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.     

Effect of Microstructure on Tensile Behavior and Mechanical Stability of Retained Austenite in a Cold-Rolled Al-Containing TRIP Steel

In the present study, a quenching treatment prior to two-stage heat treatment was conducted on a Fe–0.28 C–1.55 Mn–2.06 Al transformation-induced plasticity steel to tailor the final microstructure. Compared with the microstructure of the ferrite, bainite and blocky retained austenite obtained by conventional two-stage heat treatment, the microstructure subjected to quenching plus two-stage heat treatment was composed of the ferrite, lath bainite and film-like retained austenite. The corresponding tensile behavior and mechanical stability of retained austenite were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the mechanical stability of blocky retained austenite grains is lower and most of them transform to martensite during the tensile deformation, which leads to higher ultimate tensile strength and instantaneous work hardening exponent. Film-like retained austenite has relatively higher stability, which could cause sustained work hardening and high ductility as well as product of strength and elongation.     

氮对半高速钢奥氏体冷却转变动力学的影响

研究加氮和不加氮的两种半高速钢（低合金高速钢）的奥氏体冷却转变动力学行为,建立并对比分析两种钢的连续冷却转变图（CCT图）。采用图象分析仪和扫描电镜以及能谱仪（EDS）分析了淬火加热的碳化物数量及成分的变化规律。结果表明,加氮的半高速钢中氮化物相对含有较多的V、W、Mo和较少的Cr元素,淬火加热时表现出较高的稳定性;氮的添加使Ac1和Ms／Mf温度下降,显著提高了淬透性。     

Decomposition of Retained Austenite in a High-Speed Steel GPM A30

The effect of tempering on the decomposition of retained austenite in a powder metallurgy (PM) high-speed steel, GPM A30, has been monitored with a high-speed dilatometer. The corresponding microstructures of specimens with different tempering cycles have been investigated by a combination of scanning electron microscopy and analytical transformation electron microscopy. The as-quenched structure of the steel studied is composed of retained austenite, untempered martensite, and carbides. The results indicate that the complete transformation of retained austenite can be more nearly accomplished by double or triple tempering cycles than by a single long-time cycle. The possible transformation mechanism for the decomposition of retained austenite during multiple tempering cycles is attributed to the invariant-plane-strain of the prior martensitic transformation extending accommodation defects to the adjacent retained austenite, which favors further transformations in the subsequent tempering operations.

含Ti微合金钢动态再结晶行为及模型研究

采用Gleeble 3800热力模拟实验机,研究了一种Ti微合金钢在850～1 150℃、0.1和1.0 s-1条件下的变形奥氏体的动态再结晶行为及再结晶后奥氏体晶粒尺寸的演化规律。结果表明:形变激活能与变形条件相关,随着应变速率的增加而增加;Avrami指数m同样与变形条件有关;对实验数据进行回归分析后建立的Ti微合金钢动态再结晶的特征应变、特征应力、动力学以及稳定状态晶粒尺寸的数学模型精度较高。     

Dependency of Deformation Behavior of Retained Austenite in TRIP Steels on Microstructural and Chemical Homogeneity

The room-temperature stability of the retained austenite against strain-induced martensitic transformation, its deformation behavior, the response to the bainitic isothermal treatment, the appearance of yield point elongation and other peculiarities of plastic flow, and the mechanical properties of transformation-induced plasticity(TRIP) steel were tailored based on the chemical homogeneity and the relative distribution of the retained austenite, bainite, and ferrite in the microstructure. The presence of ferritic-pearlitic banded structure in the initial microstructure resulted in an inhomogeneous TRIP microstructure, in which the retained austenite and bainite were confined to some bands and it was found to be responsible for the resultant inferior mechanical properties. The appearance of discontinuous yielding for the chemically inhomogeneous material was related to the martensitic transformation of unstable retained austenite at the initial stage of tensile deformation. These results are essential for better understanding of the behavior of advanced high-strength steels and their applications.     

