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.     

The effect of temperature and nitrogen content on the partitioning of alloy elements in duplex stainless steels

The partitioning of the alloy elements chromium, nickel, and molybdenum that occurs in conventional duplex stainless steels has been measured for various nitrogen contents and annealing temperatures. The experimental results have been combined with data available in the literature, and the behavior is summarized by use of a statistically derived empirical model. An increase in the nitrogen content significantly decreases the degree of chromium partitioning but has a lesser effect on nickel and molybdenum. However, the partitioning of nickel can be significantly reduced by an increase in the annealing temperature. The usual measures of partitioning were found to be unsuitable for statistical analysis, and an alternative measure of the degree of partitioning is proposed.     

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.     

Lowcycle fatigue behavior on duplex stainless steels

Stainless steels are used predominantly for their corrosion resistance in moderate to highly aggressive environments. For construction purposes, engineers normally select carbon steel due to low cost, long experience, applicable design rules and a large variety of strength classes. However, different stainless steel types can also provide a very wide range of mechanical properties and they have the advantage of not needing surface protection. Duplex Stainless Steels (DSSs) in particular, are austeno-ferritic steels with twice the mechanical strength of conventional austenitic and ferritic stainless steels and have a potential use in construction. In the early 1980’s, a ‘second generation’ of duplex steels was introduced with better weldability mainly through nitrogen alloying. The most common duplex grade today is the UNS S32205/S31803, which is used in a great number of applications in a wide variety of product forms. This grade was the basis for the development of a ‘third generation’ of duplex steels. These higher alloys are called super-duplex stainless steels and identified as UNS S32750/S32760. The cyclic hardening-softening response, the cyclic stress-strain curve and the microstructure evolution of a high nitrogen duplex stainless steel S32750 have been evaluated and the results compared with reference to low and medium nitrogen duplex stainless steels, S32205 and S32900 grades, respectively. The beneficial effects of nitrogen on the cyclic properties of most modern alloys have been analyzed in terms of the flow stress components, i.e. the back and the friction stress. A phenomenological model is proposed to explain the influence of nitrogen atoms on the cyclic behavior of these steels.     

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.     

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.     

合金元素对双相不锈钢2101耐点蚀性能的影响

研究了合金元素对双相不锈钢2101耐点蚀性能的影响规律。结果显示,2101系列合金的浸泡点蚀腐蚀速率在1.9～7.0 g/（m^2.h）之间,与304不锈钢在同一数量级;Mo是提高2101系双相不锈钢耐腐蚀性的关键元素,而N对耐腐蚀性的影响不大;点蚀起源和Thermo-Calc计算结果显示2101成分体系中,铁素体相是耐点蚀性较弱相,提高铁素体相耐蚀性是提高合金整体耐蚀性的关键;当Cr含量固定在21.5%时,Mo作为铁素体形成元素将在铁素体相中富集,提高铁素体相的耐点蚀性能,从而提高合金整体耐蚀性。     

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.     

The critical amount of nitrogen on the formation of nitrogen gas pores during solidification of 25Cr-7Ni duplex stainless steels

The effects of casting thickness, nitrogen contents, cooling rate, and Mn contents on the formation of nitrogen gas pores during solidification of 25Cr-7Ni-1.5Mo-3W duplex stainless steels (DSS) were quantitatively investigated. In the case of a sand mold, the formation of nitrogen gas pore was not affected by the thickness of castings, which ranged from 13 to 52 mm, and the critical initial nitrogen content for the formation of gas pore was 0.30 wt pct. In the case of the molds made of a stainless steel (STS) and water-cooled Cu, the critical initial nitrogen content did not change much compared to the sand mold. The amount of nitrogen gas pores increased with initial nitrogen contents of castings. The segregation of nitrogen and alloying elements was calculated with Thermo-Calc. The calculated data and the experimental results were compared to estimate the critical nitrogen partial pressure in the residual melt for the nucleation of gas pores. The effect of Mn content on the formation of gas pores was also investigated. The increase of Mn content from 1 wt pct to 2.6 wt pct changed the critical initial nitrogen content 0.30 wt pct to 0.40 wt pct.     

