冷变形316L不锈钢在高温高压水中的应力腐蚀

采用恒应力强度因子K=33 MPa·m1/2的加载方法,利用直流电压降方法在线监测核辅管道316L不锈钢在高纯水中应力腐蚀裂纹扩展速率.对比200、250、280和325℃温度下,氩气除氧和含有2 mg·L-1溶解氧的水化学环境中材料的裂纹扩展速率发现:溶解氧为2 mg·L-1时的裂纹扩展速率明显比氩气除氧时的裂纹扩展速率高.氩气除氧时,裂纹扩展速率在250℃时有一个最高点;溶解氧为2 mg·L-1的条件下,裂纹扩展速率随温度的升高而升高.      

GH4169合金高温疲劳裂纹扩展的微观损伤机制

空气环境对高温合金在高温下的损伤行为有显著影响.为了研究标准热处理态GH4169合金在高温疲劳裂纹扩展过程中的微观损伤机制,在空气环境中进行650℃、初始应力强度因子幅ΔK=30 MPa·m1/2和应力比R=0.05的低周疲劳裂纹扩展试验.使用扫描电镜(SEM)及能谱(EDS)对试样的断口、外表面和剖面进行观察和分析.实验结果表明:疲劳主裂纹以沿晶方式萌生并扩展,随后沿晶二次裂纹出现,并且其数量和长度沿主裂纹方向逐渐增加,进入快速扩展阶段后,断口呈现韧窝组织形貌;在裂纹扩展过程中,δ相与基体的界面发生氧化,使得沿晶二次裂纹沿界面扩展并产生偏折,从而起到阻碍二次裂纹扩展的作用;试样外表面的主裂纹周围出现晶界氧化损伤区,其尺寸和晶界开裂程度沿主裂纹扩展方向逐渐增大.      

加载方向对Al—Zn—Mg合金型材应力腐蚀开裂行为的影响

采用恒载荷拉伸应力腐蚀试验和电化学试验研究取向对Al-Zn-Mg合金型材的应力腐蚀(SCC) 开裂的影响, 腐蚀介质采用质量分数3. 5%的Na Cl溶液, 容器温度维持在50±2℃, 并通过光学金相显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD) 等研究不同取向试样应力腐蚀前、后的微观形貌.结果表明横向试样在315 h时断裂, 而纵向试样在整个加载过程中未发生断裂, 纵向试样有更好的抗应力腐蚀开裂性能; 纵截面(L-S面) 的腐蚀电流密度为0. 980 m A·cm-2, 约为横截面(T-S面) 的5倍, 腐蚀倾向于沿挤压方向发展; 相比T-S面, L-S面晶粒间取向差较大, 大角度晶界多, 容易被腐蚀产生裂纹; 在应力腐蚀加载过程中, 试样先发生阳极溶解, 形成腐蚀坑, 聚集的腐蚀产物所产生的楔入力和恒定载荷的共同作用促使裂纹在腐蚀介质中加速扩展, 两种取向试样均发生了明显的晶间腐蚀, 存在应力腐蚀开裂的倾向.      

合金元素对铁素体不锈钢耐应力腐蚀开裂性能的影响

通过42%沸腾氯化镁U型弯曲试验,结合扫描电子显微镜、X射线衍射分析,研究了合金元素Ni、Cu、Mo对铁素体不锈钢耐氯化物应力腐蚀开裂性能的影响规律。结果表明,Ni、Cu元素能够提高铁素体不锈钢应力腐蚀开裂的敏感性,单独添加Mo元素不会降低铁素体不锈钢耐应力腐蚀开裂的能力,但是钢中Mo、Cu元素同时存在时,应力腐蚀开裂的敏感性大大增加。钢中析出的ε-Cu在氯离子环境中形成点蚀,引起了铁素体不锈钢应力腐蚀开裂。     

B级船板钢形变断裂过程的原位研究

 在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明：在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。     

