Grain-Size Effects on the High-Temperature Oxidation of Modified 304 Austenitic Stainless Steel

The high-temperature oxidation behavior of modified 304 austenitic stainless steels in a water vapor atmosphere was investigated. Samples were prepared by various thermo mechanical treatments to result in different grain sizes in the range 8–30 μm. Similar Σ3 grain boundary fraction was achieved to eliminate any grain-boundary characteristics effect. Samples were oxidized in an air furnace at 700 °C with 20 % water vapor atmosphere. On the fine-grained sample, a uniform Cr₂O₃ layer was formed, which increased the overall oxidation resistance. Whereas on the coarse-grained sample, an additional Fe₂O₃ layer formed on the Cr-rich oxide layer, which resulted in a relatively high oxidation rate. In the fine-grained sample, grain boundaries act as rapid diffusion paths for Cr and provided enough Cr to form Cr₂O₃ oxide on the entire sample surface.     

Investigation of Susceptibility to Intergranular Corrosion of Tin-Added Austenitic Stainless Steel

The intergranular corrosion(IGC) character of tin-added B316 LX and the influence of tin addition on IGC susceptibility were investigated by DL-EPR, oxalic acid etch test and transmission electron microscopy. IGC susceptibility of B316 LX is mainly caused by the precipitation of M23C6 carbide and intermetallic Laves phase. DL-EPR test is unsuitable to evaluate the IGC susceptibility of B316 LX with long-time sensitization, because more carbides and intermetallic phases are formed at grain boundaries and inside the grains, which induce more severe IGC and pits attacks. Tin addition increases the IGC susceptibility, maybe due to diffusion of tin toward grain boundaries.     

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。     

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.     

Anisotropic Electroplastic Effects on the Mechanical Properties of a Nano-Lamellar Austenitic Stainless Steel

Effects of electropulsing treatment(EPT) on the microstructural evolution and mechanical properties of a cold-rolled 316 L austenitic stainless steel with nano-lamellar structure were investigated.The EPT experiments were carried out with the electric current direction along the rolling direction(RD) and the transverse direction(TD) of the samples,respectively.Significant anisotropic electroplastic effects for the RD and TD specimens,i.e.,reduced hardness/strength and enhanced ductility,were obtained owing to the different recrystallization behaviors of the RD and TD specimens during EPT.The RD specimens after EPT with larger recrystallized grain size and higher volume fraction of recrystallization show lower strength and higher elongation than that of the TD specimens.The main reason might be attributed to the change of the current direction in the two kinds of samples,which results in the different sensitivity of the microstructures to thermal and athermal effects during EPT.     

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.     

不同参数脉冲电流对不锈钢Cr18Ni9Ti凝固组织的影响

采用储能电容为1200μF的脉冲电源，在不同的脉冲电压下，对奥氏体不锈钢1Crl8Ni9Ti进行脉冲电流处理。实验结果表明：脉冲电流能够细化奥氏体不锈钢的凝固组织，使奥氏体一次枝晶长度缩短，二次枝晶间距减小。在一定范围内，脉冲电压升高和脉冲重复率增大都有利于奥氏体不锈钢凝固组织的细化和改善。     

高硼奥氏体钢的抗氧化性能

研究了高硼奥氏体钢在850℃下的高温抗氧化性能,采用SEM研究了材料的组织状态。结果表明,不含硼的奥氏体耐热钢850℃保温9 h和20 h的抗氧化评级均为GB/T 13303-1991中的4级"弱抗氧化性",材料抗高温氧化性能优于ESR-H13的5级"不抗氧化性"。添加硼后,氧化过程中硼有利于形成致密的氧化薄膜CrBO₃,有效阻止了氧化行为的深入,提高了材料的抗氧化性能。含硼0.3%、碳0.5%的奥氏体耐热钢850℃保温9 h和20 h抗氧化评级均达到2级"抗氧化性"。高硼奥氏体耐热钢的含碳量从0.2%提高到0.4%时,材料均具备良好的抗氧化性能,850℃下保温9 h和20 h的氧化速度均达到2级"抗氧化性"标准。较高的含碳量提高了奥氏体基体稳定性,并有利于形成CrBO₃。高硼奥氏体钢的高温抗氧化性能优于ESR-H13钢。     