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     

氮对新型无镍高氮奥氏体不锈钢塑性流变行为的影响

利用Ludwigson模型研究了两种氮含量不同的无镍奥氏体不锈钢18Cr-12Mn-0.55N(质量分数/%)和18Cr-18Mn-0.63N在室温快速拉伸时的塑性流变行为.结果表明,由于N含量的增大,实验钢18Cr-18Mn-0.63N的加工硬化能力明显强于实验钢18Cr-12Mn-0.55N.N促进CrMnN奥氏体...     

Sulfide stress cracking susceptibility of nickel containing steels

A systematic evaluation of the sulfide-stress cracking (SSC) behavior of five steels with nickel contents ranging from 0 to 3 pct was conducted in an acidified chloride solution saturated with H₂S at room temperature (NACE solution). All of the steels were low-alloy, structural, or pressure vessel steels that are heat treatable to high strength levels with high toughness. All of the steels were heat treated to yield strength of approximately 690 MPa (100 ksi) and evaluated by identical test methods. The relative cracking susceptibility of the steels was determined from threshold stresses in constant-load tension tests and threshold stress intensities shown by precracked double-cantilever-beam specimens. Tempering treatment was a decisive factor in SSC susceptibility of low-nickel steels. When double tempered, low-Ni steels with greater than 1 pct Ni can be equivalent in SSC resistance to nominally nickel-free 4130 steel.     

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.     

Effect of strain rate on stress corrosion cracking in duplex type 304 stainless steel weld metal

The influence of strain-rate on the stress-corrosion cracking properties of wholly austenitic Type 304 base metal and duplex austeno-ferritic Type 304 weld metal in boiling MgCl₂ was investigated using constant extension rate tensile testing techniques. Transgranular SCC in both base and weld metals is preferred at low strain-rates, while intergranular cracking in the base metal and interphase cracking along the austenite-ferrite interface in the weld metal are preferred at higher strain-rates. Promotion of the intergranular stress-corrosion cracking in the base metal and “interphase-interface” stresscorrosion cracking in the weld metal with increases in strain-rate may be mechanistically analogous. Stress-induced alterations in the grain or interphase boundary defect structure may make these regions preferentially susceptible to dissolution. W. A. BAESLACK III, Lt., USAF, formerly with Rensselaer Polytechnic Institute, Troy, New York     

Solidification and solidification cracking in nitrogen-strengthened austenitic stainless steels

The solidification behavior of three heats of nitrogen-strengthened austenitic stainless steel was examined and was correlated with solidification mode predictions and with hot cracking resistance. The heat of NITRONIC* 50 solidified by the austenitic-ferrite mode, and the NITRONIC 50W and NITRONIC 50W - Nb heats solidified by the ferritic-austenitic mode. This behavior was in good agreement with predictions based on Espy’s formulas for Cr and Ni equivalents. Both the NITRONIC 50W and NITRONIC 50W + Nb welds contained primary delta-ferrite, with the latter weld and the NITRONIC 50 weld also containing some eutectic ferrite. Solute profiles in austenite near the eutectic ferrite showed decreasing Fe and increasing Cr, Ni, Mn, and Mo relative to austenite in the dendrite cores. Numerous Nb-rich precipitates were found on the eutectic ferrite/austenite interfaces and within the eutectic ferrite. The precipitates were mainly Nb(C, N), with some Z-phase, a Nb-rich nitride, also detected. One instance of the transformation of eutectic ferrite to sigma-phase was observed to have occurred during cooling of the NITRONIC 50 weld. Hot cracking was seen in the NITRONIC 50 and NITRONIC 50W + Nb welds and resulted from the formation of a niobium carbonitride eutectic in the interdendritic regions. In the absence of Nb, the NITRONIC 50W heat formed no observable eutectic constituents and did not hot crack. The presence of hot cracks in the NITRONIC 50W + Nb weld indicates that solidification by the ferritic-austenitic mode did not counteract the effects of small Nb additions.     

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。     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.