Hydrogen-assisted cracking of sensitized 316L stainless steel

Thin tensile specimens of 316L-type austenitic stainless steel were tested either at room temperature after cathodic charging or whilst undergoing cathodic charging. Throughout this study we have compared solution-annealed samples with samples given the additional sensitization treatment. The results of the tensile tests show that the room temperature yield and ultimate strengths were not much affected by sensitization, whilst significant reduction in the ductility was observed depending on the heat treatment and the method of charging. The specimens tested while undergoing cathodic charging showed 21% reduction of elongation at-fracture for the annealed specimens and 49% reduction of elongation for the sensitized specimens. The fracture surfaces of the specimens tested while cathodically charged show considerable differences between the annealed and the sensitized specimens. The sensitized specimens were predominantly intergranular, while the fracture of annealed specimens showed massive regions of microvoid coalescence producing ductile rupture. The results were correlated with the presence or absence of the martensite phases determined by means of a ferrite detector and transmission electron microscopy studies.     

The behaviour of 316L stainless steel in hydrogen

Hydrogen embrittlement of 316L type austenitic stainless steel has been studied by charging thin tensile specimens (0.2 mm thick) with hydrogen through cathodic polarization. The effects of hydrogen on the phase transition and the relative role of the metallurgical variables is discussed. Room temperature cathodic charging of unstressed specimens produces intergranular and secondary transgranular cracks along crystallographic planes. Severe grain boundary spelling has been observed at longer times of charging indicating that high stresses were formed. The surface cracking that was observed during the ageing is consistent with the development of high tensile surface stresses. TEM studies of the fracture surfaces of both annealed and sensitized, fine and coarse grain size, have revealed high dislocation structure. Thin plates of hydrogen induced (h c p)-martensite was observed. These plates appear in a heavily faulted region. The evidence of faults within-plates indicates that the overlapping stacking fault mechanism for the austenite to transformation is in agreement with strain induced. The results of the tensile tests while undergoing cathodic charging show that the additional sensitization treatment and coarse-grained samples, lower the mechanical properties. The fracture surfaces of the sensitized steel contains regions of intergranular fracture where the micromechanism of the failure is microvoid nucleation and coalescence along grain boundaries. Finally, the microstructures are connected to various modes of cracking.     

Effect of Hydrogen Charging on the Tensile and Constant Load Properties of an Austenitic Stainless Steel Weldment

The effect of cathodic hydrogen charging on the tensile and constant load properties was determined for an austenitic stainless steel weldment comprising a 304L steel in the solution treated condition as a base metal and a 308L filler steel as a weld metal. Part of the 304L solution treatedsteel was separately given additional sensitization treatment to simulate the microstructure that would develop in the heat affected zone. Tests were performed at room temperature on notched round bar specimens. Hydrogen charging resulted in a pronounced embrittlement of the tested     

Effects of Deformation (Strain) and Heat Treatment on Grain Boundary Sensitization and Precipitation in Austenitic Stainless Steels

Sensitization, particularly the degree of sensitization (DOS) in type 316 stainless steel pipe is critically dependent upon the solution anneal of the mill-annealed or commercial material, and is particularly sensitive to low-temperature aging when the starting material is solution annealed between about 1,000°C and 1,100°C. It is observed that when the DOS is above about 10 C/cm² (quantitative electrochemical potentiokinetic reactivation units in Coulombs/cm²), noticeable carbide precipitation occurs in the grain boundaries and increases with increasing DOS. Transmission electron microscopy (TEM) examination of precipitation occurring in type 316 stainless steel pipe grain boundaries has shown them to exhibit many microstructural features that seem to be coincident with grain boundary microstructures, particularly ledges. Uniaxial, tensile strain prior to aging of solution annealed pipe was observed to have a dramatic effect on DOS of type 316 stainless steel pipe; DOS increases with increasing strain. This effect appears to be correlated with the propensity of dislocations to be generated near grain boundaries, and for grain boundary microstructural changes to be related with strain, as observed in type 304 stainless steel. The effects of strain on DOS, combined with instabilities connected with annealing of type 316 stainless steel, indicate that in the practical utilization of pipe, it is imperative to reduce deformation prior to service and during service applications, to avoid sensitization and, thus, minimize the probability of grain boundary cracking.     

