Stress Induce Martensitic Transformations in Hydrogen Embrittlement of Austenitic Stainless Steels

In austenitic type stainless steels, hydrogen concentration gradients formed during electrochemical charging and followed by hydrogen loss during aging, at room temperature, surface stresses, and martensitic phases α′-BCC and ε-HCP developed. The basic relationship between the X-ray diffraction peak broadening and the hydrogen gradients, formed during charging and aging at room temperature in such austenitic stainless steels, were analyzed. The results demonstrate that the impact of stresses must be considered in the discussion of phase transformations due to hydrogenation. Austenitic stainless steels based on iron-nickel-chromium, have relatively low stacking fault energy γ_SFE and undergo: quenching to low temperatures, plastic deformation, sensitization heat treatments, high pressure (≥3–5 × 10⁹ Pa) by hydrogen or other gases, electrochemical charging (when the sample is cathode) and when is irradiation by various ions the samples in vacuum. All the above mentioned induce formation of ε and α′ in the face-centered cubic (FCC) austenite γ matrix. The highest stresses cause formation of mainly α′ phase and ε-martensite, and both are involved in plastic deformation processes and promoting crack propagation at the surface. In 310 steel, the crack propagation is based on deformation processes following ε-martensitic formation only. Formations of ε- and α′-martensites were noted along the fracture surfaces and ahead of the crack tip. The cracks propagated through the ε-martensitic plates, which formed along the active slip planes, while α′ phase was always found in the high-stress region on the ends of the ligaments from both sides of the crack surfaces undergoing propagation.     

钛稳定化超纯铁素体不锈钢的屈服行为

以钛稳定化的超纯Crl7铁素体不锈钢为研究对象,系统研究了精轧温度对铁素体不锈钢薄板屈服行为的影响。经高温精轧和低温精轧后,两种工艺的热轧板采用相同的退火及冷轧工艺得到成品薄板。采用低温精轧后,拉伸过程中薄板的Ltiders应变降低至0．5％左右,与高温精轧相比减少大约66．7％。通过金相显微组织观察、X射线衍射检测、...     

提高铁素体不锈钢质量的主要措施

介绍了易于在铁素体不锈钢中出现的主要缺陷,包括室温敏感性高和起皱。总结了改善铁素体不锈钢质量的主要措施。     

Paraequilibrium Carburization of Duplex and Ferritic Stainless Steels

AISI 301 and E-BRITE stainless steels were subjected to low-temperature (743 K) carburization experiments using a commercial technology developed for carburization of 316 austenitic stainless steels. The AISI 301 steel contained ~40 vol pct ferrite before carburization but had a fully austenitic hardened case, ~20-μm thick, and a surface carbon concentration of ~8 at. pct after treatment; this “colossal” paraequilibrium carbon supersaturation caused an increase in lattice parameter of ~3 pct. The E-BRITE also developed a hardened case, 12- to 18-μm thick, but underwent a more modest (~0.3 pct) increase in lattice parameter; the surface carbon concentration was ~10 at. pct. While the hardened case on the AISI 301 stainless steel appeared to be single-phase austenite, evidence for carbide formation was apparent in X-ray diffractometer (XRD) scans of the E-BRITE. Paraequilibrium phase diagrams were calculated for both AISI 301 and E-BRITE stainless steels using a CALPHAD compound energy-based interstitial solid solution model. In the low-temperature regime of interest, and based upon measured paraequilibrium carbon solubilities, more negative Cr-carbon interaction parameters for austenite than those in the current CALPHAD data base may be appropriate. A sensitivity analysis involving Cr-carbon interaction parameters for ferrite found a strong dependence of carbon solubility on relatively small changes in the magnitude of these parameters. This article is based on a presentation given at the “International Conference on Surface Hardening of Stainless Steels,” which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS.

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Ridging Control in Transformable Ferritic Stainless Steels

An alloy design concept leading to an improved ridging resistance in the transformable ferritic stainless steels is introduced. It is based on achieving a small γ-phase fraction at the ingot soaking temperature. The γ-phase fraction is then increased to a maximum value during the early stages of hot rolling. The nucleation of γ-phase islands in the ferritic matrix increases the fraction of transformed ferrite. The multiplicity of crystallographic orientations resulting from the α→γ and γ→α transformations leads to a pronounced weakening of the as-cast texture and an increased ridging resistance.     

