High-Temperature Oxidation Studies of Low-Nickel Austenitic Stainless Steel. Part II: Cyclic Oxidation

The cyclic-oxidation behavior of a low-nickel austenitic stainless steel (LNiSS) at 873 and 973 K has been investigated up to 500 cycles. A wide range of experimental techniques, including SEM, EDS, and XRD have been applied to examine the oxide scales. After cyclic oxidation at 873 and 973 K, the new LNiSS alloy showed good oxidation resistance. XRD and EDS analysis show that composition of oxide scales developed during cyclic oxidation at 873 and 973 K are the same. The oxide scales formed a cubic oxide-type M₂O₃ and a cubic spinel-type M₃O₄ adhered well to the substrate.     

High-Temperature Oxidation Studies of Low-Nickel Austenitic Stainless Steel. Part I: Isothermal Oxidation

The oxidation behavior of low-nickel austenitic stainless steel (LNiSS) in air at 873 and 973 K was investigated for 500 hr. The oxide scales formed during the process were examined by a wide range of experimental techniques including SEM/EDS, XRD, and EPMA, in order to determine their influence on kinetics behavior. Kinetics laws were close to parabolic at both temperatures, but the morphology of scales showed important differences with temperature. At 873 K the oxide scale was thinner, with irregular growth, intrusions, and without spallation. It was concluded that slower kinetics and advantageous scale morphology suggest that LNiSS is a suitable material for isothermal oxidation in air at 873 K. At higher temperatures, uniformly thick scales plus iron-rich nodules were observed with different composition regions. The most destructive feature was the formation of Fe-rich nodules, which were vulnerable to spalling during cooling.     

The Effects of Small Additions of Yttrium on the High-Temperature Oxidation Resistance of a Si-Containing Austenitic Stainless Steel

The oxidation characteristics of a 19Cr–10Ni–1.5Si alloy with and without 0.03Y have been examined using scanning-electron microscopy, secondary-ion mass spectroscopy and transmission-electron microscopy. Y has been found to segregate to oxide grain boundaries and thereby to inhibit outward-cation diffusion and promote the internal oxidation of Si. The synergism between the beneficial effects of Y and Si is described, and the results are discussed in relation to the de-oxygenating and de-sulphurizing influences of Y in steel production.     

Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.     

Grain-Size Effects on the High-Temperature Oxidation Behaviour of Chromium Steels

Oxidation of two low-Cr (Cr content 1.5 wt% and 2.25 wt%) and three high-Cr (Cr content 9 wt%, 12 wt% and 18 wt%) boiler steels was investigated at temperatures between 550 °C and 830 °C in laboratory air. Thermogravimetry (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied for evaluation of the oxidation kinetics, oxides phase identification and scale structure analysis. Particular attention was paid to the phenomenon of inward oxidation and its relationship with the Cr content of the steels under investigation. The results showed that the inward oxidation of the Cr steels is governed by grain boundary diffusion. Two different Cr-depending tendencies concerning the effect of alloy grain size on the inward oxidation were observed. For low-Cr steels (less than 2.25 wt% Cr), an increase in the grain size improved the oxidation resistance, while steels with high Cr content (18 wt% Cr) can form a thin and protective chromia scale on the surface more easily at finer grain size. In the latter case an increase in grain size deteriorates the oxidation resistance by the formation of a thicker scale composed of both an outer and an inner Fe-oxide-based layer.     

304奥氏体不锈钢固溶渗氮的研究

在氮气中对304奥氏体不锈钢进行固溶渗氮，用电子探针测定了渗氮层的氮浓度分布，用扫描电子显微镜观察了渗氮层的金相组织。结果表明，固溶渗氮可以使304奥氏体不锈钢获得高氮奥氏体层；表面氮浓度随渗氮温度的升高和氮气压力的增大而增加，渗氮层深度随渗氮温度的升高和保温时间的延长而增加；渗氮后空冷和水冷时氮为固溶状态，炉冷时会析出氮化物。     

铣削加工参数对304奥氏体不锈钢耐蚀性的影响

利用电化学动电位极化法测试了不同铣削加工参数下304奥氏体不锈钢的腐蚀行为。结果表明:铣削加工后材料的耐蚀性高于原始材料的。耐点蚀能力随着铣削加工进给率和切削速率的上升而下降,不同加工参数之间材料的点蚀电位差高达138mV,表明铣削加工参数对于奥氏体不锈钢的耐蚀性有着强烈的影响。     

Comparison of the High-Temperature Oxidation of Uncoated and CVD–FBR Aluminized AISI-304 Stainless Steel

