【作者】 刘大双；

【导师】 魏艳红； 刘仁培；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2013， 博士

【摘要】 无渣自保护药芯焊丝无需保护气源、焊剂，焊后无渣，熔敷速度明显高于普通的药芯焊丝和实芯焊丝，这些优点契合了大型设备如辊压机对修复效率的内在需求，为大型、超大型构件的高效、自动化堆焊修复提供了新的有效途径。本文针对辊压机辊面的磨损失效及堆焊修复现状，提出并按照气体-金属联合保护的思想，研制了一种环境友好型无渣自保护药芯焊丝。该焊丝焊接工艺性能较佳，堆焊层硬度在58～67HRC范围，耐磨性能优良。在此基础上，本文围绕无渣自保护药芯焊丝工艺性能和耐磨性能展开了系统的研究。研究认为，无渣自保护药芯焊丝的飞溅主要有四种类型：电弧斥力飞溅、电爆炸飞溅、气体析出飞溅和蒸汽阻力飞溅。利用一次回归正交设计试验方案，建立了药芯组分硼铁、锰粉、石墨、铝镁合金和硅铁对飞溅组成的数学模型。并从表面张力、焊接电弧导电性、气体析出、焊接反应放热程度等四个方面对无渣自保护药芯焊丝飞溅行为机理展开了系统的研究，认为添加能够降低熔滴表面张力、提高电弧导电性、减小气体析出或焊接反应放热程度的药芯组分，有利于降低飞溅率。通过药芯组分调整飞溅率最小仅0.57%。分析了无渣自保护药芯焊丝的环保特性。与目前常用的手工焊条或CO2气体保护堆焊相比，它节约了大量的能耗，并减少排放。依托自主搭建的高速摄影观察平台和电弧物理监测系统，在国内外首次对无渣自保护药芯焊丝熔滴过渡行为展开系统研究。发现无渣自保护药芯焊丝熔滴过渡过程中频繁的熔渣飞溅，即“渣溅”这一特有的现象；认为其熔渣与熔融金属高温下线膨胀系数相差大，熔渣本身的体积较小，熔滴在过渡过程中自身快速旋转的事实，是形成““渣溅”的根本原因。“渣溅”的发生成功解释了合金元素脱氧保护之后氧化产物的去向问题，它是导致焊后焊道表面没有熔渣的基本原因。建立了“渣溅”的简单模型。提出了熔滴过渡的几种形态：排斥过渡、表面张力过渡、颗粒过渡和爆炸过渡。排斥过渡和表面张力过渡是无渣自保护药芯焊丝的主要过渡方式。并对其形成机理以及熔滴过渡形态对焊接电弧的影响进行了研究，从焊接工艺参数和药芯组分两个方面探讨熔滴过渡形态的调控方法。通过试验确立了无渣自保护药芯焊丝的基本合金系：Fe-Cr-B-C。分别调整药芯组分中石墨、硼铁含量，研究了C、B及B/C对Fe-Cr-B-C系堆焊合金凝固组织的影响规律。石墨促进了堆焊层组织先析碳化物的形成，同时抑制了共晶碳化物的生长，并使得先析碳化物趋于垂直于母材表面生长。随着石墨含量的增加，洛氏硬度值逐渐增加；当石墨含量超过6%后，硬度值增幅放缓。在具体Fe-Cr-C合金系下，不足5%的B添加量就可以使得组织中获得90%以上的碳化物。随着硼铁含量从0增加至12%，Fe-Cr-Ti-C合金组织中先析碳化物的直径从9um增加至20um，同时碳化物体积分数从14.10%增加至36.00%，合金硬度从55HRC增加至65HRC。在药芯中硼铁和石墨添加总量一定的条件下，随着硼铁的比重增加，硬质相的尺寸趋于减小。在Fe-Cr-B-C合金系的基础上继续添加含量变化范围较宽的钛铁及铌铁，系统的研究了Ti和Nb对无渣自保护药芯焊丝堆焊合金组织和耐磨性的影响，并揭示了堆焊合金的耐磨机制。钛铁的添加促进堆焊层中高硬度TiC碳化物的形成。TiC可作为M7(C, B)3(M=Cr, Fe, Mn)碳化物的形核核心，并减少M7(C, B)3碳化物的数量。由于形成TiC的过程中消耗了一定量的C，当钛铁含量增至24%时，堆焊合金组织由过共晶组织转变为亚共晶组织。钛铁的添加使得合金具有更高的硬度和细化的组织，因而有利于改善堆焊合金的耐磨性能。当钛铁添加量增加至24%时，磨损量14.9mg，达到最小值。随着药芯中铌铁的添加，M7(C, B)3碳化物的数量逐渐减少，且NbC的数量增多。当铌铁含量添加至18%时，堆焊合金由过共晶转变为共晶组织；当铌铁含量继续添加到24%时，堆焊合金的组织继续转化为亚共晶组织。不加铌时堆焊合金的硬度值为58.9HRC，当药芯中铌铁含量增加至18%时堆焊合金的硬度值达到64.3HRC。当药芯中铌铁添加量继续增加至24%，堆焊合金的硬度值有所回落，为62.7HRC。另一方面，药芯中铌铁含量从0增加至18%时，磨损失重随着药芯中铌铁含量的增加而不断减小；继续增加铌铁含量至24%，磨损失重值不再有明显变化。随着药芯中钛铁或铌铁的添加，堆焊合金的组织出现了尺寸较小、硬度极高的MC (M=Ti/Nb)型碳化物，同时堆焊合金中先析M7(C, B)3碳化物的数量和尺寸均不断减少，并且基体组织得到了Cr的固溶强化。这一成分和组织变化导致了堆焊合金磨损机理的变迁：由强化前的微裂纹机制转变为得到Nb或Ti强化堆焊合金时的微切削和经历多次塑形变形所导致的微犁耕机制。利用有限元辅助设计了无渣自保护药芯焊丝辊压机辊面堆焊制造工艺。在考虑焊接残余应力的情况底下，利用有限元方法展开对堆焊辊体工作应力分布的模拟分析，结果表明在堆焊四层的情况下，较佳的堆焊工艺为：缓冲层堆焊3层(厚度为60mm)，耐磨层堆焊1层(厚度为20mm)。此时，辊压机工作时的最大应力值出现在距离堆焊表面约15mm处，且为558MPa。该结果为成功将无渣自保护药芯焊丝应用于辊压机堆焊修复制造打下了良好的基础。更多还原

【Abstract】 Slag-free self-shielded flux cored wires have several advantages, such as simple weldingprocedure (not need shielding gas, flux and slag clearing), high deposition rate as compared to thecommon flux-cored wires and solid wires, which offer effective support to the automatic hardfacingfor heavy equipments. In this paper, a new type of slag-free self-shield flux-cored wire was developed.This wire possessed excellent processing properties and wear resistance. The hardness were among58～67HRC. On this basis, the processing properties and wear resistance of slag-free self-shieldedflux