROFINING REACTORS

Based on the theory of grain boundary segregation, a kinetics model of temper em-brittlement caused by long-term service for hot-wall hydrofining reactors was studied.The kinetics model was applied to phosphorus (P) segregation in 2.25Cr-1Mo steelused for a hot-wall hydrofining reactor, and the kinetics of grain boundary segrea-tion of impurity P in the steel exposed to the process environment of the hydrofiningreactor was calculated on the basis of the model. The Auger electron spectroscopytest was performed in order to determine the grain boundary concentration of P. Theexperimental result is agreement with the theoretical calculated data. The results showthat the kinetics equation is reasonable for predicting the levels of grain boundarysegregation of impurity P in 2.25Cr-1Mo steel used for hot-wall hydrofining reactors.

BInfluence of Cerium and Phosphorus onEmbrittlement in Manganese Steels

The influence of Ce and P on embrittlement transition temperature in manganesesteels has been investigated by using impact test , SEM and Auger spectrometer. The results showthat the segregation of phosphorus along the austenite grain boundaries causes a rise in the embrit-tlement transition temperature. Ce doped in the steels can minimise the degree of segregation ofphosphorus and raises the impact toughness and, therefore, reduces the embrittlement transitiontemperature.

Effect of Rare Earths on Tempered Martensite Embritlement of Steel

The mechanical properties of SiMnCr and SiMnMo steels tempered in lowtemperature range were studied. The results show that there is no notable effect of RE on material strength during lowtemperature tempering. There are toughness troughs of tempered martensite embrittlement(TME) at 350 ℃ and 400 ℃ for steel SiMnCr and SiMnMo respectively. RE raises the toughness of TME troughs to some extent by refining grains and restraining embrittlment of austenite grain boundary, although it does not change TME temperature.

Relationship Between Grain Boundary Segregation of Antimony and Temper Embrittlement in Titanium-Doped Nickel-Chromium Steel

The antimony segregation at grain boundary was observed and the temper embrittlement in titanium-doped nickel-chromium steel was analyzed. It is concluded that the antimony segregation at grain boundary is nonequilibium and the kinetics of temper embrittlement agrees well with those of nonequilibrium antimony segregation at grain boundary. Besides, the mechanism of nonequilibrium antimony segregation at grain boundary proved to be the most satisfactory one among the existing mechanisms to interpret the antimony induced embrittlement kinetics in the nick- el-chromiunl steel. Based on these, the activation energy and frequency factor of diffusion of antimony vacancy complexes were obtained according to the concept of critical time in nonequilibrium grain boundary segregation theory.

Effect of Mg addition on temper embrittlement in 2.25Cr–1Mo steel doped with 0.056% P–Mg segregation behavior at grain boundary

To verify the microalloying function and segregation behavior of trace Mg at grain boundary in steel,the 2.25Cr–1Mo steel doped with 0.056%P containing different Mg contents was refined with a vacuum-induction furnace.The effects of trace Mg addition on the temper embrittlement susceptibility of 2.25Cr–1Mo steel were studied by step-cooling test and the segregation behavior of Mg at grain boundary was explored by Auger electron spectroscopy.It is shown that P-induced temper embrittlement susceptibility can be reduced after subjecting to step-cooling treatment with trace Mg addition,mainly benefited from the segregation of Mg at grain boundary.This segregation can decrease the segregation amounts of P and S,especially for P,and increase the grain boundary cohesion,reducing the adverse effect on temper embrittlement caused by P and S.

Effect of isothermal temper embrittlement and subsequent hydrogen embrittlement on tensile properties of 2.25Cr-1Mo-0.25V base metal and welded metal

2.25Cr–1Mo–0.25V base metal(BM)and welded metal(WM)with different temper embrittlement states were obtained by isothermal temper embrittlement test.The ductile–brittle transition temperature and the carbide size of temper embrittled 2.25Cr–1Mo–0.25V BM and WM increased with the isothermal tempering time.The increase in temper embrittlement time leads to a decrease in yield strength(YS)and ultimate tensile strength(UTS).Hydrogen embrittlement(HE)can decrease the ductility and increase YS and UTS of the material.The hydrogen embrittlement sensitivity and microstructure analysis both show a combined effect of HE and temper embrittlement.The deeper the temper embrittlement,the more sensitive the material to HE.When the hydrogen content in the material is low,the WM is less susceptible to HE due to its welding defects.