超级马氏体不锈钢的等离子渗氮和氮碳共渗

超级马氏体不锈钢(SMSSs)是典型的新一代13%Cr马氏体钢,含碳量较低,并含镍和钼,故具有更好的可焊性和低温韧性。研究表明,不锈钢低温等离子渗氮或氮碳共渗可形成硬的表层从而提高耐磨性。本文对SMSS试样分别在400℃、450℃和500℃进行了等离子渗氮和氮碳共渗,并对处理后的SMSS试样分别采用光学显微镜、显微硬度、XRD和干磨损试验进行了表征。X射线衍射分析证明,氮化铬含量随着渗氮和氮碳共渗温度的升高而增加,也显示出铁和铬的碳化物含量随着处理温度的升高而增加。不同温度下处理后的试样均随着处理温度的升高,磨损体积减小、耐磨性提高。对经等离子处理和未处理的试样所观察到的主要磨损机制都是凿削磨损。     

Plasma nitriding and nitrocarburising of a supermartensitic stainless steel

Abstract

Supermartensitic stainless steels (SMSSs) are a new generation of the classic 13%Cr martensitic steels, lower in carbon and with additional alloying of nickel and molybdenum offering better weldabilty and low temperature toughness. Several works have shown that plasma nitriding and nitrocarburising of stainless steels at low temperatures produces a hard surface layer which results in increased wear resistance. In this work, SMSS samples were plasma nitrided and nitrocarburised at 400, 450 and 500°C. The plasma treated SMSS samples were characterised by means of optical microscopy, microhardness, X-ray diffraction and dry wear tests. The thickness of the layers produced increases as temperature is raised, for both plasma nitriding and nitrocarburising. X-ray diffraction demonstrates that the chromium nitride content grows with temperature for nitriding and nitrocarburising, which also showed increasing content of iron and chromium carbides with temperature. After plasma treating, it was found that the wear volume decreases for all temperatures and the wear resistance increased as the treatment temperature was raised. The main wear mechanism observed for both treated and untreated samples was grooving abrasion.     

Development of delta ferrite on the weld and HAZ produced by pulsed plasma arc welding on a supermartensitic stainless steel

Delta ferrite is considered an undesirable phase in the microstructure of martensitic stainless steels, given its power to influence the performance of fatigue strength, toughness and corrosion under stress. This study aimed to investigate the effect of pulsed plasma welding on the distribution and amount of delta ferrite on a supermartensitic stainless steel (SMSS). Weld beads were obtained by surface melting on a sample of SMSS alloy using conventional and pulsed current. Then, the microstructural characterization was performed, in particular of the delta ferrite via metallographic analyses by optical and scanning electron microscopy. The experiments showed that for the same heat input, the pulse current promoted the formation of delta ferrite in the form of bands along the weld bead and a significant reduction of this phase in the heat-affected zone compared to conventional welding.     

Influence of Prior Austenite Grain Size on the Degree of Temper Embrittlement in Cr-Mo Steel

Reversible temper embrittlement has been frequently observed in many different low alloy steels serving at high temperature, e.g. order of 500 °C. This type of embrittlement can change the brittle transgranular fracture mode to intergranular decohesion with subsequent change in fracture stress and fracture toughness. The present paper deals with the influence of the prior austenite grain size and isothermal aging time on the degree of embrittlement of 2.25Cr-1Mo steel, which is very popular for its use in power generating and other petrochemical industries. In this research work, the specimens of 2.25Cr-1Mo steel were treated in three different austenitizing temperatures along with different isothermal embrittling time periods. Then the induced degree of embrittlement was characterized by the fracture stress values at −196 °C and area fraction of intergranular failure. The outcome of the experimental results shows that the increase in austenite grain size and/or isothermal embrittling time severely weakens the grain boundary cohesive strength leading to brittle intergranular failures and thus to a greater degree of temper embrittlement.     