cored wires were studied.The spatter of slag-free self-shielded flux cored wires typically classified into four types as arcrepulsion force spatter，electrically exploding spatter, gas precipitation spatter and metal vaporpressure. The mathematics model of quantitative relation between spatter and cored-wirecompositions was established using an orthogonal regression design. The mechanism of spatter wasanalyzed from surface tension, electrical conductivity of arc, gas precipitation and exothermicreaction. The cored-wire compositions which reduced surface tension, increased electricalconductivity of arc, reduced the amount of gas precipitation and exothermic reaction contributed tothe decrease of spatter rate. The smallest spatter rate (0.57%) was got by adjusting flux-coredcomposition. For the environmentally aware, the slag-free self-shielded flux cored wires were alsostudied. By comparising the shielded metal arc welding and CO2gas shielded arc welding, slag-freeself-shielded flux-cored contributed to energy saving and emission reduction.The dropt transfer of slag-free self-shielded flux cored wires was studied using self-made highspeed photography apparatus and arc physics monitoring system. The results showed that the slagspattered continually in the welding process, which named “slag spatter”. It was discussed that theformation of slag spatter was caused by the great difference of coefficience of linear expansionbetween slag and droplet, small volume of slag and rapid rotation of droplet. The feature of slagspatter was successful in explaining oxidation products where they gone, it was what caused weldbond without slag.Moreover, the droplet transfer classified into four types, namely repelled transfer, surface tensiontransfer, drop transfer and explosive transfer. Among them, repelled transfer and surface tensiontransfer were the main droplet transfer. In addition, the effect of mode of droplet transfer on arc wasstudied. Finally, the effects of welding parameter and cored-wire compositions on the mode of droplettransfer were also studied. The alloy system of slag-free self-shielded flux cored wire was determined as Fe-Cr-B-C.Especially, the effects of graphite, Fe-B and Fe-B/graphite on the microstructure of Fe-Cr-B-Chardfacing alloys were studied. The graphite promoted the growth of primary carbides perpendicularto the surface of base metal and restrained the growth of eutectic carbides. The hardness wasincreased rapidly with the carbon content increased to6%, and then increased more slowly. When theB content was about5%, the volume of carbides exceeded90%. Moreover, the average diameter ofM7(C, B)3(M＝Cr, Fe mainly) carbide was increased from9to20um and carbide volume fraction(CVF) was increased from14.10%to36.00%with the boron content increasing from0to1.4%inFe-Cr-Ti-C alloy. When the total