Fracture Of Austenitic Stainless Steel Research Articles

Effect of Mo and Cr on S-Induced Intergranular Fracture in γ-Fe

S is a common corrosion medium for austenitic stainless steels. The severe intergranular fracture of austenitic stainless steels occurs in sulfur environments. In this paper, the permeation of S at different atomic positions for three symmetric tilt grain boundary types, i.e., Σ5(210), Σ5(310), and Σ9(114) have been computed using first-principles calculations. S has the strongest segregation tendency in the Σ5(210) grain boundary. A high content of S at the grain boundary indicates harm to the grain boundary. Sulfur segregation in the grain boundaries can weaken the strength of the metallic bond. When Mo and Cr are present at the Σ5(210) grain boundary, the sulfur-induced embrittlement is inhibited. With increased S concentration at the grain boundary, the coexistence of Mo and Cr can suppress the intergranular fracture of S on the grain boundary. The reason why high-Mo stainless steel has excellent sulfur-induced intergranular corrosion resistance is explained at the atomic level.

Flow and fracture of austenitic stainless steels at cryogenic temperatures

In this paper, we have characterized the microstructural evolution and the plastic flow and fracture behaviours of AISI 304L and AISI 316LN stainless steel grades at liquid nitrogen temperature (77K) and at liquid helium temperature (4K). Uninterrupted tensile experiments, where the sample is continuously deformed under quasi-static loading conditions until fracture, have been carried out with a Single-Section Sample to obtain the stress–strain characteristics of the two grades. Interrupted tensile experiments, in which the sample is unloaded before fracture, have been performed with a novel Double-Section Sample to later characterize the strain-induced martensitic transformation at different levels of deformation. The content of martensite has been determined post-mortem, using magnetic induction, electron backscatter diffraction and quantitative light optical micrography. The results obtained with the three methods show quantitative agreement, and reveal that the martensitic transformation in AISI 304L occurs faster and to a greater extent than in AISI 316LN both at 77K and at 4K. To the authors’ knowledge, in this paper we provide the first experimental results for the evolution of the content of strain-induced martensite in AISI 304L and AISI 316LN samples tested at liquid helium temperature. In addition, the experimental data for the evolution of the martensite volume fraction with the strain have been used to identify the temperature-dependent parameters of the martensitic transformation kinetic model proposed by Olson and Cohen (1975). Moreover, Mode I fracture tests with fatigue-precracked Compact Samples have been carried out to determine the fracture properties of the two investigated materials using the “resistance curve procedure” (ASTM-E1820-20a). The crack-growth resistance curves have been obtained with four different methods here referred to as ASTM Compliance Method, W–N Compliance Method, Modified W–N Compliance Method and ASTM Normalization Method, which is an original methodological contribution of this paper. While the four approaches yield similar results for the fracture toughness, only the W–N Compliance Method and the Modified W–N Compliance Method, the latter being proposed in this paper, fulfil all the requirements of the standard ASTM-E1820-20a so that the calculated fracture toughness can be accepted as a material property. The comparison of results for both materials and testing temperatures shows that AISI 316LN displays higher fracture toughness than AISI 304L. Moreover, post-mortem microstructural analysis of the Compact Samples near the fracture surface has revealed that the content of martensite is greater in AISI 304L than in AISI 316LN. Furthermore, for AISI 304L more martensite is formed in the sample tested at 77K because the plastic deformation near the crack is more than at 4K.

Effect of microstructural and environmental variables on ductility of austenitic stainless steels

Austenitic stainless steels are used extensively in harsh environments, including for high-pressure gaseous hydrogen service. However, the tensile ductility of this class of materials is very sensitive to materials and environmental variables. While tensile ductility is generally insufficient to qualify a material for hydrogen service, ductility is an effective tool to explore microstructural and environmental variables and their effects on hydrogen susceptibility, to inform understanding of the mechanisms of hydrogen effects in metals, and to provide insight to microstructural variables that may improve relative performance. In this study, hydrogen precharging was used to simulate high-pressure hydrogen environments to evaluate hydrogen effects on tensile properties. Several austenitic stainless steels were considered, including both metastable and stable alloys. Room temperature and subambient temperature tensile properties were evaluated with three different internal hydrogen contents for type 304L and 316L austenitic stainless steels and one hydrogen content for XM-11. Significant ductility loss was observed for both metastable and stable alloys, suggesting the stability of the austenitic phase is not sufficient to characterize the effects of hydrogen. Internal hydrogen does influence the character of deformation, which drives local damage accumulation and ultimately fracture for both metastable and stable alloys. While a quantitative description of hydrogen-assisted fracture in austenitic stainless steels remains elusive, these observations underscore the importance of the hydrogen-defect interactions and the accumulation of damage at deformation length scales.

