热输入对低碳贝氏体焊缝组织性能的影响

发布时间：2018-05-20 18:55

本文选题：热输入 + 粒状贝氏体　；参考：《兰州理工大学》2017年硕士论文

【摘要】：随着科学技术的进步,工程结构向大型化和轻量化发展。综合力学性能优异的高强钢可大大降低工程结构重量,从而减少在生产运输过程中的经济成本及碳化物的排放,进一步降低对能源的消耗,这使得高强钢在工程结构中愈发得到重视。但是在诸多高强钢结构中存在焊缝金属低温韧性低于母材的问题,限制了高强钢进一步推广应用。而焊接接头的机械性能是由接头的组织来决定,当焊材和母材被确定后,接头的组织和性能则是由焊接工艺来决定。而热输入则是焊接工艺中较为重要的因素之一,因此研究热输入对焊缝金属组织性能的影响十分必要。本文采用金属粉芯焊丝,以焊缝金属作为研究对象,研究了热输入对低碳贝氏体焊缝组织性能的影响规律;在最佳热输入下,研究了Ni元素对焊缝金属组织性能的影响;并分析了Ni含量为6%的焊缝金属产生热裂纹的原因及解决方法。研究结果表明:(1)Ni0、Ni2的原始焊缝(盖面焊缝)、焊缝内再热区组织是由粒状贝氏体和准多边形铁素体组成,而Ni4的原始焊缝(盖面焊缝)、焊缝内再热区组织是由板条贝氏体(LB)和准多边形铁素体构成。并且,随着热输入的逐渐增大,三种Ni含量的焊缝金属的原始焊缝、焊缝内再热区的粒状贝氏体或板条贝氏体含量呈现出先升高后降低的变化趋势。在热输入为17.9kJ/cm时,粒状贝氏体或板条贝氏体的含量达到最大值。焊缝金属的屈服强度、抗拉强度则随着热输入的增加先增大后减小,在热输入为17.9kJ/cm时,屈服强度、抗拉强度均达到峰值;冲击韧性则是随着热输入的增大而逐渐降低。(2)在15.8kJ/cm的最佳热输入下,当焊缝金属中Ni含量由0逐渐增加到4%时,原始焊缝、焊缝内再热区的显微组织由粒状贝氏体(GB)+准多边形铁素体逐渐转变为板条贝氏体(LB)+准多边形铁素体;随着焊缝金属中Ni含量的逐渐增大,焊缝金属的抗拉强度、屈服强度、冲击韧性、维氏硬度均呈现出逐渐增大的趋势。(3)在Ni含量为6%的第一层焊缝金属中,由于Fe、Ni、Si、Mn、S等合金元素的存在,特别是Ni元素的偏析,易于形成较多的低熔点共晶,在焊缝金属冷却凝固过程中形成的拉伸应力作用下产生热裂纹,并逐渐向其它焊道扩展;无论是增大热输入还是降低热输入到8.2kJ/cm,焊缝金属中均出现了大量的热裂纹,故在当前的焊接工艺和选用的焊材下热裂纹的萌生扩展是无法避免的。
[Abstract]:With the progress of science and technology, engineering structure develops to large-scale and light-weight. High strength steel with excellent comprehensive mechanical properties can greatly reduce the weight of engineering structure, thus reducing the economic cost and the emission of carbides in the process of production and transportation, and further reducing the consumption of energy. This makes high-strength steel more and more important in engineering structure. However, in many high strength steel structures, the low temperature toughness of weld metal is lower than that of base metal, which limits the further application of high strength steel. The mechanical properties of welded joints are determined by the structure of the joints, and the microstructure and properties of the joints are determined by the welding process when the welding materials and base materials are determined. Heat input is one of the most important factors in welding process, so it is necessary to study the effect of heat input on microstructure and properties of weld metal. In this paper, the effect of heat input on microstructure and properties of low carbon bainite weld metal was studied by using metal powder cored wire, and the effect of Ni element on microstructure and properties of weld metal was studied under optimum heat input. The causes and solutions of hot cracks in weld metal with Ni content of 6% were analyzed. The results show that the original weld (cover weld), the reheat zone microstructure in the weld is composed of granular bainite and quasi polygonal ferrite, and the microstructure of the weld is composed of granular bainite and quasi polygonal ferrite. The original weld of Ni4 is composed of lath bainite and quasi polygonal ferrite. Moreover, with the increase of heat input, the content of granular bainite or lath bainite in the reheat zone of three kinds of weld metal with Ni content increased first and then decreased. When the heat input is 17.9kJ/cm, the content of granular bainite or lath bainite reaches the maximum. The yield strength and tensile strength of weld metal first increase and then decrease with the increase of heat input. When the heat input is 17.9kJ/cm, the yield strength and tensile strength reach the peak value. The impact toughness decreases gradually with the increase of heat input.) under the optimum heat input of 15.8kJ/cm, when Ni content in weld metal increases gradually from 0 to 4, the original weld seam, The microstructure of the reheating zone in the weld gradually changed from granular bainite (GB) quasi-polygonal ferrite to lath bainite / LB) quasi-polygonal ferrite, and the tensile strength and yield strength of the weld metal increased with the increase of Ni content in the weld metal. The impact toughness and Vickers hardness of the first layer of weld metal with Ni content of 6% are gradually increasing. Due to the presence of Fe Ni Si Si mn S and other alloy elements, especially the segregation of Ni elements, it is easy to form more low melting point eutectic. Under the action of tensile stress formed during the cooling and solidification of the weld metal, hot cracks are produced and gradually extended to other welds, whether the heat input is increased or decreased to 8.2 kJ / cm, a large number of hot cracks occur in the weld metal. Therefore, the hot crack initiation and propagation under the current welding process and the selected welding material is unavoidable.
【学位授予单位】：兰州理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG441.7

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