Creep and creep rate curves of a 10cr-30mn austenitic steel during carbide precipitation

The effect of carbide precipitation on creep and creep rate curves was investigated for 10Cr-30Mn austenitic steel containing 0.003 to 0.55 wt pct carbon. After solution annealing, the specimens were subjected to creep testing at 873 K for up to 30 Ms (8300 hours). In the low-carbon steels containing below 0.1 wt pct carbon, where carbide precipitation scarcely occurred, the decrease in creep rate with time in the transient creep region was described by log έ = A - (1/3) log t, where A is a constant depending on stress and carbon concentration. On the other hand, in the high-carbon steels containing above 0.2 wt pct carbon, where extensive precipitation of M₂₃C₆ occurred, the creep rate decreased significantly at long times above 3 to 5 ks (1 hour), deviating from the preceding equation for the low-carbon steels. The Johnson-Mehl equation with the time exponent n = 2/3 provided a reasonable approximation for the significant decrease in creep rate at long times. This resulted from a stress-induced precipitation of M₂₃C₆ on dislocation lines produced by creep deformation. The rate constant of the Johnson-Mehl equation depended on carbon concentration but not on stress levels examined.     

18-18-0.5N高氮奥氏体不锈钢冷轧变形过程中的组织演变和形变强化

利用XRD、TEM、SEM和拉伸实验等分析测试方法研究了18-18-0.5N高氮奥氏体不锈钢在冷轧变形过程中的组织演变和形变强化规律.结果发现:18-18-0.5N高氮奥氏体不锈钢在冷变形过程中并未发生形变诱导相变,微观组织和力学性能在变形量为43.8%附近出现转折.提出了屈服强度随应变变化的预测方程.     

Precipitation of carbide and its effects on the properties of nickel-saving austenitic stainless steel

In this work,the effects of temperature and cooling rate on the precipitation of carbides in nickel-saving metastable austenitic stainless steel were studied.The test results show that the temperature range of carbide precipitation in the test steel was 500-950℃,and 750℃was the most sensitive temperature.However,when completely solution treated samples were cooled from high to room temperature at a cooling rate of more than 50 K/s,no carbides precipitated.The carbide precipitates increased the yield strength but decreased the corrosion resistance of the steel,with little impact on toughness.     

Influence of alloying by nitrogen on the strength and austenite stability of X18H10 steel

The strengthening and change in austenite stability when steel of X18H10 type alloyed by nitrogen are investigated, for applications where corrosion-resistant structural steel must operate satisfactorily at both high and cryogenic temperatures. Alloying by nitrogen may be regarded as a promising means of increasing the strength and stability of austenitic stainless steel. Preliminary cold or hot working increases the likelihood of martensite formation under load and consequently limits the working temperature range of the steel. High-strength nonmagnetic nitrogen steel based on X18AH10 steel with up to 0.22% N may be used for undeformable components at cryogenic temperatures. Without nitrogen, deformational martensite is always formed in such steel at temperatures below ?70°C. High strength, plasticity, and ductility may only be ensured in such steel by means of the TRIP effect or reduction in the grain size. Nitrogen effectively strengthens the solid solution in the high-temperature state. The use of combined high-temperature deformation and moderate-temperature deformation permits additional strengthening of the steel during thermomechanical treatment, including strain aging, which is effective where thermostable steel is required.     

The microstructural,mechanical, and fracture properties of austenitic stainless steel alloyed with gallium

The mechanical and fracture properties of austenitic stainless steels (SSs) alloyed with gallium require assessment in order to determine the likelihood of premature storage-container failure following Ga uptake. AISI 304 L SS was cast with 1, 3, 6, 9, and 12 wt pct Ga. Increased Ga concentration promoted duplex microstructure formation with the ferritic phase having a nearly identical composition to the austenitic phase. Room-temperature tests indicated that small additions of Ga (less than 3 wt pct) were beneficial to the mechanical behavior of 304 L SS but that 12 wt pct Ga resulted in a 95 pct loss in ductility. Small additions of Ga are beneficial to the cracking resistance of stainless steel. Elastic-plastic fracture mechanics analysis indicated that 3 wt pct Ga alloys showed the greatest resistance to crack initiation and propagation as measured by fatigue crack growth rate, fracture toughness, and tearing modulus. The 12 wt pct Ga alloys were least resistant to crack initiation and propagation and these alloys primarily failed by transgranular cleavage. It is hypothesized that Ga metal embrittlement is partially responsible for increased embrittlement.     

