Strength Austenitic Stainless Steel Research Articles

Influence of grain size on fatigue strength of austenitic stainless steel (Investigation of ultimate strength dependency of fatigue strength)

Grain size dependency of the fatigue strength was investigated using Type 316 stainless steel. The main interest of this study was to clarify the root cause of the ultimate strength dependency of fatigue strength, particularly in the high-cycle fatigue regime. Four kinds of heat treatment were applied to obtain different grain sizes. The increase in the grain size caused smaller ultimate strength. The strain-controlled fatigue test and the crack growth test were conducted at room temperature. It was shown that the materials of larger grain size tended to have a shorter fatigue life, although the grain size had little influence on the crack growth rate. The reduced fatigue life due to large grain size was deduced to have been brought about by an increase in initial crack size after an incubation period before crack initiation. It was concluded that the grain size was not the dominant factor which caused the ultimate strength dependency of the fatigue strength, although the grain size surely affected both the fatigue strength and the ultimate strength.

Ultra-flash annealing constructed heterogeneous austenitic stainless steel with excellent strength-ductility

The application of austenitic stainless steel is seriously restricted due to its relatively low yield strength. To significantly improve the strength of austenitic stainless steel with an acceptable decrease in ductility, the as-received austenitic stainless steel is cold-rolled with 90% thickness reduction to obtain the hybrid structures including retained austenite, deformation-induced martensite, and subgrains. Subsequently, an ultra-flash annealing strategy is proposed to produce the heterostructured austenitic stainless steel consisting of recrystallized austenite grains and non-recrystallized austenite zones (diffusionless-reversion formation), and the hybrid microstructure formed by ultra-flash annealing shows superior strength-ductility synergy. The thorough investigation reveals that the origin of the high yield strength is not only grain boundary strengthening, but also the back-stress hardening induced by the constraint of hard domains to soft domains. The consequent high uniform elongation is provided by the synergistic effect of dislocation hardening, back-stress hardening and twinning, which is different from the coarse austenitic stainless steel with twinning-induced plasticity and transformation-induced plasticity.

Characterization of Microstructure and High Temperature Compressive Strength of Austenitic Stainless Steel (21-4N) through Powder Metallurgy Route

Exposure of the engine valve to high temperatures led to the degradation of the valve material due to microstructural instability and deteriorating mechanical properties. Performance enhancement and alteration in microstructures can be attained through the powder metallurgy route which is a viable method to produce near net shape components. In this current study, the development of austenitic stainless steel (21-4N) through the powder metallurgy route as an alternate material for engine valves was investigated. Mechanical alloying was carried out for the pre-alloyed mixtures and consolidated using vacuum hot pressing. Sintering parameters were fixed at 1200 °C, 50 MPa and at a vacuum level of 10-3 Torr. A scanning electron microscope was used to analyze the morphology of the milled powders. Densities for the hot pressed powders were compared with theoretical densities and found to be around 98–99%. Observations regarding grain size, the presence of austenitic grain, heterogeneous distribution of metal carbides and analysis of chemical composition along the metal matrix were determined using both optical and electron microscopes. X-ray diffraction was carried out for both the consolidated and powder samples. The hot pressed samples exhibited a hardness value of 410 ± 10 Hv. An isothermal compression test for the sintered samples was carried out at a temperature of 650 °C and strain rate of 0.001 s−1. It is showed that the compressive strength of 1380 MPa. An analysis between the room temperature yield strength obtained from hardness measurement and the strengthening mechanism based on the microstructure was conducted. Grain size, dislocation and solid solution are the major strengthening mechanisms which strengthen the material. Overall, the development of valve steel material through the powder metallurgy route exhibited improved metallurgical and mechanical properties in comparison to the corresponding cast product.