Corrosion Protection of Low-Nickel Austenitic Stainless Steel by Yttrium and Erbium-Ion Implantation Against Isothermal Oxidation

The beneficial effect of ion-implanted yttrium and erbium on the oxidation behavior of low-nickel austenitic stainless steel (LNiSS) at 973 K has been investigated up to 500 hr of oxidation test in air. The resulting oxide scales were examined by a wide range of experimental techniques, including SEM/EDS, XRD, and EPMA. The results indicate that both reactive elements have similar effects. The most significant effects have been to significantly reduce the corrosion rate and to improve the oxide scale adhesion. It is concluded that ion implantation is a powerful tool as surface-modification process introducing reactive elements in the top surface.     

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.     

化学氧化处理对不锈钢表面性质的影响

研究了化学氧化处理对不锈钢（1Cr18Ni9Ti）表面亲水性、表面自由能及表面化学结构的影响,并提出了此三者之间的关系。研究结果表明：化学氧化处理可以有效改善不锈钢的表面亲水性,且亲水性随处理时间的延长和处理温度的升高而升高,最佳工艺条件是处理温度为75℃,处理时间为8min;经化学氧化处理后,不锈钢的表面自由能γgs及其极性分量γgs^p显著增大,使不锈钢表面呈现出极强的亲水性;多功能电子能谱分析表明,表面自由能γgs及其极性分量γgs^p的增加与表面及一定厚度范围内含氧极性基团的增加有关。     

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。     

Effect of Cerium on High-Temperature Oxidation Resistance of 00Cr17NbTi Ferritic Stainless Steel

The influence of cerium addition on the isothermal oxidation behavior of 00Cr17 NbTi ferritic stainless steel was studied at temperature up to 1,000 °C for 100 h in air. The results show that cerium additions can reduce the grain size of this ferritic stainless steel, improve the diffusion of chromium and decrease the critical concentration of chromium to form protective Cr2O3 layer. With the increasing of cerium addition, the oxide particles become smaller and this can increase the rupture strength and spalling resistance of oxide layers. The transport mechanism through the oxide layer is varied from metal transport outward from steel to principally oxygen transport inward with the increase of cerium content,which leads to the lower oxidation rate and the better scale adherence of 00Cr17 NbTi ferritic stainless steel.     

High-Temperature Creep Behavior and Microstructural Evolution of a Cu-Nb Co-Alloyed Ferritic Heat-Resistant Stainless Steel

The creep behavior of Fe–17 Cr–1.2 Cu–0.5 Nb–0.01 C ferritic heat-resistant stainless steel was investigated at temperatures ranging from 973 to 1123 K and stresses from 15 to 90 MPa. The evolution of precipitates after creep deformation was analyzed by scanning electron microscopy, energy dispersion spectrum, and transmission electron microscopy. The minimum creep rate decreased with the decrease in the applied load and temperature, thereby extending the rupture life. Cu-rich phase and Nb-rich Laves particles were generated in dominant quantities during the creep process, and the co-growth relationship between them could be detected. Creep rupture was featured by ductile fracture with considerable necking. As increasing the temperature and decreasing the stress, the softening of the metal matrix was accelerated, showing more obvious plastic fl ow. The true stress exponent and activation energy were 4.9 and 375.5 kJ/mol, respectively, indicating that the creep deformation was dominated by the diffusion-controlled dislocation creep mechanism involving precipitate-dislocation interactions. Based on the creep rupture data obtained, the Monkman–Grant relation and Larson-Miller parameter were established, which described the creep rupture life for the studied steel well.     

不锈钢离子渗碳耐磨性的研究

为进一步提高不锈钢的耐磨性以更好地适应既需耐蚀,又需耐磨的现代工业中.本试验用离子渗碳的方法对其进行了改性,处理后获得了无共晶莱氏体组织的均匀细小的碳化物,从而改善了组织,提高了性能.与滚轮淬火钢相比,其耐磨性提高了8倍.     

