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。     

The effects of H on the mechanical behavior of austenitic stainless steels at room temperature

A stable face-centered cubic (fcc) stainless steel was H charged by exposing it to 9.8 MPa H gas at 300 °C for 14 days. The concentration of H was determined to be 65 ppm. The mechanical testing at room temperature has shown that after H charging, the yield strength of the steel increases, and the behavior of the transient creep and the short-term stress relaxation becomes more intensive. All the results can be interpreted qualitatively, and part of the results can be quantitatively calculated by the theory of mobile dislocation density proposed by Alden. The analysis indicates that the essential effect of H is to obstruct the movement of dislocations, so that the strength and inelastic viscousness of the steel increase.     

Elevated temperature mechanical properties of shock-strengthened austenitic stainless steel

The mechanical properties of explosively shock-strenghened (320 kbar) Type 304 stain-less steel were investigated. Tensile tests were performed on the shock-strengthened material at temperatures ranging from room temperature to 650°C. The shock hardening caused large increases in yield strength, moderate increases in ultimate strength, and reductions in ductility. Annealing studies on the shock-strengthened stainless steel indicated that considerable amount of the shock-induced strength stainless steel indicated that considerable amount of the shock-induced strength increase is retained after long time anneals at 700 and 750°C. However, metallographic studies on the annealed material indicated the presence of fine carbide particles which may partly account for the strength of the material. Creep studies indicated that the minimum creep rate at 650°C was reduced by a factor of 1000 due to shock strengthening.     

Stress relaxation and mechanical equation of state in austenitic stainless steels

Stress relaxation experiments have been performed on types 304 and 316 stainless steel at room temperature. The stress-strain rate data show that these materials exhibit the behavior of mechanical equation of state. The experimental results are discussed using the concept of internal stress.     

Damping properties of austenitic stainless steels containing strain-induced martensite

Damping properties of two austenitic stainless steel grades, EN 1.4318 and EN 1.4301, were investigated. The test materials were cold rolled to different reductions and damping capacity was measured as a function of temperature with an internal friction method. Microstructures of the test materials were studied by means of X-ray diffraction (XRD) and magnetic measurements. The results showed that damping capacity of the studied materials depended on the amounts of strain-induced ε- and α′-martensite phases. At temperatures around 0 °C, highest damping capacity was achieved with cold rolling reduction of 10 to 15 pct. This behavior is related to the existence of ε-martensite and stacking faults. Internal friction peak due to α′-martensite phase was present at the temperature of 130 °C. Strain aging heat treatment at 200 °C for 20 minutes decreased the damping capacity in the entire studied temperature range.     

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.     

Stability and mechanical properties of some metastable austenitic steels

The relation between austenite stability and the tensile properties, as affected by testing temperature and processing, was studied for a series of alloys of increasing compositional complexity, viz., the Fe-Ni, Fe-Ni-C, and Fe-Ni-Cr-Mn-C systems. The “stress” and “strain induced” modes of transformation to martensite differed significantly in their influence on the shape of the stress-strain curve. Under certain testing conditions, unusually low yield strengths and high work hardening rates were observed in some of these alloys. Maxima in yield strengths were observed for all austenitic alloys containing carbon that were processed at deformation temperatures between 200° and 300°C. Evidence gleaned from electron microscopy and magnetic and mechanical testing suggested that the maxima were due to the formation of carbon atmospheres on dislocations during processing. The influence of austenite stability on the mechanical properties of steels, varied by systematic changes in test temperature (22° to -196°C), composition (8 pct, 12 pct, 16 pct, and 21 pct Ni) and deformation temperature (25° to 450°C), was evaluated quantitatively. An erratum to this article is available at .     

The effect of high temperature low cycle fatigue on the corrosion resistance of austenitic stainless steels

The effect of high temperature (650 °C) low cycle fatigue on the corrosion behavior of five austenitic stainless steels (Types 304, 316L, 321, and Incoloy Alloys 800 and 800H) has been investigated. For comparison, corrosion tests were also performed on samples of as-received material as well as material which had been solutionized and material which was sensitized at 650 °C. It was observed that cyclic loading at high temperature reduces the corrosion resistance to a much greater extent than does just the exposure of unstressed material to elevated temperatures. Formation of chrome carbides during cycling and depletion of chromium from the matrix is responsible for the decrease in corrosion resistance. Of the alloys tested, Type 304 exhibited the lowest corrosion resistance. Superior corrosion resistance of the other alloys was due to the following: (a) a lower carbon content, (b) a higher chromium content, and (c) the presence of a strong carbide forming element (stabilized material).     

Microstructure and mechanical properties of microwave sintered austenitic stainless steel

The objective of this work is focused on understanding the effect of microwave heating on sintering of 316L powders. The stainless steel samples were prepared from prealloyed powders of 316L. The powder samples were compacted at a pressure of 560 MPa and sintered at 1300°C in a microwave furnace of 2.4 GHz and 2KW capacity in nitrogen atmosphere. The sintering time was varied from 10 to 20 minutes in order to study the effect of sintering time on sintering behavior and mechanical properties of the sintered samples. The sintered samples were subjected to optical metallography, hardness testing, tensile testing and fractogrphy. The average density of sintered stainless steels was 92% of the theoretical density, approximately 18% increment from green density for 20 min. sintering time. Microstructural analysis showed the regularly distributed porosity with very small grains. The hardness value was in the range of 365VHN to 396VHN and tensile strength, in the range of 255MPa to 580 MPa. Fractographs for these steels revealed mixed mode of fracture.     