Influence of inclusions and heat treated microstructure on hydrogen assisted fracture properties of aisi 316 stainless steel

An attempt has been made to assess the influence of nonmetallic inclusions and heat treated microstructure on hydrogen assisted cracking of AISI 316 austenitic steel. The steel obtained in two levels of cleanliness was given solution annealing treatment in the temperature range of 1173–1473 K, and additional sensitization treatment of 973 K for 26 hours. Hydrogen embrittlement of this steel has been studied by charging Charpy and Compact Tension specimens of ASTM specification, with hydrogen through cathodic polarization. It is found that hydrogen embrittlement susceptibility increases with the presence of large size inclusions, larger grain and sensitized microstructure.     

Some factors exerting an influence on the coaxing effect of austenitic stainless steels

The present paper describes some factors exerting an influence on the coaxing effect of austenitic stainless steels. Particularly, the influence of prestrain was investigated in detail. The materials used were austenitic stainless steels, type 304 and 316. Type 304N2 was also used to examine the properties of the stabilized austenitic phase in type 304. Two types of rotating bending fatigue tests, i.e. the conventional constant amplitude tests and stress‐incremental tests, were performed using the specimens subjected to the several tensile‐prestrain levels. Under the constant amplitude tests, the fatigue strengths of type 304 and 316 increased with increasing prestrain. Under the stress‐incremental tests, type 304 showed a remarkable coaxing effect, where the fatigue failure stress significantly increased regardless of the prestrain level. The coaxing effect in the unprestrained specimens was larger than those of the prestrained ones. Type 304N2 showed lower coaxing effect than type 304. In addition, the strain‐induced martensitic transformation did not occur because of the higher stability of austenitic phase in type 304N2. In type 316, the coaxing effect was dependent on the prestrain level, i.e. below 15% prestrain the coaxing effect became smaller with increasing prestrain, whereas above 25% prestrain the coaxing effect reappeared. Based on the tests results, it was considered that the coaxing effect in austenitic stainless steel was due to the mechanisms such as work hardening, strain ageing and strain‐induced martensitic transformation. The contribution of these mechanisms to the coaxing effect was different among type 304, 304N2 and 316.     

Hydrogen permeation behaviour and associated phase transformations in annealed AISI304 stainless steels

The effective diffusion coefficient and subsurface concentration of hydrogen in annealed AISI304 austenitic stainless steels have been measured by the electrochemical permeation method. The effects of different cathodic current densities on the effective diffusion coefficient, hydrogen concentration beneath the cathodic surface and steady state permeation current density have been studied. The value of the effective diffusion coefficient for the permeated specimens increases with increasing charging current density. The hydrogen subsurface concentration and steady state permeation current density first increase with increasing charging current density, then decrease with increasing current density. X-ray diffraction analyses were used to investigate the phase transformation during hydrogen charging. The results revealed that cathodic charging resulted in the formation of a considerable amount of ε and α′ martensites, which will increase with charging time.     

Evaluation of EB-PVD deposited Ni–Cr–Al alloy foil

Ni–Cr–Al alloy foils were deposited at 900 °C from a target of Ni–Cr–Al alloy by EB-PVD, and then annealed at 760 °C in a high vacuum furnace for 16 h. Microstructure of as-deposited foil consisted of a large amount of fine grains with mean size of 90 nm. However, after annealing the grains grew obviously to an average size of 4 μm. In addition, long range chemical ordering of L1₂ type emerged in the annealed specimens, while no evidence of ordering structure were found in the deposited samples. The as-deposited foil exhibited a strong {111} texture on the evaporation side. Twin substructure was dominated within grains of as-deposited and annealed samples. The mechanical properties of the foils were examined using room tensile tests. The ultimate tensile strength of as-deposited and annealed samples is 780 and 767 MPa, respectively. Investigation of fracture surfaces from samples revealed that brittle fracture took place along boundaries of columnar crystals formed on the substrate side, while the equiaxed dimples were dominant on the evaporation side.     

Grain boundary engineering of titanium-stabilized 321 austenitic stainless steel

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized 304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized 321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular corrosion during GBE.     

