Grain-size and heat-treatment effects in hydrogen-assisted cracking of austenitic stainless steels

Hydrogen embrittlement of 304L and 316L types austenitic stainless steels has been studied by charging thin tensile specimens with hydrogen through cathodic polarization. Throughout this study we have compared solution-annealed samples, having various prior austenite grain size, with samples given the additional sensitization treatment. The results of the tensile tests while undergoing cathodic charging show that the additional sensitization treatment and coarse-grained samples together, lower the mechanical properties in both 304L and 316L types, and the sensitized steel is more susceptible to hydrogen-assisted cracking. However, the room-temperature yield and ultimate strengths, and the elongation of type 316L, were much less affected depending on the heat treatment and prior austenitic grain size. The fracture surfaces of the specimens tested while cathodically charged show considerable differences between the annealed and the sensitized specimens. The sensitized coarse-grained specimens were predominantly intergranular in both 304L and 316L types, while the annealed 316L type specimens fracture shows massive regions of microvoid coalescence producing ductile rupture and the annealed 304L type specimens fracture were primarily transgranular and cleavage-like. Sensitization seems both to facilitate the penetration of hydrogen along the grain boundaries into the steel and to introduce susceptibility to fracture along grain boundaries while refined grain size improves resistance regardless of the failure mode.     

Some effects of heat treatment on grain boundary chemistry and precipitation in type 316 steel

Abstract

The effects of solution treatment temperature and cooling rate, carbon content, and aging at 600°C on the precipitation and grain boundary composition of AISI type 316 stainless steel were investigated. No matrix precipitation was observed even in a high carbon cast aged for 1000 h. Carbides of M₂₃C₆ type were the only precipitates observed and these formed on grain boundaries. Nucleation of a second phase in these solution treated specimens was difficult, despite the carbon supersaturation. The grain boundary regions were investigated using high spatial resolution X-ray analysis to show that chromium depleted and nickel enriched zones formed when grain boundary carbides occurred. The behaviour of molybdenum was more complex; it segregated strongly to grain boundaries either during a slow cool from the solution treatment temperature or during aging, but was also incorporated into any carbides that formed.

MST/890     

Suppression of chromium depletion and sensitization in austenitic stainless steel by surface mechanical attrition treatment

The effects of surface nanocrystallization via surface mechanical attrition treatment (SMAT) on degree of sensitization (DOS) of an austenitic stainless steel were investigated by means of double loop electrochemical potentiokinetic reactivation (DLEPR) test. The treated sample with grain size about 10 nm showed very low degree of sensitization value which can be considered as the non-sensitized material. This is mainly due to the formation of twin boundaries in the microstructure of the SMATed sample which weren't susceptible to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy as compared with those of other grain boundaries.     

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.     

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.     

Comparative evaluation of strain controlled low cycle fatigue behaviour of solution annealed and prior cold worked 316L(N) stainless steel

The effect of 20% prior cold work on low cycle fatigue (LCF) behaviour of type 316L(N) stainless steel (SS) was studied at 873 K by conducting total axial strain controlled tests in air with strain amplitudes in the range ±0.25% to ±1.0%. The cyclic deformation behaviour of 20% prior cold worked (PCW) material was compared with the LCF response of solution annealed (SA) alloy tested under similar conditions. The cyclic stress response (CSR) of 316L(N) SS in the PCW condition was characterized by a short period of hardening followed by prolonged softening prior to failure, whereas SA material exhibited a significant hardening regime followed by stress saturation. Interrupted tests on PCW material were carried out at different stages of CSR in order to determine the underlying mechanisms as reflected in substructural changes. The fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes of testing (≥±0.5%) while at lower strain amplitudes, the PCW material exhibited longer life.     

Intergranular attack during crevice corrosion of stainless steels

The crevice corrosion resistance of AISI type 316 stainless steel was investigated as a function of carbide precipitation and grain size in 0.5 M NaCl solution using a potentiodynamic technique and a specially designed crevice assembly. It has been found that if there is chromium depletion along the grain boundaries (for sensitized material) or if there is a large grain boundary area per unit volume (for fine grained material), the intergranular attack takes place in the crevice due to the presence of more active areas along the grain boundaries.     

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.     