电化学充氢对Cr15铁素体不锈钢和304奥氏体不锈钢氢脆敏感性的影响

研究了充氢时间、充氢电流密度、晶体结构对不锈钢氢脆敏感性的影响。结果表明:对于铁素体不锈钢,随着充氢时间的延长、电流密度的增大,塑性显著降低,氢脆敏感性大幅度增加;通过SEM观察实验钢断口形貌,断裂类型由韧性断裂转变为脆性断裂。而相同条件下,奥氏体不锈钢氢脆敏感性较低,抗氢脆性能较好。充氢后实验钢表面存在大量H,且氢含量随试样深度逐渐降低,晶界可能作为氢陷阱影响实验钢的氢脆敏感性。     

Effect of phosphorus segregation on fracture properties of 2.25Cr-1Mo pressure vessel steel

Phosphorus is a very common trace element that can segregate at prior austenite grain boundaries and/or carbide/matrix interfaces of low alloy steels at high temperature (e.g., order of 500 °C) and adversely affect the fracture properties. This paper investigates segregation of P during reversible temper embrittlement (96 h at 520 °C) of quenched and fully tempered 2.25Cr-1Mo steel by Auger electron spectroscopy and describes the segregation mechanism. This paper also describes the effect of P segregation on fracture resistance and fracture mode of unembrittled steels, respectively, by fracture toughness testing over a temperature range of −196 °C to 20 °C and fractography in scanning electron microscopes. During temper embrittlement phosphorus segregation has been attributed due to the mechanism of “carbide rejection”. This segregation caused a reduction in fracture toughness values of the quenched and tempered steels at all test temperatures and an increase in the transition temperature. Phosphorus segregation also changed the brittle fracture micromechanism of quenched and fully tempered samples from one of transgranular cleavage to a mixed mode of fracture (transgranular cleavage and intergranular decohesion). The micromechanism of fracture at temperatures from the upper shelf, however, remained almost unchanged.     

铌和钛微合金化超级马氏体不锈钢在海水中点蚀机制的研究(英文)

超级马氏体不锈钢(SMSS)在较高温度下具有良好的力学性能,并具有高的应力腐蚀断裂抗力和良好的可焊性,因此其在苛刻环境下的应用正日趋广泛。用于石油勘探的材料要求其具有优良的综合性能,双相不锈钢和超级双相不锈钢尽管价格昂贵,但仍在该领域得到了广泛应用,而具有技术和经济优势的SMSS能取代上述两种钢在石油勘探领域获得应用。通常,SMSS在许多加工行业也有望得到应用。本文研究了添加与未添加Nb或Ti的SMSS的点蚀机制和形貌。添加Nb和Ti是为了使敏化的影响减至最小,促进晶粒细化,并研究其对钢在海水中点蚀的影响,重点在于点蚀的形貌和腐蚀电位。添加与未添加Nb和Ti的钢具有相近的显微硬度和类似的回火马氏体组织。阳极极化曲线表明,用Nb微合金化的钢的腐蚀电位高于其他钢。对不同电位下点蚀的形成和扩展进行了重复试验,并采用光学显微镜进行观察。在工业用钢中,添加Ti的钢显示出了最佳的耐蚀性能——具有最高的腐蚀电位和较低的点蚀电位。添加Nb的钢尽管腐蚀电位较高,但其点蚀电位比未添加Nb的钢低。     

Fracture Toughness of Functionally Graded Steels

In this study, fracture toughness of functionally graded steels in both crack divider and crack arrester configurations has been studied. Spot-welded plain carbon steel and austenitic stainless steel with different thicknesses and arrangements were used as electrodes of electroslag remelting to produce functionally graded steels. Fracture toughness of the specimens in crack divider configuration was found to depend on the arrangements of the primary electrodes’ pieces together with the type of the containing phases. In crack arrester configuration, the fracture toughness was found to depend on the crack tip position and the distance of the crack tip with respect to the bainitic or martensitic intermediate layers.     

Environmentally assisted cracking of 18%Ni maraging steel

Slow strain rate testing of notched cylindrical specimens of 18Ni2400 maraging steel has been carried out in air with 30% relative humidity and synthetic seawater environments. Peak-aged condition has been chosen, considering the relevance to engineering applications. Studies have also been carried out with different notch geometries to understand the effect of stress concentration factor. It is concluded from the study that (i) degree of stress concentration at the notch influences the notched tensile strength (ii) mild hydrogen embrittlement seems to occur in air environment, (iii) synthetic seawater environment drastically brings down the notched tensile strength and time to fracture (iv) environmentally assisted cracking occurs in air tests in quasicleavage and microvoid coalescence modes and in seawater tests in intercrystalline mode.     

