Effect of pre-quenching on martensite-bainitic microstructure and mechanical properties of GCr15 bearing steel

In order to improve the strength and toughness of steel GCr15 (52100), the effect of different amounts of pre-transformed martensite on the kinetics of isothermal bainitic transformation and the strength and toughness of martensite-bainitic (MB) duplex microstructure has been studied by using pre-quenching after conventional 850°C heating to different temperatures (220, 200, 180°C) below M _s and then isothermal treatment at 240°C. The experimental results show that the accelerating effect of pre-quenched martensite on isothermal bainitic transformation principally depends upon the pre-quenching temperature (the amount of pre-quenched martensite). The MB duplex microstructure with 33% pre-transformed martensite has the optimum combination of strength and toughness.     

MARTENSITE FORMATION AND THE RELATED TOUGHNESS IN 304 STAINLESS STEEL DURING LOW TEMPERATURE FATIGUE

Abstract— Effect of transformed martensite during low temperature fatigue in metastable 304 stainless steel on low temperature toughness were investigated. Low temperature fatigue was mainly conducted at R = 0.1 with maximum stress of 1030 MPa at −196°C. The amount of martensite during fatigue cyclic deformation was related to the reduction of impact toughness and to the apparent fracture toughness. Beyond 30% of martensite formation, a strong decrease of low temperature impact toughness was found. This phenomenon is related to the phase stability of the metastable austenitic phase.     

304不锈钢箔材在不同应变速率下的拉伸性能研究

304不锈钢是一种常用的奥氏体不锈钢.在拉伸应变过程中,应变速率的变化会诱发马氏体转变量和转变速率,以及内部组织滑移线、位错、层错、形变孪晶密度的转变量和转变速率的不同,从而表现出不同的应变硬化行为.本文针对0.1 mm厚度304奥氏体不锈钢箔材,从断后伸长率,断面收缩率,屈服强度,抗拉强度及硬化指数5个方面,研究了室温条件下不同应变速率对其拉伸性能的影响.实验结果表明：马氏体转变理论同样适用于304奥氏体不锈钢箔材, 且0.1 mm厚度304不锈钢存在“越薄越脆,越小越强”的尺寸效应现象;同时,0.1 mm厚度304奥氏体不锈钢箔材拉伸力学性能随应变速率的变化主要表现在以下几方面：断后延伸率和断面收缩率均随着应变速率的增加而降低;低应变速率时,随着应变速率的增加屈服强度增大,而抗拉强度随应变速率的提高呈现减弱的相反规律;高应变速率下,304奥氏体不锈钢的强度主要由材料本身性能决定,应变速率的改变对强度的影响较小;准静态低应变速率下,硬化指数随应变速率增大而升高,较高应变速率下,硬化指数与应变速率变化无关.     

Effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel

The effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel have been investigated. Yield strength is reduced by sensitisation, but ultimate tensile strength is nearly unaffected. Strain-hardening behaviour is changed by sensitisation, too. Although sensitisation may induce martensite formation near grain boundary, twin boundary, and austenite/martensite interface, the sensitisation-induced martensite does not exert a great influence on tensile behaviour in the 304 steel. In the unsensitised condition, martensitic transformation in the steel bulk induced by prior deformation and liquid-nitrogen immersion also does not change strain-hardening behaviour in the present steel.     

Radial distribution of martensitic phase transformation in a metastable stainless steel under torsional deformation: A synchrotron X-ray diffraction study

The strain-induced martensitic phase transformation in a metastable 304 L stainless steel under torsional deformation was investigated using synchrotron X-ray diffraction. The measured radial distribution of the martensite phase fraction in a solid cylindrical specimen agrees well with the prediction based on a combination of transformation kinetics and a radial plastic strain distribution equation.     

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

The effect of Ni content (8.3-12 wt.%) 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 8.3-9.0 wt.% 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 12 wt.% Ni STS because only the ?-martensite transformation was produced at the low strains.     

不锈钢冷加工形变诱发马氏体相变及其腐蚀行为

在实验室和现场对经过不同温度，不同方式，不同程度冷加工的奥氏体304不锈钢进行马氏体相变量的检测，研究了冷加工与马氏体相变的关系。通过浸泡试验和金相显微镜研究了304不锈钢在腐蚀过程中马氏体含量的变化。结果表明，冷加工能产生不同程度的马氏体相变；在腐蚀过程中，马氏体相存在优先溶解。     

Deformation induced martensitic transformation in a 201 modified austenitic stainless steel

The production of low nickel austenitic stainless steels has increased considerably mainly due to nickel price evolution in the last years. In the present work, the susceptibility to deformation induced martensitic transformation of a 201 modified stainless steel was evaluated. The results were compared to existing results of traditional AISI 304 steel. The variation of martensite volume fraction against true strain was modeled by a sigmoidal equation and the transformation rate was also determined.     