Microstructures and mechanical properties of melt-quenched ferritic stainless steels

Using the twin-roller type melt-quenching technique, the effects of rapid solidification on the microstructures and mechanical properties of ferrite have been examined in 17 pct Cr and 17 pct Cr-M steels where M = Ti, Nb, Si, or Al. The melt-quenched 17 pct Cr steels have the columnar and equiaxed grains which become finer with decreasing of specimen thickness. But the yield strength of these steels, which is about 0.48 GPa, remains almost unchanged with grain size refinement, contrary to predictions based on Hall-Petch’s relationship. The columnar structure of the melt-quenched 17 pct Cr-M steels is very fine on alloying with Ti and Si, while it is slightly coarse on alloying with Nb and Al. The hardness, yield strength, and fracture strength of these 17 pct Cr-M steels increase with increased amount of alloying element, these maximum values being about 460 DPN, 1.0 GPa, and 1.35 GPa, respectively, for 17 pct Cr- 6 pct Si steel, in the ductile compositional range. These are suitable values for a fine-gauge material having high hardness and strength as well as good ductility.     

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.     

High temperature phase chemistries and solidification mode prediction in nitrogen-strengthened austenitic stainless steels

Nitronic 50 and Nitronic 50W, two nitrogen-strengthened stainless steels, were heat treated over a wide range of temperatures, and the compositions of the ferrite and austenite at each temperature were measured with analytical electron microscopy techniques. The compositional data were used to generate the (γ + δ phase field on a 58 pct Fe vertical section. Volume fractions of ferrite and austenite were calculated from phase chemistries and compared with volume fractions determined from optical micrographs. Weld solidification modes were predicted by reference to the Cr and Ni contents of each alloy, and the results were compared with predictions based on the ratios of calculated Cr and Ni equivalents for the alloys. Nitronic 50, which contained ferrite and austenite at the solidus temperature of 1370 °C, solidified through the eutectic triangle, and the weld microstructure was similar to that of austenitic-ferritic solidification. Nitronic 50W was totally ferritic at 1340 °C and solidified as primary delta ferrite. During heat treatments, Nitronic 50 and Nitronic 50W precipitated secondary phases, notably Z-phase (NbCrN), sigma phase, and stringered phases rich in Mn and Cr.     

氮含量对无镍奥氏体不锈钢理化性能的影响

高氮无镍奥氏体不锈钢在多个领域有着广泛的应用前景,然而氮质量分数对其物理性能和化学性能的影响规律尚不十分清晰。设计并冶炼了氮质量分数为0.02%~1.20%的无镍奥氏体不锈钢,对不同氮质量分数钢的晶格常数、线膨胀系数、电阻率、耐汗液腐蚀性能、自腐蚀电位进行了检测和分析。结果表明,氮质量分数的增加使材料的晶格常数随着线性增大,获得了氮质量分数与晶格常数的定量关系。氮质量分数越高,材料的膨胀系数和电阻率越大。不同氮质量分数试验钢经720 h人工汗液腐蚀后,氮质量分数最低(0.02%)和最高(1.20%)时,试验钢严重腐蚀,而其他钢未被腐蚀。试验钢的自腐蚀电位首先随着氮质量分数的增加而升高,峰值出现在氮质量分数为1.05%时,氮质量分数为1.20%的钢其自腐蚀电位下降至低于氮质量分数为0.60%钢的水平。     

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.     

Analysis of the microstructure and mechanical properties of 309L austenitic stainless steel

309L is a kind of austenitic stainless steel.This paper discusses 309L,which was smelted and rolled in a lab.Its microstructural characteristics in states of as-cast,hot-rolling,cold-rolling and solid solution were investigated respectively by optical microscope (OM).It is found that 309L is composed of austenite and residual δ-ferrite under these conditions,and the δ-ferrite cannot be eliminated easily by heat-treatment.Its hot plasticity and deformation resistance were investigated in a series of heat simulation tests.Its mechanical properties under different cold reductions were studied.All these provide valuable experimental data for the industrial production.     

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.     

The effect of combined low-cycle fatigue and creep on the life of austenitic stainless steels

Experiments regarding the effect of simultaneous fatigue and creep on life have been performed for a 20 pct Cr-35 pct Ni stainless steel at 700°C. It was found that a life fraction rule, comprising an interaction term besides the terms of pure fatigue and creep damage, adequately accounts for the observed lives. The interaction term contains the product of the fatigue and creep damage, and may physically be interpreted as the damage generation due to the interaction of the fatigue and creep processes. There is evidence that the surface crack density represents a measure of the total damage. The cracks created during the combined fatigue and creep test were both transgranular and intergranular. They were probably nucleated intergranularly and later, during their growth, were transformed to transgranular cracks. As the creep damage of the test was increased gradually, more cracks remained intergranular throughout the test.