Hot deformation behavior of a Nb-containing 316LN stainless steel

A Nb-containing 316LN stainless steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s^?1. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. The microstructural evolution was observed and the mechanism of flow instability was discussed. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate. On the contrary, the efficiency of power dissipation increased with them; Flow instability was manifested as cracking and flow localization; The hot deformation equation and the relationships between deformation condition and dynamic recrystallization grain size and fraction were obtained; For Nb-containing 316LN stainless steel, the favorite nucleation sites for dynamic recrystallization are in sequence of triple point, grain boundary, twin boundary and intragranular deformation band; The suggested processing window is given.     

S32750双相不锈钢相界与晶界特征对其力学性能和耐蚀性能的影响EI北大核心CSCD

通过固溶处理获得不同初始组织状态的S32750双相不锈钢样品,然后进行厚度压下量80%的冷轧变形和1050℃的退火处理,采用SEM-EBSD和XRD技术研究合金相界与晶界特征以及相组成分布情况,并利用拉伸实验、纳米压痕和双环电化学动电位再活化法(DL-EPR)分析不同初始状态样品的组织对力学性能与耐晶间腐蚀性能的影响规律。结果表明:高温固溶处理的合金样品经冷轧退火后晶粒细小均匀,两相分布接近1∶1,且相界占内界面(晶界+相界)比例较高,同相晶粒团簇程度最低,表现出优异的综合力学性能。合金样品经敏化处理后,σ相易沿α相晶界析出,高温固溶并经轧制退火后的样品中,由于α晶界比例较少且满足K-S取向关系的相界比例较高则又表现出良好的晶间腐蚀抗力。因此,通过适当的工艺来调控合金的相界与晶界分布可以实现材料强度和晶间腐蚀抗力的同步改善。     

Hot flow behavior of boron microalloyed steels

This research work studies the effect of boron contents on the hot flow behavior of boron microalloyed steels. For this purpose, uniaxial hot-compression tests were carried out in a low carbon steel microalloyed with four different amounts of boron over a wide range of temperatures (950, 1000, 1050 and 1100 °C) and constant true strain rates (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Experimental results revealed that both peak stress and peak strain tend to decrease as boron content increases, which indicates that boron additions have a solid solution softening effect. Likewise, the flow curves show a delaying effect on the kinetics of dynamic recrystallization (DRX) when increasing boron content. Deformed microstructures show a finer austenitic grain size in the steel with higher boron content (grain refinement effect). Results are discussed in terms of boron segregation towards austenitic grain boundaries during plastic deformation, which increases the movement of dislocations, enhances the grain boundary cohesion and modificates the grain boundary structure.     

Effects of aging temperature and grain size on the formation of serrated grain boundaries in an AISI 316 stainless steel

The effects of aging temperature and grain size on the formation of serrated grain boundaries have been investigated in an AISI 316 stainless steel. Grain size increased slightly over aging temperature ranges of 650–870 °C, resulting in predominantly serrated grain boundaries. However, when the temperature exceeded 880 °C, the grain size significantly increased, and grain boundary serration was not observed. The initial grain size also had an influence on the occurrence of grain boundary serration. For specimen having a large initial grain size of about 200 μm, no serrated grain boundary formed after aging treatment at 760 °C. Serrated grain boundaries were observed when “normal” initial grain sizes 55 μm were employed. It was found that the frequency of low angle boundaries markedly increased as the initial grain size increased from 55 to 200 μm. From the results obtained, it is possible to describe that the grain boundary serration could be considered as a spontaneous reaction that aims to reduce the total free energy of the system, and form a new interface of lower free energy. We proposed that the competition between grain growth and grain boundary serration during aging treatment reduces the total free energy of the alloy system: at temperatures exceeding 880 °C, the dominant process is the grain growth, while grain boundary serration predominates over the range of 650–870 °C.     

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.     

Hall–Petch behaviour of 316L austenitic stainless steel at room temperature

Abstract

The room temperature plastic deformation behaviour of two different batches (with differences in chemical composition) of 316L austenitic stainless steel has been studied. By thermomechanical treatments, a wide range of grain sizes varying from 2·7 to 64·0 νm was obtained in this study. The different microstructural parameters, such as grain size, distribution of grain size and shape, dihedral angle distribution, and grain aspect ratio were measured for annealed and deformed specimens of the two batches. The Hall–Petch behaviour of batch 1 showed two distinctly different linear regions, one in the fine grain size range (d≤6νm) and the other in the coarse grain size range (d6νm). The Hall–Petch parameter K _H (?) was significantly higher in the fine grain regime than coarse grain regime at all strains. Hardness measurements were also performed across the grain at different strain levels. The applicability of the Hall–Petch relationship was assessed in batch 1 and batch 2. It was observed that the Hall–Petch relationship was applicable in the coarse grain regime and Kocks composite relationship in the fine grain regime of batch 1. In batch 2 of 316L austenitic stainless steel, a single linear Hall–Petch relationship could describe the deformation behaviour over the entire range of grain size (from 2.9 to 46 νm) studied. The variation of the Hall–Petch and Kocks composite parameters with strain was discussed in terms of changes in the microstructural parameters.     

