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.     

Effect of surface roughness on low-cycle fatigue behavior of type 304 stainless steel

The effects of surface roughness on the low-cycle fatigue life of Type 304 stainless steel at 593°C in air have been investigated. It is observed that, at a strain rate of 4 × 10^?3 s^?1 and a total strain range of 1 pct, the fatigue life (N _f cycles) decreases with an increase in surface roughness. Information on crack growthvs strain cycles has been generated, as a function of surface roughness, by the measurement of striation spacing on fractured surfaces of specimens tested to failure. Crack propagation follows the Ina ∞N (wherea is the crack length afterN strain cycles) relation for longer specimen fatigue lives (N_f > 2700 cycles) and departs from Ina ∞N for shorter fatigue lives. A quantitative estimate is made of the number of cycles N_o(r) to generate a crack length equal to 0.1 mm (≈ 1 grain diam). The initial surface roughness significantly affects only the initiation component of specimen life time. The effect of roughness on crack initiation is described byN ₀ (R) = 1012R^?0.21, whereR is the surface roughness (root-mean-square value) in microns.     

Effect of surface roughness on low-cycle fatigue behavior of type 304 stainless steel

The effects of surface roughness on the low-cycle fatigue life of Type 304 stainless steel at 593°C in air have been investigated. It is observed that, at a strain rate of 4 × 10⁻³ s⁻¹ and a total strain range of 1 pct, the fatigue life (N _f cycles) decreases with an increase in surface roughness. Information on crack growthvs strain cycles has been generated, as a function of surface roughness, by the measurement of striation spacing on fractured surfaces of specimens tested to failure. Crack propagation follows the Ina ∞N (wherea is the crack length afterN strain cycles) relation for longer specimen fatigue lives (N_f > 2700 cycles) and departs from Ina ∞N for shorter fatigue lives. A quantitative estimate is made of the number of cycles N_o(r) to generate a crack length equal to 0.1 mm (≈ 1 grain diam). The initial surface roughness significantly affects only the initiation component of specimen life time. The effect of roughness on crack initiation is described byN ₀ (R) = 1012R^−0.21, whereR is the surface roughness (root-mean-square value) in microns.     

The effects of cold working on sensitization and intergranular corrosion behavior of AISI 304 stainless steel

The effects of prior cold rolling of up to an 80 pct reduction in thickness on the sensitization-desensitization behavior of Type AISI 304 stainless steel and its susceptibility to intergranular corrosion have been studied by electrochemical potentiokinetic reactivation (EPR) and Strauss-test methods. The results indicate that the prior deformation accelerated the sensitization as compared to the undeformed stainless steel. The deformed Type 304 stainless steel experienced desensitization at higher temperatures and times, and it was found to be enhanced by increased cold deformation. This could be attributed to the increased long-range chromium diffusion, possibly brought on by increasing pipe diffusion and vacancies. The role of the deformation-induced martensite (DIM) and texture, introduced by uniaxial cold rolling, on the sensitization-desensitization kinetics has also been discussed. This study could not reveal any systematic relationship between texture and the degree of sensitization (DOS) obtained. The effect of DIM on DOS seems to be pronounced at 500 °C when the steel retained significant amounts of DIM; however, the retained DIM is insignificant at higher sensitization times and temperatures.     

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.     

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.     

Prediction of fatigue crack formation in 304 stainless steel

Strain-controlled fatigue tests have been conducted on center-holed 304 stainless steel specimens. The fraction of total fatigue life spent until formation of an “engineering” crack ranged from about 15 to 85 pct, indicating the potential importance of being able to predict the fatigue crack formation life. A “just formed engineering crack,” as defined here, is a through crack long in the thickness direction, which has just emerged from the center hole. An energy based parameter, ΔσrΔε,, has been shown to correlate with the appearance of fatigue cracks in the center-holed 304 stainless steel specimens. This parameter is suggested to be more useful in predicting fatigue crack formation life than Δσ or Δε, alone. A good correlation was found over the limited range of data for two types of 304 stainless steel, a powder metallurgy (PM) stainless steel with higher than normal strength prop-erties and an ingot metallurgy (IM) stainless steel with normal strength properties. A better correlation was found for strain-controlled fatigue tests which did not go into compressive strain than for com-pletely reversed fatigue. Formerly a graduate student with the Materials Science and Engineering Department, Northwestern University, is     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

Fatigue-crack propagation behavior of type 304 stainless steel in a liquid sodium environment

The fatigue crack growth of Type 304 stainless steel in a liquid sodium environment at 800 and 1000°F was characterized using a linear-elastic fracture mechanics approach. Tests were conducted in low (< 2.0 ppm) and high (20-40 ppm) oxygen sodium and the results were compared to those of tests conductedin vacua and in air. Similar crack propagation characteristics in air at room temperature and at elevated temperatures in sodium and vacuum is evidence that the thermally activated component of crack propagation observed in air is an environmental effect. Related work on environmental effects on fatigue or crack propagation properties are reviewed and discussed.     

Hydrogen assisted cracking of type 304 stainless steel

This paper reports a study of hydrogen assisted cracking in type 304 stainless steel. It shows that the most detrimental effect in increasing the susceptibility of the material to hydrogen cracking is the formation of martensite upon deformation. This is particularly damaging if the martensite is localized at the grain boundaries. With martensite present intergranular impurities such as phosphorus play a secondary role. As martensite becomes more difficult to form, the importance of impurities increases.     

Corrosion fatigue and stress corrosion cracking of type 304 stainless steel in boiling NaOH solution

Results are reported for corrosion fatigue of Type 304 stainless steel in boiling (140 C) 17.5M NaOH (46 wt pct) solution. Specimens, of the smooth round bar type, were cycled sinusoidally at 1.0 Hz in tension-tension about mean stresses of 248 MPa (36 ksi) and 124 MPa (18 ksi). Both solution annealed and sensitized specimens cracked readily in a transgranular mode. Sensitization did not increase the environmental effect. The caustic solution drastically shortened cyclic life and eliminated the endurance limit observed in air. Cyclic stress was a more important variable than mean stress as the lower mean stress did not significantly improve life. Anodic passivation did not increase cyclic life as it did for constant load SCC. Comparison of the SCC tests results with those of corrosion fatigue indicates that cyclic stresses, even when confined to the elastic region, accelerate failure more than sustained loads in the plastic region; this accelerative effect was most intense under anodic passivation. R. W. STAEHLE, formerly Professor of Metallurgical Engineering, and Director of The Fontana Corrosion Center, Ohio State University     

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.     

Interpretation of high-temperature creep of type 304 stainless steel

The elevated-temperature creep behavior of Type 304 stainless steel is examined in terms of the measured effective and internal stresses. Results show that the mean effective stress is related to the applied stress by a power law of the form σ^* = α(σ)^β, where the constants α and β are functions of temperature. The dependence of creep rate on applied stress follows a power law, and the stress exponent is dependent on temperature. The latter behavior arises from the variation in the mean effective stress with applied stress and temperature. The creep rates are also described as a function of effective stress. The dislocation velocity-stress exponent obtained from stresschange tests is higher than the effective stress exponent evaluated from creep data. The dependence of creep rate on temperature at various values of effective stress yields a total activation energy of approximately the same magnitude as self-diffusion.