Low temperature martensitic transformations in In/Tl alloys

The fcc to fct martensitic transformation in In/Tl solid solutions has been studied by interference contrast microscopy over the range of temperature from room temperature down to 9.5° K. The form of the initial martensite bands, and the subsequent progress of the transformation, both depend on whether the M _s of the alloy lies above or below 0.5 melting point (° K). The transformation is reversible on heating, and the associated thermal hysteresis is found to increase when the amount of Tl in the alloy lowers its M _s temperature below 0.5 of the melting point. These effects are discussed in terms of the effect of temperature on the strains inherent in the transformation.     

Effects of aging temperature on the microstructure and mechanical properties of 1.8Cu-7.3Ni-15.9Cr-1.2Mo-low C, N martensitic precipitation hardening stainless steel

The effects of aging temperature on the microstructure and mechanical properties of a newly designed martensitic precipitation hardening stainless steel, which is 1.8Cu-15.9Cr-7.3Ni-1.2Mo-low C, N steel, for improving the toughness, ductility and corrosion resistance of stainless steel of 1000 MPa grade tensile strength were experimentally investigated. The specimen aged at 753 K for 14.4 ks has a typical lath martensitic structure with about 12% interlath austenite, while the specimens aged at 813 K and 853 K for 14.4 ks have the lamellar duplex microstructure of the reverted austenite and the aging hardened martensite. The formation process of reverted austenite is controlled by diffusion of Ni in martensite. The mean size of precipitates which are enriched with Cu increases with rising aging temperature, however, it is about 30 nm even after aging at 853 K for 14.4 ks. The specimens aged at 813 K and 853 K for 14.4 ks, in which the reversion of martensite to austenite is observed, have the excellent combinations of strength, ductility and toughness.     

The influence of tempering temperature on the reversed austenite formation and tensile properties in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steels

The influence of tempering temperature on the reversed austenite formation and tensile properties are investigated in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steel in the temperature range of 550-950 °C. It is found that at the temperatures below 680 °C, the reversed austenite formation occurs by diffusion. Amount of the reversed austenite is determined by the tempering temperature and the holding time. The segregation of Ni is the main reason for the stability of the reversed austenite. When the temperatures are above 680 °C, the reversed austenite formation proceeds by diffusionless. The reversed austenite will transform back to martensite after cooled to room temperature. The tensile properties are most strongly influenced by the amount of the reversed austenite obtained at room temperature. The excellent combination of good strength and ductility is at 610 °C.     

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.     

Effect of aluminium on TRIP Fe---Mn---Al alloy steels at room temperature

Fe---Mn---Al alloy steel has undergone many significant developments during the past few years, and it is considered to be a steel of high strength and toughness. Studies and test results have shown that Fe---Mn---Al is a good potential replacement for conventional stainless steels which contain expensive nickel and chromium additions. Recently, it has been found that strain-induced martensitic transformation at room temperature is possible in austenitic Fe---Mn---Al alloy. Further study has shown that the ′ martensite can improve the mechanical properties of these Fe---Mn---Al alloys. However, as the aluminium content increases, the strain-induced phase transformation is inhibited. Moreover, the mechanical properties of these materials are very dependent on the aluminium content.     

Stress induced martensite transformation texture in AISI 304 austenitic stainless steel

Abstract

The phenomenological theory of martensitic transformation was applied to a tension induced martensitic transformation in an AISI 304 austenitic stainless steel in order to estimate the transformation texture. Input data were obtained from the published literature. Calculated pole figures were constructed assuming a variant selection process based on Patel and Cohen’s theory, which emphasises that a mechanical component of free energy is the driving force for martensitic transformation at temperatures above martensite start M_s. The results showed a remarkably good match between the calculated and published measured data.     

Investigation of the evolution of retained austenite in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering

The microstructure and amount of retained austenite (the austenite remained at room temperature) evolved in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering at 620 °C have been investigated. The amount of retained austenite showed a parabolic trend with increase in tempering time, which can be attributed to the gradual decrease in the thermal stability of the reversed austenite (the austenite formed at high temperature). The influences of chemical composition, morphology of reversed austenite, and mechanical constraints originating from tempered martensite matrix on the thermal stability have been discussed. The precipitation and growth of M₂₃C₆ in reversed austenite dilute the carbon concentration in reversed austenite. The spheroidization of lathy reversed austenite during tempering decreases the interfacial energy barrier to the phase transformation of reversed austenite to martensite. Furthermore, the decrease in the strength of martensite matrix lowers the strain energy associated with the transformation of reversed austenite to martensite. All these factors during tempering weaken the thermal stability of reversed austenite and facilitate the phase transformation of reversed austenite to martensite during the cooling step of intercritical tempering.     