The Effect of Phase Transformation on the Tensile Fracture of Austenitic Stainless Steel

The tensile fracture of two austenitic stainless steels with different degrees of stability for low temperature strain induced martensitic transformation was investigated. A stable AISI type 310 stainless steel displayed typical tensile stress-strain curves with decreasing work hardening rate at temperatures in the interval of 25 to-196°C, in which no martensitic transformation occurred. By contrast, a metastable type 302 stainless steel with martensitic transformation from 25 to-196°C showed a range of plastic deformation with increasing work hardening rate. The fracture of the stable 310 steel presented the characteristic cup and cone ductile aspect with micro dimples and sparse evidence of intergranular cracks. On the contrary, the martensitic transformation in the metastable 302 steel causes a neckless fracture with generally brittle appearance, despite the relatively high strain attained at fracture. At-80 and-196°C, associated with higher amount of transformed martensite, the fracture surface was covered with micro-craters formed around inclusions enveloped by thin strain induced martensite lamellae.

Micro-mechanical investigation for effects of helium on grain boundary fracture of austenitic stainless steel

Effects of helium (He) on grain boundary (GB) fracture of austenitic stainless steel were investigated by micro-tensile tests. Micro-bicrystal tensile specimens were fabricated for non-coincidence site lattice boundaries of He ion-irradiated 316 stainless steel by focused ion beam (FIB) micro-processing. Micro-tensile tests were conducted in a vacuum at room temperature in the FIB system. Specimens containing more than 2at.% He fractured at GBs. The criteria for brittle fracture occurrence on GBs were: (1) He concentrations higher than 2at.%; (2) formation of He bubbles on the GBs with less than a 5nm spacing; and (3) matrix hardening to more than 4.6GPa (nano-indentation hardness). The fracture stress of GB brittle fracture was lower for a specimen with higher He concentration while the size and areal density of the GB He bubbles were the same. The specimens that contained 10at.% He and had been annealed at 923K after irradiation fractured at the GB nominally in a brittle manner; however the inter-bubble matrix at the GB experienced ductile fracture. The annealing caused He bubbles to grow but decreased the areal density so that the spacing of the GB He bubbles widened and the hardness decreased, therefore the fracture mode changed from brittle to ductile. The findings revealed that He promotes GB fracture by weakening the GB strength and hardening the matrix due to the formation of He bubbles both on GBs and in the matrix. In addition, the findings suggested that GB segregated He atoms may have a role in GB fracture.

オーステナイトステンレス鋼の高温高サイクルにおける2段S-N曲線と内部破壊

オーステナイトステンレス鋼の高温高サイクルにおける2段S-N曲線と内部破壊

Increasing Behavior of Strength for Fatigue Fracture of Austenitic Stainless Steel during Fatigue Tests at Elevated Temperature

In order to clarify notch effect on fatigue strength of an austenitic stainless steel, rotating bending fatigue tests were carried out for smooth and notched specimens at 573K. Clear endurance limits were recognized and the fatigue strength did not decrease continuously with increasing stress concentration factor Kt, notwithstanding any non-propagating crack was not recognized at notch roots for specimens endured at 108 stress cycles. According to hardness tests, an area of notch root hardened during fatigue test by dynamic strain aging. The hardening behavior occurred remarkably with increasing Kt. Effects of Kt and the hardening behavior on fatigue strength canceled each other, and then fatigue strength became insensitive to Kt. Fatigue strength σSIT obtained by stress increment fatigue tests became higher than the ordinary fatigue strength for each specimen of Kt, that is, the coaxing effect was recognized clearly. This result indicated that the strength for fatigue fracture increased gradually during fatigue test. σSIT had a peak value at Kt of 2.0 and it decreased with increasing Kt sensitively. The increasing behavior of strength for fatigue fracture depended on Kt and fatigue fracture was controlled by circumstances of hardened material and stress amplitude considering Kt.

