Time-independent mechanical properties of a chromium-manganese austenitic steel weldment

Room and elevated (450° C) temperature tensile data and room temperature notch impact data on weld metal and a welded joint of 10 wt % Cr-17.5 wt % Mn austenitic steel, a candidate material for thermonuclear fusion reactor first wall and blanket structures, are reported and discussed in terms of the microstructural features observed.     

Discontinuous precipitation of Cr2N in a high nitrogen,chromium-manganese austenitic stainless steel

The main purpose of the present work is to study the effect of a high nitrogen content (1 wt% N), on the microstructural evolution of a Cr-Mn austenitic stainless steel aged over the [400–900 °C] temperature interval. Thermal treatments carried out between 700 and 900 °C lead to the decomposition of the nitrogen supersaturated austenitic matrix by discontinuous precipitation of Cr₂N particles. The microstructural features of the reaction are described and analysed. In the present case, the cellular precipitation of Cr₂N is a peculiar and complex phenomenon which involves two diffusion mechanisms: the diffusion of an interstitial element (nitrogen) and the diffusion of a substitutional one (chromium). The nucleation of the discontinuous precipitation arises from a reduction of the surface energy of the precipitates. Furthermore, the precipitation growth is a non-steady state process, because the reaction is governed at first by the intergranular diffusion of chromium, and then tends to be controlled by its bulk diffusion. Consequently, the features of this discontinuous precipitation do not fit in with the assumptions of usual theories, which have been established for binary substitutional systems that transform in steady state conditions. This discontinuous precipitation brings about a slight hardening. Then, the hardness of the aged samples can be described by an additive relationship between the hardness of the precipitation cells and that of the untransformed matrix. Beside the discontinuous precipitation of Cr₂N, sigma phase forms with significant volume fractions.     

耐酸性高Mn奥氏体钢的实验研究

为适应特殊油气开采环境的复杂工况条件,提高设备的使用寿命和安全性,降低开采成本,不同于常规管线钢低C低Mn的合金设计思路,采用高C高Mn成分体系获得了综合性能优异的新型耐酸性奥氏体钢.通过拉伸实验、冲击试验以及氢致开裂实验等方法对其综合性能进行研究,并利用光学显微镜、扫描电镜、透射电镜等手段对高Mn奥氏体钢的组织进行了观察分析.研究结果表明:实验钢抗拉强度达到1 153 MPa,屈强比仅为0.46,伸长率高达50%,-40℃冲击功达到123 J,同时A溶液条件下经96 h浸泡未发现氢鼓泡及裂纹.实验钢显微组织为单相奥氏体组织,组织中存在大量位错、层错以及孪晶.与常规管线钢相比较,实验钢具有低屈强比、高均匀塑性变形的优点.此外,奥氏体组织的溶氢能力极强,本实验钢具有优良的抗氢致开裂腐蚀性能.     

Development of an austenitic structural steel

In this investigation a new low alloy austenitic structural steel has been developed. This steel after melting and casting and hot rolling has a pearlitic microstructure which can be spherodize annealed to attain good machinability. After austenitizing at 1150 °C (2100 °F) and water quenching, it has a fully austenitic (100%) microstructure. Because of the fully austenitic structure, it is non-ferromagnetic. The initial study on this experimental alloy indicates that this new steel has reasonably good strength, fracture toughness and ductility. A major application of this steel is expected to be in power generation devices such as turbines, generators, etc. There will be very little power loss in power generation devices due to low permeability and the non-magnetic nature of austenite.     

Effect of low temperature on resistance of stable austenitic steel to embrittlement by hydrogen

Abstract

Tensile straining of a ‘stable’ austenitic stainless steel at subambient temperatures has revealed deformation induced transformation to martensite reaching a maximum at about 200 K. Although the particular steel concerned is only marginally embrittled by hydrogen charging at ambient temperature, the transformation to martensite coincides with increasing embrittlement at lower temperatures. The recovery of a resistance to embrittlement below 215 K is attributed to the decreasing transport of hydrogen by moving dislocations as the temperature is further decreased.

MST/1701     

Corrosion-cracking resistance of austenitic stainless steel under stress in an iodine medium

The critical iodine concentration (C_cr) is determined for steel KhNS-4 (steel OKH16N15M3B after vacuum remelting and and microalloying with Sc) at 923°K to assess the feasibility of its use as a jacket material for the fuel cells of fast reactors. The value of C_cr is established from criteria of residual deformation at failure, longevity, and creep rate under a given stress. Experiments are conducted under ordinary and in reactor conditions. It is noted that creep rate is the most sensitive criterion. It increases sharply as the iodine concentration reaches values of 10–15 mg/cm², depending on the stress level, whose growth is accompanied by an increase in the value of C_cr It is demonstrated that radiation damage improves resistance to corrosion cracking. It is concluded that the C_cr values obtained exceed the corrodent concentration actually obtained by many times.Translated from Problemy Prochnosti, No. 12, pp. 12–17, December, 1993.     