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Friction stir welding of AISI 316LN high strength austenitic stainless steel for cryogenic application

Friction stir welding (FSW) is a solid state joining process that uses the heat generated by the friction of a rotating tool and the base material to join materials together. Due to the fact that the material is never melted, and that extensive plastic deformation is introduced in the weld seam, a unique set of properties is achieved. The technique has been extensively used to join aluminium and aluminium alloys, but very few developments are reported on high strength austenitic stainless steel, which is the material of choice for many high energy physics and fusion magnets. This paper contains a comprehensive microstructural and mechanical characterization, including at cryogenic temperature, of an 8 mm thick high strength austenitic stainless steel plates. The steel grade is the high alloy version of AISI 316LN (identified as 1.4429 or -X2CrNiMoN17-13-3 according to European standards). Special attention was given to cryogenic elastic – plastic J – integral testing of the weld seam. To the authors’ knowledge, this is the first time fracture toughness at cryogenic temperature on friction stir welded 1.4429 has been measured.

Direct energy deposition of high strength austenitic stainless steel matrix nanocomposite with superior ductility: Microstructure, tensile properties, and deformation behavior

Direct energy deposition (DED) was used to additively manufacture 22–13-5 austenitic stainless steel (SS), of which the mechanical properties and corrosion resistance are superior to those of SS316L and SS304L. Additionally, in-situ formed oxide-driven strengthening was utilized in this study. The DED-processed austenitic SS matrix nanocomposite (SSMNC) exhibited unique microstructural features of heterogeneous grains and dislocation networks. The nano-sized precipitates existed at the sub-structure boundaries and decorated the dislocation network. The results of the transmission electron microscopy (TEM)-energy loss spectroscopy (EELS) analyses revealed nano-sized precipitates with an average size of 21.1 nm that were identified as (Mn,Cr)-rich oxides. This means that, during the DED process, the oxygen in the powder feedstock transformed into nano-sized oxide particles by rapid solidification. The DED-processed SSMNC revealed a yield strength of 705.4±5.3 MPa, which is higher than those of reported AM-processed stainless steels. In addition, the elongation-to-failure was measured as 41.8±3.5%. This suggests that the DED-processed SSMNC has an excellent combination of strength and ductility at room temperature. The high ductility of the alloy developed in this work was found to be achieved by a twinning-induced plasticity (TWIP) mechanism that operated during the deformation. Based on the above findings, the relationships between the microstructure, mechanical properties, and deformation mechanism are also discussed.

Detection Method for Liquid Metal Embrittlement Cracks Inside the Intermediate Sheet Zone of Dissimilar Resistance Spot Welds

Liquid metal embrittlement (LME) is a phenomenon where a liquid metal damages a solid bulk metal. Notwithstanding previous investigations, LME remains an actual as well as a still not fully understood topic. A so far not yet investigated area can be defined with ultra‐high strength austenitic stainless steels for passenger cars. The most commonly used joining procedure in car body engineering is still resistance spot welding. During this vehicle assembly step, the uncoated surface of an austenitic stainless steel is because of the process‐related lap joint configuration in direct contact with zinc‐coated surface‐finished steels as a dissimilar material combination. During welding, liquid zinc could penetrate inside the intermediate sheet zone in the grain boundaries of the austenitic steel and is therefore able to initiate cracks in the heat‐affected welding zone. Herein, the radiographic inspection is introduced, which is a process‐reliable, nondestructive detection method for the inaccessible intermediate sheet zone area, which is usable in automotive big‐industrial scale. With the implemented detection method, liquid metal‐induced cracks can be detected in the intermediate sheet zone down to a crack length of 50 μm. Subsequently, the radiographic inspection helps to analyze the crack characteristic depending on surrounding conditions and welding parameters.

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Relationship between hardness and tensile strength of austenitic stainless steel

Relationship between hardness and tensile strength of austenitic stainless steel

Evaluation of mechanical properties distribution for irradiated stainless steels simulated by utilizing the gradient of the cold working ratio