钢的氢脆的新研究方向

简要回顾了人们对钢的氢脆问题的认识过程和研究历史,概述了钢中氢的存在状态与氢脆的关系,钢的塑性变形与氢脆的关系,其中包括断口形貌和断裂过程。此外还论述了奥氏体不锈钢的氢脆及氢引起的断裂现象等共性问题。     

时效对JN1奥氏体钢超低温断裂韧度的影响

ＪＮ１ 奥氏体钢１３４８Ｋ固溶处理与９２３ 、９７３ 、１０７３Ｋ 时效５ｈ 后,４Ｋ 及７７Ｋ 断裂韧度测定结果表明,该钢有明显的时效脆化倾向。组织观察及Ｘ 射线衍射结果确认,时效脆化是Ｍ２３Ｃ６ 型碳化物沿晶界及退火孪晶界析出造成的。     

Stress corrosion and hydrogen embrittlement of two precipitation hardening stainless steels

Abstract

Straining of smooth tensile specimens of two precipitation hardening stainless steels over a wide range of controlled potentials in natural sea water has revealed regimes of low ductility above about — 200 mV(SCE) and below — 600 mV(SCE). The apparent loss inductility in the anodic region was attributable to dissolution and was found to be associated with pitting and crevice corrosion effects that were exacerbated when straining was conducted in acidified sea water. The embrittlement observed at more cathodic potentials, which gave rise to transgranular or intergranular failure depending upon the material and its heat treatment, is believed to be due to hydrogen that is evolved at such potentials. The occurrence of embrittlement by hydrogen in these materials, to produce similar modes of failure, was confirmed by testing smooth specimens ajtercathodic charging and both smooth and precracked specimens in gaseous hydrogen. Cracking of precracked specimens in gaseous hydrogen is complicated by the presence of δ ferrite stringers in the microstructure, which can give rise to delamination perpendicular to the principal plane of fracture if the stresses ahead of the crack are not reduced significantly by hydrogen embrittlement.     

Use of stainless steels in bus coach structures

This study focuses on weld integrity of stainless steels in bus coach applications. Safety aspects have been studied based on fracture mechanics and impact toughness testing. Fatigue resistance of welded rectangular hollow section (RHS) profiles was evaluated according to the Eurocode 3 (1992) fatigue standard. Corrosion resistance was studied by salt-spray chamber tests in a deicing salt atmosphere and by field testing for 3 years under an urban bus. The mechanical tests show that austenitic stainless steel EN 1.4310 (AISI 301) is a superior material, and a low-C 12% Cr alloyed stainless steel EN 1.4003 is also a competitive material in bus coach applications. According to the life cycle cost (LCC) calculations, stainless steels are competitive compared with carbon steels or aluminum.     

Improved impact toughness of 13Cr martensitic stainless steel hardened by laser

The impact toughness of AISI 403 martensitic stainless steel plate and laser-hardened specimens tempered at various temperatures were examined. Phosphorus was the primary residual impurity responsible for tempered embrittlement of this alloy. The experimental result also indicated that AISI 403 stainless steel was very sensitive to reverse-temper embrittlement. The improved impact toughness of the laser-hardened specimen was attributed to the refined microstructure in the laser-hardened zone.     

Corrosion study of SS304 and SS316 alloys in atmospheric,underground and seawater splash zone in the Arabian Gulf

Bare stainless steel type 304 and 316 alloys were exposed to atmospheric, underground and seawater splash conditions in order to evaluate their corrosion properties. The exposure was undertaken for 15 months at the coastal and industrial city of Jubail located in the Arabian Gulf. The corrosivity of the environment was determined by testing the soil, groundwater, seawater and air quality. Corrosion rate of the exposed coupons was determined by weight loss method. The experimental results indicate that the both types of stainless steels corroded moderately in this environment. Primary factors influencing the corrosion process are thought to be high degree of variation in temperature and humidity combined with high chloride and sulphate concentrations. The corrosion rate of stainless steel is also compared with those reported for other international locations.     

Fracture Toughness in Niobium Treated Steels

While niobium can significantly improve the yield strength of microalloyed steels, such additions frequently result in low toughness and heat-affected-zone embrittlement. A direct correlation between fracture surface appearance, fracture profile, microstructure and precipitation behavior may be used to explain differences in toughness as a function of niobium content in steels containing 0.03, 0.06 and 0.21 wt.% niobium.     

