Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.     

Stress-Corrosion Fracture of 12Kh18AG18Sh Steel in Chloride-Containing Media with Copper Cations

We studied the corrosion and corrosion-mechanical resistance of new austenitic high-nitrogen 12Kh18AG18Sh steel for the retaining rings of turbogenerators in aqueous solutions containing chlorides and copper (II) cations. A significant decrease in the corrosion resistance of this steel (P constitutes 0.52–14.50 mm/year) and in its resistance to corrosion cracking and corrosion-fatigue fracture in CuCl₂ solutions was established. We also calculated the coefficients of linear parts of the kinetic diagrams of fatigue fracture of the steel under study in air with a humidity of 40% and in 22% NaCl and CuCl₂ solutions.     

Modulated structures of Fe–10Mn–2Cr–1.5C alloy

In Fe–10Mn–2Cr–1.5C alloy the superlattice diffraction spots and satellite reflections have been observed by transmission electron microscopy, these results show that the ordering structure and modulated structure have taken place in this alloy. X-ray diffraction proved that austenitic steel in this alloy is more stable than in traditional austenitic manganese steel. Based on this investigation, we consider that the C–Mn ordering clusters were existing in austenitic manganese steel and the chromium could strengthen this effect by linking the weaker C–Mn couples together. These structures may play an important role in the work hardening of austenitic manganese steel.     

Mechanical and Structural Properties of Similar and Dissimilar Steel Joints

The mechanical properties of specimens from similar and dissimilar weld joints were examined. A ferritic steel (St37-2) and an austenitic stainless steel (AISI 304) were joined by the gas tungsten arc weld (GTAW) process using an austenitic filler metal. Mechanical and metallographic properties of the specimens were obtained by means of microhardness testing, tensile testing, bending fatigue testing, and light optical and scanning electron microscopy. The highest microhardness values were recorded on the ferritic–austenitic dissimilar weld joint, whereas the highest tensile strength and bending fatigue life were obtained with the austenitic–austenitic joints. Ferritic and pearlitic structures were observed in the microstructure of the ferritic–ferritic joint. The microstructures of austenitic–austenitic and austenitic–ferritic joints showed small recrystallization grains in addition to the typical austenitic and ferritic structures. Scanning electron microscopy was used to observe the fracture surfaces of the specimens and the origins of the fatigue cracks.     

Measurements of the temperature rise ahead of a fatigue crack

Summary The temperature rise ahead of a fatigue crack has been measured using a Thermovision camera. Observations were made on three kinds of polymer and an austenitic stainless steel. The maximum temperature difference in macro-scale between the zone ahead of the crack and the bulk of the material was found to be, for the polymers, about 30°C at 725 N/min and amplitude o.1–0.8 kp/mm² and, for the steel, 14°C at 6000 N/min and amplitude 3.0–24.0 kp/mm².     

Evaluation of Mn substitution for Ni in alumina-forming austenitic stainless steels

There has been increasing interest in the substitution of low-cost Mn for Ni in austenitic stainless steels due to the rising price of Ni. This paper investigates the possibility of such a substitution approach for the recently developed alumina (Al₂O₃)-forming austenitic (AFA) class of heat-resistant stainless steels. Computational thermodynamic tools were utilized to predict the alloy composition range to maintain an austenitic matrix microstructure when Mn is substituted for Ni in the presence of Al, which is a strong body-centered-cubic (BCC) phase stabilizer. Phase equilibria, oxidation behavior, and creep properties of Fe–(10–14)Cr–(5–15)Mn–(4–12)Ni–(2.5–3)Al–Cu–Nb–C–B (in weight percent) based alloys were studied. The alloys based on Fe–14Cr–2.5Al–(5–9)Mn–(10–12)Ni exhibited the best balance of oxidation and creep resistance, which represents approximately 50% reduction in Ni content compared to previously developed AFA alloys. These low-Ni, high-Mn AFA alloys formed protective Al₂O₃ scales up to 973–1073 K in air and at 923 K in air with 10% water vapor. Creep-rupture lives of the alloys under a severe screening condition of 1023 K and 100 MPa were in the 7.2 × 10⁵–1.8 × 10⁶ s (200–500 h) range, which is comparable to or somewhat improved over that of type 347 stainless steel (Fe–18Cr–11Ni base).     

Effects of hydrogen on stress cracking in unheated thermal powder station pipes

Summary Reasons are examined for damage to unheated bends in thermal power station pipes made of steel 20 and steel 12Kh1MF. Corrosion and thermal fatigue are accompanied by creep and by a substantial contribution from the development of brittle cracks on account of hydrogen occluded by the steel from the two-phase or one-phase medium acting on the inner surfaces of the tubes.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 1, pp. 31–36, January–February, 1991.     