content of graphite and Fe-B remained the same, the size of thecarbides was reduced, the shape of the carbides became grain-like and the distribution of the carbidesbecame diffuse with the increase of the proportion of Fe-B.Fe-Nb and Fe-Ti were added into Fe-Cr-B-C slag-free self-shielded flux-cored wire and theeffects of Ti and Nb on the alloy microstructure and wear resistance were investigated. TiC existed inthe iron-based hardfacing alloy due to the addition of Fe-Ti, and it acted as the nucleus of the primaryM7(C, B)3carbides (M=Cr, Fe, Mn). TiC decreased the amount of M7(C, B)3carbides when titaniumwas added into the alloys. When24wt.%Fe-Ti was added, the alloy microstructure was changedfrom a hypereutectic structure to a hypoeutectic one due to the formation of TiC, which consumed amass of carbon. The addition of titanium into iron-based hardfacing alloy was beneficial to the wearresistance with respect to the higher hardness and refined microstructure. The wear loss of the samplewith24wt.%Fe-Ti was the smallest (14.9mg). On the other hand, NbC acted as the nucleus of theprimary M7(C, B)3carbides and decreased the amount of M7(C,B)3carbides when niobium was addedinto the alloys. When18wt.%Fe-Nb was added, the microstructure of hardfacing alloy transformedfrom a hypereutectic structure to a eutectic one due to the formation of NbC, which consumed a massof carbon. Moreover, the microstructure changed into a hypoeutectic structure when the Fe-Nbcontent was up to24wt.%. The hardness of Nb-free hardfacing alloy was58.9HRC. When theFe-Nb content was increased to18%, the hardness was64.3HRC. However, its hardness wasdecreased to62.7HRC when the Fe-Nb content reached to24%. The wear loss of the hardfacingalloy with18wt.%Fe-Nb addition was the smallest among all the alloys.With the addition of Fe-Ti or Nb-Fe, the MC-type carbide (M=Ti/Nb) with small size and highhardness was precipitated, the amount and size of M7(C, B)3carbides were both reduced, and thematrix was enhanced by solid solution strengthening of Cr. The main abrasive wear mechanismchanged from microcracking to microcutting and microploughing resulting from times of plastic deformation due to the formation of MC and the reduction of primary M7(C, B)3carbides.Hardfacing processing of roller press was simulated by the finite element method (FEM). Theworking stress distribution in roller press was analyzed considering welding residual stress. Theresults showed that when the hardfacing layer has three buffer layers (60mm in thickness) and onewearing layer (20mm in thickness), the maximum stress was558MPa and occurred at the region15mm below the hardfacing surface. It was the optimal hardfacing processing. The results providefundamental base for the further application for hardfacing roller press by using slag-free self-shieldedflux cored wire.更多还原