Low-cycle fatigue behavior of Mn/N stainless steels

The low-cycle fatigue behavior of AISI 201 and 202, Mn/N stabilized austenitic stainless steels has been investigated. The fatigue life under controlled strain cycling conditions was determined and correlated with microstructural and fractographic observations. It was determined that both α′ (BCC) and ε (HCP) martensitic phases formed as a result of either monotonie or cyclic deformation in both steel compositions, but the two differed considerably in the amount of martensites formed. AISI 201 steel formed up to 90 vol. pet of α′ and was found to have a shorter fatigue life than AISI 202, which only formed a few tenths of a percent of α′. Comparisons with three other austenitic stainless steels are made.

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Formerly a Research Assistant     

Low-cycle fatigue behavior of Mn/N stainless steels

The low-cycle fatigue behavior of AISI 201 and 202, Mn/N stabilized austenitic stainless steels has been investigated. The fatigue life under controlled strain cycling conditions was determined and correlated with microstructural and fractographic observations. It was determined that both α′ (BCC) and ε (HCP) martensitic phases formed as a result of either monotonie or cyclic deformation in both steel compositions, but the two differed considerably in the amount of martensites formed. AISI 201 steel formed up to 90 vol. pet of α′ and was found to have a shorter fatigue life than AISI 202, which only formed a few tenths of a percent of α′. Comparisons with three other austenitic stainless steels are made.     

双相不锈钢超塑性变形机理

从材料的晶体结构出发,研究了双相不锈钢超塑性变形的机理.利用背散射电子衍射花样分析系统(EBSD),获得了双相不锈钢变形过程中的ODF图、极图和取向与转轴分布等晶体取向分布规律.结合透射电镜对微观组织的观察结果进行了综合分析.研究表明,双相不锈钢超塑性变形的机理为形变诱导析出和动态再结晶、晶界滑移以及变形中的晶粒转动.     

Role of nitrogen in the cyclic deformation behavior of duplex stainless steels

The role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It can be attributed to the greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with increasing nitrogen content because austenite dominantly takes part in plastic deformation, being more strain hardened due to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change because ferrite, more dominantly accommodating plastic strain, rarely shows a change of strain-hardening behavior due to the similar nitrogen content in ferrite.     

Influence of microstructure on the flow behavior of duplex stainless steels at high temperatures

Three kinds of duplex stainless steel, with different ferrite-to-austenite ratios, were deformed in torsion over the temperature range 900 °C to 1200 °C; the corresponding microstructural evolution was observed and correlated with the deformation conditions. The shapes of the high-temperature flow curves depend strongly on the volume fractions of the phases, the characteristics of the ferrite-austenite interface, and the active softening mechanism. At low volume fractions of austenite, the mechanical behavior is determined by the ferrite matrix and the flow curves are typical of materials that soften by continuous dynamic recrystallization. When the volume fraction of austenite is increased, coherent γ particles distributed within the grains and at the grain boundaries hinder the deformation of the softer α matrix, increasing both the yield and the peak stress. These peaked flow curves are characterized by rapid work hardening followed by extensive flow softening; under these conditions, the hard austenite particles become aligned with the deformation direction after large strains. At high volume fractions of austenite (∼50 pct), the material tends to form a duplex structure, with the flow curves displaying extended work-hardening and work-softening regions; however, a drastic decrease is observed in ductility because of the dissimilar plastic behaviors of the two phases.     

High density sintering of duplex stainless steels

Abstract

Duplex stainless steels prepared from mixes of elemental powders with martensitic 410L steel have been shown to possess stable dual phase microstructures. Following heat treatment, the steels possess good mechanical properties. The addition of boron as a sintering aid reduces compressibility but promotes densification through transient liquid phase sintering; its effect on corrosion resistance is particularly favourable.     

High-temperature fatigue crack growth behavior of 17-4 PH stainless steels

The fatigue crack growth (FCG) behavior was investigated for 17-4 PH stainless steels in three heat-treated conditions, i.e., unaged (condition A), peak-aged (condition H900), and overaged (condition H1150), at temperatures ranging from 300 °C to 500 °C. The high-temperature fatigue crack growth rates (FCGRs) of condition H1150 were increased with an increase in temperature. However, for conditions A and H900 tested at 500 °C, the FCGRs were lower than the lower temperature ones. At 300 °C and 400 °C, H1150 and H900 generally showed the lowest and highest FCGRs, respectively, with condition A demonstrating behavior between the two. At 500 °C, the FCGR curves for all material conditions merged together. The anomalous FCG behavior of 17-4 PH stainless steels at 500 °C was mainly caused by an in-situ precipitate-coarsening effect during test. This work was funded by the National Science Council of the Republic of China (Taiwan) under Contract Nos. NSC-90-2216-E-008-007 and NSC-91-2216-E-008-007.     