Hydrogen Embrittlement of Automotive Advanced High-Strength Steels

Advanced high-strength steels (AHSS) have a better combination between strength and ductility than conventional HSS, and higher crash resistances are obtained in concomitance with weight reduction of car structural components. These steels have been developed in the last few decades, and their use is rapidly increasing. Notwithstanding, some of their important features have to be still understood and studied in order to completely characterize their service behavior. In particular, the high mechanical resistance of AHSS makes hydrogen-related problems a great concern for this steel grade. This article investigates the hydrogen embrittlement (HE) of four AHSS steels. The behavior of one transformation induced plasticity (TRIP), two martensitic with different strength levels, and one hot-stamping steels has been studied using slow strain rate tensile (SSRT) tests on electrochemically hydrogenated notched samples. The embrittlement susceptibility of these AHSS steels has been correlated mainly to their strength level and to their microstructural features. Finally, the hydrogen critical concentrations for HE, established by SSRT tests, have been compared to hydrogen contents absorbed during the painting process of a body in white (BIW) structure, experimentally determined during a real cycle in an industrial plant.     

铁素体不锈钢410S与430高温相组织的Thermo-calc计算与试验研究

通过金相观察与DSC试验,对410S和430两种铁素体不锈钢的高温组织进行了研究,并通过定量分析,分别获得了高温相组织的体积分数及与温度的变化关系.利用Thermo-calc热力学软件,计算了两钢种的高温相图,计算结果和试验结果吻合较好.由于高温下铁素体不锈钢存在两相组织,钢的热塑性会降低,导致轧制时产生边裂,故探讨了通过热加工工艺优化来避免边裂的问题.     

439不锈钢析出行为对耐冷凝液腐蚀性能的影响

利用OM、SEM、EM和EDS研究了单钛稳定439不锈钢和Nb-Ti双稳定439不锈钢的析出行为对耐冷凝液腐蚀性能的影响,确定了两种钢中各种析出物的大小、成分、显微结构以及取向关系。对两种钢分别进行了5个周期、10个周期和20个周期的冷凝液腐蚀评价试验,并使用极值分析方法对冷凝液腐蚀试验后样品的最大点蚀深度进行了统计分...     

新型汽车排气系统用铁素体不锈钢的冷凝液腐蚀

为了开发既经济又具有良好耐冷凝液腐蚀性能的汽车排气系统用不锈钢,采用冷凝液腐蚀试验方法,对一系列的铁素体不锈钢进行了10周期的腐蚀评价研究。研究结果表明:Cr当量高于17%的铁素体不锈钢与Cr的质量分数为17%的铁素体不锈钢耐冷凝液腐蚀性能相当,且平均腐蚀失重量均小于6 g/m2,平均最大腐蚀深度均小于0.03 mm。在此试验结果的基础上,对新开发的439M型铁素体不锈钢和409L型铁素体不锈钢进一步开展5、10、20周期的冷凝液腐蚀试验,并使用极值分析方法对三种周期冷凝液腐蚀试验后样品的最大点蚀深度进行统计分析,研究结果表明,新开发的439M型铁素体不锈钢的预测寿命是409L的1.6倍。     

Embrittlement in CN3MN Grade Superaustenitic Stainless Steels

Superaustenitic stainless steels (SSS) are widely used in extreme environments such as off-shore oil wells, chemical and food processing equipment, and seawater systems due to their excellent corrosion resistance and superior toughness. The design of the corresponding heat treatment process is crucial to create better mechanical properties. In this respect, the short-term annealing behavior of CN3MN grade SSS was investigated by a combined study of Charpy impact tests, hardness measurements, scanning and transmission electron microscopy. Specimens were heat treated at 1200 K (927 °C) for up to 16 minutes annealing time and their impact strengths and hardnesses were tested. The impact toughness was found to decrease to less than the half of the initial values while hardness stayed the same. Detailed fracture surface analyses revealed a ductile to brittle failure transition for relatively short annealing times. Brittle fracture occurred in both intergranular and transgranular modes. SEM and TEM indicated precipitation of nano-sized intermetallics, accounting for the intergranular embrittlement, along the grain boundaries with respect to annealing time. The transgranular fracture originated from linear defects seen to exist within the grains. Close observation of such defects revealed stacking-fault type imperfections, which lead to step-like cracking observed in microlength scales.     

