Comparison of load relaxation data of type 316 austenitic stainless steel with Hart's deformation model

Load relaxation experiments have been performed on Type 316 stainless steel at temperatures up to 650°C. The resulting stress-plastic strain rate relationships below 500°C show the behavior of a plastic equation of state. The experimental results suggest that at lower temperatures the deformation is controlled by dislocation glide and at higher temperatures dislocation glide becomes less important. Hart's phenomenological model based on a plastic equation of state is used to account for the experimental data. F. V. ELLIS, formerly Post-Doctoral Associate, Department of Materials Science and Engineering, Cornell University, is now     

Low temperature aging behavior of type 308 stainless steel weld metal

The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M₂₃C₆ carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.     

Relationship between anelastic and nonlinear viscoplastic behavior of 316 stainless steel at low homologous temperature

At low homologous temperature the plastic strain rate seems to be controlled largely by dislocation glide friction. However, since a sizeable fraction of the applied stress σ is dissipated in overcoming the strong barriers due to dislocation tangles generated by strain hardening, only a portion of the applied stress is actually expended against the frictional resistance. A recent model for this process, proposed by Hart, includes the role of dislocation pile-ups at the strong barriers. The pile-ups provide a mechanism for producing the internal back stress that reflects the barrier penetration stress. They also appear in the deformation as a stored anelastic strain component. The resultant behavior at low temperature and high stress is similar to that proposed by Gupta and Li. The same model also predicts an anelastic behavior at low stress. Measurements at both high and low stress levels on 316 Stainless Steel have now shown that the predictions of the model are quantitatively consistent at both stress levels.     

316L不锈钢动态再结晶行为

在Gleeble-1500热模拟试验机上,通过高温压缩实验对316L不锈钢的动态再结晶行为进行了系统研究.结果表明:316L不锈钢热变形加工硬化倾向性较大,在真应力应变曲线上没有出现明显的应力峰值σ_p;316L不锈钢在热变形过程中发生了动态再结晶,但只是在局部区域观察到了动态再结晶晶粒.对动态再结晶的实验数据进行拟合,得到316L不锈钢的热激活能和热变形方程,并给出了发生动态再结晶的临界应变和临界应力以及Zener-Hollomon参数和稳态应力的关系.     

316L不锈钢中氮气合金化行为

通过FactSage热力学计算软件和实验室实验研究了在常压和真空条件下温度、氮分压和碳含量对316L不锈钢中氮溶解度的影响.结果表明：钢中氮的溶解度随着温度的降低而升高,随着氮分压的增大而增大,随着钢液碳含量的增加而减少,其中氮分压对钢液氮溶解度的影响最大.不同吹氮条件下氮溶解度实测值与FactSage热力学软件计算值较吻合.生产控氮型316L不锈钢可以在吹氧脱碳阶段实现,生产氮质量分数大于0.10%的中氮型316L不锈钢,只能在氮分压大于30kPa的加料阶段以及破真空后大气微调阶段实现.     

The aging behavior of homogenized type 308 and 308CRE stainless steel

The aging behavior of type 308 stainless steel and type 308 stainless steel with controlled residual elements (308CRE) was studied. The alloys were initially homogenized, resulting in a fully austenitic structure in the type 308 alloy and a duplex, austenite plus ferrite structure in the type 308CRE alloy. The materials were subsequently aged at 550, 650, 750, and 850 °C for times up to 10,000 hours. Aging of the type 308 steel resulted in precipitation of chromium-rich M₂₃C₆, primarily along grain boundaries, followed by sigma-phase formation after long aging times (≥5000 hours). The aging response of the duplex type 3O8CRE steel was somewhat more complicated. Although the titaniumrich carbides, nitrides, and sulfides produced during homogenization remained stable during subsequent aging, some additional precipitation was found. G-phase formed within the residual ferrite but eventually redissolved. The ferrite partially dissolved during early aging and later transformed to sigma phase. The sigma-phase formed significantly faster in the aged type 308CRE alloy. The results are summarized in the form of TTT diagrams. A comparison is made with a similar aging study on the same alloys but starting with the as-welded condition.     

