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     

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.     

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.     

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.     

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.     

Microstructural stability of thermal-mechanically pretreated type 316 austenitic stainless steel

Studies of the microstructural stability of Type 316 austenitic stainless steel were performed for a wide range of thermal-mechanical pretreatments in the limited aging temperature range of 550° to 760°C. The pretreatments were selected in order to investigate the effects of varying solution treatment temperature, amount of cold reduction by rolling, initial grain size, and initial precipitate distribution. Large variations in both phase stability and recrystallization behavior can be effected by appropriate pretreatments. Cold work accelerates precipitation of M₂₃C₆ carbide and the intermetallic compounds (Laves, χ, and σ phases). Both the amount and kinetics of σ phase formation are especially enhanced by recrystallization occurring in the aging temperature range. It is suggested that this occurs due to ready σ nucleation at slowly moving (recrystallizing) grain boundaries together with enhanced growth rates due to diffusion along the boundary. Fine grain size enhances phase instability by providing additional nucleating sites and decreased diffusion paths for precipitate forming elements, but in the grain size range studied (ASTM No. 3.5 to No. 13) the effect is not as significant as the effect of cold work, particularly when recrystallization occurs during the aging treatment. Fine grain size and pretreatments which precipitate the carbides prior to the final cold working step enhance recrystallization kinetics relative to solution treated and cold-worked materials. This is apparently due to stabilization of the cold-worked substructure in the solution treated samples by precipitation of carbide and Laves phases on the dislocations and stacking faults.     

Tensile and fracture properties of type 316 stainless steel after creep

The effects of creep on the mechanical properties of type 316 stainless steel were studied. Tensile and Charpy specimens were machined from the oversize specimens crept at 750 °C and 103 MPa. The ambient fracture energy was found to deteriorate rapidly after creep. The ambient yield stress was increased moderately, but the tensile ductility was severely reduced. The effects of intergranular carbides alone on mechanical properties were studied with specimens thermal aged without load. These carbides were shown to cause a moderate reduction in fracture energy and tensile ductility but had little effect on yield stress. Extensive grain boundary separations were observed on the fracture surfaces. SEM studies showed that these grain boundaries were covered with micro voids initiated by the dense intergranular carbides. Frequently, large dimples on grain boundary joined up and initiated shear fracture into the grain. In the crept specimens additional microstructural changes in the form of intragranular carbides and subgrain boundaries were observed. Both are responsible for the increase in yield stress and the further reduction in tensile ductility and fracture energy. The intragranular carbides also modified the size and density of the dimples on the fracture surfaces. Formerly with Metallurgy and Materials Science Division, Brookhaven National Laboratory, Upton, NY 11973.     

Discontinuous creep deformation in a type 316 stainless steel casting

A static cast ingot of type 316 stainless steel was tested at 811, 866, and 922 K. Creep curves for this material under certain temperature and stress conditions show instantaneous increase in strain. In the open literature this increase has been called “strain burst” or “jump”. The number of “bursts” per curve varied from 0 to 3, and the instantaneous strain was as large as 2 pct. The bursts generally occurred at high stresses and at the two lower test temperatures, 811 and 866 K. The rupture time for specimens showing bursts was lower than for specimens without them. The creep-rupture strength of the static casting was approximately 40 MPa lower than that for wrought material. The rupture time of the casting was a strong function of specimen location. The fine grained structure near the ingot edge produced longer rupture time compared with the coarse grain structure. Microprobe analysis showed significant variation in chromium, nickel, and molybdenum across the dendritic regions. The inhomogeneous distribution of the potent creep-strengthening element molybdenum has been suggested as responsible for the strain bursts.     

Fatigue-crack propagation behavior of type 304 stainless steel at elevated temperatures

The fatigue-crack propagation behavior of Type 304 stainless steel was investigated within the framework of linear elastic fracture mechanics at temperatures of 75‡, 600‡ 1000‡ and 1200‡F. The cyclic frequency for the elevated temperature tests was 4 cpm. It was found that, in general, fracture mechanics concepts may be used to describe the crack propagation behavior at these temperatures, and that increasing the temperature had a significant effect in increasing the fatigue-crack growth rate.