Aluminide coatings were obtained by means of the CVD–FBR technique at 525°C for 1.5 hr under a reactive-gas mixture composed of 10 vol.% H₂+1 vol.% HCl, the rest being Ar as the fluidizing inert gas. Subsequent heat treatment at temperatures to 900°C was conducted to enhance interdiffusion of the components. As a result, substoichiometric Ni–Al phases were found to form. Uncoated and coated plus heat-treated specimens were then subjected to 950°C oxidation for up to about 200 hr under isothermal conditions. The coated plus heat-treated specimens had much lower oxidation rates than the uncoated ones because of the formation of protective alumina scales. Loss of protective behavior occurred only by spalling of the alumina scales upon cooling from the oxidation temperature. The higher oxidation rates of uncoated specimens have been attributed to nodule formation and minor subsequent spalling of the oxide scale.     

奥氏体304不锈钢钢管A-TIG焊接工艺研究

以Φ45 mm×8 mm规格奥氏体304不锈钢钢管为研究对象,采用A-TIG焊(添加活性剂的钨极氩弧焊)进行对接焊接工艺试验,研究不同焊接起始点对焊缝成型的影响,并检测外观质量符合要求的焊件的性能和微观组织等。分析认为:采用A-TIG焊接工艺焊接Φ45 mm×8 mm规格奥氏体304不锈钢钢管时,可在不开坡口、不填丝的情况下,实现一次性焊透,并达到单面焊双面成型的目的;焊缝组织为奥氏体+铁素体,其组织与未添加活性剂时的一致,但添加活性剂可明显改善组织性能;焊接接头的性能满足标准要求。     

Dynamic Recrystallization during Hot Deformation of 304 Austenitic Stainless Steel

The kinetics of dynamic recrystallization (DRX) during hot compression of 304 austenitic stainless steel was studied over the temperature range of 900-1200 °C and strain rate range of 0.002-0.1 s^?1. The initiation and evolution of DRX were investigated using the process variables derived from flow curves. By the regression analysis for conventional hyperbolic sine equation, the activation energy for DRX was determined as Q = 475 kJ mol^?1. The temperature and strain rate domain where DRX occurred were identified from the strain rate sensitivity contour map. The critical stress (and strain) for the initiation of DRX was determined from the inflection point on the work hardening rate (θ = dσ/dε) versus flow stress (σ) curve. The saturation stress of the dynamic recovery (DRV) curve was calculated from the θ-σ plot at the same condition at which DRX occurred. Progress of fraction recrystallization was determined from the difference between the generated DRV curve and the experimental DRX curve. In addition, the microstructural evolution at different strain levels during DRX was characterized and compared with the calculated fraction recrystallization.     

奥氏体耐热不锈钢310S的抗高温氧化性能研究

采用增重法研究了奥氏体耐热不锈钢310S在700、900和1000℃空气中高温氧化动力学,并结合X射线衍射(XRD)、扫描电镜(SEM)及能谱分析(EDS)等手段,对氧化膜的形貌和组成进行了分析。结果发现,700℃时氧化速率比较稳定且氧化增重较小,其余温度下氧化增重较大且遵循抛物线规律。该钢中Cr在高温时容易形成FeO·Cr2O3、FeO·Fe2O3和尖晶石结构(FeCr2O4,NiCr2O4)等保护性氧化膜,是310S钢具有良好的抗高温氧化性能的重要原因。     

奥氏体耐热不锈钢310S的抗高温氧化性能研究

采用增重法研究了奥氏体耐热不锈钢310S在700、900和1 000℃空气中高温氧化动力学,并结合X射线衍射(XRD)、扫描电镜(SEM)及能谱分析(EDS)等手段,对氧化膜的形貌和组成进行了分析.结果发现,700℃时氧化速率比较稳定且氧化增重较小,其余温度下氧化增重较大且遵循抛物线规律.该钢中Cr在高温时容易形成FeO·Cr2O3、FeO·Fe2O3和尖晶石结构(FeCr2O4,NiCr2O4)等保护性氧化膜,是310S钢具有良好的抗高温氧化性能的重要原因.     

