Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic stainless steels

The effects of deformation-induced formation of martensite have been studied in metastable austenitic stainless steels. The stability of the austenite, being the critical factor in the formation of martensite, was controlled principally by varying the amounts of carbon and manganese. The formation of martensite was also affected by different test and rolling temperatures, rolling time, and various reductions in thickness. The terms “stress-induced” and “strain-induced” formation of martensite are defined. Experimental results show that low austenite stability resulted in stress-induced formation of martensite, high work-hardening rates, high tensile strengths, low “yield strengths,” and low elongation values. When the austenite was stable, plastic deformation was initiated by slip, and the work-hardening rate was too low to prevent early necking. A specific amount of strain-induced martensite led to an “optimum” work-hardening rate, resulting in high strengthand high ductility. For best results processing should be carried out aboveM _d and testing betweenM _d andM _s. Mechanical working aboveM _d had a negligible effect on the yield strength betweenM _d andM _s when the austenite stability was low, but its effect increased as the austenite became, more stable. Serrations appeared in the stress-strain curve when martensite was strain induced.     

The effects of deformation induced martensite on the sensitization of austenitic stainless steels

This paper compares the effects of deformation which induces martensite in austenitic stainless steel with deformation which does not on the sensitization and corrosion susceptibility of these alloys. We show that deformation which induces martensite causes rapid sensitization at temperatures below 600 °C, leads to extensive transgranular corrosion, and can produce rapid healing. The martensite is also an area of extensive carbide precipitation. Deformation alone noticeably increases the kinetics of sensitization only at temperatures where undeformed samples are readily sensitized. Without the presence of martensite, intergranular corrosion is always the predominant corrosion path, rapid healing is not observed, and most carbides precipitate along the grain boundaries.     

Effect of strain-induced martensitic phase transformation on the formability of nitrogen-alloyed metastable austenitic stainless steels

Strain-induced martensitic phase transformation and its influence on the formability of newly developed nitrogen-alloyed metastable austenitic stainless steels were systematically investigated. Yield strength for the asreceived steels bearing lownickel content was around 300 MPa and their elongation ratios varied from 55. 2% to61. 7%. Erichsen numbers of these samples differed from 13. 82 to 14. 57 mm. Although its Cu content was lower than that of other samples,steel D2 exhibited better plasticity and formability,which was attributed to γ→α'martensitic phase transformation. EBSD,XRD,and magnetism tests showed that increases in deformation ratio gradually increased the α' martensite phase of a sample,thereby contributing to its strain and inducing the optimal transformation-induced plasticity effect. An M_(d30/50) temperature of around 20 ℃,which is close to the deformation temperature,provided the austenite with adequate stability and gradually transformed it into martensite,thereby endowing lean ASS with better formability.     

Duplex grain boundary precipitation in austenitic stainless steels containing aluminium and titanium

This paper considers the grain boundary precipitation reactions which may occur during the ageing of stainless steels based on the composition Fe-20%Cr-25%Ni which contain additions of aluminium and titanium. It is shown that the formation of the equilibrium precipitate phase, associated with the composition of the alloy, causes the subsequent formation of adjacent regions of chromium-rich ferrite. In addition, it is shown that this ferrite is unstable, and may decompose to a more stable intermetallic phase. The choice of final intermetallic phase, together with the multiphase crystallographic orientation relationships obeyed during the precipitation reaction and the local precipitate phase compositions, are discussed with respect to the overall reaction sequence.     

Creep-fatigue interaction in austenitic stainless steels

Low cycle fatigue failures occur by the initiation and controlled growth of a surface crack. The development of crack propagation models, based on continuum mechanics, have enabled successful predictions of fatigue life at both room and elevated temperatures. This paper attempts to extend such models to cover the situations in which creep damage, introduced during periods of stress relaxation, influences the rate of growth of the surface fatigue crack. Equations predicting fatigue life as a function of hold period are in good agreement with experimental data, for Type 304 stainless steel, Type 316 stainless steel and Incoloy-800.     