氮对00Cr18Ni5Mo3Si2双相不锈钢耐局部腐蚀性能影响的研究

本文主要研究了在氯化物介质中氮对00Cr18Ni5Mo3Si2(18-5)双相不锈钢耐孔蚀和耐应力腐蚀性能的影响。结果表明,双相不锈钢中奥氏体相的耐孔蚀性较铁素体相差,故优先受到腐蚀,氮使钢的耐孔蚀性提高,实际上是增加了奥氏体相的耐孔蚀性。研究还表明,氮增加18-5钢在40％CaCl_2溶液中的应力腐蚀敏感性,是由于位错的共平面性滑移引起钝化膜的局部破裂。在中性的25％NaCl+1％K_2Cr_2O_7溶液中,应力腐蚀裂纹始于孔蚀坑,含氮的18-5钢耐孔蚀性提高,从而导致钢的耐应力腐蚀性能的增加。利用俄歇电子谱分析了钝化后钢的表面膜成分,结果表明氮在膜中靠基体侧富集,从而改善了表面膜的性能,这可能是氮提高18-5双相不锈钢耐孔蚀性能的主要原因,     

300M超高强度钢电化学性能及应力腐蚀开裂

采用动电位扫描技术和慢应变速率拉伸试验研究了超高强度钢300M在3.5%NaCl溶液中的应力腐蚀行为,并利用扫描电镜观察了不同外加电位下的断口形貌.300M钢在3.5%NaCl溶液中开路电位下的应力腐蚀开裂机制为阳极溶解型,Cl-的存在明显地增加了材料的应力腐蚀开裂敏感性.阳极电位-600 mV下300M钢溶解速率加快,表现出较高的应力腐蚀开裂敏感性,断面收缩率损失由开路电路下的52.6%升高至99.5%,裂纹起源于表面点蚀坑处,应力腐蚀开裂为阳极溶解型机制.阴极电位-800 mV下材料处于阴极保护电位范围,表现出较低的应力腐蚀开裂敏感性,强度和韧度与空气中拉伸的数值相近,开裂机制为阳极溶解和氢致开裂协同作用.在更低电位(低于-950 mV)下,300M钢的应力腐蚀开裂机制为氢致开裂,在氢和拉应力的共同作用下表现出很大的应力腐蚀开裂敏感性.      

T225NG钛合金在高温高压水介质中应力腐蚀行为的研究

研究T2 2 5NG钛合金在高温高压水中的应力腐蚀情况 ,通过将双悬臂梁试样预加应力后放入高温高压的回路水中 ,试样经过 2 0 0 0h的试验 ,观察试样应力腐蚀和裂纹扩展情况。结果发现 ,双悬臂梁试样的预制裂纹经试验后发生了很小的扩展。经过计算 ,在 5 0 0h内裂纹扩展速度da/dt<1× 10 - 9m·s- 1 ,而且KISCC/KIC>0 .75 ,说明T2 2 5NG钛合金在高温高压的回路水中具有良好的耐应力腐蚀性能。同时也发现在试样表面生成了氧化膜 ,研究表明 ,表面氧化膜主要是由白色的TiO2 颗粒组成 ,由于表面氧化膜的存在 ,阻碍了阴极析氢产生的氢进入钛合金中 ,防止了氢在钛合金中的聚集而产生的氢化物及氢脆 ,这进一步证明了T2 2 5NG钛合金在高温高压水溶液中具有良好的耐应力腐蚀性能。     

管线钢裂纹萌生及其扩展的研究

采用硫化氢腐蚀试验、金相显微镜和扫描电镜(SEM)观察及EDS分析研究了非金属夹杂物和微观组织对X65管线钢氢致裂纹(HIC)萌生和扩展的影响.研究表明:晶界、相界、脆硬的非金属夹杂物是钢中的氢陷阱.夹杂物的尺寸、形态及类型影响管线钢的HIC敏感性.HIC不易在小尺寸球形夹杂物上萌生,而长条和具有尖角的非金属夹杂物使钢具有高HIC敏感性,Ca处理能使夹杂形态趋于分散的球形,降低钢的HIC敏感性;HIC裂纹易在脆硬的富Si夹杂物处萌生和扩展.与铁素体组织相比,HIC更易在脆硬的贝氏体组织中扩展,HIC可沿着晶界或穿过晶界扩展.     

固溶处理对2205双相不锈钢组织与疲劳裂纹扩展规律的影响

采用光学显微镜、扫描电镜、EDS能谱分析和力学性能测试等方法,研究了固溶处理对2205双相不锈钢显微组织与疲劳裂纹扩展规律的影响。结果表明：与原始热轧态相比,在950~1150℃范围固溶处理的试样的疲劳裂纹门槛值显著提高,稳态裂纹扩展速率均有所减小。在950℃固溶处理时,组织中析出少量的σ相,试样的稳态裂纹扩展速率显著降低;随着固溶温度的升高,组织中α相含量逐渐增加,σ相溶解,试样的稳态裂纹扩展速率的变化呈现为先增大后减小的趋势;当固溶温度达到1150℃时,组织中α相含量最高且两相组织明显粗化,试样的稳态裂纹扩展速率达到最小,呈现出最高的抗疲劳裂纹扩展能力。固溶处理引起σ相的析出与溶解、α相含量的增加及组织粗化是引起2205双相不锈钢试样疲劳裂纹扩展性能非单调变化的原因。     

