抗晶间腐蚀奥氏体不锈钢的晶界工程

Sensitization by chromium depletion due to chromium carbide precipitation at grain boundaries in austenitic stainless steels can not be prevented perfectly only by previous conventional techniques, such as reduction of carbon content, stabilization-treatment, local solution-heat-treatment, etc. Recent studies on grain boundary structure have revealed that the sensitization depends strongly on grain boundary character and atomic structure, and that low energy grain boundaries such a~ coincidence-site-lattice (CSL) boundaries have strong resistance to intergranular corrosion. The concept of grain boundary design and control has been developed as grain boundary engineering (GBE). GBEed materials are characterized by high frequencies of CSL boundaries which are resistant to intergranular deterioration of materials, such as intergranular corrosion. A thermomechanical treatment was tried to improve the resistance to the sensitization by GBE. A type 304 austenitic stainless steel was cold-rolled and solution-heat-treated, and then sensitization-heat-treated. The grain boundary character distribution was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction The frequency of CSL boundaries indicated a maximum at the small reduction. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material. A high density of annealing twins were observed in the thermomechanical-treated specimen. The results suggdst that the therrmomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface. The effects of carbon content and other minor elements on optimization in grain boundary character distribution (GBCD) and thermomechanical parameters were also examined during GBE.     

Grain boundary segregation in austenitic stainless steels and its effect on intergranular corrosion and stress corrosion cracking

This paper reports a study of grain boundary segregation, intergranular corrosion, and intergranular stress corrosion cracking in austenitic stainless steels. The results show that phosphorus, nitrogen, and sulfur all segregate to grain boundaries in these materials and that they can affect one another's segregation through site compctition. In particular, the results demonstrate that phosphorus segregation can be lowered by the presence of nitrogen and sulfur in the steel. Also, if manganese is present in the steel, sulfur segregation will be greatly decreased as a result of formation of manganese sulfides. Phosphorus, sulfur, and nitrogen will not initiate intergranular corrosion in the modified Strauss test, although if corrosion is initiated by chromium depletion, these elements might enhance the corrosion process. Phosphorus segregation does enhance corrosion in the Huey test, even in steels that have not undergone grain boundary chromium depletion, although there does not appear to be a precise correlation between the depth of corrosion penetration and phosphorus segregation. Intergranular stress corrosion cracking in 288 °C water at a pH of 2.5 and electrochemical potential of OV_SHE can occur in these steels even in the absence of chromium depletion if sulfur is present on the grain boundaries. Phosphorus segregation appears to have very little effect.     

硅对含强氧化性离子沸腾硝酸中奥氏体不锈钢耐蚀性的影响

 超低碳Cr Ni奥氏体不锈钢在含强氧化性离子的硝酸溶液中会发生过钝化腐蚀和晶间腐蚀,而高硅不锈钢在强氧化性浓硝酸中具有优良的耐蚀性。笔者以高纯级000Cr25Ni20和00Cr14Ni14Si4钢作为对比材料,发现在纯稀硝酸中000Cr25Ni20钢具有更优良的耐蚀性,在不同含量的40%沸腾硝酸溶液中,00Cr14Ni14Si4钢可有效抑制过钝化腐蚀,而且不发生晶间腐蚀。     

Stress corrosion cracking of stainless steels in NaCl solutions

The metallurgical influences on the stress corrosion resistance of many commercial stainless steels have been studied using the fracture mechanics approach. The straight-chromium ferritic stainless steels, two-phase ferritic-austenitic stainless steels and high-nickel solid solutions (like alloys 800 and 600) investigated are all fully resistant to stress corrosion cracking at stress intensity (K₁) levels ≤ MN • m-^3/2 in 22 pct NaCl solutions at 105 °C. Martensitic stainless steels, austenitic stainless steels and precipitation hardened superalloys, all with about 18 pct chromium, may be highly susceptible to stress corrosion cracking, depending on heat treatment and other alloying elements. Molybdenum additions improve the stress corrosion cracking resistance of austenitic stainless steels significantly. The fracture mechanics approach to stress corrosion testing of stainless steels yields results which are consistent with both the service experience and the results from testing with smooth specimens. In particular, the well known “Copson curve” is reproduced by plotting the stress corrosion threshold stress intensity (AT_ISCC) vs the nickel content of stainless steels with about 18 pct chromium. Formerly with the BBC Brown Boveri Company, Baden, Switzerland     