00Cr30Mo2铁素体不锈钢475 ℃脆性的研究

通过热处理调整00Cr30Mo2铁素体不锈钢的固溶碳含量，利用拉伸实验、扫描和透射电镜观察等研究了钢在475℃时效不同时间后的拉伸性能、变形和断裂行为。结果表明：固溶在基体中的碳能够抑制富Cr区的形成，推迟475℃脆性的发生，时效后钢的变形方式以位错的平面滑移为主，同时伴有一定的孪晶变形。     

铌、钛对445M铁素体不锈钢韧性的影响

 通过真空冶炼、锻造、热轧和不同固溶处理温度试验制备出含有不同微合金元素含量的445M铁素体不锈钢,结合其冲击试验,运用金相显微镜、扫描电子显微镜及能谱仪等分析方法,探讨了微合金化对其韧性的影响。结果表明：化学元素铌和钛的加入,在晶粒内部和晶界上形成了TiN、NbC和(Nb,Ti)(C,N)析出相,稳定了钢中的碳、氮元素,防止由铬的碳氮化物析出而引起的晶间腐蚀,但对钢的冲击韧性有一定影响。采用单铌作为稳定化元素钢的冲击韧性优于单钛稳定钢;添加少量的钛,采用Nb+Ti双稳定钢,也可获得优异的冲击韧性,并且能够降低生产成本。     

B443超低碳氮铁素体不锈钢的研究

为开发价格昂贵的SUS304奥氏体不锈钢的替代产品,试验研究了不同含量C N和Nb,Ti稳定化对B443系列铁素体不锈钢再结晶组织、力学性能、点蚀电位的影响.试验结果表明,Nb,Ti双稳定化提高了B443铁素体不锈钢的再结晶温度和点蚀电位.在试验室研究基础上,宝钢不锈钢分公司试制了B443NT超低碳氮铁素体不锈钢.该产品不仅具有与SUS304相当的耐腐蚀性能,而且具有优良的深拉伸性能和满意的焊接性能,可用于运输、建筑、厨具等众多行业.     

Inclusions for Ultra-pure Ferritic Stainless Steels Containing 21% Chromium

As stabilizing elements added into ultra-pure ferritic stainless steels, niobium and titanium react with carbon and nitrogen to form carbonitrides and have great effects on the ratio of equiaxed zone and the grain size of solidification structure of ingots, which remarkably affect the quality of cold-rolled sheets. Combined with thermodynamic calculation, style and precipitation progress of inclusions in ultra-pure ferritic stainless steels were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy. The results indicate that the inclusions are mainly Ti-Al-N-O system inclusions in ultra-pure ferritic stainless steels. Al₂O₃ starts to precipitate firstly and then TiO_x and TiN precipitates sequently. The inclusions are mainly single TiN particles and complex inclusions with Al₂O₃-Ti₂O₃ as cores and covered with TiN under the condition of 0.31% titanium addition and mainly Al₂O₃ under the condition of 0.01% titanium addition. A few (Nb, Ti) N particles precipitate because of no enough titanium to react with nitrogen when titanium addition is 0.01%. In addition, fine Nb(C, N) particles with size of less than 500 nm precipitate at relatively low temperature.     

稀土元素在铁素体不锈钢中的作用和应用前景

铁素体不锈钢(11%～30%Cr)的抗点蚀、抗应力腐蚀和抗高温氧化性能均优于奥氏体不锈钢。但普通铁素体不锈钢晶间腐蚀敏感性较高,塑性和韧性较低,焊接裂纹倾向较大。经分析得出,铁素体不锈钢加稀土元素可改善钢的凝固组织,影响碳、氮化物析出形态,细化晶粒,改变钢中硫化物形态和夹杂物成分,从而改善钢的横向韧性、焊接性能和疲劳性能。因此,稀土元素在铁素体不锈钢中的应用研究工作有广阔的前景。