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.     

Effect of hydrogen on fatigue crack growth of metals

The present paper shows several important phenomena obtained by investigations of the effect of hydrogen on fatigue crack growth behaviour, including the measurement of the hydrogen content in various materials such as low-carbon, Cr-Mo and stainless steels. Particularly important phenomena are the localization of fatigue slip bands, strain-induced martensite in Types 304, 316 and even 316L, and also strong frequency effects on fatigue crack growth rates. For example, with a decrease in frequency of fatigue loading down to the level of 0.2 Hz, the fatigue crack growth rate of a Cr-Mo steel is accelerated by 10-30 times. The same phenomenon also occurs even in austenitic stainless steels at the frequency of the level of 0.001 Hz. Striation morphology is also influenced by hydrogen. It has been revealed by re-analysing the results of the authors’ separately published reports that this basic hydrogen embrittlement mechanism is essentially the same throughout all the materials, i.e. low-carbon, Cr-Mo and stainless steels. Thus, the coupled effects of hydrogen content, hydrogen diffusion coefficient (for BCC or FCC), load frequency, localization of fatigue slip bands and strain-induced martensite must be always considered in fatigue test and analysis of hydrogen embrittlement.     

Hydrogen-induced change in microstructure and properties of steels: 18Cr10Mn–0.4N vis-à-vis 18Cr10Ni

ABSTRACT

Cr–Mn–N stainless steels have a cost and strength advantage over conventional Cr–Ni stainless steels. In this study microstructure and mechanical property of hydrogen-charged 18Cr10Mn-0.4N was compared with 18Cr10Ni austenitic stainless steel. This is the first such study for 18Cr10Mn–0.4N austenitic stainless steel. Electron microscopy was used to compare the deformed microstructure of the uncharged and hydrogen-charged specimens. The results are discussed in view of the current knowledge on hydrogen embrittlement. The 18Cr10Mn–0.4N steel suffered higher embrittlement mainly because it absorbed moref hydrogen.     

Crack initiation and propagation in torsional fatigue of circumferentially notched steel bars

Circumferentially notched bars of austenitic stainless steel, SUS316L, and carbon steel, SGV410, with three different notch-tip radii were fatigued under cyclic torsion without and with static tension. The torsional fatigue life of SUS316L was found to increase with increasing stress concentration under the same nominal shear stress amplitude. Electrical potential monitoring revealed that the crack initiation life decreased with increasing stress concentration, while the crack propagation life increased. This anomalous notch-strengthening effect was ascribed to the larger retardation of fatigue crack propagation by sliding contacts of fracture surfaces. The superposition of static tension on cyclic torsion causes notch weakening. The notch-strengthening effect in torsional fatigue was not found in carbon steels, SGV410. The difference in the crack path of small cracks near notch root between stainless steel and carbon steel gives rise to the difference in the notch effect in torsional fatigue. The factory-roof shape observed on fracture surfaces of SUS316L became finer with higher stress amplitude and for sharper notches. The superposition of static tension makes the factory-roof shape less evident. Under higher stresses, the fracture surface was smeared to be flat. The fracture surfaces of SGV410 became smoother with increasing stress amplitude and notch acuity. The three-dimensional feature of fracture surfaces clearly showed the difference of the topography of fracture surfaces. The topographic feature was closely related to the amount of retardation of crack propagation due to the sliding contact of fracture surfaces.     

Evaluation of Equibiaxial Compressive Stress Introduced into Austenitic Stainless Steel Using an Eddy Current Method

Equibiaxial compressive residual stress is introduced into steel after peening in order to improve both its resistance to stress corrosion cracking and its fatigue strength. Thus, a nondestructive and relatively quick method to evaluate the equibiaxial compressive residual stress in a surface layer modified by peening is required in order to evaluate the peening intensity needed to enhance the integrity of structural components. The purpose of the work reported here is to establish an eddy current method to evaluate equibiaxial compressive stress which can be applied to the residual stress introduced into various non-ferromagnetic materials after peening. To this end, hydraulic jacks were used to elastically deform specimens of the austenitic stainless steel, Japanese Industrial Standard (JIS) SUS316L, thereby introducing an equibiaxial compressive stress. In the case of SUS316L steel, stress-induced martensitic transformation is rare. The electromagnetic properties of these specimens were then measured. In addition, the eddy current signals from peened specimens were compared with these. The results demonstrate that it is possible to establish a method for evaluating the equibiaxial stress utilizing eddy current signals.     