Sensitization of 12 Wt Pct chromium,titanium-stabilized ferritic stainless steel

The sensitization behaviors of 19 heats of titanium stabilized, 12 Wt Pct chromium, ferritic stainless steel were evaluated by measuring the intergranular corrosion resistance of samples given a two-step heat treatment. The latter consisted of a one-hour high temperature exposure followed by a water quench and one of 19 different low temperature treatments. The results indicate that heat treating at temperatures ≳ 1000 °C produces a microstructure which can be sensitized by subsequent aging at temperatures ≲ 600 °C. The amount of carbon and not the amount of nitrogen nor the amount of carbon plus nitrogen dictated the sensitization resistance of the titanium-stabilized, 12 Wt Pct chromium alloy. The proposed mechanism of sensitization in these alloys suggests that during the high temperature exposure, the titanium carbonitrides decompose, freeing carbon or carbon and nitrogen into interstitial solid solution. During subsequent heat treating at temperatures ≲600 °C, chromium-containing carbides precipitate intergranularly, and a chromium-depleted zone is formed along the grain boundaries. The presence of the chromium-depleted zone results in susceptibility to intergranular corrosion, α′ precipitates form after the intergranular precipitates during heat treating at 600 °C. The presence of α′ enhances the intergranular corrosion rate.     

Stress corrosion cracking of stainless steels

The similarities and differences in the stress corrosion cracking response of ferritic and austenitic stainless steels in chloride solutions will be examined. Both classes of materials exhibit a cracking potential: similar transient response (to loading) of the potential in open circuit tests or the current in potentiostatic tests and similar enrichment of chromium and depletion of iron in the film associated with localized corrosion processes. The ferritic steels are more resistant to localized corrosion than are the austenitic steels, which is responsible for the difference in the influence of prior thermal and mechanical history on cracking susceptibility of the two types of steel. Similarities in the fractography of stress corrosion cracks and those produced by brittle delayed failure during cathodic charging of the ferritic steels indicate that hydrogen embrittlement is involved in the failure process.     

晶粒尺寸对Cr-Ni奥氏体不锈钢在硝酸中晶间腐蚀敏感性的影响

通过在含400 mg/L Cr6++40%硝酸溶液中浸没试验和电化学试验,发现随晶粒尺寸的减小,Cr-Ni奥氏体不锈钢晶间腐蚀敏感性随之增加,但当晶粒尺寸小于临界晶粒尺寸时,晶间腐蚀敏感性开始降低。过钝化电位数值与晶粒尺寸无直接关系,与材料本身特性有关,极化后的表面腐蚀形貌与浸没试验结果相一致。     

Corrosion Behavior of High Nitrogen Nickel-Free Fe-16Cr-Mn-Mo-N Stainless Steels

The purpose of the current study is to develop austenitic nickel-free stainless steels with lower chromium content and higher manganese and nitrogen contents. In order to prevent nickel-induced skin allergy, cobalt, manganese, and nitrogen were used to substitute nickel in the designed steel. Our results demonstrated that manganese content greater than 14 wt pct results in a structure that is in full austenite phase. The manganese content appears to increase the solubility of nitrogen; however, a lower corrosion potential was found in steel with high manganese content. Molybdenum appears to be able to increase the pitting potential. The effects of Cr, Mn, Mo, and N on corrosion behavior of Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were evaluated with potentiodynamic tests and XPS surface analysis. The results reveal that anodic current and pits formation of the Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were smaller than those of lower manganese and nitrogen content stainless steel.     

