Amount Of Strain-induced Martensite Research Articles

Evaluation of fatigue crack propagation behavior of 316LN TIG weldments at liquid helium temperature

In the present work, fatigue crack propagation behavior of 316LN tungsten inert gas (TIG) weldment and weldment with post weld heat treatment (PWHT) was investigated at liquid helium temperature. Results indicates that the weldment with PWHT results in the highest fatigue crack propagation resistance, whereas the base material demonstrates the lowest fatigue crack propagation resistance. Microstructure evolution reveals that the fatigue crack propagation is characterized as transgranular fracture for the base material as well as the as-welded weldments and the weldment with PWHT. Moreover, the crack propagation path of the weldments shows a zigzag crack extension, especially for the weldment with PWHT, whereas the crack extension of the base material is relative straight. Electron backscattered diffraction reveals more amount of strain-induced martensite near the crack tip in the base material compared to in the as-welded weldment and in the weldment with PWHT, which is considered to be a crucial factor in the fatigue crack propagation behavior at cryogenic temperature.

A Study of the Structure and Properties of Austenitic Metastable Steels under Strain

The structure and mechanical properties of the corrosion-resistant metastable austenitic steels 03Kh14N11К5М2YuТ and 03Kh14N11КМ2YuТ have been investigated. The steels have been deformed by tagging them at room and negative temperatures. It has been established that the amount of martensite and the strength properties are higher at a negative temperature than at room temperature with an equal degree of strain. The investigated austenitic steels are strain-metastable. The higher the strain degree and the lower the deformation temperature, the greater the amount of strain-induced martensite and, correspondingly, the higher the strength properties. Martensite is not observed upon cooling to the temperature of liquid nitrogen.

Effect of Drawing Strain on Development of Martensitic Transformation and Mechanical Properties in AISI 304L Stainless Steel Wire

AISI 304L stainless steel wires are cold drawn to different strains for examining the effect of plastic deformation on the mechanical properties change, the microstructural evolution, and martensitic transformation. Microstructures of drawn wires are characterized using optical microscopy, X-ray diffraction and electron backscatter diffraction (EBSD) techniques. It is shown that deformed structure consists of austenite, ά-martensite, and ϵ-martensite. The ultimate strength of wires drawn to strains of 0.56, 1.13, and 1.7 are found to be equal to to 1028, 1492, and 1743 MPa, respectively, from intial value of 735 MPa. These strengths are correspond to the strain induced martensite of 34, 62, and 100%. Also, it is shown that elastic modulus of wires are changed due to the strain induced transformation during cold drawing. For accessing the strain distribution during wire drawing, FEM simulations are carried out according to the experimental investigations. By help of simulation results and applying different criteria, it is shown that regions near the wire surface are suitable area for initiation of strain induced martensite. Moreover, microstructural observations and hardness measurements reveal that the amount of strain-induced martensite is higher just below the surface.

Eddy-current control of the phase composition and hardness of metastable austenitic steel after different regimes of nanostructuring frictional treatment

The possibilities are studied for using an electromagnetic eddy-current technique of monitoring the phase composition and hardness of austenitic chromium–nickel steel that has been subjected to hardening frictional treatment in different technological regimes. It is shown that an eddy-current structuroscope and a Vektor eddy-current defectoscope make it possible to control the amount of straininduced martensite and the microhardness of the surface of metastable austenitic steel after nanostructuring frictional treatment with different numbers of gliding indenter scans (passes). The Vektor eddycurrent defectoscope allows one to monitor the phase composition of austenitic steel that has been subjected to nanostructuring frictional treatment in a temperature range from −196 to +250°C.