Dynamic Compression Behavior and Microstructure of a Novel Low-Carbon Quenching-Partitioning-Tempering Steel

A 0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb （wt%） steel is treated respectively by novel quenching-partitioning-tempering （Q-P-T） process and traditional quenching and tempering （Q＆T） process for comparison. X-ray diffraction analysis indicates that Q-P-T steel has about 10% retained austenite, but Q＆T steel hardly has one. With the increase of com- pression strain rate from 7 × 10^2 to 5 × 10^3 s^-1, the flow stress of Q-P-T steel increases, which demonstrates the positive strain rate effect, but does not exist in Q＆T steel. The characterization of scanning electron microscopy indicates that a large number of long, straight martensite laths in Q-P-T steel will bend or be destroyed by large compressive strain of 35% at 5 × 10^3 s^-1. However, relative small compressive s~xain of about 5% at 7× 10^2 s^-1 almost does not have any effect on the original lath morphology. The characterization of transmission electron microscopy further reveals the origin of the positive strain rate effect and the microstructural evolution during dynamic compressive deformation.     

碳锰含量对低碳(锰)钢过冷奥氏体形变过程转变动力学的影响

提高低碳(锰)钢中碳含量，低碳(锰)钢形变强化相变孕育期明显延长，转变动力学曲线整体向高应变方向移动．提高锰含量，相变孕育期有所延长，转变动力学过程明显变缓．提高碳、锰含量，钢中铁素体形核率增大，晶粒细小，碳的影响程度比锰显著．过冷奥氏体形变过程铁素体转变分三个阶段，第一阶段符合Cahn的“位置饱和”机制，第二、三阶段不符合“位置饱和”机制。     

Positive Strain Rate Sensitivity and Deformation Behavior of a Fe-29Mn-3Al-3Si TWIP Steel

The Fe-29Mn-3Al-3Si twin-induced plasticity (TWIP) steel is used to conduct quasi-static compression and dynamic impact deformation with strain rates ranging from 8.3 × 10^-4 to 3800 s^-1. The microstructures and properties of deformed samples under different strain rates were investigated comparatively. These results show that positive strain rate sensitivity was observed with the increase in strain rates and that there was a significant difference in strain rate sensitivity factor (ṁ) between quasi-static compression (ṁ = 0.029) and dynamic impact deformation (ṁ = 0.190). Compared to the quasi-static compression, the dynamic impact deformation exhibited higher yield strength. Microstructural examination reveals that the primary twins were frequently found during the quasi-static compression process, and the secondary twins were rarely observed. However, the secondary and multi-fold deformation twins were florescent in the dynamic impact samples. At the initial stage of dynamic impact deformation, partial dislocations and staking faults on multiple conjugate {111} planes were simultaneously activated and produced a large number of Lomer-Cottrell dislocations, resulting in a large increase in yield strength during dynamic impact.     

Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel

The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850–1050 ℃ and strain rates of 0.05–10 s~(-1).The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization(DRX).The flow behavior was characterized by significant flow softening at deformation temperatures of 950–1050 ℃ and lower strain rates of 0.05–5 s~(-1), which was attributed to heating during deformation, DRX and flow instability.A step-by-step calculating procedure for constitutive equations is proposed.The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel.Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium Mn steel should be carried out at 1050 ℃, and the strain rate of 0.05–10 s~(-1) resulted in significantly recrystallized microstructures in the in steel.The flow localization is mainly flow instability mechanism for experimental steel.     

Influence of Aging Time on Microstructure and Corrosion Behavior of a Cu-Bearing 17Cr-1Si-0.5Nb Ferritic Heat-Resistant Stainless Steel

17Cr-1Si-0.5Nb-1.2Cu ferritic heat-resistant stainless steel was aged at 750 °C from 10 min to 30 h to simulate time aging and study the microstructural evolution and its effect on corrosion behavior by using optical microscopy, scanning electron microscopy, transmission electron microscopy, potentiodynamic polarization, electrochemical impedance spectroscopy, and the Mott-Schottky approach. Four types of precipitates were discovered, including ε-Cu, NbC, Fe₃Nb₃C, and Fe₂Nb-type Laves phase. The nano-sized ε-Cu phase forms first, and its fraction follows the parabolic law change and is the largest. Compared to NbC and Fe₃Nb₃C particles, the coarsening of the Laves phase is the most pronounced. The aging process is divided into three parts: early-aged (0-5 h), peak-aged (5 h), and over-aged (5-30 h). However, the corrosion resistance is reduced in the early-aged stage of 0-2 h. Further extending the aging time to 30 h, the corrosion resistance is gradually improved. This change may be related to the competitive relationship between the beneficial effects of the Cu-rich phase and the harmful effects of Nb-containing particles. The dissolved Cu on the surface becomes more effective for the suppression of the anodic dissolution by the formation of ionic compounds of chlorine, thereby reducing the deterioration of corrosion resistance caused by Nb-rich precipitation.     