Antibacterial Property and Precipitation Behavior of Ag-Added 304 Austenitic Stainless Steel

A 304 stainless steel with the addition of 0.27 wt% Ag was found to exhibit excellent antibacterial properties. Based on Thermo-Calc calculation, a special heat treatment was introduced to obtain Ag precipitate in this steel. Anti- bacterial experiments show that the alloy can kill the adhering Escherichia coli effectively. Since the Ag element plays a key role in killing the adhered bacteria, microstructures of Ag and Ag-rich compounds were characterized by scanning electron microscopy and transmission electron microscopy. Ag-rich compounds with the size of a few microns were found to be embedded within the matrix and along grain boundaries, Moreover, pure Ag particles with a lattice parameter of 0.422 nm were found within the austenite matrix. The orientation relationship between the matrix and Ag particles was identified. Nano-sized Ag particles were precipitated during heat treatment, and the interfacial energy between Ag precipitates and matrix was determined by Becket＇s model calculation.     

稀土钇对FeCrNi耐热钢循环氧化行为的影响

采用增重法在1100℃、1150℃和1200℃条件下对Fe-21Cr-11Ni试验钢进行循环氧化试验,结合X射线衍射、扫描电镜及能谱等方法研究了钇元素在耐热钢抗高温氧化中的作用机理。结果显示,添加微量稀土钇元素能显著提高耐热钢的抗高温氧化性能。在氧化过程中,钇元素能促进铬元素向氧化层的扩散,在含钇耐热钢表面,形成了致密的粘附性好的Cr2O3层和FeCr2O4尖晶石外氧化物层,起到了很好的保护作用,在内氧化层中,钇元素不仅加强了SiO2钉扎,而且直接参与和强化了钉扎作用。     

High-Temperature Plasticity Enhanced by Multiple Secondary Phases in a High-Si Austenitic Stainless Steel

An austenitic stainless steel with 6 wt% Si and multiple secondary phases was produced with the aim to achieve enhanced plasticity during hot deformation. The microstructure of the steel after fracture was characterized via electron back-scattered diffraction, transmission Kikuchi diffraction and scanning transmission electron microscopy. From the tail of the gage to the necking region, the microstructure of the material evolved from low-angle grain boundaries (LAGBs) to mixtures of LAGBs and high-angle grain boundaries (HAGBs), and fine equiaxed recrystallized grains. The elongation to failure in the tensile test exceeds 167%. During the hot deformation, continuous dynamic recrystallization of the austenitic matrix was promoted by the multiple secondary phases. The dislocations introduced by the secondary phases were rearranged and continuously transformed into HAGBs. The initially coarse grains (30.5 μm) were refined into ultra-fine equiaxed grains (1 μm), which contributed significantly the enhanced plasticity during hot deformation of the steel. In the necking area of the sample, twins were nucleated in the stress concentration regions and accommodated the local strain by discontinuous dynamic recrystallization, which was also beneficial to improving the plasticity.     

SUS430不锈钢纳米表层的高温氧化行为

通过高能喷丸方法在SUS430不锈钢表面制备纳米表层,利用X射线衍射(XRD)、热分析(TGA)、扫描电镜(SEM)及能谱(EDS)进行测试,研究了纳米表层在不同气氛中的高温氧化行为,并分析了表面自身纳米化对SUS430不锈钢耐氧化性能的影响。结果表明:纳米表层在氧化初期氧化强烈,呈线性规律迅速增重,但达到钝化的时间短,氧化膜薄且致密,厚度仅为原始表面氧化膜的1/4,使钝化以后的氧化速率比原始表面低,总的氧化增重仅为原始表面的1/3,耐氧化性大大提高。其原因是表面自身纳米化处理增加了表层的晶界面积,提高了氧化物的形核几率,也为元素的扩散提供了更多通道,因而促进了Cr元素的选择性氧化。在最表层形成的是疏松的富Fe氧化物,该层氧化物对耐氧化性的提高贡献不大,在靠近基体的底层形成致密的富Cr氧化物,能很好地抵抗基体的进一步氧化;纳米表层比原始表面更容易形成连续的氧化膜,该氧化膜的内应力小,韧性好,与基体的结合力强。