固溶处理温度对304奥氏体不锈钢敏化与晶间腐蚀的影响

采用电化学动电位再活化法(EPR)研究了经950℃和1050℃固溶处理304奥氏体不锈钢晶间腐蚀敏化度Ir/Ia、敏化时间t和敏化温度T之间关系,根据腐蚀速率Rmpy与微观腐蚀形貌绘制了304不锈钢敏化的TTS曲线,探讨了固溶处理温度改变对TTS曲线的影响。结果表明,1050℃固溶处理试样的耐晶间腐蚀性能优于950℃固溶处理试样。     

Studies on Stress Corrosion Cracking of Super 304H Austenitic Stainless Steel

Stress corrosion cracking (SCC) is a common mode of failure encountered in boiler components especially in austenitic stainless steel tubes at high temperature and in chloride-rich water environment. Recently, a new type of austenitic stainless steels called Super304H stainless steel, containing 3% copper is being adopted for super critical boiler applications. The SCC behavior of this Super 304H stainless steel has not been widely reported in the literature. Many researchers have studied the SCC behavior of steels as per various standards. Among them, the ASTM standard G36 has been widely used for evaluation of SCC behavior of stainless steels. In this present work, the SCC behavior of austenitic Fe-Cr-Mn-Cu-N stainless steel, subjected to chloride environments at varying strain conditions as per ASTM standard G36 has been studied. The environments employed boiling solution of 45 wt.% of MgCl₂ at 155 °C, for various strain conditions. The study reveals that the crack width increases with increase in strain level in Super 304H stainless steels.     

板坯连铸304奥氏体不锈钢结晶器保护渣的研制

在分析304奥氏体不锈钢凝固特性的基础上,研制开发了板坯连铸304奥氏体不锈钢用TD601结晶器保护渣。TD601结晶器保护渣有较强的吸附夹杂物的能力,具有较好的稳定性、适用性,能满足生产现场的工艺要求。现场试验,铸坯修磨率降低、成品材质量优级率提高。     

304NG奥氏体不锈钢在超临界水环境中的腐蚀行为

采用腐蚀增重法研究了304NG奥氏体不锈钢在550~650℃/25 MPa的超临界水(SCW)中的腐蚀行为。使用SEM和EDS分析了材料的氧化动力学、氧化膜表面形貌、氧化膜截面形貌和合金元素分布。结果表明:304NG奥氏体不锈钢在SCW中的腐蚀增重服从抛物线生长规律;在550℃的SCW中具有较好的抗腐蚀性能,当温度升高到650℃时,腐蚀增重速率急剧升高;304NG奥氏体不锈钢在SCW中腐蚀初期形成薄而致密的氧化膜,之后则会出现疖状腐蚀,并且腐蚀岛的尺寸随着腐蚀时间的延长而逐渐增大,650℃时尤为明显;腐蚀生成的氧化膜形态为典型的双层结构。     

304奥氏体不锈钢拉深过程有限元模拟

不锈钢制品由于其美观的外表和良好的使用性能,在国民经济各行各业被广泛使用,但不锈钢在拉深过程中硬化严重,易出现起皱、破裂现象,使成品率降低。本文应用ABAQUS有限元软件模拟了304奥氏体不锈钢圆筒件在不同摩擦条件下的拉深成形过程,分析了摩擦系数对304不锈钢圆筒件拉深成形过程的影响。结果表明,304奥氏体不锈钢圆筒件拉深过程中,较适宜的摩擦系数为0.08～0.15。当摩擦系数过小时,将在坯料与凹模圆角处发生破裂;而摩擦系数过大,则严重影响坯料的拉深极限。     

激光熔凝对304奥氏体不锈钢拉伸力学性能的影响

采用5 kW横流CO2激光器,分别在常温空气冷却、冰水快速冷却条件下对304奥氏体不锈钢的上下表面进行熔凝试验,并对比分析其显微组织和拉伸力学性能.结果表明:经激光熔凝处理后,熔凝层组织形态由里及表依次为平面晶、胞状晶、树枝晶和等轴晶;组织明显得到细化,常温空气冷却和冰水快速冷却的熔凝层组织分别比基体组织细化约14倍和18倍;经两种不同的处理方式得到的试件,其拉伸力学性能均得到增强,且随熔凝层冷却速度的增加,相应的拉伸力学性能指标提高的幅度也增加.其中,经冰水快速冷却激光熔凝处理后的试件屈服强度、抗拉强度和伸长率分别可提高约22.4％、16.4％和15.7％     

SUS304-2B不锈钢薄板退火工艺研究

采用单向拉伸实验研究了SUS304-2B不锈钢薄板的加工硬化规律；采用退火实验研究了该合金硬化后的退火软化规律和机理，确定了其最佳的退火工艺参数。实验表明，冷加工后该合金强度明显增加，塑性降低；并且随着形变量的增加，组织中形变孪晶数目增多，加工硬化的程度增加；对不同加工硬化程度的试样，在低温(100～490℃)下退火35min(空冷)后再拉伸，其力学性能基本不变，退火软化效果不明显；在高温(900～1050℃)下退火3～10min(快冷)，该合金组织发生完全再结晶，且晶粒大小较均匀，退火软化效果明显。由此确定，SUS304-2B不锈钢最佳退火工艺为：在1020～1150℃下退火3min(快冷)。