激光选区熔化成形工艺对304L不锈钢冲击韧性的影响

目的 了解激光选区熔化(SLM)成形工艺参数对304L不锈钢冲击韧性的影响,从而得到304L不锈钢的最佳成形工艺参数。方法 对激光功率300~340 W,激光扫描速度800~1 500 mm.s^?1条件下的激光选区熔化成形304L不锈钢开展冲击试验,通过表面硬度、微观组织及断口形貌观察对冲击韧性的影响规律进行分析。结果 SLM成形304L不锈钢微观组织为跨越熔池生长形成的不规则柱状晶粒,成形工艺参数对试样表面硬度影响不显著;随着激光功率的增大和激光扫描速度的降低,304L不锈钢断面致密程度提高,孔洞类缺陷尺寸减少且数量减少,冲击韧性增大,冲击功最大值为141.9 J。结论 基于冲击试验结果,在激光体能量密度为100~140 J/mm³的条件下,304L冲击韧性稳定在138 J左右,为SLM成形304L材料的最佳成形参数区间。     

The characterisation of PM 304 stainless steel sintered in the presence of a copper based additive

Samples of 304 stainless steel powders alloyed with a range of copper-based additive (2-8 wt.%) have been compacted and sintered at two different temperatures (1250 and 1350 °C) in an Argon atmosphere. The effect of copper-based ternary alloy on the microstructure and hardness values of a sintered stainless steel has been investigated. It was found that the quantity of copper-based additive as well as the sintering cycle has a profound impact on the microstructure and hardness values of sintered stainless steel. The residual porosity in the sintered specimens was substantially reduced in samples compacted from powder mixtures containing 2-8 wt.% of additives. The microstructural characterisation of PM 304 stainless steel alloyed with different amounts of copper addition shows that copper is homogenously diffused into the base stainless steel. The microhardness values of samples increase with an increase in the amount of additive due to the elimination of porosity.     

Development of carbon—Low alloy steel grades for low temperature applications

Low alloy steels are processed to fulfill the requirements of low temperature applications. Besides the chemical composition, the steel should receive a suitable heat treatment to ensure the targeted mechanical properties at low temperature. In other words, the steels are designed to delay the ductile to brittle transition temperature to resist dynamic loading at subzero temperatures. Steel alloys processed for liquefied gas pipeline fittings are examples for applications that need deep subzero impact transition temperature (ITT).The main purpose of the present work was to find a suitable heat treatment sequence for alloys LC2 and LC2-1. Further, it aimed to correlate the impact toughness with the microstructure and the fracture surface at different sub-zero temperatures.The steels under investigation are carbon-low alloy grades alloyed with Ni, Cr and Mo. LC2 steel alloy has been successfully processed and then modified to LC2-1 alloy by addition of Cr and Mo. Oil quenching from 900 °C followed by tempering at 595 °C was used for toughness improvements. Hardness, tensile and impact tests at room temperature have been carried out. Further impact tests at subzero temperatures were conducted to characterize alloys behavior. Metallographic as well as SEM fractographic coupled with XRD qualitative analysis are also carried out.Non-homogenous martensite-ferrite cast structure in LC2 was altered to homogeneous tempered martensite structure using quenching-tempering treatment, which is leading to shift the ITT down to −73 °C. Addition of Cr and Mo creates a very fine martensitic structure in LC2-1 alloy. Quenching-tempering of LC2-1 accelerates ITT to −30 °C. It is expected that the steel was subjected to temper embrittlement as a result of phosphorus segregation on the grain boundary due to Cr and Mo alloying, as it was concluded in reference no. [6].     

Monitoring of Martensitic Transformation in Austenitic Stainless Steel 304 L by Eddy Currents

A nondestructive eddy current technique has been applied to cyclic-strain induced martensitic transformation (MT) monitoring in austenitic stainless steel 304 L. During plastic deformation, the austenite transforms locally in martensite. Transformed martensite distribution depends on the geometry of the specimen under test (rectangular section in this work): transformation occurs first in the corners, later close to the flat free surfaces, and finally in the bulk. The obtained value is then representative of a small volume of the sample. Local measurements with an eddy current probe proved to be sensitive to this kind of transformation and even quantitatively after calibration. This article proposes a procedure to calibrate the eddy current response in order to determine the fraction of martensite formed under the probe without the need of X-ray measurements.     

Transformation of stress-induced martensite in 304 stainless steel by ion implantation

Type 304 stainless steel made largely martensitic by rolling was implanted with nitrogen, nickel, and neon. Transformation of the martensite to austenite was found by transmission election microscopy for the nitrogen and nickel implantations, but not for the neon. The transformation was found to be confined to the implanted layer. Lattice expansion was measured for nitrogen implantation, and an unusual electron-diffraction effect apparently due to surface strain was observed for the highest nitrogen concentration.