On Microstructure and Microhardness of Isothermally Aged UNS S32760 and the Effect on Toughness and Corrosion Behavior

This paper investigates the microstructure and secondary phase precipitations obtained in UNS S32760 super duplex stainless steel and their effect on impact toughness and corrosion resistance. The heat treatment included first solution annealing at 1150 °C followed by water quenching, then isothermal heating at different temperatures from 350 to 950 °C for different times, ranging from less than 1 min to 600 min, followed by water quenching again. Microscopic investigation, microhardness tests, and x-ray diffraction (XRD) analysis were used to identify the microstructure and secondary phase precipitations formed by heat treatment. The study indicates a fair correlation between the microscopic observations and microhardness results, while XRD analysis defined the phase’s chemistry and confirmed the microscopic and hardness results. In addition to the austenite (γ) and ferrite (α) phases of the duplex structure, secondary phases of (σ, χ, and chromium nitrides) are observed at a high temperature range, while (?) and (aged ?) are observed at a lower temperature range. It is concluded that the microhardness test can be used to identify the phases appearing in the microstructure, which results in fair prediction for the TTT diagram and σ-phase range. The variation of toughness and corrosion resistance by heat treatment differs depending on the secondary phase formation.     

Influence of Step Annealing Temperature on the Microstructure and Pitting Corrosion Resistance of SDSS UNS S32760 Welds

In the present work, the influence of step annealing heat treatment on the microstructure and pitting corrosion resistance of super duplex stainless steel UNS S32760 welds have been investigated. The pitting corrosion resistance in chloride solution was evaluated by potentiostatic measurements. The results showed that step annealing treatments in the temperature ranging from 550 to 1000 °C resulted in a precipitation of sigma phase and Cr₂N along the ferrite/austenite and ferrite/ferrite boundaries. At this temperature range, the metastable pits mainly nucleated around the precipitates formed in the grain boundary and ferrite phase. Above 1050 °C, the microstructure contains only austenite and ferrite phases. At this condition, the critical pitting temperature of samples successfully arrived to the highest value obtained in this study.     

Influence of sigma-phase formation on the localized corrosion behavior of a duplex stainless steel

Because of their austenitic-ferritic microstructures, duplex stainless steels offer a good combination of mechanical and corrosion resistance properties. However, heat treatments can lower the mechanical strength of these stainless steels as well as render them susceptible to intergranular corrosion (IGC) and pitting corrosion. In this study, a low-carbon (0.02%) duplex stainless steel is subjected to various heat treatments at 450 to 950 °C for 30 min to 10 h. The heat-treated samples then undergo ASTM IGC and pitting corrosion tests, and the results are correlated with the microstructures obtained after each heat treatment. In the absence of Cr₂₃C₆ precipitation, σ-phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic σ-phase precipitates are deleterious for IGC resistance. Longer-duration heat treatments (at 750 to 850 °C) induce chromium diffusion to replenish the chromium-depleted regions around the σ-phase precipitates and improve IGC resistance; pitting resistance, however, is not fully restored. Various mechanisms of σ-phase formation are discussed to show that regions adjacent to σ-phase are depleted of chromium and molybdenum. The effect of chemical composition (pitting resistance equivalent) on the pitting resistance of various stainless steels is also noted.     

固溶温度对2507双相不锈钢力学性能及耐蚀性的影响

利用光学显微镜、扫描电镜、XRD、拉伸试验机和电化学综合测试仪等研究了不同固溶温度对2507超级双相不锈钢组织、力学性能和耐蚀性的影响。采用Thermo-Calc热力学软件计算了2507双相不锈钢的热力学平衡相图,并与测试结果进行了对比。研究结果表明,经1050 ℃及以上温度固溶后,σ相溶解;随着固溶温度的升高,铁素体相含量增加,奥氏体相含量降低,α/γ相体积分数比增加;1050~1100 ℃固溶30 min并水冷时,双相不锈钢具有较好的综合力学性能,屈服强度、抗拉强度和伸长率分别大于600 MPa、840 MPa和35%。1050 ℃固溶30 min时,双相钢可获得较好的耐蚀性能。     