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.     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

Abstract

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Super austenitic stainless steels — a promising replacement for the currently used type 316L stainless steel as the construction material for flue-gas desulphurization plant

Potentiodynamic anodic cyclic polarization experiments on type 316L stainless steel and 6Mo super austenitic stainless steels were carried out in simulated flue-gas desulphurization (FGD) environment in order to assess the localized corrosion resistance. The pitting corrosion resistance was higher in the case of the super austenitic stainless steel containing 6Mo and a higher amount of nitrogen. The pit-protection potential of these alloys was more noble than the corrosion potential, indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed that the super austenitic stainless steels have a little tendency for leaching of metal ions such as iron, chromium and nickel at different impressed potentials. This may be due to surface segregation of nitrogen as CrN, which would, in turn, enrich a chromium and molybdenum mixed oxide film and thus impedes the release of metal ions. The present study indicates that the 6Mo super austenitics can be adopted as a promising replacement for the currently used type 316L stainless steel as the construction material for FGD plants.     

A new high temperature life correlation model for austenitic and ferritic stainless steels

Low-cycle fatigue tests on 429EM ferritic stainless steel and 316L austenitic stainless steel were carried out in a wide range of temperatures from room temperature to 750 °C. The Tomkins fatigue life model was applied to correlate the fatigue life with crack propagation rate and this model matched well with the fatigue life of 429EM stainless steel but not for the 316L stainless steel. A new life prediction model was developed to consider the temperature effect on fatigue life. The predictions show good agreement with experimental results for both materials. The predicted lives were within a±2X scatter band at all test temperatures.     

Cyclic deformation behavior of austenitic Cr–Ni-steels in the VHCF regime: Part II – Microstructure-sensitive simulation

In Part I – Experimental study, the cyclic deformation behavior of two austenitic stainless steel grades (AISI 304, AISI 316 L) were experimentally investigated at low stress amplitudes in the very high cycle fatigue (VHCF) regime. The observations indicate that during VHCF the metastable austenitic stainless steel (304 grade) performs a pronounced localization of plastic deformation in shear bands followed by a deformation-induced martensitic phase transformation. The 316 grade undergoes only a very limited local plastic deformation in shear bands with almost no phase transformation. Consequently, both materials exhibit distinctly different cyclic softening and hardening characteristics during VHCF. In order to provide a more detailed knowledge about the individual deformation mechanisms and their effect on the cyclic softening and hardening behavior the experimental study is extended by microstructure-sensitive modeling and simulation. Two-dimensional (2-D) microstructures consisting of several grains are represented using the boundary element method and plastic deformation within the microstructure is considered by a mechanism-based approach. Specific mechanisms of cyclic plastic deformation in shear bands and deformation-induced martensitic phase transformation – as documented by experimental results and based on well-known model approaches – are defined and implemented into the simulation. The fatigue behavior at low stress amplitudes observed in experiments can be well represented in simulations so that the underlying model helps to understand the cyclic deformation behavior of austenitic stainless steels at low stress amplitudes in the regime of VHCF strength. In a comparative study based on the resonant behavior the effect of certain deformation mechanisms on the global cyclic softening and hardening characteristics is pointed out for both materials.     

基于磁性测定法研究奥氏体不锈钢波纹管的形变马氏体含量

根据形变诱发马氏体磁性的变化,针对SUS304和SUS316L奥氏体不锈钢分步机械胀压成型波纹管以及未经固溶处理与经固溶处理SUS304奥氏体不锈钢液压成型波纹管,采用MP30E—S型铁素体测定仪定量测定了波纹管母材区及焊缝区的形变马氏体含量。结果表明：形变马氏体含量的大小与波纹管材料、相对变形量以及热处理状态等均有很大关系;在相同变形量条件下SUS316L不锈钢的形变马氏体含量比SUS304不锈钢要小得多;相对变形量越大,形变马氏体含量也越大,且波峰处的形变马氏体含量较波谷处的要大得多;与未固溶处理波纹管相比,经固溶处理后波纹管的形变马氏体含量显著减小。     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

Classification of metallic alloys for fatigue damage accumulation: A conservative model under strain control for 304 stainless steels

Fully reversed uniaxial tests performed under total strain and stress control on 304 stainless steels specimens show that, under strain control the fatigue damage for High–Low (H–L) cycling is more significant than that using Miner’s rule, but under stress control opposite results are obtained. This has been attributed to opposite effects of pre-hardening under strain and stress control. Classical non linear damage accumulation models are not able to take into account this difference in sequence effect. Smith–Watson–Topper (SWT) and Fatemi–Socie (FS) criterion combined to linear damage accumulation can take into account this difference in sequence effect through the presence of maximum stress. However these models require an elastic–plastic constitutive law which is difficult to propose due to the presence of high cycle secondary hardening observed on 304 stainless steel. A conservative model for damage accumulation under variable amplitude strain control loading is thus proposed, which does not require a constitutive law. Linear damage accumulation is used, while sequence effect is taken into account using the elastic–plastic memory effect through cyclic strain–stress curves (CSSC) with pre-hardening. This modeling classifies metallic alloys in two groups for damage accumulation, with a stable (independent to pre-hardening) CSSC as for aluminum alloys and with an unstable (dependent to pre-hardening) one as for austenitic stainless steels. For the former case the modeling is identical to Miner’s rule. The modeling is approved based on a large number of tests on 304 stainless steel and is compared with SWT and FS models. In presence of mean stress the modeling permits in a qualitative way to explain the fact that tensile mean stresses in constant amplitude strain control tests are more detrimental than for constant amplitude stress control tests. Moreover it is shown that the SWT model is not always able to predict accurately the fatigue life in presence of a mean stress. Finally, it is concluded that for a 304 stainless steel, in order to take into account the mean stress in fatigue life, the mean stress effect has to be decomposed into two parts: maximum and “intrinsic” mean stress effects.     