Instabilities in stabilized austenitic stainless steels

The effect of aging on the precipitation of grain boundary phases in three austenitic stainless steels (AISI 347, 347AP, and an experimental steel stabilized with hafnium) was investigated. Aging was performed both on bulk steels as well as on samples which were subjected to a thermal treatment to simulate the coarse grain region of the heat affected zone (HAZ) during welding. Aging of the bulk steels at 866 K for 8000 hours resulted in the precipitation of Cr₂₃C₆ carbides, σ, and Fe₂Nb phases; the propensity for precipitation was least for the hafnium-stabilized steel. Weld simulation of the HAZ resulted in dissolution of the phases present in the as-received 347 and 347AP steels, leading to grain coarsening. Subsequent aging caused extensive grain boundary Cr₂₃C₆ carbides and inhomogeneous matrix precipitation. In addition, steel 347AP formed a precipitate free zone (PFZ) along the grain boundaries. The steel containing hafnium showed the best microstructural stability to aging and welding. Formerly with Exxon Research and Engineering Company.     

Formation of nanosized martensite particles in stainless steels

Martensitic transformations occur in steels and other metals when the material is quenched or cooled. During the transformation, a metastable phase transforms in an autocatalytic way into a thermodynamically stable phase. Thus, it has been considered to be impossible to control the size of martensite in the order of nanometers. Here, we report the formation and dispersion of nanosized martensite in stainless steels by controlling the dislocation density and temperature. Electron microscopy revealed that martensite particles of approximately 5 nm in diameter were dispersed. Since both work hardening and precipitation hardening are effective in this material, this would be an additional, newly discovered strengthening mechanism for steels.     

Effect of technological factors on the properties of forged austenitic stainless steels

The conditions for the production of austenitic stainless steel Kh18N10, Kh18N12M2, and Kh23N18 (i.e., Cr18Ni10, Cr18N12Mo2, and Cr23Ni18) by forging porous blanks have been studied and optimized. In many cases, the properties of these steels are determined by the production technology: the duration, temperature, and heating medium used in the presintering and the duration and temperature of the heating for forging the blanks. In each specific case the choice of fabrication conditions depends on the shape and purpose of the part. Simple parts with plane-parallel ends without changes in section along the height can be produced by hot forging without presintering of the powder blanks. Complex parts require hot forging at high temperatures (1150–1200°C) of presintered blanks. If the stainless steels do not have to meet stringent ductility requirements but must possess high strength, the blanks can be heated at 950–1050°C without a protective gas atmosphere.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 5–6, pp. 40–45, May–June, 1994.     

Dispersion strengthening austenitic stainless steels by nitriding

The internal nitridation of thin sections of austenitic Fe−Cr−Ni−Ti alloys containing up to 2 pct Ti was studied over the temperature range 1600° to 2210°F in order to develop a method of strengthening the alloys through the introduction of a dispersoid of stable titanium nitrides. The interparticle spacing (IPS) of the nitrides was found to increase linearly with depth from the external surface; the effects of various parameters on the rate of change of IPS vs depth are presented. The mechanical properties of these alloys at room and elevated temperatures were markedly improved by internally nitriding. Useful mechanical properties were obtained up to 2200°F, with typical properties at 2000°F of 10 to 20 ksi 0.2 pct offset yield strength and 15 to 25 ksi ultimate tensile strength, but section thickness was limited to about 10 mils because of the increase in IPS with depth and the long nitriding times needed for thicker material. In order to produce a small interparticle spacing in a heavier section, internally nitrided 5 mil strip was consolidated by hot roll bonding and evaluated at a 60 mil thickness by tensile and rupture testing at 2000°F. It is demonstrated that the approach taken in this work offers a feasible technique for making a high temperature alloy having useful engineering properties.     

Hydrogen-related phase transformations in austenitic stainless steels

The effect of hydrogen and stress (strain) on the stability of the austenite phase in stainless steels was investigated. Hydrogen was introduced by severe cathodic charging and by elevated temperature equilibration with high pressure H₂ gas. Using X-ray diffraction and magnetic techniques, the behavior of two “stable” type AISI310 steels and an “unstable” type AISI304 steel was studied during charging and during the outgassing period following charging. Transformation from the fcc γ phase to an expanded fcc phase, γ*, and to the hcp ε phase occurred during cathodic charging. Reversion of the γ* and e phases to the original γ structure and formation of the bcc α structure were examined, and the kinetics of these processes was studied. The γ* phase was shown to be ferromagnetic with a subambient Curie temperature. The γ⇆ε phase transition was studied after hydrogen charging in high pressure gas, as was the formation of a during outgassing. These results are interpreted as effects of hydrogen and stress (strain) on the stability of the various phases. A proposed psuedo-binary phase diagram for the metal-hydrogen system was proposed to account for the formation of the γ* phase. The relation of these phase changes to hydrogen embrittlement and stress corrosion cracking of stainless steel is discussed.     