Hot Deformation Behavior and Processing Map of a Cu-Bearing 2205 Duplex Stainless Steel

The hot deformation behavior and processing map of Cu-bearing 2205 duplex stainless steel(2205-Cu DSS) were investigated at temperatures of 950-1150℃and strain rates of 0.01-10 s~(-1).The effects of Cu addition and different deformation parameters on deformation behavior were,respectively,characterized by analyzing flow curves,constitutive equations and microstructures.The results indicated that the shapes of flow curves strongly depended on the volume fraction of two phases.When deformed at low strain rate,DRV in ferrite was prompted with increase in the temperature and was further developed to continuous DRX.At high strain rate,flow localization preferentially occurred in ferrite at low deformation temperature due to the strain partitioning and relatively less fraction of ferrite.The activation energy for 2205-Cu DSS was 452 kJ/mol and was found to connect with the variation of strain,strain rate and deformation temperature.The optimum hot deformation parameters for 2205-Cu DSS were obtained in the temperature range of 1100-1150℃and strain rate range of 0.1-1 s~(-1)with a peak power dissipation efficiency of 41%.Flow localization was the main way to lead to flow instability.Meanwhile,the Cu-rich precipitates were generated within a few ferrite grains when deformed at temperature lower than 1000℃.The interaction between dislocations and Cu-rich precipitates at high strain rate,as well as the limited DRV in ferrite and DRX in austenite,contributed to the complex microstructure and flow behavior.     

一种含Re单晶镍基合金的中温蠕变行为及影响因素

通过对合金进行不同温度的固溶处理、蠕变曲线测定及组织形貌观察,研究了热处理工艺对4.5%Re镍基单晶合金中温蠕变行为的影响。结果表明:随着固溶温度提高,可降低元素的偏析程度,提高合金的蠕变性能。在760℃/800MPa条件的蠕变期间,合金中γ′相不形成筏状组织,但在近断口区域,立方γ′相的扭曲程度增加。合金在蠕变期间的变形特征是位错在基体中运动和剪切γ′相,其中,切入γ′相的<110>超位错可由{111}面交滑移到{100}面,形成K-W锁,而切过γ′相的<110>超位错在{111}面发生分解,可形成(1/3)<112>超肖可莱不全位错+层错的位错组态,阻碍位错运动和抑制位错的交滑移。     

不锈钢纤维及其织物的电阻与温度关系研究

研究了不锈钢纤维及其织物的电阻与温度之间的关系。通过实验观察到不锈钢纱线电阻随温度增加而线性增加，不锈钢织物的电阻随温度增加而下降，且在一定范围内成线性关系，并大大提高了其电阻变化的灵敏度。织物电阻随温度变化的机理主要是温度变化过程中纱线间接触电阻的变化引起织物电阻的相应变化。     

Hot Deformation Behavior and Processing Maps of 0.3C-15Cr-1Mo-0.5N High Nitrogen Martensitic Stainless Steel

Hot deformation behavior of 0.3 C-15 Cr-1 Mo-0.5 N high nitrogen martensitic stainless steel(HNMSS) was investigated in the temperature range of 1173-1473 K and at strain rates of 0.001-10 s~(-1) using a Gleeble 3500 thermal-mechanical simulator.The true stress-strain curves of the studied HNMSS were measured and corrected to eliminate the effect of friction on the flow stress.The relationship between the flow stress and Zener-Hollomon parameter for the studied HNMSS wsa analyzed in the Arrhenius hyperbolic sine constitutive model by the law of Z=3.76×10~(15) sinh(0.004979σ_p)~(7.5022).The processing maps at different strains of the studied HNMSS were plotted,and its flow instability regions in hot working were also confirmed in combination with the microstructure examination.Moreover,the optimal hot deformation parameters of the studied HNMSS could be suggested at T=1303-1423 K and ε=5-10 s~(-1) or T=1273-1473 K and ε=0.005-0.04 s~(-1).     