特超级双相不锈钢的发展现状及趋势

概述了世界范围内特超级双相不锈钢的发展现状及趋势,重点对特超级双相不锈钢的成分设计思路、组织特点进行了分析,并结合其性能优势介绍了国外在相关领域的应用范例。强度更高、耐蚀性能更优的特超级双相不锈钢正在成为新一代的双相不锈钢材料,其将在海洋工程、石油石化、化工等领域获得广泛的应用。     

宝钢双相不锈钢焊接技术

讨论了B2101、B2304、B2205以及B2507双相不锈钢的焊接性.针对双相不锈钢的焊接实践推荐了合理的焊接方法、填充金属、热循环及焊接保护气体参数,并列出了双相不锈钢在不同焊接方法条件下的典型接头性能;针对特殊焊接方法推荐了合适的焊后热处理工艺.结果表明,双相不锈钢同常规奥氏体不锈钢一样具有较好的焊接性,但焊接过程须避免过大或过小的热输入以免造成接头腐蚀性能的下降;焊接保护气中添加一定含量的氮较纯氩气保护可以使接头获得更高耐蚀性能,同时适当温度范围内的短时热处理也可以明显改善接头耐点腐蚀性能.实践表明,通过合适的焊接方法和焊接工艺控制,可以获得具有优异综合性能的双相不锈钢焊接接头.     

Room-temperature aging of manganese-alloyed high nitrogen duplex stainless steels

The room-temperature aging behavior of two duplex stainless alloys with different austenite stability was investigated. Both alloys readily aged at room temperature. Even for aging times as short as 30 seconds, the originally continuous yielding behavior becomes discontinuous upon reloading after prestraining. The magnitude of the stress increase due to aging was higher in the presence of strain-induced martensite, even though it was shown that aging also occurred in the austenite phase. The aging response was shown to be thermally activated, with increasing age hardening associated with increasing aging times. The results could be explained by the combination of aging phenomena in the bcc phases by interstitials and the aging by interstitial-vacancy complexes in the fcc phase, where the interstitials are thought to be immobile during the short aging times used and aging would occur due to short-range migration of vacancies instead.     

固溶处理对2205双相不锈钢组织与疲劳裂纹扩展规律的影响

采用光学显微镜、扫描电镜、EDS能谱分析和力学性能测试等方法,研究了固溶处理对2205双相不锈钢显微组织与疲劳裂纹扩展规律的影响。结果表明：与原始热轧态相比,在950~1150℃范围固溶处理的试样的疲劳裂纹门槛值显著提高,稳态裂纹扩展速率均有所减小。在950℃固溶处理时,组织中析出少量的σ相,试样的稳态裂纹扩展速率显著降低;随着固溶温度的升高,组织中α相含量逐渐增加,σ相溶解,试样的稳态裂纹扩展速率的变化呈现为先增大后减小的趋势;当固溶温度达到1150℃时,组织中α相含量最高且两相组织明显粗化,试样的稳态裂纹扩展速率达到最小,呈现出最高的抗疲劳裂纹扩展能力。固溶处理引起σ相的析出与溶解、α相含量的增加及组织粗化是引起2205双相不锈钢试样疲劳裂纹扩展性能非单调变化的原因。     

Effect of environment on crack growth behavior in austenitic stainless steels under creep and fatigue conditions

Crack growth behavior at high temperatures under cyclic, static, and combined loads was studied in annealed and 20 pct cold-worked Type 316 and 20 pct cold-worked Type 304 austenitic stainless steels in air and vacuum. Under cyclic load, crack growth rates in annealed Type 316 steel are slightly lower in vacuum than in air, but this difference decreases with increase in crack growth rate. Most importantly, the effect of temperature on crack growth is present even in vacuum and arises mostly from the variation of elastic modulus with temperature. In the cold-worked Type 316 steel, the pronounced hold-time effects on fatigue crack growth in air reported in the literature persists even in vacuum. This implies that at high crack growth rates these hold-time effects arise mostly from creep-fatigue interaction rather than environment fatigue interaction. Environment has a negligible effect also on crack growth under static load. Thus, time dependent crack growth in these steels is due to creep processes. Crack growth behavior in annealed and cold-worked materials are compared and reasons for the enhanced time dependent crack growth in cold-worked material are discussed in detail.     

