纯镍板带等离子拼焊关键问题的研究

发布时间：2019-04-29 20:05

【摘要】：我国镍及镍合金板带加工水平整体落后,仅能生产单张板,尚不能生产宽幅、大卷重镍及镍合金板带产品。因此,全面提升镍及镍合金板带加工水平,加快产品结构调整和技术进步显得非常紧迫。在各方努力下建成了一条5000吨/年镍及镍合金板带生产线,虽然填补了装备方面的空白,但受制于工艺技术与纯镍的焊接难度,目前仍无法生产合格的宽幅、大卷重镍及镍合金板带产品。存在的主要问题:镍及镍合金板带生产线上没有专用拼焊设备,也没有成熟可靠的拼焊工艺技术,同时纯镍焊接性较差,焊接过程中极易出现缺陷,导致拼焊焊缝在酸洗、冷轧工序大量断带,影响生产效率及产品质量。换言之,在线拼焊是保证冷轧、酸洗等工序生产效率及镍及镍合金板带产品成材率的最关键工艺,拼焊质量直接决定了宽幅、大卷重镍及镍合金板带产品的质量。基于上述问题,本文重点对纯镍N6的焊接方法、焊接工艺、焊缝组织与性能、焊缝的碾压形变热处理以及焊缝断带机理等问题进行了试验研究并提出解决方案,彻底地解决了纯镍N6板带在线拼焊的问题。首先,探讨了纯镍等离子弧自熔焊焊缝缺陷形成机理。结晶裂纹的形成是基于Borland拉伸液膜理论。在等离子弧焊接时的较小熔池中,纯镍热导率高、固液相温度区间小、液态金属粘度大、流动性差,致使结晶速度加快,气泡逸出速度小于结晶速度,便在焊缝中形成气孔。分析纯镍焊接性及气孔特征,气孔主要是氢气孔、一氧化碳气孔、氮气孔。焊缝与近缝区晶粒粗化与母材化学成分、焊接方法及焊接热输入有关,焊接热输入增大,晶粒长大;纯镍N6化学成分中几乎没有可以形成碳、氮化物的元素,因此焊接过程中产生的第二相质点少,基本没有第二相质点阻碍奥氏体晶粒的长大,晶界可以自由移动至平衡晶粒尺寸。而且等离子弧自熔焊过程中,熔池中少量的碳、氮化物粒子大部分发生溶解,减弱了粒子对晶界迁移的钉扎作用。总之,缺陷导致焊缝成形差、削弱力学性能,影响产品成材率。其次,针对等离子弧自熔焊焊缝缺陷,提出采用等离子弧填丝焊焊接纯镍N6板带。通过分析焊缝组织、性能,研究并揭示了填充焊丝消除自熔焊焊缝缺陷的机理。等离子弧填丝焊实现了6mm厚纯镍N6板带的单面焊双面成形;焊缝成形美观,无气孔、裂纹、咬边、凹陷等缺陷;等离子弧填丝焊过程稳定,拓宽了工艺参数规范区间;虽然纯镍填丝焊焊缝、自熔焊焊缝及母材基体均为奥氏体,但填丝焊焊缝晶粒较自熔焊焊缝细小;填丝焊焊缝抗拉强度、断后伸长率、冲击功比自熔焊焊缝分别提高了11.75%、84.8%、31.7%;填丝焊焊缝在不同腐蚀介质中的耐蚀性也优于自熔焊焊缝。第三,针对填丝焊焊缝存在较大焊接残余应力且焊缝晶粒较粗大的问题,提出对焊缝进行碾压形变热处理工艺,探讨了碾压形变热处理工艺对焊缝组织、性能的影响。结果显示,随碾压热处理温度升高,焊缝抗拉强度逐渐升高,断后伸长率呈现出先增大后减小的趋势;合理的碾压形变热处理工艺可降低焊接残余应力;通过碾压作用破碎焊缝粗大的柱状晶,细化焊缝晶粒;适合6mm厚纯镍焊缝的碾压形变热处理工艺为:总碾压力21368.97N,碾压温度400℃,碾压速度3mm/s,碾压次数1次,压下率20%~25%。碾压形变热处理工艺对优化纯镍焊缝组织、改善性能有积极作用,特别对提高焊缝塑性十分有效,能够得到焊缝高强度、高塑性的良好配合。最后,针对非真空熔炼工艺生产的纯镍N6板带焊缝强度低、断带率高的问题开展研究,明确了焊缝断带原因:焊缝强度低、断带率高与熔炼工艺有关,即不同熔炼工艺生产的纯镍N6中同种化学成分含量有差别;非真空熔炼工艺生产的纯镍中非金属夹杂物的数量、尺寸均高于真空熔炼工艺生产的纯镍;非金属夹杂物降低焊缝力学性能与工艺性能的根本原因是:脆性不变形夹杂物与纯镍N6基体属于简单的机械结合,结合力弱。非金属夹杂物与纯镍的弹、塑性及线膨胀系数有较大差别,变形过程中它们的变形程度不同,非金属夹杂物使应力发生再分布引起应力集中,非金属夹杂物与纯镍界面处形成空隙或裂纹,当持续加载时裂纹不断扩展,导致焊缝在低于正常拉伸强度时断裂。同时提出该生产线应全部使用真空熔炼工艺生产的纯镍N6板带。
[Abstract]:The processing level of nickel and nickel alloy strip in China is backward and can only produce a single sheet. It is not yet possible to produce wide-width, large-volume heavy nickel and nickel alloy plate and strip products. Therefore, it is very urgent to improve the processing level of nickel and nickel alloy plate, and to speed up the structural adjustment and technological progress of the product. In the effort of the parties, a production line of 5000 tons/ year of nickel and nickel alloy is completed, although the blank in the equipment is filled, the welding difficulty of the process technology and the pure nickel is limited, and the qualified wide-width, large-volume heavy nickel and nickel alloy plate and strip products can not be produced at present. the main problems are that no special welding equipment is arranged on the nickel and nickel alloy strip production line, And the production efficiency and the product quality are influenced. In other words, on-line welding is the most critical process to ensure the production efficiency of cold rolling and acid washing and the yield of nickel and nickel alloy strip products, and the welding quality directly determines the quality of wide-width, large-volume heavy-nickel and nickel-alloy strip products. Based on the above-mentioned problems, this paper focuses on the welding method of pure nickel N6, the welding process, the microstructure and the performance of the weld, the rolling and deformation heat treatment of the weld, the mechanism of the seam breakage, etc., and the solution is put forward to thoroughly solve the problem of on-line welding of the pure nickel N6 strip. First, the formation mechanism of pure nickel plasma arc self-welding seam is discussed. The formation of the crystal crack is based on the Borland tensile membrane theory. In the small molten pool at the time of plasma arc welding, the pure nickel has the advantages of high thermal conductivity, small solid-liquid phase temperature range, large liquid metal viscosity and poor fluidity, so that the crystallization speed is accelerated, the air bubble escape speed is less than the crystallization speed, and air holes are formed in the welding seam. The welding and air hole characteristics of pure nickel are analyzed, and the air holes are mainly hydrogen, carbon monoxide and nitrogen. the grain coarsening of the weld and the near-seam area is related to the chemical composition of the base material, the welding method and the welding heat input, the welding heat input is increased, the crystal grain is grown, Substantially no second phase particles impede the growth of the austenite grain and the grain boundaries can be freely moved to the balance grain