不同组织X80钢在高pH土壤模拟溶液中的点蚀电化学行为

将水淬、空冷方式热处理的X80钢与供货状态(原始组织)X80钢相比较,用动电位极化曲线和动电位交流阻抗谱研究不同组织X80钢在0.5 mol/L Na_2CO_3+1 mol/L NaHCO_3高pH值土壤模拟溶液中的点蚀电化学行为。结果表明,水淬热处理后的X80钢更易钝化,点蚀敏感性低,但维钝电流较其他两种组织的都大;三种组织的X80钢极化电阻和电极表面状态常数n值随电位的变化规律基本一致,但水淬组织的n值在不同电位区间分别高于或低于其他两种组织;这些电化学行为的差异与不同组织试样表面钝化膜的非均匀性和致密程度有关;动电位极化法和动电位电化学阻抗谱法所得结论一致,腐蚀电流密度随电位的变化及电极极化电阻随电位变化都可用来反映电极的点蚀行为。     

Effect of annealing temperature on pitting behavior and microstructure evolution of hyper‐duplex stainless steel 2707

Hyper‐duplex stainless steel (HDSS) 2707 is a competitive material for application in extremely caustic environments. In this study, different annealing temperatures ranging from 1020°C to 1200°C were examined by electrochemical tests and microstructure analysis. The microstructure characterization indicated that precipitations were detected when the annealing temperature was below 1050°C and a relatively balanced austenite–ferrite phase structure was obtained at 1100°C. Through electrochemical measurements in NaBr solution, it was revealed that with the increase of temperature the pitting resistance of HDSS 2707 first rose then declined, peaking at 1100°C. The highest critical pitting temperature was about 67°C. In addition, the pitting position shifted from austenite phase to austenite–ferrite boundary and finally to ferrite interior with the annealing temperature increasing, which was in agreement with the pitting resistance equivalent values (PREN) of the two phases.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Effect of prior heat treatment on microstructure and mechanical properties of 0·23% carbon dual phase steel

Abstract

The present study is aimed at understanding the effect of prior heat treatment on the microstructure and mechanical properties of 0·23% carbon dual phase steel. The as received steel was subjected to annealing, normalising and hardening treatment before actual dual phase heat treatment. These steels along with the as received steel were intercritically annealed at 745°C followed by water quenching in order to produce a dual phase microstructure consisting of varying quantities and morphologies of ferrite and martensite. Mechanical properties were evaluated by tensile testing and hardness measurement. It was found that prior heat treatment affects the final dual phase microstructure in terms of size and relative amount of ferrite and martensite. It was also found that dual phase treatment resulted in improved mechanical property. Optimum combination of strength and ductility was obtained with the steel subjected to prior hardening (DPLA-3) treatment. The maximum in ultimate tensile strength (762 MPa) was obtained with the steel subjected to prior annealing treatment (DPLA-1). The X-ray diffraction study confirmed the presence of carbides in DPLA-1 steel, which contribute to the increased strength by precipitation hardening.     

Sensitization Behavior of Type 409 Ferritic Stainless Steel: Confronting DL-EPR Test and Practice W of ASTM A763

Stainless steels employed for manufacturing automotive exhaust systems must withstand severe thermal cycles, corrosive environment due to urea decomposition, and welding operations. AISI 409 ferritic stainless steel can be considered a low-cost alternative for this application. However, depending on the manufacturing conditions during welding cycles, this material can be sensitized due to the precipitation of chromium carbides at grain boundaries. In this work, the intergranular corrosion resistances of the AISI 409 ferritic stainless steel were evaluated after annealing at 300, 500, and 700 °C for 2, 4, and 6 h. Solution-annealed samples were also tested for comparison purposes. Two methodologies were used to assess the sensitization behavior of the 409 stainless steel samples: the first one was based on the ASTM A763 (practice W), while the second one was based on the double-loop electrochemical potentiodynamic reactivation test. It was possible to identify that the annealing treatment performed at 500 °C was more critical to the occurrence of intergranular corrosion.     

Electrochemical corrosion response of a low carbon heat treated steel in a NaCl solution

Dual‐phase (DP) steels are produced from a specific heat treatment procedure and have recently emerged as a potential class of engineering materials for a number of structural and automobile applications. Such steels have high strength‐to‐weight ratio and reasonable formability. The present study aims to investigate the effects of four different and conventional heat treatments (i.e., hot rolling, normalizing, annealing, and intercritical annealing) on the resulting microstructural patterns and on the electrochemical corrosion behavior. Electrochemical impedance spectroscopy (EIS) and Tafel plots were carried out on heat treated steel samples in a 0.5 M NaCl solution at 25 °C with neutral pH. An equivalent circuit analysis was also used to provide quantitative support for the discussions. The normalizing and the annealing heat treatments have provided the highest and the lowest corrosion resistances, respectively. The intercritical annealing and as‐received (hot rolled) low carbon steel samples have shown similar corrosion behavior. Although a deleterious effect on the corrosion resistance has been verified for DP steel due to the residual stress from the martensite formation, it combines good mechanical properties with intermediate electrochemical corrosion resistance.     