Role of strain-induced martensite on microstructural evolution during annealing of metastable austenitic stainless steel

Metastable austenitic stainless steel of type AISI 304L was cold rolled to 90% with and without inter-pass cooling. Inter-pass cooling produced 89% of strain-induced martensite whereas no inter-pass cooling resulted in the formation of 43% of martensite in the austenite matrix. The cold-rolled specimens were annealed at various temperatures in the range of 750–1000 °C. The microstructures of the cold-rolled and annealed specimens were studied by the electron microscope. The grain size and low angle boundaries were determined from the orientation maps recorded by the scanning electron microscope-based electron backscattered diffraction technique. The observed microstructural changes were correlated with the reversion mechanism of martensite to austenite and volume fraction of martensite. It was noted that large volume fractions of martensite at low annealing temperatures, below 900 °C, were most suitable for the formation of fine grains. On the contrary, reversion of small volume fractions of martensite at critical annealing temperature of 950 °C resulted in grain refinement.     

Tensile behaviour of type 304 austenitic stainless steels in hydrogen atmosphere at low temperatures

Abstract

The tensile behaviour of solution annealed type 304L, solution annealed type 304, and solution annealed and sensitised type 304 stainless steels was investigated in hydrogen and helium under a pressure of 1·1 MPa over the temperature range 300–80 K at strain rates ranging from 4·2×10^-5 to 4·2×10^-2 s^-1. For 304L steel, hydrogen environment embrittlement (HEE) increased with decreasing strain rate. For 304L and 304 steels, HEE increased with decreasing temperature, reached a maximum, and then decreased with further decrease in temperature: the decrease was particularly rapid near the minimum temperature for HEE. Sensitisation enhanced the HEE of 304 steel. Above the maximum HEE temperature, the HEE behaviour was similar to the hydrogen embrittlement behaviour of materials in previous studies, but near the minimum temperature for HEE it was different. Three types of hydrogen induced brittle fracture were observed as a result of HEE: transgranular fracture along strain induced martensite laths and twin boundary fracture on the fracture surfaces of solution annealed 304L and 304 steels, and grain boundary fracture on the sensitised 304 steel. It was found that from room temperature to the maximum HEE temperature, the HEE of the materials depended on the transformation of strain induced martensite and below the maximum HEE temperature it depended on the diffusion of hydrogen.     

Influence of the starting condition on the kinetics of sensitization and loss of toughness in an AISI 304 steel

This work describes the investigation of the embrittlement of AISI 304 steel sensitized at 650°C by Charpy impact test, comparing two starting conditions: (1) mill annealed and machined (MA-M); and (2) solution treated at 1050°C by 1 h followed by oil quenching (ST). The degree of sensitization for both samples was assessed by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and by Double Loop Electrochemical Potentiodynamic Reactivation test. The results showed that MA-M samples undergo more severe and rapid embrittlement than ST ones and a higher kinetics of sensitization due to small strains concentrated in grain boundaries and martensite phase produced during the machining operations. The martensite phase is found to be quite stable at the sensitization treatment at 650°C. The increase of microvoids nucleation at the grain boundaries seems to be the mechanism of embrittlement in the sensitized 304 steel.     

Formation of ultrafine grained microstructure in the austenitic stainless steel and its impact on tensile properties

Commercial grade AISI 316L austenitic stainless steel was heavily cold rolled to 90% of thickness reduction. The cold rolled material was subjected to repetitive annealing treatment for short duration of 45-60 s at various temperatures. The microstructure of the cold rolled and after annealing was studied by optical as well as transmission electron microscope. The microstructural examination of the specimens after repetitive annealing process revealed the formation of ultrafine grain size microstructure. It was also noted that depending on the processing condition the grain size distribution varied widely. The tensile testing of the annealed specimen showed that the yield strength increased by 4-5 times that of the coarse grained material. However, a loss in the strain hardening ability was observed in these specimens. A good combination of yield strength and ductility for ultrafine grained stainless steel as compared to the coarse grained material could be obtained by the optimization of the microstructure.     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

沉淀强化奥氏体合金的氢致断裂行为

研究了未充氢和热充氢沉淀强化奥氏体合金的拉伸断裂行为,分析了其氢脆敏感性与拉伸断裂行为间的联系,研究了氢对合金局部塑性变形及微裂纹形核的影响。结果表明:氢使沉淀强化合金由单一的韧窝断裂转变为韧窝断裂、沿晶断裂和滑移带开裂的混合断裂方式。其原因是:一方面,氢促进位错平面化滑移趋势、加剧局部塑性变形;另一方面,滑移带被晶界、孪晶界以及不同取向的滑移带所阻碍,引起了位错塞积和氢聚集。     

