共查询到18条相似文献,搜索用时 156 毫秒
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在钢的连铸过程中,钢水在结晶器内的凝固对铸坯的产量和质量均有很大影响,几乎所有的铸坯表面缺陷均形成于结晶器内。近年来,随着连铸拉速的增加及对铸坯表面质量要求的提高,有关结晶器冷却、传热对钢水的初始凝固及表面纵裂纹影响的研究成为连铸科学研究的重点。结晶器壁热流不均是纵裂纹产生的有利环境,保护渣控制传热为常用的措施。薄板坯浇铸时由于拉速高,为获得表面无缺陷铸坯,对保护渣控制传热的要求更高,同时也需协调保护渣的润滑功能。通过生产试验,研究比较3种碱度保护渣(CaO/SiO2分别为1.06、1.26和1.48)对薄板坯结晶器平均热流量的影响,发现与低碱度保护渣相比,使用高碱度保护渣时,结晶器热流量最低,有利于实现弱冷却,形成均匀凝固坯壳,在一定拉速条件下浇铸裂纹敏感钢种时有助于获得良好表面质量的铸坯。 相似文献
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Mn13高锰钢连铸坯有纵向裂纹。采用扫描电镜、高温拉伸试验和高温激光共聚焦显微镜、电子探针分析等检测了裂纹的形貌、Mn13钢的高温力学性能以及Mn13钢凝固过程中的组织演变。试验结果表明:纵向裂纹深度达40 mm,裂纹附近无保护渣卷入; Mn13钢在1 200℃以下温度的塑性较差,较易开裂。在1 200℃左右Mn13钢奥氏体晶界开始析出碳化物,进一步增大了铸坯的开裂倾向;由于结晶器内保护渣熔化不均匀,导致凝固坯壳厚度不均匀,从而使铸坯产生凹陷和纵向裂纹。提高凝固坯壳厚度和结晶器热流的均匀性是避免Mn13钢连铸坯纵向开裂的重要途径。 相似文献
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针对大方坯含硫钢表面纵裂纹问题,对裂纹缺陷进行了金相、能谱观察,发现纵裂纹在铸坯出结晶器后发生;通过高温热塑性测试和组织析出区间的确定,分析了硫加入对钢的高温热塑性和组织析出区间的影响,确认在900 ℃以下第三脆性区间,奥氏体晶界形成的先共析铁素体弱化了晶界强度,在热应力、组织应力、机械矫直应力的作用下发生晶界开裂是导致含硫钢表面纵裂纹的根本原因。在此基础上开展了保护渣优化、二冷区改造、铸坯加热和堆冷过程保温等工艺研究。研究结果表明,提高保护渣碱度、降低保护渣熔点和黏度、延长二冷区长度、规范加热和堆冷工艺,大方坯含硫钢铸坯表面纵裂纹得到了较好控制,因纵裂纹导致的圆钢钢质原因修磨率从20%降低到5%以下。 相似文献
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为探究涟钢连铸纵裂纹产生的原因,对钢水质量、过热度、进出水温、保护渣性能、液面波动及结晶器铜板等进行了检测、分析.认为其主要原因是:钢中部分硫含量处于较高水平,使w(Mn) /w(S)比为28.8;停产后结晶器进水温度下降幅度达到25℃,恢复生产后水温又快速上升到30~32℃;保护渣碱度波动较大,最低时达到1.1,使渣玻璃性强、导热性较好;液面波动高达6~7mm,保护渣不能均匀流入,影响结晶器的传热;另外,结晶器铜板划伤造成初生坯壳与铜板接触不良,局部温度偏高.并提出了相对应的措施,经过调整后,涟钢1号机表面纵裂纹发生率由0.50%下降到0.024%. 相似文献
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在生产Q235B、Q345B等包晶钢时,铸坯表面产生了大量的纵裂纹,通过对该缺陷的宏观形貌、金相组织以及微观形貌分析,证实该类缺陷形成于结晶器内,根本原因在于δ相向γ相转变时体积收缩引起结晶器内初生坯壳生长不均,而保护渣是其关键的影响因素。为减缓铸坯向结晶器的传热,对现有保护渣进行了优化,将保护渣碱度由1.39增加到1.53,提高保护渣结晶性能以强化弯月面钢水的缓冷,同时将熔化温度由1 171降低到1 130 ℃,1 300 ℃时黏度保持在0.08 Pa·s,确保保护渣消耗量以保证对铸坯的润滑。生产实践表明,采用优化的保护渣后,板坯表面质量明显改善,纵裂纹缺陷发生率由原来的10.58%降低到1.85%。 相似文献
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316L焊管在生产过程中产生沿管壁圆周方向的裂纹。采用宏观形貌观察、金相显微镜观察、扫描电镜和能谱分析等方法对焊管开裂原因进行了分析。结果表明:焊管断裂方式为沿晶断裂,在断裂晶面和附近晶界上有铜元素富集,管壁内侧有富铜颗粒粘附。开裂主要原因为液相铜扩散进入奥氏体晶界引起晶界脆化,在应力作用下引起焊管开裂。 相似文献
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综述了焊缝金属对SUPER 304H钢焊接性的影响。结果表明,SUPER 304H钢不同成分GTAW奥氏体焊缝的热裂纹倾向较大;焊缝中凝固裂纹倾向主要是受A凝固模式控制,而HAZ液化裂纹倾向的主要原因则与晶界析出相,以及铜的富集等因素有关。不同成分奥氏体焊缝接头的力学性能各异;合适的焊缝化学成分和优化的焊接工艺是获得满意接头综合力学性能的重要技术手段。不同成分形成的"同质"或"异质"焊缝,其微观组织皆为奥氏体+析出相。熔化焊焊缝为A凝固模式,非熔化焊的摩擦焊焊缝为AF凝固(相转变)模式。 相似文献
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用扫描电镜观察了受应变疲劳载荷作用的Cu双晶物的表面形貌,发现晶界是疲劳形变双晶是有利的裂纺萌生地点,在滑移带撞击晶界的地方,特别是在几条粗滑移带共同撞击晶界的地方观察到许多疲劳微裂纹;并且发现与平行晶界双晶相比,垂直晶界双晶有有利于疲劳裂纹沿晶界作早期扩展。 相似文献
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As a result of the phase transformation of austenite to martensite during steel quenching, weakened structural regions, specifically the boundaries of the original austenite grains, have been formed. They are weakened because of microstructural factors, such as the residual internal microstrains and segregation of embrittling impurities. The joint effect of microstructural factors, namely, residual microstrains and segregation of phosphorus and carbon at grain boundaries, on reducing the local strength of the boundaries of the initial austenite grains in martensitic steels is quantitatively evaluated, and the impacts of these microstructural