Fracture in stress waves and recompaction during the convergence of austenitic steel 12Kh18N10T shells

Austenitic steel 12Kh18N10T shells subjected to explosive loading are studied by metallographic methods. It is found that two spalls, which stop during convergence before focusing, form in loading according to the first experimental version. In the second version of loading, the flight of the explosion products is confined, and the convergence of a shell is provided up to a deep radius. Nevertheless, spall and shear damages are only partly recompacted. The detected changes in the microstructure and properties of the shells are related to the γ α ′ phase transformation during tension and to dynamic recrystallization and melting during subsequent high-rate deformation.     

Correlation between precipitation reactions and bulk density changes in type 18-12 austenitic stainless steels

Dimensional changes in austenitic stainless steels exposed to fast neutron fluxes have been attributed mainly to void formation and thermal and irradiation induced creep. In this in-vestigation, immersion bulk density measurements were performed on variously preheated AISI Types 304, 316L and 316 stainless steels. The results show that the density changes accompanying sequential precipitation of the various carbide (M₂₃C₈, M₆C) and intermetallic phases (sigma, chi, eta) during thermal aging can be comparable to those attributed to void formation at low fast neutron fluences. Vacuum melted Type 316 stainless steel always den-sified with aging at 725°C or 810°C and exhibited higher rates of precipitation and recovery of cold work during aging than the equivalent air melted steel. The direction of the density change in the air melted material was dependent upon the prior thermomechanical treat-ments. This behavior can be rationalized on the basis of a higher impurity content in the air melted material. A mathematical model was constructed which successfully explains the variation of bulk density due to sequential precipitation in Type 18-12 austenitic stain-less steels.     

Effects of tantalum on austenitic transformation kinetics of RAFM steel

The RAFM (reduced activation ferritic/martensitic) steels containing different tantalum contents (0 wt.％,0.027 wt.％,0.073 wt.％) were designed and cast.Differential scanning calorimetry and optical microscopy were employed to explore the influence of tantalum content on the austenitic transformation of RAFM steels.The austenitic transformation kinetics was described by a phase-transformation model.The model,involving site saturation nucleation,diffusion-controlled growth and impingement correction,was established based on the classical Johnson-Mehl-Avrami-Kolmogorov model.The phase-transformation kinetics parameters,including D0 (pre-exponential factor for diffusion) and Qd (activation energy for diffusion),were calculated by fitting the experimental data and the kinetic model.The results indicated that the average grain size is decreased with the increase of tantalum.The values of Ac1 and Ac3 (onset and finish temperature of austenitic transformation,respectively) are increased by increasing the tantalum content.The increase of tantalum caused the decrease of D0.However,Qd is increased with the increase of tantalum.In addition,as a carbides forming element,tantalum would reduce the carbon diffusion coefficient and slow down the austenitic transformation rate.     

Effects of tungsten on the oxidation resistance and high-temperature strength of 18CrNbTi ferritic stainless steel

18CrNbTi ferritic stainless steel is a low-cost material mainly used for the fabrication of manifolds,which usually work at temperatures below 950 ℃.With the development of engine technology,exhaust manifolds tend to work above 1 000 ℃ and this may be even higher in the future.For developing a new kind of steel to satisfy these requirements,the effects of tungsten(W) addition on the high-temperature strength and oxidation resistance of18 CrNbTi ferritic stainless steel are discussed in this study.The test results show that W enhances high-temperature strength at 1 000 ℃ and significantly improves oxidation resistance.However,W addition tends to degrade oxide layer adhesion,causing spalling during alternate hot and cold conditions.     

