Cr12型钢模具失效及其热处理工艺的改进

分析了Cr12型钢模具的失效原因,并指出提高冷作模具的质量,关键在于其硬度与韧性的最佳配合,即延长微细崩刃和疲劳崩刃的周期,针对这点,又对Cr12型钢的常规热处理工艺提出了改进意见。     

Ti含量和轧制工艺对AH36奥氏体晶粒大小的影响

本文利用Gleeble－1500热模拟试验机，研究了Ti含量对Ah36钢粗化温度的影响，同时利用二辊轧机研究了轧制温度、变形量对AH36钢的奥氏体晶粒大小的影响。该钢合理的加热温度范围为1200－1250℃。在轧制温度一定时，加大变形量，奥氏体晶粒尺寸减小；钛含量增加，奥氏体晶粒尺寸减小。     

Hydrogen effect on nanoindentation creep of austenitic stainless steel: A comparative study between primary creep stage and steady-state creep stage

The hydrogen effect on room temperature creep of 310 S stainless steel was investigated by using nanoindentation. Hydrogen enhanced the primary creep of 310 S steel much more significantly compared with the steady-state creep, which was attributed to the interaction between hydrogen and dislocation. Hydrogen obviously decreased the indentation stress so as to promote the primary creep, while hydrogen had little effect on the steady-state creep due to the evolution from mobile dislocation to immobile dislocation.     

The influence of intergranular and interphase boundaries and δ-ferrite volume fraction on hydrogen embrittlement of high-nitrogen steel

We study the effect of grain size of austenitic and ferritic phases and volume fraction of δ-ferrite, which were obtained in different solution-treatment regimes (at 1050, 1100, 1150 and 1200 °C), on hydrogen embrittlement of high-nitrogen steel (HNS). The amount of dissolved hydrogen is similar for the specimens with different densities of interphase (γ-austenite/δ-ferrite) and intergranular (γ-austenite/γ-austenite, δ-ferrite/δ-ferrite) boundaries. Despite, the susceptibility of the specimens to hydrogen embrittlement, depth of the hydrogen-assisted surface layers, hydrogen transport during tensile tests and mechanisms of the hydrogen-induced brittle fracture all depend on grain size and ferrite content. The highest hydrogen embrittlement index I_H = 32%, the widest hydrogen-affected layer and a pronounced solid-solution hardening by hydrogen atoms is typical of the specimens with the lowest fraction of the boundaries. Even though fast hydrogen transport via coarse ferritic grains provides longer diffusion paths during H-changing, the width of the H-affected surface layer in the dual-phase structure of the HNS specimens is mainly determined by the hydrogen diffusivity in austenite. In tension, hydrogen transport with dislocations increases with the decrease in density of boundaries due to the longer dislocation free path, but stress-assisted diffusion transport does not depend on grain size and ferrite fraction. The contribution from intergranular fracture increases with an increase in the density of intergranular and interphase boundaries.     

Hydrogen embrittlement behavior in interstitial Mn–N austenitic stainless steel

The susceptibility to hydrogen embrittlement behavior was investigated in an interstitial Mn–N austenitic steel HR183 and stainless steel 316L. Hydrogen was introduced by cathodic hydrogen charging at 363 K. HR183 has stronger austenite stability than 316L despite its lower nickel content, the addition of manganese and nitrogen inhibited martensitic transformation during the slow strain rate tensile deformation. Due to the diffusion of hydrogen being delayed by the interstitial solution of nitrogen atoms and the uniform dislocation slips, hydrogen permeates more slowly in HR183 than 316L, contributing to an 84.79 μm thinner brittle fracture layer in HR183 steel. Hydrogen charging caused elongation losses in both 316L and HR183 steels associated with the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanism. However, the hydrogen embrittlement susceptibility of HR183 is 3.4 times lower than that of 316L according to the difference in elongation loss between the two steel after hydrogen charging. Deformation twins trapped a lot amount of hydrogen leading to brittle intergranular fracture in 316L. The multiple directions of slip in HR183 steel suppressed the strain localization inside grains and delayed the adverse effects conducted by HELP and HEDE mechanism, eventually inhibiting server hydrogen embrittlement in the HR183 steel. This study is assisting in the development of low-cost stainless steel with excellent hydrogen embrittlement resistance that can be used in harsh hydrogen-containing environments.     

Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel

The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material’s susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material’s ductility compared to other alloying elements.     

The effect of age-hardening mechanism on hydrogen embrittlement in high-nitrogen steels

The effect of age-hardening regime on peculiarities of hydrogen-assisted fracture and tensile properties in two high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N and Fe-19Cr-22Mn-1.5V-0.3C-0.9N steels was studied. A large number of intergranular (austenite/austenite) and interphase boundaries (austenite/coarse particle) provides high fraction of trapping sites for hydrogen atoms in V-alloyed steel. This leads to a change in fracture regime from transgranular brittle mode in coarse-grained V-free steel to intergranular brittle fracture of hydrogen-assisted surface layers in fine-grained V-alloyed steel with coarse (V,Cr)(N,C) particles. The formation of cells (Cr₂(N,C) particles and austenite) along the grain boundaries due to discontinuous precipitate-hardening reaction facilitates predominantly interphase hydrogen-assisted fracture for both steels. The complex reaction of the particle-strengthening mechanisms including discontinuous precipitation with formation of austenite/Cr₂(N,C)-plates interfaces or homogeneous nucleation of coherent (V,Cr)(N,C) particles in austenite (age-hardening regime 700 °C, 10 h) promotes mainly transgranular cleavage-like fracture mode under hydrogen-charging. The structural scheme is proposed to describe a change in hydrogen-assisted fracture micromechanisms and tensile properties of the steels with different density and distribution of interphase and intergranular boundaries.     

Effect of Rare Earth Elements on Growth Dynamics of Austenite in 60CrMnMo Steel

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Modelling the Influence of Dynamic Strain Ageing on Deformation Behaviour

A model for the flow stress of low‐alloy steels is presented. In addition to work hardening and dynamic recovery the model includes dynamic strain ageing. Special attention is paid to the validity of the model at larger strains and to the mutual relationship between work hardening, dynamic recovery and dynamic strain ageing. This leads to a model consisting of three evolution equations, one for each of the three state variables, being the free dislocation density, the locked dislocation density and the free solute density. The model is consistent with previous work on work hardening at low temperatures. The model was applied to austenite. Experiments have been performed to measure the flow stress of austenite as a function of the strain rate and temperature. The dependence of the flow stress on temperature and strain rate has been investigated for various low‐alloy steels. It can be explained by assuming climb of dislocations by bulk diffusion of vacancies as the appropriate mechanism. The relation between the coefficients in the model and the chemical composition is analysed and discussed.     

Variation in electric resistivity in metastable alloys during thermomechanical loading: Effects of temperature,elasticity, plasticity and phase transformation

301L metastable stainless steel is a ductile material in which there is strong coupling between plasticity and phase transformation, leading to strongly non-linear thermomechanical behaviour. To determine phase transformation kinetics, isothermal tensile tests at different temperatures were carried out and in situ voltage (electric resistance) was measured. We then chose an appropriate post-processing method based on observed voltage using strain and temperature. It was thus possible to determine the effect of temperature, elasticity, plasticity and phase transformation on the electric resistivity of the studied material. After identifying the effect of each strain mechanism, a volume phase fraction determination method based on electric resistivity variation was developed and used to determine the kinetics of phase transformation. Finally, we compared our results with those of two classic methods: the neutron diffraction method and the magnetic method.