A Study of Microstructure and Mechanical Properties for the Autogenous Single-Pass Butt Weldment of a Ferritic/Martensitic Steel Using Gas Tungsten Arc Welding

A 12%Cr ferritic/martensitic steel,HT-9,has been used as a primary core material for nuclear reactors.The microstructure and mechanical properties of gas tungsten arc butt welded joints of HT-9 in as-welded,and as-tempered conditions have been explored.In as-welded condition,the fusion zone (FZ) contained a fresh martensite matrix with delta (δ)-ferrite.Theδ-ferrite was rich in Cr and depleted in C compared with the matrix.The heat-aff ected zone (HAZ) could be divided into three areas as the distance from the fusion line increased:δ-ferrite/martensite duplex zone,fully recrystallized zone,and partly recrystallized zone.Prior austenitic grains did not coarsen in theδ-ferrite/martensite duplex zone due to the newly nucleatedδ-ferrite grains and incompletely ferritizing (δ-ferrite) during the welding thermal cycle.The weldment microhardness distributed heterogeneously with values above 600 HV _(1.0 )in the HAZ and FZ and 250 HV _(1.0 )in the base metal (BM).Solute C in the matrix,induced by the dissolution of carbide during the welding process,dominated the microhardness variation.Low toughness was observed in the FZ with a quasi-cleavage fracture tested from-80 to 20℃.The tensile fracture occurred in the relatively soft BM tested from 20 to 600℃.In as-tempered condition (760℃ for 1 h),M _(23 )C _6-type carbides precipitated within the martensitic laths,the lath boundaries,and theδ-ferrite/martensite interfaces.Moreover,V,Cr,Mo-rich nitrides with very small size also precipitated in theδ-ferrite/martensite interface.The tempering treatment improved the homogenous distribution of weldment hardness significantly.Tensile fracture still occurred in the BM of the weldment specimens tested from 20 to 600℃.The impact toughness improved significantly,but the ductile–brittle transaction temperature was-12℃ which was higher than that of the normalized and tempered (NT) BM.δ-ferrite was considered to be one of the major factors aggravating the impact toughness in the FZ.     

FV（520）B不锈钢在CO2/H2S共存条件下的腐蚀行为

目的研究长输管线压缩机叶片材料FV(520)B不锈钢在高含H_2S、H_2O、CO_2条件下的腐蚀行为。方法利用高温高压反应釜模拟特定工况,在H_2S分压为0.9 MPa、CO_2分压为0.6 MPa、温度为150℃的条件下于5 g/L的氯化钠溶液中制备硫化腐蚀层,利用XPS、SEM、XRD等手段对腐蚀层的成分及结构进行分析。结果 FV(520)B不锈钢的腐蚀速率逐渐降低,试样表面粗糙度先增大后下降,腐蚀产物主要为FeCO_3、Fe_3O_4、FeS、FeS_2、S、Cr_2S_3、Cr_2O_3和Cr(OH)_3。结论在腐蚀前期,FeS的形成速率大于FeS_2、S,腐蚀产物颗粒不断长大。形成完整的Cr_2O_3、Cr(OH)_3保护膜后,腐蚀得到抑制,此时腐蚀反应主要为FeS_2、S的生成,试样表面腐蚀产物颗粒尺寸变小,试样表面粗糙度降低。     

低焊接裂纹敏感性高能量焊接用钢的应变时效行为

研制了一种低焊接裂纹敏感性高能量输入焊接用低合金高强钢,并研究了其冷应变时效行为.结果表明,该钢经2.5%及5%拉伸冷应变和应变时效后在常温及-40℃冲击吸收功分别高于100 J和47 J,与基材的冲击吸收功相近,试验钢时效脆性倾向不显著.金相组织观察显示,2.5%及5%拉伸冷应变试样中亚晶粒发生了塑性应变,原奥氏体晶粒未发生明显应变.     