Effect of grain boundary orientation on the sensitization of austenitic stainless steel

The effect of grain misorientation on the sensitization of grain boundaries in austenitic stainless steel was investigated by sensitizing samples consisting of a large number of 50–80 μm size grains that were sintered to flat, 10 mm2 single crystals. Seven different sensitization treatments were employed and samples were intergranulary corroded in the modified Strauss test. X-ray pole figures were obtained for each sample and were used to identify the grain misorientations that were resistant to sensitization. In general, macroscopic grain boundary geometry could not explain the sensitization behaviour of most grain boundaries. Nevertheless, the Σ = 9 boundary was found to be especially resistant to sensitization. Results suggest that grain misorientation primarily affects the growth of sensitization rather than its nucleation. Finally, the crystallographic plane of the grain boundary appears to have an effect on sensitization. This revised version was published online in November 2006 with corrections to the Cover Date.     

Measurement of Poisson's ratio through the elastic-plastic transition

Poisson's ratio has been measured in solution annealed stainless steel type 316, under elastic and plastic deformation, using Moire interferometry. The aim of the study was to evaluate the validity of expressions for calculating the shift in the value of Poisson's ratio, from its theoretical elastic value to its plastic value, as a function of strain.     

The effect of heat treatment on the microstructure stability of modified 316LN stainless steel

As a conduit-sheath material for Cu–Nb–Sn wires, chemically modified 316LN steel is subjected to the same reaction heat treatment (100 h at 700 °C) used in transforming the wires into superconducting composite wires. In spite of the long annealing time at 700 °C, there was little or no change in the strength of the steel. A systematic study of the material annealed for 1, 10, 20, 50 and 100 h using orientation imaging microscopy (OIM) showed that with the exception of grain boundary precipitation at t = 100 h, the grain size and grain boundary character were stable. Our results show that twin boundaries (Σ3, Σ9, and Σ27) accounted for about 50% of the total boundaries in all the material conditions studied, suggestive that the density of twins had reached a limit. The stability of the material in spite of the prolonged heat treatment was attributed to the attainment of this maximum twin density in the as-received condition. In view of the high percentage of the twin boundaries in the microstructure, a comprehensive Hall–Petch relationship, which incorporates the contribution of the chemistry, grain size as well as twin boundaries to strengthening was developed. This upper bound theoretical strength compared favorably with the experimental value at 4 K.     

A study on the mechanism of serrated grain boundary formation in an austenitic stainless steel

Measurement of the activation energy for the formation of serrated grain boundaries (GB) has been carried out to understand its underlying formation mechanism in an AISI 316 stainless steel. The apparent incubation time necessary to initiate grain boundary serration was obtained at different aging temperatures, and the apparent activation energy for serration was carefully calculated from the Arrhenius relationship between incubation time and aging temperature. The activation energy for GB serrations in this alloy was measured to be approximately 148 ± 20 kJ mole⁻¹, which is consistent with the activation energy for lattice diffusion of carbon in γ-iron (142 kJ mole⁻¹). This result indicates that GB serration could be controlled essentially by the lattice diffusion of carbon to grain boundaries. Based on the through-thickness observation of serrated GBs, a straight boundary began to serrate from the surface at an early stage of the aging treatment, and then the serrated parts propagated throughout the entire grain boundary.     

EBSD analysis of a Ti-IF steel subjected to hot torsion in the ferritic region

The effect of grain size on the warm deformation behaviour of a titanium stabilized interstitial free steel was investigated using hot torsion. Tests were performed at temperatures between 765 °C and 850 °C at strain rates between 0.003 s⁻¹ and 1 s⁻¹ for samples with grain sizes of 25 μm, 75 μm and 150 μm. The structures were observed using EBSD analysis and are consistent with those expected for materials dominated by dynamic recovery. Some evidence was found for small amounts of thermally induced migration of pre-existing boundary (bulging) and for the generation of new segments of high angle boundaries by continuous dynamic recrystallization. The early onset of a steady-state flow stress in the finer grained samples is attributed to one or a combination of thermally induced boundary migration and enhanced rates of recovery near subgrain (and grain) boundaries.