S32205双相不锈钢冷轧退火组织转变及强塑化机理分析

通过设计一种特殊的冷变形退火工艺,实现了S32205双相不锈钢强度和塑性同时提升。通过室温拉伸试验、显微组织观察、背散射电子衍射分析、热力学计算等手段研究了显微组织特征及新组织结构的强塑化机理。结果表明,该工艺处理后S32205双相不锈钢的晶粒尺寸呈双态分布,两相组织混合分布且细晶的新生奥氏体均匀弥散分布在铁素体相中;在细晶强化和两相间变形不协调引发的背应力强化的共同作用下,试验钢强度和塑性相较于传统固溶热处理工艺同时得到了提升。     

深冷处理对440C马氏体不锈钢组织和耐蚀性的影响

研究了深冷处理对440C马氏体不锈钢组织和耐蚀性的影响。研究表明,液氮深冷处理后440C不锈钢硬度可提高2.3 HRC,残留奥氏体含量降低了11.7%,72 h中性盐雾试验表面无明显点蚀。440C不锈钢淬火后室温停留2 h以上残留奥氏体含量明显增加,硬度值、耐蚀性下降。实际生产中,淬火与液氮深冷处理时间间隔应不超过2 h。     

循环冷却水中 2507 双相不锈钢微生物腐蚀研究

目的研究2507双相不锈钢在循环冷却水模拟溶液(添加微生物SRB+IOB)中的腐蚀特征,确定其腐蚀机理。方法模拟某炼油厂循环冷却水溶液,采用SEM,EDS和电化学测试等手段分析2507双相不锈钢在SRB+IOB循环冷却水中浸泡不同时间后的腐蚀产物形貌及其电化学腐蚀情况。结果2507双相不锈钢的腐蚀速率很低,属于轻度腐蚀;在循环冷却水模拟溶液中的阳极极化曲线具有明显的钝化区,且钝化膜具有良好的自修复能力;腐蚀倾向随时间增加先增大后减小,腐蚀速率随时间增加先减小后增大。结论 SRB和IOB及其代谢活动与Cl-协同作用是点蚀的主要原因;2507双相不锈钢具有良好的钝化性和耐微生物腐蚀性能。     

Use of SECM to study the electrochemical behavior of DIN 1.4575 superferritic stainless steel aged at 475 °C

In order to lower the excessive costs of metallic prosthesis materials alternatives to Ti and Ti alloys have been searched. In this study, the corrosion resistance of the DIN 1.4575 superferritic stainless steel, either solution annealed or solution annealed and aged at 475 °C for periods varying from 100 to 1080 h, was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization methods in Hanks' solution. The solution annealed and the aged for 1080 h samples were also tested using scanning electrochemical microscopy (SECM) in a 0.1 mol/L NaCl solution at 25 °C. The EIS results showed that the corrosion resistance of the DIN 1.4575 steel decreases with heat treatment time at 475 °C probably due to alpha prime formation. Besides the diminution of the overall impedance values, the low frequency limit of the Nyquist diagrams show a progressive change from an almost capacitive response to a resistive behavior as the heat treatment time increases. Pitting corrosion resistance also decreased with aging time at 475 °C.     

喷丸和盐酸酸洗对2205双相不锈钢热轧板氧化层微观结构的影响

利用光学显微镜(OM)、X射线衍射(XRD)、扫描电镜(SEM)配合能谱仪(EDS)和X射线光电子能谱(XPS)研究了2205双相不锈钢热轧板经过退火和喷丸处理后的氧化层组成和结构,并分析了盐酸及混酸(HNO3+HF)酸洗后表面钝化膜微观组织结构,利用电化学测试手段分析了酸洗后钢的耐蚀性,进一步提出了热轧退火2205双...     