size. And the plasma arc self-welding process, a small amount of carbon and nitride particles in the molten pool are dissolved, and the pinning effect of the particles on the grain boundary migration is reduced. In short, that defect cause the weld to form poor, weaken the mechanical property, and affect the yield of the product. Secondly, aiming at the defects of the plasma arc self-welding seam, the pure nickel N6 strip is welded by using the plasma arc welding. By analyzing the structure and properties of the weld, the mechanism of eliminating the defect of the self-welding seam by the filler wire is also discussed. The single-side welding of the 6-mm thick pure nickel N6 strip is realized by the plasma arc-filling welding, and the welding seam is beautiful in shape, free of air holes, cracks, undercut, depression and the like, and the plasma arc-filling welding process is stable, and the specification section of the process parameters is widened; and the welding seam of the pure nickel is welded, The self-welding seam and the base metal matrix are of the austenite, but the grain of the welded seam is smaller than that of the welded seam, and the tensile strength, the elongation at break and the impact power of the welded seam are improved by 11.75%, 84.8% and 31.7%, respectively. The corrosion resistance of the fillet weld in different corrosive media is also superior to the self-welding seam. Thirdly, for the problem of large welding residual stress in the welding seam of the filler and the coarse grain size of the weld, the process of rolling and deformation heat treatment of the weld is put forward, and the effect of the rolling deformation heat treatment process on the microstructure and the performance of the weld is discussed. The results show that the tensile strength of the weld gradually increases with the increase of the temperature of the rolling heat treatment, the elongation at break exhibits a tendency to decrease after the increase, and the reasonable rolling and deformation heat treatment process can reduce the residual stress of the welding, and the coarse columnar crystal of the weld is broken by the rolling action. The grain of the welded seam is refined; the rolling deformation heat treatment process suitable for 6mm thick pure nickel welding line is as follows: the total rolling pressure is 21368.97N, the rolling temperature is 400 DEG C, the rolling speed is 3 mm/ s, the number of rolling is 1 time, and the reduction ratio is 20-25%. The rolling deformation heat treatment process has a positive effect on the optimization of the pure nickel weld tissue and the improvement of the performance, in particular to the improvement of the plasticity of the weld, and can be well matched with high strength and high plasticity of the weld. In the end, the research on the problems of low weld strength and high band-breaking rate of the pure nickel N6 plate and strip produced by the non-vacuum melting process is studied, the reason of the welding seam is clear: the weld strength is low, the band-breaking rate is high and the smelting process is related, in other words, the content of the same chemical components in the pure nickel N6 produced by the different smelting processes is different; the quantity and the size of the non-metal inclusions in the pure nickel produced by the non-vacuum smelting process are higher than that of the pure nickel produced by the vacuum smelting process; The root cause of the non-metallic inclusion in reducing the mechanical and technological properties of the weld is that the brittle non-deformed inclusion and the pure nickel N6 matrix belong to a simple mechanical bond, and the bonding force is weak. the non-metallic inclusions are different from the elasticity, the plasticity and the linear expansion coefficient of the pure nickel, the deformation degree of the non-metal inclusions is different in the deformation process, the non-metallic inclusion causes the stress concentration, the non-metal inclusions and the pure nickel interface to form voids or cracks, The crack continues to expand as it is continuously loaded, causing the weld to break when under normal tensile strength. At the same time, it is proposed that all the pure nickel N6 plates produced by the vacuum melting process should be used in the production line.
【学位授予单位】：兰州理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG457.1

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