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响.当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大.单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象.两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象.用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为.     

Sliding wear of austenitic and austenitic-ferritic stainless steels

The dry sliding behavior of a 304L austenitic stainless steel and a duplex 2205 austenitic-ferritic stainless was investigated. The evolution of wear was characterized by the existence of a sliding-distance transition. In particular, wear passed from delamination to tribo-oxidation, with a reduction in wear rate. The occurrence of such a transition was interpreted with reference to a theory of sliding wear based on the formation, by subsurface plastic deformation, of a tribological layer and its detachment during sliding. It has been found that the transition is controlled by the ability of the tribological system to form, on its outer part, a protective oxide-rich scale. This introduces a kinetic limitation, which is particularly important in the case of the duplex 2205, because of its lower ductility in comparison to the 304L steel. In this frame, the influence of sliding velocity, the particular frictional behavior, the role of chromium in the oxidative wear, and the surface temperature evolution during sliding could be explained.     

Stacking fault energies of seven commercial austenitic stainless steels

The stacking fault energies of seven commercial austenitic Fe-Cr-Ni, Fe-Cr-Ni-Mn and Fe-Mn-Ni alloys have been determined by X-ray diffraction line profile analysis. From comparison with existing data on laboratory alloys with similar compositions, it is concluded that both Ni and C increase γ while Cr, Si, Mn, and N decrease γ. Regression analysis of data produced in this study provides an expression relating γ to commercial alloy composition in terms of Ni, Cr, Mn, and Mo alloy concentrations.     

晶粒细化对奥氏体不锈钢高温力学性能的影响

摘要：对粗晶201LN奥氏体不锈钢采用60%冷变形结合700℃退火120s工艺制备超细晶奥氏体不锈钢,研究晶粒细化对奥氏体不锈钢高温力学性能的影响。利用光学显微镜、扫描电子显微镜、透射电子显微镜、电子背散射衍射技术对粗晶和超细晶奥氏体钢进行了组织表征,并使用万能试验机测试20和650℃环境下力学性能。结果显示粗晶奥氏体不锈钢经过冷变形结合退火工艺处理,平均晶粒尺寸由18μm细化为0.9μm,屈服强度由383MPa提高到704MPa,而伸长率由63.8%下降到46.3%,表明晶粒细化能有效提高奥氏体不锈钢屈服强度的同时较小损害塑性,TEM证实其形变机制均为形变诱导马氏体和孪生协同作用。当温度由20℃提高到650℃时,粗晶奥氏体不锈钢屈服强度和伸长率分别下降到180MPa和28.1%,超细晶奥氏体不锈钢屈服强度和伸长率分别为384MPa和24.2%。这表明在650℃高温环境下细晶强化作用仍然有效,粗晶和超细晶奥氏体不锈钢也有较好的塑性,其形变机制分别变为位错滑移和位错滑移+层错+孪生。     

Microstructural modification of austenitic stainless steels by rapid solidification

The microstructural modifications in three austenitic stainless steels (types 308, 310, and 312) were evaluated after rapid solidification. These three steels are commonly used weld filler metals. Two methods of rapid solidification were investigated, autogenous laser welding and arc-hammer splat quenching. The structure of 310 stainless steel was found to be 100 pct austenite, and did not vary over the range of conditions studied. On the contrary, the structures of types 308 and 312 steels were very sensitive to the cooling rates and solidification conditions. With the highest cooling rates, the type 308 structure was fully austenitic while the type 312 structure was fully ferritic. At lower cooling rates, the structures were duplex ferrite plus austenite. The results were interpreted in terms of faster kinetics of solidification of austenite compared to ferrite under the conditions examined. A comparison of the structures produced by the two rapid solidification techniques indicated the cooling rates are comparable.     

Stacking fault energies of seven commercial austenitic stainless steels

Metallurgical and Materials Transactions A - The stacking fault energies of seven commercial austenitic Fe-Cr-Ni, Fe-Cr-Ni-Mn and Fe-Mn-Ni alloys have been determined by X-ray diffraction line...