Effect of Strain-Induced Martensite on Tensile Properties and Hydrogen Embrittlement of 304 Stainless Steel

Room temperature tensile tests have been conducted at different strain rates ranging from 2 × 10−6 to 1 × 10−2/s on hydrogen-free and hydrogen-charged 304 stainless steel (SS). Using a ferritescope and neutron diffraction, the amount of strain-induced martensite (SIM) has been in situ measured at the center region of the gage section of the tensile specimens or ex situ measured on the fractured tensile specimens. The ductility, tensile stress, hardness, and the amount of SIM increase with decreasing strain rate in hydrogen-free 304 SS and decrease in hydrogen-charged one. Specifically, SIM that forms during tensile tests is beneficial in increasing the ductility, strain hardening, and tensile stress of 304 SS, irrespective of the presence of hydrogen. A correlation of the tensile properties of hydrogen-free and hydrogen-charged 304 SS and the amount of SIM shows that hydrogen suppresses the formation of SIM in hydrogen-charged 304 SS, leading to a ductility loss and localized brittle fracture. Consequently, we demonstrate that hydrogen embrittlement of 304 SS is related to hydrogen-suppressed formation of SIM, corresponding to the disordered phase, according to our proposition. Compelling evidence is provided by the observations of the increased lattice expansion of martensite with decreasing strain rate in hydrogen-free 304 SS and its lattice contraction in hydrogen-charged one.

Effect of Strain Induced Martensite for 304M2 Austenitic Stainless Steel Wire

The impact of deformation and strain rate on the strain induced martensite (SIM) from 304M2 austenitic stainless steel was studied by using X-Ray diffraction, transmission electron microscopy. The results indicate that 304M2 is easy to have SIM because of the chemical component and microstructural characteristic. The amount of SIM has great relationship with the deformation and strain rate. It is found that the amount of SIM is reduced during high speed deformation. The obvious SIM can be observed with higher deformation, but the growth rate slows down. When the deformation rates are 8.3%, 55.0% and 67.3%, the contents of martensite are 6.55%, 15.35% and 16.21%, respectively. Compared with the slow stretching, the quick stretching leads to less martensite transformation. Moreover, the elongation of the specimens decreases. At the stable deformation stage, the temperature increases are 72.8 and 91.9℃, respectively, when the strain rates are 2×10-2s-1 and 5×10-4s-1. Therefore, the martensitic transformation and the deformation behavior of the austenitic stainless are affected by heating.

Effect of Ni content on the tensile properties and strain-induced martensite transformation for 304 stainless steel

The effect of Ni content (8.3–12wt.%) on the tensile properties and strain hardening behavior was studied on type 304 stainless steels (STS) used for the membrane of LNG storage tanks. The tensile test temperature was varied from 25°C to −196°C. At room temperature, the hardening and ductility indices (tensile strength, strain hardening exponent and elongation) increased with decreasing Ni content. For the 8.3–9.0wt.% Ni STS, a lower yield point was observed at temperatures below −60°C. It was due to the dynamic strain softening and/or transformation-induced plasticity (TRIP) that accompanied the rapid increase in the amount of strain-induced martensite (α′) at low strains. Neither dynamic strain softening nor TRIP was observed for the 12wt.% Ni STS because only the ɛ-martensite transformation was produced at the low strains.

Effect of Mechanical Pretreatment on Gas Nitriding Behavior of Austenitic Stainless Steels

In order to develop a method of gas nitriding of austenitic stainless steels without chemical treatment such as pickling, three types of steels, SUS304, SUS316 and SUS310, were pretreated under various mechanical processes (as-cut with resinoid blade, polishing with emery papers, grinding with CBN wheel, and shot peening), followed by nitriding in NH3 gas with a flow rate of 2.8 m 3 /h at 843 K for 20 h. The influence of the mechanical pretreatment on nitriding behavior was examined in connection with two factors of surface roughness and strain-induced martensite. Nitriding reaction depends primarily on surface roughness in SUS310 and SUS316. This nitriding reaction is markedly enhanced with increasing surface roughness when its value exceeds 6 and 1 μm for SUS310 and SUS316, respectively, while no reaction is recognized for the surface roughness less than these values. In SUS304, martensitic transformation is readily induced by the mechanical pretreatment, and not only the surface roughness but also the strain-induced martensite contribute to nitriding behavior. It is confirmed from the present results that mechanical pretreatment enables the gas nitriding of austenitic stainless steels without pickling, when the surface roughness and the amount of strain-induced martensite are controlled properly.