Mechanical properties of austenitic stainless steel with high niobium contents

To better understand the influence of the microstructural characteristics on the mechanical properties of Fe-Cr₂₂-Ni₂₅-Nb_x austenitic stainless steel (ASS-Nb_x), the mechanical properties were investigated. Nb addition was conducted with four different amounts: 0, 0.29, 0.58 and 0.86%. With the increasing Nb content, the mean grain size for the Fe-Cr-Ni-based alloys decreased, while the size of Nb precipitate increased. Owing to the different microstructural characteristics, their mechanical properties were altered. The highest tensile strength was obtained for ASS-Nb_0.29 alloy. However, with the increasing Nb content in ASS-Nb_0.29–0.86 alloys, the tensile strength decreases despite the grain refinement. The mechanical properties of the ASS-Nb alloys were influenced by the Nb precipitation hardening and the grain boundary cohesive strength, arising from the size of Nb precipitations.     

Intergranular corrosion resistance of nanostructured austenitic stainless steel

Nanostructured metals and alloys possess very high strength but exhibit limited plasticity. Enhancement of the strength/ductility balance is of prime importance to achieve wide industrial applications. However, post-deformation heat treatment, which is usually used to improve plasticity, can lead to a decrease in other properties. In the case of austenitic stainless steels, heat treatment in the range from 480 to 815 °C can increase their susceptibility to intergranular corrosion. The aim of the work reported in this paper was to determine if nanostructured austenitic stainless steel is susceptible to intergranular corrosion if heat treated for 1 h at 700 °C. Samples of 316LVM austenitic stainless steel were hydrostatically extruded, in a multi-step process with the total true strain of 1.84 to produce a uniform microstructure consisting of nanotwins. These nanotwins averaged 21 nm in width and 197 nm in length. Subsequent annealing at 700 °C produced a recrystallised structure of 68-nm-diameter nanograins. The heat treatment improved the ductility from 7.8 to 9.2 % while maintaining the ultimate tensile strength at the high level of 1485 MPa. Corrosion tests were performed in an aqueous solution consisting of 450 ml concentrated HNO₃ and 9 g NaF/dm³ (according to ASTM A262-77a). The evaluation of the corrosion resistance was based on transmission and scanning electron microscopic observation of the microstructure and chemical analyses. The results revealed that both the as-received and HE-processed samples are slightly susceptible to the intergranular corrosion after annealing at 700 °C for 1 h.     

Microstructural thermostability of high nitrogen austenitic stainless steel

A high nitrogen austenitic stainless steel, Fe–24Mn–18Cr–3Ni–0.62N, was isothermally annealed from 750 to 900 °C for different times to investigate the thermostability of its microstructures. Results show that the precipitates were Cr₂N and initially formed along the grain boundary. The time–temperature–precipitation diagram was established, according to which the critical cooling rate of this material with less than 0.1–0.5 vol.% of precipitated Cr₂N was specified as 30 °C/min. The microhardness of the matrix in samples annealed at different temperatures decreased with the diffusion of nitrogen from the matrix to grain boundaries.     

Dependence of corrosion resistance on grain boundary characteristics in a high nitrogen CrMn austenitic stainless steel

Processing schedules for grain boundary engineering involving different types of cold deformation(tension, compression, and rolling) and annealing were designed and carried out for 18Mn18Cr0.6N high nitrogen austenitic stainless steel. The grain boundary characteristic distribution was obtained and characterized by electron backscatter diffraction(EBSD) analysis. The corrosion resistance of the specimens with different grain boundary characteristic distribution was examined by using potentiodynamic polarization test. The corrosion behavior of different types of boundaries after sensitization was also studied.The fraction of low-∑ boundaries decreased with increasing strain, and it was insensitive to the type of cold deformation when the engineering strain was lower than 20%. At the strain of 30%, the largest and smallest fractions of low-∑ boundaries were achieved in cold-tensioned and rolled specimens, respectively. The fraction of low-∑ boundaries increased exponentially with the increase of grain size. The proportion of low-∑ angle grain boundaries increased with decreasing grain size. Increasing the fraction of low-∑ boundaries could improve the pitting corrosion resistance for the steels with the same grain size.After sensitization, the relative corrosion resistances of low-∑ angle grain boundaries, ∑3 boundaries, and ∑9 boundaries were 100%, 95%, and 25%, respectively, while ∑27 boundaries, other low-∑ boundaries and random high-angle grain boundaries had no resistance to corrosion.