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.     

Effect of strain wave shape on low-cycle fatigue crack propagation of SUS 304 stainless steel at elevated temperatures

Effect of strain wave shape on strain-controlled low-cycle fatigue crack propagation of SUS 304 stainless steel was investigated at 600 and 700 °C. It was found that the rate of crack propagation in a cycle-dependent region was successfully correlated with the range of cyclicJ-integral, ΔJf, regardless of the strain wave shape, frequency, and test temperature. It was also shown that the rate of crack propagation gradually increased from cycle-dependent curve to time-dependent one with decreasing frequency and slow-fast strain wave shape, and that one of the factors governing the rate of crack propagation in such a region was the ratio of the range of creepJ-integral to that of totalJ-integral, ΔJ _c/ΔJT. Based on the results thus obtained, an interaction damage rule proposed semi-empirically was interpreted, with regard to crack propagation. Furthermore, fatigue crack initiation mechanism in slow-fast strain wave shape was studied, and it was shown that grain boundary sliding took an important role in it.     

Elastoplastic deformation of 316 stainless steel under tensile loading at elevated temperatures

The response of 316 stainless steel has been examined under uniaxial tensile loading during a range of tests carried out between 20 °C and 650 °C. In-situ neutron diffraction was used to measure internal elastic strain in subsets of differently oriented crystallites within the polycrystal aggregate. This allowed the determination of diffraction elastic constants. Further, results have been compared with predictions from a slip-based elasto-plastic self-consistent model. Good agreement is obtained during both conventional slip and when dynamic strain aging (DSA) is evidenced. The quality of agreement was reduced in the higher temperature regime, where it is expected that other mechanisms become active.     

The effect of one slow-fast strain cycle on the fatigue crack growth behavior of SUS 304 stainless steel at elevated temperature

Fatigue crack growth behavior of SUS 304 stainless steel when one slow-fast strain cycle was introduced to the fast-fast strain cycles was investigated at 600 °C. It was found that the fatigue crack growth rate in the fast-fast strain cycles was accelerated by the introduction of one slow-fast strain cycle. It was also found that there was a linear relationship between the size of the region where the acceleration occurred and the value of creepJ-integral range, ΔJ_C, which was produced just when one slow-fast strain cycle was introduced. To investigate the above acceleration, the longitudinal section through the specimen was observed. It was shown that the wedge-type cracks were produced at the grain boundaries and that they contributed to the above acceleration. Based on the results thus obtained, the predicting procedure of crack growth curve when one slow-fast strain cycle was introduced to the fast-fast strain cycles was proposed. As the result, it was shown that the predicted crack growth curve was in good agreement with the experimental one.     

Cyclic creep of type 304 stainless steel during unbalanced tension-compression loading at elevated temperature

Samples of Type 304 stainless steel were subjected to cyclic stresses with a positive mean stress at 300 and 560°C. Very rapid net elongation was observed whenever the stress limits were such as to produce a plastic strain amplitude of the same order of magnitude as the elastic strain at the peak stress. The maximum mean strain-rate, or cyclic creep rate, for a given peak tensile stress was achieved when the mean stress was just slightly above zero. Increasing the mean stress caused the mean strain rate to de-crease. The sensitive dependence of the mean strain-rate on the plastic strain ampli-tude and inverse dependence on the mean stress indicates that remobilization of disloca-tions by the reverse strain is an important mechanism for cyclic-creep acceleration. Although rapid cyclic creep was observed at both temperatures, a measurable mean strain rate was found for a much narrower range of stress conditions at 560 than at 300°C. The strain accumulated during cyclic creep did not produce any strain hardening, but did influence the shape of the stress-strain curve in a subsequent tensile test.     

Failure behavior of 304l stainless steel in torsion

The workability of 304L austenitic stainless steel has been investigated using torsion testing at temperatures from 20 °C to 1200 °C and strain rates of 0.01 and 10.0s ^-1 For the lower strain rate, temperature changes due to deformation heating were minimal, and failure was found to be fracture controlled at all temperatures. As for many other metals, the 304L exhibited a ductility minimum at warm-working temperatures. For the higher strain rate, failure was controlled by flow-localization processes at 20 °C and 200 °C. At these temperatures, flow softening resulting from deformation heating was deduced to be the principal cause of flow localization. A model to predict the strain at the onset of localization was developed and applied successfully to the 304L results. For high strain-rate torsion tests at 400 °C and above, failure was fracture controlled as in the low strain-rate tests, and the ductilities were shown to be correlated to those at the lower strain rate through the Zener-Hollomon parameter by employing an activation energy derived from flow-stress data.     