Characteristics of steels in annealing combined with hot-rolling preheating

The rates of transformation and the morphologies after full annealing and industrial rolling without reheating were determined to establish the degree of similarity. Steels with 0.1–1% C and almost nil to 17% alloying addition (M2 steel) were isothermally transformed in a dilatometer after various thermomechanical processing (TMP) sequences. For annealing, the heat treatment was performed at about 880 °C, depending on C content, and then cooled for transfer to the dilatometer. In some cases, specimens were rolled to 50% reduction in 3 passes before cooling. The rolling schedule was simulated by preheating to 1150 °C followed by slow cooling to 950 °C for finish rolling as above before transfer to the dilatometer. Some specimens were directly cooled to transformation. The high preheat thoroughly homogenized the austenite (except for the M2 steel) and slowed the transformation by a factor >10 as the alloying content rose. In all cases, the hot rolling accelerated the transformation. After the high preheat the rate and microstructure were restored to those of standard annealing for hypo-eutectoid and M2 steel.     

对相钢在流动中性含砂氯化物中的磨损腐蚀

研究了双相钢在流动含砂的3．5％NaCl溶液中的磨损腐蚀规律,测定、分析了流动体系中的电化学阻抗谱,揭示了双相钢磨损腐蚀过程中电化学的作用及其机制。结果表明,腐蚀曜 磨损腐蚀过程中起主要作用,流体力学因素只是加速了腐蚀电化学过程,阻抗谱在低频区出现一直线段和低频收缩现象。分别是双相钢在磨损腐蚀过程中电极处于自钝化状态,并受离子在钝化膜中的扩散、迁移过程控制和电极表面局部遭受破坏的特征,对于流动体系中电化学阻抗谱的分析,曹氏阻抗理论同样适用。     

Accessing the full spectrum of corrosion behaviour of tempered type 420 stainless steel

Bipolar electrochemistry produces a linear potential gradient between two feeder electrodes, providing access to the full spectrum of anodic-to-cathodic electrochemical behaviour. A type 420 martensitic stainless steel has been used to investigate microstructure evolution and corrosion behaviour with application of different tempering heat treatments. Tempering treatments at 250°C, 400°C and 700°C revealed the occurrence of pitting corrosion, with treatments at 550°C resulting in general and intergranular corrosion. Cr₂₃C₆ was present in all tempering conditions, with Cr₇C₃ and CrC only observed for tempering at 550°C. The 250°C tempering treatment had the highest corrosion resistance with a hardness value much higher than 500 HV.     

热处理工艺对2205双相不锈钢耐蚀性能的影响

对2205双相不锈钢采用不同温度进行热处理,然后用光学显微镜和电子扫描电镜观察其在0.33mol/L FeCl3+0.05 mol/L HCl溶液中腐蚀后的形貌;测试其显微硬度的变化、在沸腾的65%的硝酸溶液中浸蚀24 h的腐蚀速率和在25℃的3.5%NaCl溶液中的点蚀电位。研究表明:2205双相不锈钢在750~900℃保温4h有σ相析出,材料的显微硬度增大。同时随着热处理温度的升高,2205双相不锈钢的点蚀电位降低,腐蚀速率增大。     

Effect of small additions of ruthenium on pitting corrosion resistance of LDX2101 duplex stainless steel

Four stainless steel alloys with ruthenium compositions of 0·16, 0·24, 0·32 and 0·64 wt-% were produced from pieces cut from commercial LDX2101 duplex stainless steel plate with the manufacturer’s composition of 0·03C–21·5Cr–1·5Ni–0·3Mo–5·0Mn plus pressed ruthenium powder with purity of 99·8%. After solution annealing the samples, the actual chemical composition was analysed using X-ray fluorescence analysis, and then, ASTM A923 (01·03) test method A – sodium hydroxide etch test for classification of etch structures of duplex stainless steel was used to analyse their microstructure. Corrosion potential and pitting potential of these samples were evaluated using a potentiodynamic polarisation technique, and the results were compared to corrosion and pitting potentials of the control alloy LDX2101. The tests for both produced and control alloys were performed in naturally aerated 3·56%NaCl aqueous solution at 25±2°C. The results indicated that small additions of ruthenium significantly improved the pitting potentials of the resulting alloys. The results also indicated that ruthenium additions have no detrimental effect to the microstructure of the resulting alloys. In addition, if such small additions of ruthenium will not improve the general corrosion of the resulting alloy, it will at least not have any detrimental effect on the resulting alloy. Ruthenium will also lower the current required to maintain the passive state of LDX2101 stainless steel. In addition to reduced current to maintain the passivity of LDX2101 stainless steel, ruthenium also increased the passive range of LDX2101 stainless steel.