The effect of helium on the stress-rupture behavior of type 316 stainless steel

Flat tensile samples of type 316 stainless steels with helium contents of 1.5 × 10⁻⁶ and 4.0 × 10⁻⁵ atom fraction, were stress-rupture tested in vacuum at 700°C. The presence of helium caused large reductions in rupture life and in elongation at failure. The amount of strain produced within the grains of helium-containing samples was a small fraction of the measured total elongation. The difference in these values is accounted for by extensive intergranular cracking.     

时效温度对马氏体时效不锈钢延迟断裂行为的影响

借助于缺口慢拉伸试验研究了马氏体时效不锈钢延迟断裂倾向对时效温度的敏感性。研究结果表明,正常冶炼和加工的材料氢的质量分数在2×10-6以下,420和440℃欠时效处理的试样仍发生氢致延迟断裂;而460℃以上时效的缺口试样慢拉伸(0.001 5mm/min)的抗拉强度和断裂行为类似于快拉伸(0.15mm/min),即不发生延迟断裂。     

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M₂₃C₆ was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti₄C₂S₂ which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M₂₃C₆ and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M₂₃C₆ and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.     

Effect of LaCrO_3 coating on high temperature oxidation of type 316 stainless steel

A dense and conductive LaCrO3 coating was prepared on type 316 stainless steel (316 SS), aiming at exploring its potential appli-cations in SOFC and in other high temperature environments. Powder of LaCrO3 with perovskite structure was synthesized by sol-gel method. LaCrO3 coating on 316 SS substrate was obtained by slurry coating technique. The microstructure of the coating on 316 SS after sintering in air at 800 and 900 ℃ for 200 h was characterized. The effect of LaCrO3 coating on oxidation resistance of the steel in air was also investi-gated. The results showed that the coating was adhesive to the substrate and improved greatly the oxidation resistance of the alloy.     

Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate the relevance of crack growth data in predicting fatigue life data.     

Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Metallurgical and Materials Transactions A - Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is...     

Ferrite formation in neutron irradiated type 316 stainless steel

Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 10₂₂ neutrons/cm₂ at 425°C and 3.5 × 10₂₂ neutrons/ cm₂ (E > 0.1 MeV) at 500 and 600°C. A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetization vs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiation and post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed 100 h at 760°C before irradiation and 1 h at 500°C after irradiation at 425°C.     

Ferrite formation in neutron irradiated type 316 stainless steel

Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 10₂₂ neutrons/cm₂ at 425°C and 3.5 × 10₂₂ neutrons/ cm₂ (E > 0.1 MeV) at 500 and 600°C. A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetization vs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiation and post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed 100 h at 760°C before irradiation and 1 h at 500°C after irradiation at 425°C.     

Environmental cracking of type 316 austenitic stainless steel weldments in high temperature co2 gas

The environmental cracking of Type 316 austenitic stainless steel manual metal arc (MMA) weldments in high temperature CO₂ has been investigated. The welding thermal transient has been analyzed and used to predict the sensitization of the parent material and resulting residual stresses. Sensitization is considered to result from the local depletion of chromium about grain boundaries and this has been considered theoretically and measured using STEM combined with energy dispersive X-ray analysis. Residual stresses have been measured using the X-ray diffraction method. Flux residues over the HAZ of the weldment have been identified using X-ray diffraction analysis. Chemical changes in these residues which resulted in cracking of test samples exposed to high temperature (673 to 823 K) CO₂ gas have been analyzed by X-ray diffraction. The susceptibility of the weldments to cracking was found to be influenced by the carbon content and hardness of the material. In materials of high carbon content and high hardness, the necessary cracking parameters are satisfied by the combined contribution of the microstructural sensitization, tensile residual stresses, and the chemical interaction of weld flux residue with the CO₂ gas which provides a molten salt environment capable of allowing rapid transport of aggressive species to active crack sites.