Effect of hot working on structure and strength of type 304L austenitic stainless steel

The development of microstructure and strength during forging in a single-phase austenitic stainless steel, 304L, was investigated by means of forward extrusion of cylindrical specimens. The temperature, strain, and strain rate of deformation were varied. A low strain rate was imparted by press forging (PF), and a high strain rate by high-energy-rate forging (HERF). Low forging temperatures produced dynamically recovered microstructures and monotonic increases in strength with increasing strain for low and high strain rates. At higher forging temperatures, the high-energy-rate-forged material exhibited softening, after the application of a critical amount of strain, as a result of static recrystallization which occurred within a few seconds after cessation of deformation. Analysis of isothermal compression test data, specifically the strain-to-peak stress associated with the onset of dynamic recrystallization, confirmed that dynamic recrystallization would not be expected for the deformation conditions imposed during forward extrusion in this study. Recrystallized grain size was found to vary uniquely with strain, initial grain size, and the Zener-Hollomon parameter. Recrystallization was much less prevalent in press-forged material and may have been affected by die chilling as well as the predominance of dynamic recovery. The variation of strength, recrystallized grain size, and extent of recrystallization with the deformation parameters, temperature and strain, are presented as a set of processing-property maps for each forging technique (έ). The findings are discussed in the context of developing process design criteria for forging alloy 304L.     

Effect of cold rolling on the structure and mechanical properties of austenitic corrosion-resistant 10Kh18N8D3BR steel

The effect of section rolling of austenitic corrosion-resistant 10Kh18N8D3BR steel at room temperature on its structure and mechanical properties is studied. During section rolling, the steel acquires a lamellar-type structure consisting of α′-martensite lamellas and retained austenite, and the fraction of α′ martensite increases to 70% at a true strain ? ≈ 4. In the initial state, the yield strength of the steel is 285MPa and the relative elongation is 60%. Cold plastic deformation to ? ≈ 0.4 increases the yield strength to 1010 MPa. Further deformation is accompanied by higher hardening of the steel: the yield strength increases to 2050 MPa at ? ≈ 4, and the relative elongation decreases to 2%.     

Mechanical, tribological, and corrosion properties of stainless steel of the austenitic class grade Kh23N18 with additions of chromium carbide

The mechanical, tribological, and corrosion-resistance properties of stainless steel Kh23N18 with additions of 2.5 and 10% Cr₃C₂ are studied. It is established that chromium carbide reduces the ductility properties and in most cases steel strength, but there is an increase in hardness and tribological properties with a Cr₃C₂ content of 10%. There is a slight reduction in corrosion resistance but in some cases it may be improved.     

Z2CN19-10(N)奥氏体不锈钢超厚超重板材的开发

针对核电关键设备广泛采用的Z2CN19-10（N）超厚超重板材,设计了合理的生产工艺路径,研究了相关的成分设计、模铸工艺、锻造工艺、表面质量控制和板形控制等关键生产技术,成功开发了超厚超重核电板材。成品板材的全面评价结果表明,板材整体的晶粒度、夹杂物控制水平、力学性能、耐腐蚀性能和无损探伤性能完全满足核电行业广泛采用的RCC-M标准的各项指标要求。     

Structural strength and corrosion resistance of nanostructured steel 10

The ultrafine-grain steel 10 obtained by equal-channel angular pressing at 200°C is investigated. The structural strength of the steel samples is analyzed, with tensile tests and assessment of the impact strength and crack resistance. The strength of steel increases by a factor of 2.5 after equal-channel angular pressing. In the specified conditions, such treatment does not change the cold-brittleness threshold of steel 10. However, the interval of ductile-brittle transition becomes narrower. Calculations show that the crack resistance K _1C of steel 10 with an ultrafine grain structure is somewhat greater than that of the regular steel. The corrosion rate of uncoated steel 10 samples after equal-channel angular pressing is somewhat greater than in the initial state. However, the corrosion rate of coated steel 10 samples after equal-channel angular pressing is half that in the initial state. The significance of these findings is discussed.     

A study on fractography in the low-temperature brittle fracture of an 18Cr-18Mn-0.7N austenitic steel

The fracture mode and crack propagation behavior of brittle fracture at 77 and 4 K in an 18Cr-18Mn-0.7N austenitic stainless steel were investigated using optical and scanning electron microscopy. The fracture path was examined by observing the side surface in a partially ruptured specimen. The relationship of the fracture facets to the microstructures was established by observing the fracture surface and the adjacent side surface simultaneously. Three kinds of fracture facets were identified at either temperature. The first is a smoothly curved intergranular fracture facet with characteristic parallel lines on it. The second is a fairly planar facet formed by parting along an annealing twin boundary, a real {111} plane. There are three sets of parallel lines on the facet and the lines in different sets intersect at 60 deg. The third is a lamellar transgranular fracture facet with sets of parallel steps on it. Fracture propagated by the formation of microcracks on a grain boundary, annealing twin boundary, and coalescence of these cracks. The observation suggests that the ease of crack initiation and propagation along the grain boundary and the annealing twin boundary may be the main reason for the low-temperature brittleness of this steel. A mechanism for grain boundary cracking, including annealing twin boundary parting, has been discussed based on the stress concentration induced by impinging planar deformation structures on the grain boundaries.     