Conditions of invariance of corrosion crack resistance characteristics

Conclusions It has been established that for a number of materials the condition of invariance of fracture toughness depends not only upon the type of material but also upon its structure.For a high-strength steel with a martensitic structure the kinetics of subcritical crack growth and also the parameter K_Iscc are sensitive to the original austenitic grain size. Heat treatment for coarse grains has a favorable influence on the corrosion crack resistance of such a steel. The creation in coarse-grained steel of serrated austenitic grain boundaries leads to an additional increase in resistance to corrosion crack growth.The generally accepted criterion of invariance of fracture toughness Eq. (1) is unsuitable as a condition guaranteeing no change in corrosion crack resistance parameters. The value of the coefficient A_c in Eq. (2), which characterizes the condition of obtaining the parameter K_Iscc independent of sample thickness, is determined by the structure of the material and for the systems considered is more than 500.Corrosion cracks in steel with a martensitic structure may have a complex morphology dependent upon the subcritical crack growth mechanism, the size of the austenitic grains, and the form of their boundaries. In contrast to fine-grained and overheated steel, for which intergranular subcritical crack growth is characteristic, in steel with serrated grain boundaries the subcritical crack growth mechanism is more complex. It was also observed that in the center layers of thick samples there is primarily transgranular failure replaced by intergranular at the transition to the surface layers.To determine the effective stress intensity factor at the tip of a corrosion crack with a complex trajectory, a method based on determining the pliability of a sample with a crack propagating in a curved trajectory was found to be effective.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 17, No. 3, pp. 24–33, May–June, 1981.     

Corrosion behaviour of some stainless steel alloys in molten alkali carbonates (I)

In the present work it is aimed to study the corrosion behaviour of two types of stainless steel alloys (one ferritic and two austenitic) in molten Li₂CO₃- Na₂CO₃- K₂CO₃ mixture. This mixture is of interest in corrosion studies because of its low melting point (397°C) and good electrical properties. In this investigation the following techniques of measurements are used: (i) open circuit-potential, (ii) galvanic current, (iii) impedance, (iv) atomic absorption spectroscopy for the determination of the amount of metals dissolved in the melt (v) corrosion tests, carried out on the oxide scales formed during the oxidation of stainless steel alloys in carbonate melt. In this melt the electrode Ag/AgCl was used as a reference electrode. In molten carbonates, the oxide ions originate by self-dissociation according to the equilibrium CO₃ ^2– CO₂ + O^2–. The oxide ions, O^2–, and carbonate ions, CO₃ ^2–, play an important role in the oxidation process of these alloys and their passivation in the carbonate melt. As previously mentioned in references it can be assumed that the oxide scales formed on the alloy surface consist mainly of LiCrO₂ and LiFeO₂. The cathodic path of the corrosion process may be the reduction of CO₂ and/or CO₃ ^2–. The resistance of alloys against corrosion in melt increases with the increase of temperature. This may be due to the increase of concentration of O^2– and CO₂, enhancing both the anodic and cathodic reactions. The activation energy was calculated and found to be 91.496, 23.412 and 37.956 kJ/mol for the alloys 1, 2 and 3 respectively. The above mentioned techniques of measurements showed that the oxide scales of the austenitic stainless steel alloys (2, 3) are more passive and protective than of ferritic stainless steel alloy (1). This means that the resistance against corrosion, in the carbonate melts, of austenitic stainless steel alloys is higher than that of ferritic one.     

Fatigue Strength of Austenitic and Martensitic–Ferritic Steels for Nuclear Power Plants

We study the influence of boron-controlled media (12 g/liter H₃BO₃) with pH = 3 and 8 at temperatures of 293 and 353°K on the fatigue and cyclic crack resistance of 08Kh18N10T and 12Kh18N10T austenitic steels and 14Kh17N2 martensitic–ferritic steel. It is shown that 14Kh17N2 steel is characterized by the optimal combination of the long fatigue life with high cyclic crack resistance in inert and corrosive media at temperatures of 293 and 353°K, which gives it serious advantages over 08Kh18N10T and 12Kh18N10T austenitic steels.     

Force criterion for delayed failure of metastable austenitic steel

The phenomenon of delayed failure detected in metastable austenitic steels is connected with martensite formation during plastic deformation of austenite. In these steels a zone with a martensitic structure exhibiting a high level of residual internal microstresses develops ahead of a stress concentrator. In model representations a scheme has been suggested for distribution of deformation zones and stresses ahead of a stress concentrator. It is proposed that during delayed fracture a crack generates close to the boundary of plastically and elastically deformed martensite. A force criterion has been established for crack generation during delayed failure of metastable austenitic steel, i.e., the maximum local tensile stress (MLTS). The threshold value of the maximum local tensile stress does not depend on stress concentrator geometry and it is characterized by the tendency of metastable austenitic steel towards delayed failure.Central Scientific-Research Institute of Ferrous Metallurgy, Moscow. Translated from Problemy Prochnosti, No. 11, pp. 69–72, November, 1989.     