Forming Limit Curve (FLC) and Fracture Mechanism of Newly Developed Low‐Carbon Low‐Silicon TRIP Steel

The Forming‐Limited Diagram (FLD) of intercritically annealed 0.11C‐1.65Mn‐0.62Si TRIP‐assisted steel was investigated. The high FLD₀ value of this new low carbon TRIP steel was indicative of a superior formability. The micro‐structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro‐voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro‐voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress‐induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.     

奥氏体-贝氏体球墨铸铁断裂的微观过程及强韧化机理

用扫描电子显微镜及微拉伸台对奥氏体－贝氏体球墨铸铁裂纹萌生，扩展的微观过程进行了跟踪观察。结果发现：受拉时微裂纹首先在石墨－基体界面上萌生，并沿界面扩展，基体中的裂纹多数是沿贝氏体铁素体－奥氏体界面扩展，不同取向的基体组织可使裂纹偏转或分叉，主裂纹扩展过程中前方始终存在石墨－基体界面的开裂。此外，还根据实验结果进一步分析了奥氏体－贝氏体球墨铸铁的强韧化机理。     

Z3CN20—09M铸造奥氏体不锈钢的低周疲劳行为

采用径向应变控制研究了Z3CN20-09M奥氏体不锈钢在室温和350℃高温下的低周疲劳行为.Z3CN20-09M不锈钢表现为先硬化后软化的循环特性,但硬化的程度取决于温度和应变幅.随着应变幅的增加,Z3CN20-09M钢的低周疲劳循环寿命逐渐减短,而相同循环次数下应力幅也随之提高.温度对Z3CN20-09M钢的低周疲劳行为影响较大,与室温相比高温下的循环硬化程度更高,相同应变幅下高温的低周疲劳寿命也高于常温下的寿命.通过疲劳实验的原位观察发现,奥氏体内的滑移面、夹杂物及奥氏体和铁素体两相的界面是疲劳裂纹可能的形核位置,奥氏体和铁素体两相的不协调变形使相界处产生应力集中,导致疲劳裂纹容易沿两相界面扩展.      

Stress corrosion cracking of copper bicrystals with 〈110〉-Tilt ∑3, ∑9, and ∑11 coincident site lattice boundaries

The stress corrosion cracking (SCC) behavior of copper bicrystals with 〈HO〉-tilt ∑3(111), ∑9(221), and ∑11(311) coincident site lattice (CSL) boundaries was investigated. Stress corrosion cracking tests were carried out in 1 N NaNO₂ aqueous solution at 303 ±2 K using a slow strain rate technique (SSRT). Transgranular SCC occurred along the primary slip traces on the top surface of the bicrystal having a ∑3(111) coincidence boundary. No cracks initiated on the grain boundary except for very small and shallow corrosion pits. In contrast, for the bicrystals with ∑9(221) or ∑11(311) coincidence boundaries, corrosion pits and cracks initiated on the grain boundary and propagated into the crystal interior along {110} traces, which are almost perpendicular to the tensile axis. The SCC behavior is closely related to the activated slip systems and the degrees of crystal rotation owing to deformation. Susceptibility to intergranular SCC is affected by the angle between the Burgers vector of the primary slip system and the grain boundary plane. The susceptibility of the ∑23(111) boundary to SCC is remarkably low in comparison with the other two types of grain boundaries. Y. Nakazawa, formerly Graduate Student, Department of Mechanical Engineering, Doshisha University This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