Case reviews on the effect of microstructure on the corrosion behavior of austenitic alloys for processing and storage of nuclear waste

This article describes the corrosion behavior of special austenitic alloys for waste management applications. The special stainless steels have controlled levels of alloying and impurity elements and inclusion levels. It is shown that “active” inclusions and segregation of chromium along flow lines accelerated IGC of nonsensitized stainless steels. Concentration of Cr⁺⁶ ions in the grooves of dissolved inclusions increased the potential to the transpassive region of the material, leading to accelerated attack. It is shown that a combination of cold working and controlled solution annealing resulted in a microstructure that resisted corrosion even after a sensitization heat treatment. This imparted extra resistance to corrosion by increasing the fraction of “random” grain boundaries above a threshold value. Randomization of grain boundaries made the stainless steels resistant to sensitization, IGC, and intergranular stress corrosion cracking (IGSCC) in even hot chloride environments. The increased corrosion resistance has been attributed to connectivity of random grain boundaries. The reaction mechanism between the molten glass and the material for process pot, alloy 690, during the vitrification process has been shown to result in depletion of chromium from the reacting surfaces. A comparison is drawn between the electrochemical behavior of alloys 33 and 22 in 1 M HCl at 65 °C. It is shown that a secondary phase formed during welding of alloy 33 impaired corrosion properties in the HCl environment. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

马氏体不锈钢成分、工艺和耐蚀性的进展

介绍了马氏体铬不锈钢、马氏体铬镍不锈钢和马氏体时效不锈钢的进展和发展动态,讨论了化学成分、热加工、热处理工艺对马氏体不锈钢组织和耐蚀性的影响。当前国内开发的新型马氏体不锈钢主要为符合给定服役条件的专用钢类。优化化学成分,提高钢的纯净度,通过热加工和热处理工艺控制钢的组织,进行表面处理是提高马氏体不锈钢耐蚀性的有效途径。     

304与301B奥氏体不锈钢在线固溶热处理工艺研究

通过双道次热压缩和热轧试验研究了在线固溶热处理工艺对304和301B奥氏体不锈钢组织和性能的影响,提高终了压缩温度、冷却速度,降低卷取温度,可以改善奥氏体不锈钢微观组织,减少碳化物析出,提高抗晶间腐蚀能力和力学性能.将该工艺应用于工业生产的304奥氏体不锈钢热轧卷板,测试结果表明304各项性能得到改善,冷轧前退火工序可被在线固溶取代.     

超级高氮奥氏体不锈钢的耐腐蚀性能及氮的影响

用电化学测试、化学浸泡等方法研究了超级奥氏体不锈钢00Cr24Ni22Mo7Mn3CuN(654SMO)的耐点腐蚀和耐缝隙腐蚀的性能。通过改变氮含量,研究了氮对奥氏体不锈钢的耐点腐蚀和耐缝隙腐蚀性能的影响,结果表明,氮和适量的铬、钼结合,能显提高奥氏体不锈钢的耐点腐蚀和缝隙腐蚀的能力,并且随着氮含量的增国,砥体不锈钢的耐点腐蚀和耐缝隙腐蚀的能力也增强,对比实验表明,超级奥氏体不锈钢在耐点腐蚀,缝隙腐蚀等局部腐蚀性能方面可以和镍基合金C－276媲美,甚至优于镍基合金。     

Inhibition of Intergranular Corrosion in Austenitic Stainless Steels Through Critical Near-Surface Plastic Deformation and Aging

Metallurgical and Materials Transactions A - Inhibition of high intergranular corrosion susceptibility (HIGCS) in austenitic stainless steels can be achieved by healing Cr-depleted zones through...     