The creep of type 304 stainless steel at low stresses

The creep behaviour in vacuum of type 304 stainless steel of intercept grain size 12 ± 3 μm has been investigated for stresses up to 13 MN/m² (2 × 10⁻⁴ G), temperatures between 823 and 1023 K (0.49 to 0.61 T_m and test durations between 50 and 300 h. Creep measurements were made in situ on self-heated and self-loaded helical specimens as previously for αFe and βCo. Creep was again characterised by Bingham behaviour with dε/dσ within a factor of 3 of that sustainable by Coble diffusion creep except for the lower temperatures at which dε/dσ is significantly lower. Precipitation of M²³C₆ on grain boundaries prior to creep testing curtails primary creep consistently with earlier findings for related steels. An increment in σ₀ at higher temperatures associated with precipitation of M₂₃C₆ during testing however indicates that this is more effective than prior precipitation in inhibiting grain boundaries as vacancy sources and sinks. Extrapolation to lower stresses of published data for power law creep of this steel at higher stresses predicts transition stresses from diffusion to dislocation creep up to an order of magnitude higher than those observed in the present study.     

Inclusion and its deformation behavior in 304 stainless steel

Non-metallic inclusion is the main reason for the presence of surface defects in cold-rolled steel strip. In this study,the composition,morphology,and size of the non-metallic inclusion in hot-rolled 304 stainless steel strips are analyzed. Cold-rolled 304 stainless steel strips with different cold-rolling reduction have been prepared,and the morphology and size of inclusion in these cold-rolled strips are also analyzed. Furthermore,the deformation behavior of a non-metallic inclusion during the cold-rolling process is studied. The results showthat Ca O-SiO2-MgO-Al2O3,a kind of brittle compound oxide,is the main type of inclusion in hot-rolled 304 stainless steel strips.During the cold-rolling process,ductile deformation of this type of inclusion is not obvious,where large inclusions are crushed,and the average size of inclusions in cold-rolled strips decreased while the cold-rolling reduction increased.     

Creep-rupture behavior of iridium at elevated temperatures

The creep-rupture properties of annealed arc-cast iridium were determined between 982°C and 1288°C for times less than 1000 h. It was found possible to fit the rupture and creep results by the Sherby-Dorn parameter method using an activation energy of 70,600 calJmole. It was found that under creep deformation, failure was at least partially intercrystalline, and the failure elongation decreased with increasing temperature and time.     

Creep-rupture behavior of iridium at elevated temperatures

The creep-rupture properties of annealed arc-cast iridium were determined between 982°C and 1288°C for times less than 1000 h. It was found possible to fit the rupture and creep results by the Sherby-Dorn parameter method using an activation energy of 70,600 calJmole. It was found that under creep deformation, failure was at least partially intercrystalline, and the failure elongation decreased with increasing temperature and time.     

The martensite phases in 304 stainless steel

A detailed analysis of martensite transformations in 18/8 (304) stainless steel, utilizing transmission electron microscopy and diffraction in conjunction with X-ray and magnetization techniques, has established that the sequence of transformation is γ → ∈ → α. ε is a thermodynamically stable hcp phase whose formation is greatly enhanced as a result of plastic deformation. Comparison with the ε → α transformation in pure Fe-Mn alloys lends further support to the above sequence and suggests that a transformation line between ε and α in Fe-Cr-Ni alloys can be expected. In the 304 stainless steel used in this investigation, formation of α was induced only by plastic deformation and subsequent to formation of ε. Nucleation of α occurs heterogeneously at intersections of ε bands or where ε bands abut twin or grain boundaries (which represent unilaterally compressed regions). From electron diffraction, the Nishiyama relationship between γ and α phases appears to predominate at the start of the transformation, but then changes to that of Kurdjumov-Sachs. Based on these observations, a sequence of atom movements from the hcp structure to the bcc structure is proposed which has the basic geometric features of the martensitic transformation. Formerly with Department of Materials Science and Engineering, University of California, Berkeley, Calif.     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240±20 kJ/mol) and grain growth (285±35 kJ/mol) were in good agreement with prior results.     