It is known that irradiation damage can cause a reduction in fracture toughness and an increase in the strength of austenitic stainless steels used for the internal structures of nuclear power plant reactors. A change in the mechanical properties of material, such as fracture toughness and strength, affects the fracture behavior of the components. Actual reactor internal structures, such as the top guides and core shrouds in BWR plants, are irradiated from the core in a single direction. This causes the attenuation of neutron flux, which results in the distribution of neutron fluence in the components, meaning that the distribution of mechanical properties is in the through-wall direction of the components. It is therefore possible for the fracture mode of the components to change in the through-wall direction. It would be valuable for more accurate assessment of the structural integrity of the nuclear components to make the effect of the mechanical properties distribution on the fracture mode clear. However, it is difficult to prepare large-scale test specimens with the mechanical properties distribution induced by uni-directional neutron irradiation. In this study, the mechanical properties distribution was simulated by applying the gradient of the cold working ratio using Type 316L stainless steel plate with an uneven thickness. The targeted range of the cold working ratio was designed by assuming BWR reactor internal structures, such as the top guides. The mechanical properties distribution was evaluated based on the results of hardness, tensile and fracture toughness tests. It was found that the macroscopic mechanical properties of irradiated stainless steels can be simulated by using the cold working and the macroscopic distribution of the mechanical properties of irradiated stainless steels can also be simulated by using the cold working ratio with a gradient. Furthermore, fracture tests were performed using test plates with the mechanical properties distribution assuming irradiated components with a crack. In some cases, the maximum loads obtained by the tests were almost the same value in spite of the different cold working ratio in the ligament. Based on the results of the fracture tests, it was considered that the dominant mechanical properties for the fracture load were those at the tip of the slit, not those in the ligament.

Comparison of the Dislocation Structure of a CrMnN and a CrNi Austenite after Cyclic Deformation

In the literature, the effects of nitrogen on the strength of austenitic stainless steels as well as on cold deformation are well documented. However, the effect of N on fatigue behaviour is still an open issue, especially when comparing the two alloying concepts for austenitic stainless steels—CrNi and CrMnN—where the microstructures show a different evolution during cyclic deformation. In the present investigation, a representative sample of each alloying concept has been tested in a resonant testing machine at ambient temperature and under stress control single step tests with a stress ratio of 0.05. The following comparative analysis of the microstructures showed a preferred formation of cellular dislocation substructures in the case of the CrNi alloy and distinct planar dislocation glide in the CrMnN steel, also called high nitrogen steel (HNS). The discussion of these findings deals with potential explanations for the dislocation glide mechanism, the role of N on this phenomenon, and the consequences on fatigue behaviour.

Creep strength contribution due to precipitation hardening in copper\u2013cobalt alloys

In spite of its huge technical significance, there does not seem to be consensus about how to model the precipitation contribution to the creep strength. Most contributions in the literature are based on a constant internal stress (also called back stress or threshold stress) from the precipitation. It is well-known and it will also be demonstrated in the paper that this assumption is at variance with observations except for some ODS alloys. There is, however, one model developed by Eliasson et al. (Key Eng Mater 171–174:277–284, 2000) that seems to be able to represent experimental data without the use of any adjustable parameters. It has successfully been applied to describe the creep strength of austenitic stainless steels. Due to the fact that various mechanisms contribute to the creep strength in these steels, the model has not been fully verified. The purpose of this paper is to apply the model to published creep data for Cu–Co alloys, where the precipitation totally dominates the strength contribution to validate the model. In the paper, it is demonstrated that the model can indeed describe the influence of applied stress, alloy composition and heat treatment for the three analysed Cu–Co alloys.

The Investigation on Strain Strengthening Induced Martensitic Phase Transformation of Austenitic Stainless Steel: A Fundamental Research for the Quality Evaluation of Strain Strengthened Pressure Vessel

The manufacture of pressure vessels with austenitic stainless steel strain strengthening technology has become an important technical means for the light weight of cryogenic pressure vessels. In the process of increasing the strength of austenitic stainless steel, strain can induce the martensitic phase transformation in austenite phase. There is a quantitative relationship between the transformation quantity of martensitic phase and the basic mechanical properties. Then, the martensitic phase variables can be obtained by means of detection, and the mechanical properties and safety performance are evaluated and calculated. Based on this, the quantitative relationship between strain hardening and deformation induced martensite phase content is studied in this paper, and the mechanism of deformation induced martensitic transformation of austenitic stainless steel is detailed.