Effect of aging on high temperature brittle intergranular fracture in austenitic stainless steels

Abstract The present paper describes the effect of aging on crack growth at 550–750°C in a series of 316 and 347 based stainless steels. Crack initiation parameters and crack growth rates have been measured, and detailed fractography and microstructural characterisation carried out. The study shows that the high temperature brittle intergranular fracture mechanism operates in these alloys, as expected from incidences of cracking in austenitic stainless steels used in power plant. High temperature brittle intergranular fracture leads to lower crack tip opening displacements at initiation, and slightly higher crack growth rates than ductile intergranular failure. Susceptibility to high temperature brittle intergranular fracture is enhanced by aging. This increased susceptibility is explained in terms of the increased hardness, the reduction in dissolved C, and grain boundary precipitation. The effects of temperature, composition, and loading mode on the behaviour of the aged alloys are determined.MST/3100

Effect of aging on high temperature brittle intergranular fracture in austenitic stainless steels

The present paper describes the effect of aging on crack growth at 550–750°C in a series of 316 and 347 based stainless steels. Crack initiation parameters and crack growth rates have been measured, and detailed fractography and microstructural characterisation carried out. The study shows that the high temperature brittle intergranular fracture mechanism operates in these alloys, as expected from incidences of cracking in austenitic stainless steels used in power plant. High temperature brittle intergranular fracture leads to lower crack tip opening displacements at initiation, and slightly higher crack growth rates than ductile intergranular failure. Susceptibility to high temperature brittle intergranular fracture is enhanced by aging. This increased susceptibility is explained in terms of the increased hardness, the reduction in dissolved C, and grain boundary precipitation. The effects of temperature, composition, and loading mode on the behaviour of the aged alloys are determined.MST/3100

The effect of pre-existing epsilonmartensite on the hydrogen induced fracture of austenitic stainless steel

The effect of pre-existing epsilonmartensite on the hydrogen induced fracture of austenitic stainless steel

Effect of weld composition and microstructure on hydrogen assisted fracture of austenitic stainless steels

The effect of severe hydrogen environments on the tensile fracture behavior of a variety of austenitic stainless steel welds was investigated. In all cases, second phases or particulates common only to the weld microstructure were the origin of fracture initiation in hydrogen. These second phases formed as a result of microsegregation during solidification and/or solid state transformations during cooling or aging. In addition to second phases the weld microstructure matrix phase also influences fracture behavior. The fracture behavior is discussed in terms of localized chemical variations and the presence of second phases, and th interaction of dislocations with internal boundaries.

Acoustic emission from hydrogen saturated type 304L stainless steel

Effects of hydrogen on tensile deformation and fracture of austenitic stainless steels vary widely, depending upon the steel composition and treatment, hydrogen exposure, and loading conditions. In alloys such as Type 310 and Type 316 stainless steel, the ductility loss is small; and the fracture mode is unchanged. In contrast, Type 304L, 21-6-9, and Tenelon display large ductility losses with a change from ductile rupture to transgranular cleavage at 200 to 270 K. Intergranular fractures are observed in notched tensile tests of sensitized stainless steel at room temperature. Transgranular cleavage is observed after hydrogen exposure. The cleavage varies with test temperature, hydrogen concentration, and grain size. Microcracks form preferentially aong coherent twin boundaries throughout the deformed metal but are observable only after a 12 to 14% plastic strain. Microcracks along grain boundaries occur less frequently. The microcracks and consequent cleavage failure appear closely related to the stability of the austenite with respect to the epsilon-phase as measured by stacking-fault energy and may also be related to the strain induced martensite. The mechanism of microcrack nucleation and growth is not certain.

海水における二, 三のオーステナイトステンレス鋼の応力腐食割れ -その試験法について-

Some instruments exposed to hot sea water have showed fracture due to SCC (stress corrosion cracking). On the other hand, fracture of austenitic stainless steels due to SCC has not been observed in a marine environment. The SCC of austenitic stainless steels has been known to occur in some environment such as concentrated sulfuric acid containing sodium chloride at room temperature. This may suggest crack propagation due to SCC even in low temperature sea water. So far, Scheil solution and hot distilled water have been widely accepted as testing enviroments for SCC. But quantitative testing method of SCC in mild environment has not yet been reported. Therefore, the present study concerning a testing method of SCC in mild environment was carried out and the results were compared with those obtained by using Scheil solution.