不锈钢在海水中的耐蚀性与腐蚀电位的关系

研究了７种不锈钢在４个试验站的海水中浸泡１８０天的腐蚀电位特征,其中５种不锈钢还进行了长达４年的暴露试验。钝化能力强的不锈钢的海水腐蚀电位阴浸泡时间向正变化,其腐蚀电位趋于稳定的时间较长,稳态腐蚀电位的波动较大,钝化能力较弱的不锈钢则相反不同钢种的不锈钢在海水中的稳态腐蚀可以相差很双,在青岛、舟山和榆林,２Ｃｒ１３和ＨＲＳ－３间的稳态电位相差０．８Ｖ以上。不锈钢在海水中的稳态腐蚀电位较正,其耐蚀性     

A hydrogen embrittlement mechanism for sensitized types 304, 316 and 310 austenitic stainless steels in boiling saturated magnesium chloride solutions

We have already proposed a mechanism for intergranular hydrogen embrittlement (IG-HE) for solution annealed austenitic stainless steels (types 304, 316 and 310) in HCl solutions and in boiling saturated magnesium chloride solutions. The proposed IG-HE mechanism was based on martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS). Recently, it was reported that the fracture susceptibility and fracture mode for sensitized steels in boiling saturated magnesium chloride solution under an open-circuit condition were significantly different from those observed for solution annealed steels. In the present paper, the hydrogen embrittlement behavior of sensitized types 304, 316 and 310 in boiling saturated magnesium chloride solutions was explained in more details in terms of an inhibiting effect of chloride ions, martensite transformation, Cr depletion, HELP, the degree of corrosiveness through the comparison with those for the solution annealed steels. Furthermore, a transgranular HE (TG-HE) cracking mode that was not observed for the solution annealed steels was discussed as well as IG-HE. Then a TG-HE mechanism for sensitized austenitic stainless steels was proposed, while the IG-HE mechanism for solution annealed austenitic stainless steels which was discussed in details was applied to IG-HE of sensitized austenitic stainless steels. It was also pointed out that the occurrence of both TG-HE and IG-HE was explained with an identical concept.     

The effect of sensitization on the hydrogen-enhanced fatigue crack growth of two austenitic stainless steels

Fatigue crack growth tests were performed to evaluate the susceptibility to hydrogen-enhanced crack growth of AISI 304 and 316 stainless steels (SSs). Sensitization treatment at 650 °C 100 h played little effect on the fatigue crack growth behavior in air, regardless of testing specimens. However, hydrogen accelerated the fatigue crack growth of various specimens to different degrees; sensitized specimens were more susceptible as compared with the un-sensitized ones.

Fatigue fracture appearance of various specimens tested in air exhibited mainly transgranular fatigue fracture together with rarely intergranular fracture and twin boundary separation. Meanwhile, intergranular fracture was found for sensitized specimens tested in hydrogen. Extensive quasi-cleavage fracture related to the strain-induced martensite accounted for the hydrogen-accelerated fatigue crack growth of unstable austenitic SSs. On the other hand, the lower susceptibility of 316H specimens could be attributed to the partial austenite transformation, as evidenced by a mixture of transgranular fracture feature and quasi-cleavage.     

Effect of C,Ti, Zr and B alloying on fracture mechanisms in hot-rolled Fe–40 (at.%)Al

This paper reports a study of fracture behavior of FeAl-based intermetallic alloys with the addition of carbon, titanium, zirconium and boron (Fe–40Al–1C, Fe–40Al–1Ti and Fe–40Al–Zr–B). The alloys were prepared by modified processing technology of vacuum induction melting and hot rolling in special stainless steel sheath. Tensile and fracture toughness tests were carried out at 20 °C, 400 °C, 600 °C, 700 °C and 800 °C. The alloy showed best fracture toughness and tensile properties with Zr and B addition. The fracture toughness at 600 °C was comparable with values in stainless steels and nickel-based superalloys. The fractographic analysis revealed the change of fracture micromechanisms with temperature. Moreover, under specific conditions, the fracture micromechanisms were different in tensile and fracture toughness specimens.