合金元素对高强耐候钢耐大气腐蚀行为的影响

为了考察氧含量对钢耐蚀性能的影响,冶炼了不同氧含量（质量分数在20×10⁻⁶～200×10⁻⁶范围）的碳钢和耐候钢. 通过扫描电镜夹杂物分析、极化实验、全浸实验等方法研究了钢中夹杂物类型、形态、数量、尺寸等随氧含量变化而变化的规律,以及对耐蚀性能的影响. 结果表明,随着钢中氧含量逐渐增大,钢中夹杂物由长条状MnS、Al₂O₃向颗粒状硅酸盐转变,夹杂物总数量、平均尺寸逐渐增大,譬如氧质量分数从20×10⁻⁶、60×10⁻⁶增大到195×10⁻⁶时,MnS数量占比从69.9%、23.7%减少到5.8%,硅酸盐数量占比从3.4%、54.9%增大到73.2%,夹杂物总面积分数从0.01%、0.04%增大至0.25%,等效圆直径从0.78 µm、1.15 µm增大至4.65 µm;点蚀电位呈升高趋势,整体升高40 mV左右;腐蚀速率先下降又回升,遵从三次函数变化规律,其中氧质量分数从20×10⁻⁶～30×10⁻⁶增大到60×10⁻⁶,碳钢腐蚀速率降低53%,耐候钢腐蚀速率降低24%,耐蚀性均提高. 分析认为,氧质量分数在20×10⁻⁶～100×10⁻⁶范围,易诱发腐蚀的长条状硫化物减少以及固溶氧增多而引起基体电位升高的共同作用导致在全浸腐蚀环境下钢的耐蚀性增强;氧质量分数在100×10⁻⁶～200×10⁻⁶范围,夹杂物的数量急剧增多使得钢的耐腐蚀性降低. 适当增大氧含量,可开发经济型耐腐蚀钢.

     

Formation of a submicrocrystalline structure in austenitic 08Kh18N10T steel during equal-channel angular pressing followed by heating

The structure and mechanical properties of austenitic 08KhN10T steel subjected to equal-channel angular pressing (ECAP) at room temperature (? = 3.2) and subsequent heating are studied. In the course of ECAP, the steel undergoes a martensitic transformation; the martensite content reaches 45%. Upon heating, martensite (ferrite) transforms into austenite. The partly submicrocrystalline oriented structure of the 08Kh18N10T steel in the austenitic (55%)-martensitic (45%) state (formed upon ECAP) provides its high strain hardening (σ_0.2 = 1315 N/mm²), as compared to the initial state (σ_0.2 = 250 N/mm²), and high plasticity δ = 11%. After heating to 550°C, the steel has a predominantly submicrocrystalline austenitic (80%)-ferritic (20%) structure, σ_0.2 = 1090 N/mm², and δ = 11%.     

Improvement of production process for 08-12Kh18N10T steel

The Chelyabinsk Metallurgical Combine is one of the primary suppliers of stainless steel on the domestic market. In 2007, approximately 57000 metric tons of stainless steel were produced, of which 83.3% were austenitic steels and 10.9% were ferritic steels. The processes and equipment used support the production of cold-rolled sheet with up to 65% having surface quality M2A or M2B. The combine is continuously working to improve the quality of stainless-steel rolled products. The following measures were implemented in 2006–2007 In electric steel smelting shop No. 6 ? renovation/upgrade of the DSP-100I7 for transition to eccentric bottom tapping ? addition of a pure-argon blowdown period on the argon-oxygen carburizer, transfer of the ferronickel admixture from the DSP to the argon-oxygen carburizer ? modification of the secondary cooling zone in the continuous-casting machine (a lining was added to the crystallizer, and the crystallizer rocking amplitude was increased from ±3 to ±6 mm) In rolling shop No. 4 ? the quenching furnace for thick sheet stainless steel ? preparations are currently under way for a phased renovation/upgrade with a total capital investment of 300 million US dollars.