双相变回温温度对钢组织及其热塑性的影响

含铌钢连铸过程极易产生铸坯角部横裂纹。对连铸坯角部实施γ→α→γ双相变控冷工艺,可提高其组织的高温热塑性而减少裂纹产生。其中,α→γ相变阶段的回温温度是影响双相变控冷工艺实施效果的重要参数。通过Gleeble热模拟与金相观察、析出物透射以及断口扫描相结合的检测手段,研究分析了双相变过程回温温度对Q345D-Nb钢组织演变及其热塑性的影响规律。结果表明,回温温度为850℃时的奥氏体晶粒相比传统冷却工艺下的晶粒尺寸未产生细化,平均晶粒尺寸为502.2μm;回温温度升至900℃时,回温奥氏体出现了明显的混晶现象;当回温温度达到950℃时,晶粒细化至61.2μm;当回温温度达到1 000℃时,回温奥氏体晶粒出现了一定程度粗化,相比950℃回温温度下的奥氏体平均晶粒尺寸增加了38.07%。传统冷却工艺和不同回温温度时的双相变控冷工艺(回温温度为850、900、950、1 000℃),钢组织在700～900℃温度区内的断面收缩率最低值分别为29.6%、45.0%、56.3%、68.2%、63.2%。在传统冷却工艺下,钢组织在750℃时晶界铁素体膜的厚度为20～25μm,且碳氮化物呈大尺寸链状分布,...     

Accelerated Stress Corrosion Crack Initiation of Alloys 600 and 690 in Hydrogenated Supercritical Water

The objective of this study is to determine whether stress corrosion crack initiation of Alloys 600 and 690 occurs by the same mechanism in subcritical and supercritical water. Tensile bars of Alloys 690 and 600 were strained in constant extension rate tensile experiments in hydrogenated subcritical and supercritical water from 593 K to 723 K (320 °C to 450 °C), and the crack initiation behavior was characterized by high-resolution electron microscopy. Intergranular cracking was observed across the entire temperature range, and the morphology, structure, composition, and temperature dependence of initiated cracks in Alloy 690 were consistent between hydrogenated subcritical and supercritical water. Crack initiation of Alloy 600 followed an Arrhenius relationship and did not exhibit a discontinuity or change in slope after crossing the critical temperature. The measured activation energy was 121 ± 13 kJ/mol. Stress corrosion crack initiation in Alloy 690 was fit with a single activation energy of 92 ± 12 kJ/mol across the entire temperature range. Cracks were observed to propagate along grain boundaries adjacent to chromium-depleted metal, with Cr₂O₃ observed ahead of crack tips. All measures of the SCC behavior indicate that the mechanism for stress corrosion crack initiation of Alloy 600 and Alloy 690 is consistent between hydrogenated subcritical and supercritical water.     

Influence of austenitizing temperature on stress corrosion in 4330m steel—The role of impurity segregation in stress corrosion cracking of high strength steel

Measurements of the threshold stress intensity for stress corrosion cracking (SCC), K_ISCC, and crack growth rate,da/dt, in distilled water were made, respectively, on bolt-loaded WOL and precracked three-point-bending specimens of a 4330M steel. A significant improvement of resistance to SCC was obtained by increasing quenching temperature and it is due to a reduction of segregated impurities of P and S at prior austenite grain boundaries. Intergranular cracking tendency increases with inter-granular concentration of impurities and the fracture mode changes from intergranular separation along prior austenite grain boundaries to transgranular quasi-cleavage as the segregated impurity becomes low enough. The combined effects of hydrogen and intergranular impurities on reducing intergranular cohesion and the time for approaching the critical concentration of hydrogen are dis-cussed in terms of a dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack. Formerly with Shanghai Jiao Tong University, Shanghai, China     

The effects of microstructure,strength level,and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.     

Reducing crack formation in low-carbon rolled bar produced from continuous-cast blank

The crack formation in steel is investigated. It is found that peritectic transformation occurs during solidification by phase reactions: the formation of primary ferrite from liquid; and the formation of peritectic austenite from this ferrite and the residual liquid. The sections of anomalous structure in the boundary zones between the large dendrites correspond to the former liquational liquid. The low-melting liquid facilitates the nucleation and growth of microcracks, which penetrate to the external layer. Hot insertion of the S355J2 steel blank during the heating prior to rolling may lead to the appearance of cracks on account of temperature stress and structural stress. Warm or cold insertion is recommended, so as to avoid crack formation or development, with corresponding improvement in product acceptance and reduction in waste.     

冲击载荷对10Mn钢磨损性能的影响及耐磨机制分析

通过水韧处理配合450℃时效热处理工艺,研究10Mn钢在2和5 J冲击载荷下耐磨性能.结果表明,10Mni钢在5J冲击载荷下失重量更少,加工硬化的速率更快,并且在不同冲击载荷下磨损失重量都出现了周期性变化.磨损表面SEM结果表明,表面存在犁削磨损、凿削磨损、裂纹.通过XRD数据对亚表面奥氏体和马氏体体积分数进行定量计算...