Strength and fatigue properties in medium carbon duplex steels

The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔK_TH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness K_IC, showing that for high m values K_IC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK.     

提高双相不锈钢热塑性的实践

在国外双相不锈钢管的制造是通过热挤压的方法。而国内绝大多数制管企业采用两辊斜轧穿孔的方法来获得毛管,由于变形剧烈,对钢的热塑性要求甚高,为了适应这种情况,必须提高双相不锈钢热塑性。本文介绍了添加微量元素、铁素体控制等方法而获得的效果,即使超级双相不锈钢S32750在1180℃?1200℃ Gleeble热模拟试验,其断面收缩率也高达97%,热塑性大大提高,可顺利通过热穿孔。     

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.     

Simulation of the precipitation of sigma phase in duplex stainless steels

The precipitation of sigma phase within the ferrite component of a duplex stainless steel has been simulated using a two-dimensional computer model which takes into account the partitioning of alloy elements between ferrite and austenite. The model is based on a cellular automaton and, despite having a rather simple set of transition rules, is able to simulate changes in the volume fractions of the austenite, ferrite, and sigma phases. The microstructures produced are similar in appearance to those in the real system. Comparison of the model and the real system may assist in the assessment of the various phenomena occurring. Use is made of the model to examine many of the factors that might conceivably be harnessed to retard precipitation of the sigma phase in duplex stainless steels.     

不锈钢研发的最新进展——奥氏体、铁素体和双相不锈钢

评价了近期不锈钢的部分工业发展状况。对于奥氏体不锈钢而言,有两大引人注目的合金开发方向:低镍奥氏体不锈钢和高氮奥氏体不锈钢。这两类合金的设计目的不同,尤其是强度方面,但其合金开发原理和科研方法很相似,都围绕着氮这一合金元素对强度、塑性和耐蚀性的积极作用展开。现在人们已经相当了解氮对固溶强化、晶界强化和加工硬化的影响以及如何充分利用这些影响来开发全球市场所需的材料。在耐蚀性方面,铁素体、奥氏体和双相不锈钢彼此竞争,有关其耐蚀性比较的实验室数据也越来越多。但是,对于实际应用和合金选择而言,仅仅是实验室的数据是不够的,因此文章给出了多年来对24种工业生产的不锈钢暴露在室外海洋大气环境下的最新耐蚀性研究结果。希望在不久的将来能将更多制造商提供的钢种纳入到此项耐蚀性研究中来。这既能帮助消费者合理地选择合金,也可以帮助钢铁生产企业开发合适的钢种。     

双相不锈钢中的相变:金相和磁性质研究

用金相和不同的无损磁试验对商业低合金双相不锈钢及超级双相不锈钢因热处理和冷加工产生的显微结构过程进行了研究。在双相不锈钢(DSS)和超级双相不锈钢(SDSS)中,亚稳态的铁素体由于热处理可以分解为σ相和二次奥氏体相。这些显微结构的改变显著影响着钢材的机械性能、耐腐蚀性能和磁特性。磁性测量使得破坏性实验中获取的谐频和机械性能存在定量关系。在低合金双相不锈钢中,当低含量的Ni被N和Mn取代后,可能导致奥氏体相的不稳定性,奥氏体相容易转变为亚稳态的马氏体相,而SDSS奥氏体会更加稳定。通过室温冷轧(10%～85%压下率)的方法进行固溶退火材料的塑性变形研究,2101材料仅考察了轧制条件,而冷轧2507样本还进行了热处理。冷轧样品用光学显微镜(OM)、扫描电子显微镜(SEM)、X-射线衍射(XRD)和磁试验进行了研究。还对双相不锈钢样品进行了三种不同的磁试验,以确定其磁性能。结果表明冷轧对晶粒的大小和形状有一定影响,磁测试结果还显示2101样品中存在马氏体。