离子氮铝共渗方法及对42CrMo钢组织性能的影响

目的 研发离子氮铝共渗试验方法,达到不影响42CrMo钢基体组织性能前提下,显著提高试样表面硬度和耐磨性效果。方法 采用电解法在42CrMo钢表面沉积氢氧化铝膜,再在520 ℃/4 h工艺下进行离子氮铝共渗处理,并在相同工艺参数条件与传统离子渗氮进行对比。用光学显微镜、维氏显微硬度计、摩擦磨损测试机、X射线衍射仪及SEM对截面显微组织、截面硬度、耐磨性及物相等进行了测试和分析。结果 获得了离子氮铝共渗试验方法,在520 ℃/4 h相同工艺参数下,离子氮铝共渗形成的化合物层和有效硬化层厚度比常规离子渗氮显著增加,其中,化合物层厚度由17.24 μm增加到52.13 μm,有效扩散层从175 μm增加到1 050 μm,相当于等离子处理效率提升6倍;同时,渗层形成了AlN及FexAl强化相,大幅度提高了渗层的硬度及耐磨性能。表面硬度由750HV0.025提高到1 250HV0.025,摩擦因数由常规离子渗氮0.52下降到0.29,磨损率由常规离子渗氮3.22×10^?5 g/(m.N)下降到1.21×10^?5 g/(m.N),磨痕明显减轻。结论 采用电解硝酸铝生成氢氧化铝沉淀附着在工件表面作为预处理,获得了离子氮铝共渗试验方法,与常规离子渗氮相比,离子氮铝共渗形成了多层次渗层结构,大幅度提高常规离子处理效率、表面硬度及耐磨性。     

含La2O3奥氏体不锈钢堆焊合金层晶粒细化机制及对其耐腐蚀、磨损性能的影响

目的 通过在超低碳Cr19Ni10不锈钢堆焊合金中加入稀土氧化物La2O3,细化其微观组织,获得力学性能、耐腐蚀性能和耐磨性能等综合性能优良的堆焊合金层。方法 采用添加La2O3的超低碳Cr19Ni10不锈钢焊条制备了四种不锈钢堆焊合金。采用X射线荧光光谱、红外碳硫分析仪和X射线衍射分析仪,对堆焊合金层的元素组成和相组成进行了测定。采用金相显微镜和晶粒度统计软件,对堆焊合金层的微观组织形貌进行观察,并对晶粒度进行了统计分析。采用显微维氏硬度计和纳米压痕仪对堆焊合金层的硬度和杨氏模量进行了测定。采用电化学工作站和CSM摩擦磨损试验机对堆焊合金层的耐腐蚀性能和耐磨性能进行了评价,并且采用白光共聚焦显微镜对磨损后的磨痕形貌和尺寸进行了观察和测定。采用二维晶格错配度理论,对La2O3/γ-Fe界面间的晶格错配关系进行了计算。结果 在堆焊合金层中加入La2O3,随着La2O3加入量的增加,堆焊合金层奥氏体晶粒细化越明显。当La2O3的添加量由0%增加至1.5%时,奥氏体晶粒平均面积由400 μm²减少为210 μm²。堆焊合金层加入La2O3,可以明显提高其力学性能、耐腐蚀性能和耐磨损性能。当La2O3的添加量由0%增加至1.0%时,堆焊合金层的微观硬度由180HV增加到225HV,宏观硬度由125HBS增加到150HBS,杨氏模量由186 GPa左右增加到217 GPa,腐蚀电位由?0.4 V增加到?0.25 V,磨痕深度由50 μm减小到10 μm。La2O3(001)面和γ-Fe(110)面的二维晶格错配度为8.7%(<12%),说明La2O3可以作为γ-Fe的中等有效异质形核基底,从而细化了堆焊合金层中的奥氏体晶粒。结论 La2O3可以有效地细化奥氏体晶粒,改善堆焊合金层的力学性能,提高其耐腐蚀和耐磨损性能。但是,La2O3加入量存在一个最佳值,当La2O3的加入量为1.0%时,堆焊合金层的综合性能最好。