【摘要】：我國鎳及鎳合金板帶加工水平整體落后,僅能生產單張板,尚不能生產寬幅、大卷重鎳及鎳合金板帶產品。因此,全面提升鎳及鎳合金板帶加工水平,加快產品結構調整和技術進步顯得非常緊迫。在各方努力下建成了一條5000噸/年鎳及鎳合金板帶生產線,雖然填補了裝備方面的空白,但受制于工藝技術與純鎳的焊接難度,目前仍無法生產合格的寬幅、大卷重鎳及鎳合金板帶產品。存在的主要問題:鎳及鎳合金板帶生產線上沒有專用拼焊設備,也沒有成熟可靠的拼焊工藝技術,同時純鎳焊接性較差,焊接過程中極易出現缺陷,導致拼焊焊縫在酸洗、冷軋工序大量斷帶,影響生產效率及產品質量。換言之,在線拼焊是保證冷軋、酸洗等工序生產效率及鎳及鎳合金板帶產品成材率的最關鍵工藝,拼焊質量直接決定了寬幅、大卷重鎳及鎳合金板帶產品的質量�；谏鲜鰡栴},本文重點對純鎳N6的焊接方法、焊接工藝、焊縫組織與性能、焊縫的碾壓形變熱處理以及焊縫斷帶機理等問題進行了試驗研究并提出解決方案,徹底地解決了純鎳N6板帶在線拼焊的問題。首先,探討了純鎳等離子弧自熔焊焊縫缺陷形成機理。結晶裂紋的形成是基于Borland拉伸液膜理論。在等離子弧焊接時的較小熔池中,純鎳熱導率高、固液相溫度區(qū)間小、液態(tài)金屬粘度大、流動性差,致使結晶速度加快,氣泡逸出速度小于結晶速度,便在焊縫中形成氣孔。分析純鎳焊接性及氣孔特征,氣孔主要是氫氣孔、一氧化碳氣孔、氮氣孔。焊縫與近縫區(qū)晶粒粗化與母材化學成分、焊接方法及焊接熱輸入有關,焊接熱輸入增大,晶粒長大;純鎳N6化學成分中幾乎沒有可以形成碳、氮化物的元素,因此焊接過程中產生的第二相質點少,基本沒有第二相質點阻礙奧氏體晶粒的長大,晶界可以自由移動至平衡晶粒尺寸。而且等離子弧自熔焊過程中,熔池中少量的碳、氮化物粒子大部分發(fā)生溶解,減弱了粒子對晶界遷移的釘扎作用�？傊�,缺陷導致焊縫成形差、削弱力學性能,影響產品成材率。其次,針對等離子弧自熔焊焊縫缺陷,提出采用等離子弧填絲焊焊接純鎳N6板帶。通過分析焊縫組織、性能,研究并揭示了填充焊絲消除自熔焊焊縫缺陷的機理。等離子弧填絲焊實現了6mm厚純鎳N6板帶的單面焊雙面成形;焊縫成形美觀,無氣孔、裂紋、咬邊、凹陷等缺陷;等離子弧填絲焊過程穩(wěn)定,拓寬了工藝參數規(guī)范區(qū)間;雖然純鎳填絲焊焊縫、自熔焊焊縫及母材基體均為奧氏體,但填絲焊焊縫晶粒較自熔焊焊縫細小;填絲焊焊縫抗拉強度、斷后伸長率、沖擊功比自熔焊焊縫分別提高了11.75%、84.8%、31.7%;填絲焊焊縫在不同腐蝕介質中的耐蝕性也優(yōu)于自熔焊焊縫。第三,針對填絲焊焊縫存在較大焊接殘余應力且焊縫晶粒較粗大的問題,提出對焊縫進行碾壓形變熱處理工藝,探討了碾壓形變熱處理工藝對焊縫組織、性能的影響。結果顯示,隨碾壓熱處理溫度升高,焊縫抗拉強度逐漸升高,斷后伸長率呈現出先增大后減小的趨勢;合理的碾壓形變熱處理工藝可降低焊接殘余應力;通過碾壓作用破碎焊縫粗大的柱狀晶,細化焊縫晶粒;適合6mm厚純鎳焊縫的碾壓形變熱處理工藝為:總碾壓力21368.97N,碾壓溫度400℃,碾壓速度3mm/s,碾壓次數1次,壓下率20%~25%。碾壓形變熱處理工藝對優(yōu)化純鎳焊縫組織、改善性能有積極作用,特別對提高焊縫塑性十分有效,能夠得到焊縫高強度、高塑性的良好配合。最后,針對非真空熔煉工藝生產的純鎳N6板帶焊縫強度低、斷帶率高的問題開展研究,明確了焊縫斷帶原因:焊縫強度低、斷帶率高與熔煉工藝有關,即不同熔煉工藝生產的純鎳N6中同種化學成分含量有差別;非真空熔煉工藝生產的純鎳中非金屬夾雜物的數量、尺寸均高于真空熔煉工藝生產的純鎳;非金屬夾雜物降低焊縫力學性能與工藝性能的根本原因是:脆性不變形夾雜物與純鎳N6基體屬于簡單的機械結合,結合力弱。非金屬夾雜物與純鎳的彈、塑性及線膨脹系數有較大差別,變形過程中它們的變形程度不同,非金屬夾雜物使應力發(fā)生再分布引起應力集中,非金屬夾雜物與純鎳界面處形成空隙或裂紋,當持續(xù)加載時裂紋不斷擴展,導致焊縫在低于正常拉伸強度時斷裂。同時提出該生產線應全部使用真空熔煉工藝生產的純鎳N6板帶。
[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.
【學位授予單位】：蘭州理工大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：TG457.1

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