A comparison of sensitization kinetics in 304 and 316 stainless steels

The effects of tensile and cold rolling strain (up to 40%) over a range of grain sizes ranging from 300 m to 10 m on sensitization (and desensitization) were observed and compared for 304 and 316 stainless steel having a constant carbon content of 0.05%; at 670°C. Rapid sensitization-desensitization was observed for both materials at the smallest grain size, and plots of degree of sensitization (DOS) data with time, temperature, and tensile strain coupled with chromium diffusivity data for 304 stainless steel allowed activation energies to be calculated from corresponding Arrhenius plots utilizing supplemental data from Beltran, et al. [1] at 625°C and 775°C. Values of 1.9 and 2 kcal/mol were found for unstrained and 20% strained samples for 11 m grain size while corresponding values at 175 m grain size were 55 and 32 kcal/mol respectively. Activation energies for unstrained and 10% strained 316 stainless steel for 135 m grain size were found to be 76 and 64 kcal/mol, respectively. Sensitization was more rapid for cold-rolling versus tensile straining in both stainless steels, and there was no detectable sensitization for the largest grain size regime of the 316 stainless steel up to 10 h aging time at 670°C.     

晶粒尺寸对304奥氏体不锈钢组织演变和性能的影响

研究了初始织构相近而晶粒尺寸不同的304奥氏体不锈钢在后续10%压缩变形和热处理过程中微观组织、力学和耐蚀性的变化。结果表明,具有相似织构而晶粒尺寸不同的样品变形热处理后其织构不同,粗晶在变形中织构的变化更大;织构相近时抗拉强度对晶粒尺寸的依赖较大;织构不同时,织构对硬度和抗拉强度的影响大于晶粒尺寸和微应变的影响;变形热处理后普通大角度晶界和晶内微应变的增大降低了试样的耐腐蚀性能;初始晶粒尺寸较小的试样在变形热处理后出现四种密排面平行于外表面的织构,其耐点蚀的性能更优。     

Effect of grain refinement on strain hardening and fracture in austenitic stainless steel

The present study aims to investigate the effect of grain refinement on strain hardening behaviour and fracture surface characteristics in 316LN austenitic stainless steel (ASS). The ASSs with varying grain sizes were obtained through 90% cold rolled reduction and subsequently phase reversion annealing treatment. The results showed that the grain refinement from coarse-grained (CG) structure to ultrafine-grained (UFG) structure increased the yield strength whilst maintaining a reasonable ductility. The strain hardening curves in all the samples were divided into three stages. The fractures in all the samples were ductile fracture with dimples. The subtle differences in the strain hardening behaviour and fracture surface characteristics among the samples with various grain sizes from CG structure to UFG structure were influenced by the deformation mechanisms of austenite.     

Effect of grain refinement on the hydrogen embrittlement of 304 austenitic stainless steel

The effect of grain size (in the range from 4 μm to 12 μm) on the hydrogen embrittlement (HE) of 304 austenitic stainless steel (ASS) was studied. HE susceptibility result shows that HE resistance increases with grain refinement. Electron backscattered diffraction kernel average misorientation (EBSD-KAM) mapping shows that the strain localization can be mitigated by grain refinement. Hence, strain localization sites which act as highways for hydrogen diffusion and preferred crack initiation sites can be reduced along with grain refinement, leading to a high HE resistance. Meanwhile, grain size shows no influence on the strain induced martensite (SIM) transformation during the hydrogen charging slow strain tensile test (SSRT). Hence, the SIM formed during hydrogen charging SSRT is not responsible for the different HE resistance of 304 ASSs with various grain sizes. Hydrogen diffusion is supposed to be controlled by a competition between short-circuit diffusion along random grain boundary (RGB) and hydrogen trapping at dislocations, leading to a maximum hydrogen diffusion coefficient in the 304 ASS with an average grain size of 8 μm.     

Effects of aging at 700°C on ductile fracture of AISI 316 stainless steel

Abstract

The micromechanisms of ductile fracture have been studied in a commercial AISI 316 austenitic stainless steel. Tensile, Charpy impact, and ductile fracture toughness testing have been performed on unaged material and samples aged at 700°C for times up to 4380 h. Examination of the specimens after testing has shown that the microstructural changes occurring at grain boundaries are responsible for the observed losses of ductility and crack growth resistance. The relative magnitude of the observed changes in mechanical properties has been explained using a simple model to describe the ductile fracture process.

MST/1001