Nickel-free austenitic stainless steels for medical applications

Abstract

The adverse effects of nickel ions being released into the human body have prompted the development of high-nitrogen nickel-free austenitic stainless steels for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel in medical stainless steels, the advantages of nitrogen in stainless steels, and emphatically, the development of high-nitrogen nickel-free stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength and good plasticity, better corrosion and wear resistances, and superior biocompatibility compared to the currently used 316L stainless steel, the newly developed high-nitrogen nickel-free stainless steel is a reliable substitute for the conventional medical stainless steels.     

Fatigue life predictions for irradiated stainless steels considering void swellings effects

The objective of this study is to estimate fatigue life of irradiated austenitic stainless steels types 304, 304L, and 316, which are extensively used as structural alloys in the internal elements of nuclear reactors. These reactor components are typically subjected to a long-term exposure to irradiation at elevated temperature along with repeated loadings during operation. Additionally, it is known that neutron irradiation can cause the formation and growth of microscopic defects or swellings in the materials, which may have a potential to deteriorate the mechanical properties of the materials. In this study, uniaxial fatigue models were used to predict fatigue properties based only on simple monotonic properties including ultimate tensile strength and Brinell hardness. Two existing models, the Bäumel–Seeger uniform material law and the Roessle–Fatemi hardness method, were employed and extended to include the effects of test temperature, neutron irradiation fluence, irradiation-induced helium and irradiation-induced swellings on fatigue life of austenitic stainless steels. The proposed models provided reasonable fatigue life predictions compared with the experimental data for all selected materials.     

微观组织对0Cr17Ni4Cu4Nb马氏体不锈钢的耐点腐蚀性能的影响

采用化学浸泡法和模拟闭塞电池方法研究了固溶+时效和固溶+调整+时效处理的0Cr17Ni4Cu4Nb马氏体不锈钢的耐点腐蚀性能,并与18-8型奥氏体不锈钢(316L)耐点蚀性能进行了对比。结果表明,0Cr17Ni4Cu4Nb马氏体不锈钢组织内富Cu析出相促进了点蚀坑萌生,而点蚀坑发展则与组织形貌有关。固溶+调整+时效处理的0Cr17Ni4Cu4Nb马氏体不锈钢因组织内析出富Cu相多而大,其萌生的点蚀坑密度较高,但由于马氏体板条较细,其点蚀坑尺寸和深度较小;固溶+时效处理的0Cr17Ni4Cu4Nb马氏体不锈钢因组织内析出富Cu相少而小,萌生的点蚀坑密度较低,但粗大的板条马氏体组织导致点蚀坑尺寸和深度较大。与18-8型奥氏体不锈钢耐点蚀性能对比表明,通过对0Cr17Ni4Cu4Nb马氏体不锈钢进行合理的热处理,其耐点蚀性能可与18-8型奥氏体不锈钢相当。     

Fatigue characteristics of a type 304 austenitic stainless steel in hydrogen gas environment

The effects of a hydrogen gas environment on the fatigue characteristics of a type 304 austenitic stainless steel were investigated and the following results were obtained. The hydrogen effect is not clearly seen by judging fatigue life diagram. However, crack initiation retards and crack propagation accelerates in hydrogen gas environment. The retardation seems to be caused by the absence of oxygen and water vapour. The acceleration seems to be caused by the intrinsic hydrogen effect.     

Fatigue crack initiation and crystallographic growth in 316L stainless steel

A mesoscale model of fatigue crack formation and stress–strain behavior in crystalline alloys entitled Sistaninia–Niffenegger Fatigue (SNF) model is applied to AISI 316L austenitic stainless steel. An inelastic hysteresis energy criterion in conjunction with continuum damage modeling provides a strong tool for studying the behavior of the austenitic steel under cyclic loading. The model predictions are validated against fatigue experimental data. The results show that this microstructural-based modeling approach is capable for predicting the behavior of the steel even under complex loading conditions. It can reproduce and help to understand well known fatigue experimental facts, e.g. the effect of grain size and initial defects, by considering the anisotropic behavior of crystalline materials at the level of the microstructure.