Cryogenic deformation microstructures of 32Mn–7Cr–1Mo–0.3N austenitic steels

The cryogenic deformation microstructures of impact and tensile specimens of 32Mn–7Cr–1Mo–0.3N austenitic steel were investigated using light microscopy and transmission electron microscopy. The results show that the deformation microstructures of the impact specimens are mainly composed of stacking faults, network dislocation, slip bands, and a few mechanical twins and -martensite. These microstructures cross with each other in a crystal angle. The deformation microstructures of the tensile specimens consist only of massive slip bands, in which a few mechanical twins and -martenite are located. Because of the larger plastic deformation the slip band traces become bent. All the deformation microstructures are formed on the {111} planes and along the <110> orientation.     

Influence of a lead melt on plastic deformation of high-alloyed heat-resistant steels

The effect of liquid-metal embrittlement in lead melt is typical of Kh12MVSFBR ferrite-martensitic steel and is observed in the temperature range of 300 – 500°C at a deformation rate of 8 · 10^–4 sec^–1. The maximum decrease in the plasticity of tempered specimens occurs at a temperature of 450°C. A decrease in the amount of -ferrite in the composition (martensite hardening without high-temperature annealing) results in a substantial decrease in plasticity and in broadening of the temperature range of embrittlement. 08Kh16N11S3MB austenitic steel is not affected by liquid metal embrittlement in the investigated temperature range for the deformation rates under consideration.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 4, pp. 71–76, July – August, 1994.     

Investigation of the wear resistance of metastable austenitic steels under dry sliding-friction conditions

It was shown that steels with a metastable austenitic structure are more resistant to wear in dry sliding friction than stable austenitic steels or quenched 1Cr13-type ferritic-martensitic steels. The relatively high wear resistance of metastable austenitic steels is associated with the friction-induced formation of martensite in their surface layers.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 5, No. 5, pp. 569–572, September–October, 1969.     

Surface damage in tes high-pressure boiler steam superheater tubes

Conclusion Metal-physics and metallographic tests have been made on tubes from a TP-100 boiler steam superheater, which shows damage on the outside from surface intercrystallite cracking. It is suggested that hydrogen affects the failure in 12Kh18N12T steel.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 2, pp. 118–121, March–April, 1991.     

不锈钢炉管运行近10000h后的氧化物检测及分析

采用氧化皮检测仪对国内首座超超临界百万千瓦机组运行近10000h后2号炉过热器和再热器的材料为SUP304H和HR3C钢管壁内部的氧化物进行检测。结果表明：少量部位局部堆积磁性物质,采用射线检测方法复核并直观显示其堆积形态。又通过SEM微观形貌分析及EDS分析发现,该物质应为施工或检修过程中遗留下来的小铁屑和锈蚀物等的氧化混合产物。综合结果表明：炉管内无运行过程产生的氧化物沉积,使用的材料抗蒸汽氧化性能较好。     

Structural strength of steels of the austenitic and transition classes at low temperatures

The article investigates the structural strength of the metastable austenitic steel 03Kh16N9M2 and of the steel of the transition class (martensite + residual austenite) 05Kh14N5DM in the temperature range from 293 to 77°K for the purpose of determining their suitability for large components in cryogenics. It is shown that for their characteristics of impact toughness and low-cycle fatigue steel 05Kh 14N5DM containing up to 15% residual austenite cannot be recommended as material of cryogenic structures operating at 77°K.Translated from Probiemy Procnnosti, No. 5, pp. 43–50, May, 1993.     

Effect of loading prehistory on phase transformations in chromium-nickel steel with plastic deformation

Results are given for a study of the kinetics of phase transformations in chromium-nickel austenitic steel with repeated plastic deformation including under conditions when prior and repeated loading differ in temperature and form of stressed state. It is established that prior low-temperature deformation intensifies the process of martensite formation at room temperature whereas deformation under room temperature conditions has almost no effect on martensitic transformation kinetics at low temperature.Translated from Problemy Prochnosti, No. 10, pp. 46–50, October, 1991.     

Fretting wear study of surface modified Ni–Ti shape memory alloy

A combination of shape memory characteristics, pseudoelasticity, and good damping properties make near-equiatomic nickel–titanium (Ni–Ti) alloy a desirable candidate material for certain biomedical device applications. The alloy has moderately good wear resistance, however, further improvements in this regard would be beneficial from the perspective of reducing wear debris generation, improving biocompatibility, and preventing failure during service. Fretting wear tests of Ni–Ti in both austenitic and martensitic microstructural conditions were performed with the goal of simulating wear which medical devices such as stents may experience during surgical implantation or service. The tests were performed using a stainless steel stylus counter-wearing surface under dry conditions and also with artificial plasma containing 80 g/L albumen protein as lubricant. Additionally, the research explores the feasibility of surface modification by sequential ion implantation with argon and oxygen to enhance the wear characteristics of the Ni–Ti alloy. Each of these implantations was performed to a dose of 3 × 10¹⁷ atom/cm² and an energy of 50 kV, using the plasma source ion implantation process. Improvements in wear resistance were observed for the austenitic samples implanted with argon and oxygen. Ion implantation with argon also reduced the surface Ni content with respect to Ti due to differential sputtering rates of the two elements, an effect that points toward improved biocompatibility.