Effect of Welding on Gigacycle Fatigue Strength of Austenitic Stainless Steels

Effect of Welding on Gigacycle Fatigue Strength of Austenitic Stainless Steels

Static strength of stainless steel K- and N-joints at elevated temperatures

This paper aims to investigate the static strength of stainless steel gap and overlapped K- and N-joints at elevated temperatures. A numerical parametric study was conducted on circular hollow section (CHS) K- and N-joints under axial loads in the braces at different temperatures ranging from 22 to 760°C. A wide range of geometric parameters and chord preload ratios of CHS K- and N-joints using duplex, high strength austenitic and normal strength (AISI 304) stainless steel was investigated. The joint strength reduction was compared with reduction factors of yield stress and elastic modulus of stainless steel materials at elevated temperatures. A unified strength equation for CHS K- and N-joints at elevated temperatures was proposed by adopting the reduction factor of yield stress. The statistical analysis shows that the proposed strength equation is accurate for stainless steel K- and N-joints at elevated temperatures.

Quantitative Evaluation of Metallurgical Mechanisms Affecting Strength of Austenitic Stainless Steels

Relations predicting the mechanical properties of bars of austenitic stainless steels are already available, but no systematic work has been performed to predict the strength of cold rolled and annealed stainless steels. The tensile properties of a large number of cold rolled and annealed AISI 304 stainless steel are here correlated with their chemical composition and microstructure. Quantitative effects of various strengthening mechanisms such as grain size, δ-ferrite content and solid solution strengthening by both interstitial and substitutional solutes are described. Interstitial solutes have by far the greatest strengthening effect and, among the substitutional solutes, the ferrite-stabilizing elements have a greater effect than the austenitestabilizing elements. Regression equations are developed that predict with good accuracy the proof stress and tensile strength in AISI 304 stainless steels.

SUMITOMO SM2535-125

Abstract Sumitomo SM2535 is a very high strength austenitic stainless steel for use in oil country tubular goods as a material suitable for severe sour service. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on corrosion resistance as well as forming and heat treating. Filing Code: SS-1268. Producer or source: Nippon Steel and Sumitomo Metal Corporation.

Hall-Petch relationship for austenitic stainless steels processed by large strain warm rolling

The deformation microstructures and their effect on the yield strength of austenitic stainless steels processed by large strain warm rolling were studied. The samples of 304 L and 316 L type steels were subjected to caliber bar rolling to total strains of 2 at temperatures of 773–1173 K. The structural changes were characterized by the development of continuous dynamic recrystallization. A decrease in the rolling temperature resulted in significant grain/subgrain refinement and an increase in the yield strength at room and elevated temperatures. A power law function was obtained between the deformation grain and subgrain sizes with a grain size exponent of 0.3. Therefore, the yield strength could be expressed by a modified Hall-Petch relationship including a term of substructural strengthening, which was evaluated using the obtained size relation. The numerical factors normalized by shear modulus for both structural and substructural strengthening terms depended quite weakly on tensile test temperature in the range of 293–873 K that suggested the deformation mechanisms being invariant up to 873 K. On the other hand, the thermally activated mechanisms led to the yield strength decreasing much faster than shear modulus as the tensile test temperature increased above 873 K.

고강도 오스테나이트계 스테인리스강(STS201) 일면전단 볼트접합부의 최대내력평가

고강도 오스테나이트계 스테인리스강(STS201) 일면전단 볼트접합부의 최대내력평가

Ultimate strength of austenitic stainless steel fillet-welded connections with weld metal fracture

This study investigates ultimate behaviors such as ultimate strength and weld metal fracture mechanism of austenitic stainless steel (304 type) fillet-welded connections with TIG (tungsten inert gas) welding through experimental method. Main variables of test specimens are weld length and welding direction against applied force. Specimens are fabricated with transverse fillet weld (TFW series), longitudinal fillet weld (LFW series) and full fillet weld FW series against loading direction. Specimens of TFW series, LFW series and FW series failed by tensile fracture, shear fracture and block shear fracture in the weld metal of connections. Test ultimate strengths were compared with predictions based on the design specifications, AISC (American institute of steel construction) and ASCE (American Society of Civil Engineers) and existing researcher's equations. Current design equations underestimated excessively the strength of welded connections with weld metal fracture. Consequently, modified design formula is suggested considering actual fracture path and the effect of stress triaxiality on welded connections and its validation was verified in this paper.