The sintering of 304L stainless steel powder

An examination of the shrinkage kinetics for a 304L stainless steel powder showed that initial densification is controlled by a strain assisted volume diffusion mechanism. At temperatures above 1330 K, grain growth reduces the shrinkage rate; however, at lower sintering temperatures, the shrinkage rate is temporarily increased by the proximity of the moving grain boundaries to the interparticle necks. The activation energies of volume diffusion (240 ± 20 kJ/mol) and grain growth (285 ± 35 kJ/mol) were in good agreement with prior results.     

面内双轴循环载荷下304不锈钢的力学行为

为研究面内双轴载荷下304不锈钢材料的力学行为,运用有限元方法对十字形试样尺寸进行了设计与优化,在自主设计的面内双轴疲劳试验系统上对304不锈钢进行了单轴拉伸、双轴比例加载和非比例圆路径下的力学试验。结果表明,比例加载条件下304不锈钢的棘轮应变累积最小,圆路径下材料的棘轮应变最大,而单轴加载的棘轮应变累积介于比例载荷与圆路径之间。进入棘轮应变稳定增长阶段,圆路径对应的棘轮应变率高于单轴与比例加载,说明圆路径使304不锈钢的损伤失效进程加速。     

Proof strength values for austenitic stainless steels at elevated temperatures

The influence of the alloying elements C, N, Si, Mn, Cr, Ni, Mo, Cu, Ti on proof strength is studied in the temperature range 20–550 °C. High temperature data for the proof strength of austenitic stainless steels have been analysed. Using computational thermodynamics the amount of alloying elements in solid solution and the volume fractions of precipitates were assessed. These quantities were then applied in a regression analysis for the high temperature strength. Quantitative relationships for the proof strength as a result of the regression analysis are proposed as a function of temperature. They are to be used in materials design. The interstitial elements showed the largest effect. Si, Ni and Mo increased the strength at all temperatures. Cu and Mn reduced the strength and Cr gave an influence, which varied with temperature.     

Elevated-temperature low-cycle fatigue behavior of different heats of type 304 stainless steel

The low-cycle fatigue results of three heats of Type 304 stainless steel have been ob-tained at 593°C under selected cyclic-loading conditions. The results are compared with those generated for a reference heat of steel for which extensive low-cycle fatigue data are available. Observation of the microstructures of specimens in the pretest condition after a given heat treatment and examination of fatigue fracture surfaces were con-ducted by means of optical and scanning electron microscopy and X-ray analysis. The three heats of stainless steel, which exhibit different microstructural features, show approximately the same continuous-cycling low-cycle fatigue behavior as that of the re-ference heat. However, the three materials show improved fatigue strength during tensile hold-time conditions where significant creep occurs. The fatigue properties determined in the present study for the different heats of steel are consistent with the observed mi-crostructural features. Finally, the creep-fatigue properties of the heats as well as the microstructural observations are discussed in terms of a damage-rate approach re-cently developed by the authors.     

Dissolution behavior of 304 stainless steel in HNO3/HF mixtures

The dissolution behavior of type 304 stainless steel was studied in typical pickling environments in an effort to reduce the unnecessary loss of the critical metals nickel and chromium during the pickling process. Dissolution rates were determined for a 90-minute exposure to HNO₃/HF solutions ranging from 0.8 M to 3.5 M HNO₃ and 0.5 M to 2.6 M HF at 30°, 50°, 70°, and 90 °C and containing 0 to 0.21 M dissolved Fe, Cr, or Ni. Dissolution rates increased as a function of increasing HNO₃ concentration from 0.4 M to 1.5 M HNO₃, decreased at higher HNO₃ concentrations, and increased with increasing HF concentration. Experiments to determine the effect of temperature on the dissolution reaction gave a lower activation energy for solutions with higher HNO₃ concentrations. The dissolved Fe and Cr decreased the dissolution rate of 304 stainless steel, while dissolved Ni had essentially no effect. The Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) studies of films resulting from exposure to several HNO₃/HF solutions indicated that the fluoride did not penetrate the thin nonprotective films but remained on the outer surface of the film. The scanning Auger microscopy (SAM) studies indicated that the fluoride was uniformly distributed over the surface. Both the dissolution and surface studies are consistent with a dissolution process which is controlled by reactions occurring either in solution or at the film-solution interface.