The influence of preceding cryoforming and testing temperature on the mechanical properties of austenitic stainless steels

The TRIP effect in austenitic stainless steels leads to temperature dependent mechanical properties. As this is caused by stress or strain induced austenite/martensite transformation a predeformation at low temperatures (cryoforming) will change the microstructure and the transformation behaviour of the remaining austenite constituent. The mechanical properties in tensile tests and the J‐integral of the chromium and nickel alloyed steels 1.4301 and 1.4571 have been tested in the temperature range from 123 to 323 K in the as‐industrially supplied condition and after 10 % cryoforming at 77 K. The temperature dependence of the elongation values and the strain hardening behaviour of the undeformed steels is much more pronounced than of the yield and tensile strength. The mechanical behaviour can be explained by differences in response to the ?‐, the α_e'‐ and the α_g'‐martensite transformation. A cryoforming changes the mechanical properties of the examined austenitic stainless steels.     

The effects of deformation induced martensite on the sensitization of austenitic stainless steels

This paper compares the effects of deformation which induces martensite in austenitic stainless steel with deformation which does not on the sensitization and corrosion susceptibility of these alloys. We show that deformation which induces martensite causes rapid sensitization at temperatures below 600 °C, leads to extensive transgranular corrosion, and can produce rapid healing. The martensite is also an area of extensive carbide precipitation. Deformation alone noticeably increases the kinetics of sensitization only at temperatures where undeformed samples are readily sensitized. Without the presence of martensite, intergranular corrosion is always the predominant corrosion path, rapid healing is not observed, and most carbides precipitate along the grain boundaries.     

Recent developments in stainless steels:austenitic,ferritic,duplex

A review is given concerning some of the recent industrial developments of stainless steels. In austenitic stainless steels,two different directions of alloy development are noticeable:low nickel austenitic stainless steels and high nitrogen stainless steels.In these two cases the aims are different,particularly in terms of strength,but the philosophy of alloy development and the scientific approaches are very similar and they all revolve about the role of nitrogen as an alloying element and how this affects strength,ductility and corrosion resistance. There is now a broad and useful basis of information as to how nitrogen affects solid solution hardening,grain boundary hardening and work hardening and how to make use of these effects in developing materials required by the world market. In the field of corrosion resistance,ferritic,duplex and austenitic stainless steels compete with each other and now there is a growing body of information concerning the relative corrosion resistance based on laboratory data. However,for practical applications and for alloy selection,more than just laboratory data are needed,and thus,the first results are presented here of a many years comparison of the corrosion resistance of 24 commercial stainless steels exposed to corrosion in outdoors marine atmosphere.Hope is expressed to involve in the near future even more steels from a wider range of manufacturers in such corrosion studies.This might help consumers in appropriate alloy selection.It might also help steel makers in developing appropriate stainless steel grades.     

The Effect of Chemical Composition and Heat Treatment Conditions on Stacking Fault Energy for Fe-Cr-Ni Austenitic Stainless Steel

In order to establish more reliable formulae for calculating stacking fault energies (SFE) from the chemical compositions of austenitic stainless steels, SFE values were measured for 54 laboratory-melted heats and 2 commercial heats. The results were checked against those of a first-principle, atomistic calculation approach. More than ~20,000 data points for the width and angle of the Burgers vectors were determined from dark-field images of isolated extended dislocations in 56 heats of austenitic stainless steel using weak electron beams with g-3g diffraction conditions. Based on these numerous observations and on fundamental thermodynamic analyses, it is concluded that the SFE values for austenitic stainless steels are changed not only by chemical composition but also by heat treatment. In this paper, new formulae for calculating SFE values from the chemical compositions in three different heat treatment conditions have been proposed for austenitic stainless steels within given limited chemical composition ranges. In these formulae, the SFE values are calculated from the nickel, chromium, molybdenum, silicon, manganese, nitrogen, and carbon contents for the each heat treatment condition. The three heat treatment conditions studied were water cooling after solution heat treating (SHTWC), furnace cooling after solution heat treating, and aging after SHTWC.     

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.     

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。