Investigation of crack closure in stainless steel by laser interferometry

The effect of strain-induced martensite transformed during fatigue on the fatigue crack propagation rate near ΔKth, as well as low-cycle fatigue behaviour of three differently heat-treated stainless steels, was investigated. The heat treatments were chosen so that austenite stability during fatigue was different. The crack closure stress during fatigue crack propagation near the ΔKth region was measured using laser interferometry. The sensitized specimen showed the highest value of closure load ratio (Kcl/Kmax), which was considered to be due to the roughness-induced crack closure caused by intergranular facets. The specimen with the lowest austenite stability showing the largest amount of strain-induced martensite during fatigue, showed the highest crack growth rate. The effect of brittle fracture through the harder strain-induced martensite was larger than that of possible transformation-induced crack closure.

Variation in the amount of strain-induced martensite in a two-phase stainless steel having textures

Variation in the amount of strain-induced martensite in a two-phase stainless steel having textures

Fe-Cr-Ni系ステンレス鋼のマルテンサイト変態に対する化学組成の影響

In order to investigate the martensitic transformation in Fe-Cr-Ni stainless steel, the Ni equivalent has been introduced thermodynamically as a universal quantity which indicates stability of austenite with regard to chemical composition. The relation between the Ni equivalent and the amount of strain-induced martensite or athermal martensite has been studied by means of saturation flux density measurement. Furthermore, the mechanism of the formation of strain-induced martensite has been considered thermodynamically.The results obtained are summarized as follows:(1) The Ni equivalent, (Ni), in Fe-Cr-Ni stainless steel is expressed by\phantom(1)(Ni)=Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C\ oindentwhere Ni, Cr, etc. represent weight % of these elements in the stainless steel considered.(2) The relation between the Ni equivalent and the amount of strain-induced or athermal martensite has been obtained.(3) About 75% cold rolling of Fe-Cr-Ni stainless steel has the same effect on martensitic transformation as decreasing the Ni content by 3∼5%. In this case, the difference in the free energy at room temperature between the ferrite phase and the austenite phase increases by about 113∼188 cal/mol.(4) The deduced value of the work done by transformation strain induced by the applied stress due to cold rolling is about 120∼126 cal/mol or about 192 cal/mol by compressive or tensile stress, respectively. These values agree very well with the above mentioned change of the free energy difference calculated from the variation in Ni equivalent. Therefore, under applied stress, the stored free energy required for martensitic transformation can be lowered by this work value, 120∼192 cal/mol.

Fe-Cr-Ni系ステンレス鋼の機械的性質に対する加工温度の影響

In order to investigate the influence of working temperature on the mechanical properties of Fe-Cr-Ni stainless steel, the specimens were rolled at several temperatures between room temperature and 200°C. These specimens had such chemical compositions as to make the austenite structures stable or unstable at room temperature. Then, the amount of martensite formed in specimens were estimated by means of saturation flux density measurement, and the Federbiegegrenze, Young’s modulus, tensile strength, and elongation were also measured. The results obtained are as follows:(1) The variation in the mechanical properties with working temperature was considerably large in the specimens with chemical compositions in which the austenite structures were unstable at room temperature: Namely, the strength became greatly lowered according to a sharp decrease in the amount of strain-induced martensite with the rising working temperature.(2) A good elongation and a relatively high strength were obtained by warm working of specimens with a chemical compositions in which the austenite structures were very unstable at room temperature.(3) There was a little influence of working temperature on mechanical properties of the specimens with chemical compositions in which the austenite structures were stable at room temperature.(4) The relationships were shown between the mechanical properties and the modified Ni equivalent, which is a universal quantity deduced thermodynamically from the chemical composition and the working temperature.