factors have been separated. The dependences of the local grain-boundary strength on the ratio of various levels of residual microstrains and on the atomic concentration of phosphorus impurities at the grain boundary in segregation spots have been determined. It has been shown quantitatively that the adsorption enrichment of the austenite grain boundaries with phosphorus leads to a decrease in the intergrain adhesion and facilitates the emergence and development of cracks along the boundaries of the initial austenite grains. The quantitative dependence of the local strength of grain boundaries on the concentration ratio of carbon and phosphorus in them has been shown. Carbon in concentrations of up to 0.04% reduces the embrittlement of the boundaries due to the segregation of phosphorus and loses its neutralizing effect on the phosphorus segregation at concentrations of more than 0.04%, so the phosphorus concentration at the grain boundaries increases and the embrittlement resistance of the latter decreases. The applicability of the developed technique for the quantitative evaluation of the local strength of hardened steel grain boundaries by using tests on delayed fracture and applying the method of finite elements to determine the local strains has been shown. 相似文献
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Yifan Feng Renbo Song Zhongzheng Pei Renfeng Song Guoyu Dou 《Metals and Materials International》2018,24(5):1012-1023
The microstructure and impact toughness of the as-cast Fe–24.8Mn–7.3Al–1.2C austenitic steel after solution treatment and subsequent aging treatment were investigated in the present work. Research on the κ-carbides precipitation behavior was carried out by transmission electron microscope. The results show that nano-sized coherent κ-carbides were obtained in the as-solutionized steel after aging treatment, which produced precipitation hardening. After being aging treated at 550 °C for 1 h, the steel with regular hexagonal grain structure exhibited a good combination of yield strength (~?574 MPa) and room-temperature impact toughness (~?168 J). In the present steel, the typical cube-on-cube orientation relationship between austenite and κ-carbides was observed. However, due to the long aging isothermal time and high C content, the coarse intergranular κ′-carbide was formed and grew along the austenite grain boundary, which caused this orientation relationship to be destroyed and a dramatical increase of the coherency strain energy at grain boundary. Furthermore, serious embrittlement of grain boundaries caused that cleavage cracks trend to propagate along the grain boundaries. Accordingly, the room-temperature impact toughness decreased sharply. After aging isothermal time prolonging to 13 h, the Charpy V-notch impact toughness was only ~?5 J and fracture mode turned to fully brittle fracture accompanied with flat facets, shear cracks and well-developed secondary crack. 相似文献
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航空发动机高压涡轮工作叶片阻尼叶冠表面堆焊耐磨层后,发现焊接热影响区存在延迟裂纹。通过基体材料成分分析、金相观察、扫描电镜分析等手段分析裂纹原因,结果表明:裂纹为焊接及磨削残余应力与时效组织应力叠加导致应变集中产生的应变-时效裂纹。通过增加焊前预热、焊后缓冷、焊后立即退火、控制叶片磨削进刀量等措施有效降低裂纹故障率。冷热循环试验表明在正常工作过程中叶片裂纹扩展速率缓慢,一般不会形成封闭裂纹;叶冠边缘处裂纹有向叶冠外侧扩展倾向,可能造成掉块,影响飞行安全。 相似文献