【作者】 蒋庆磊；

【导师】 李亚江；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 博士

【摘要】 低合金高强钢因其高强度、高韧性等优点而被广泛应用于工程机械、能源、桥梁建筑和车辆船舶等重要行业。但高强钢的焊接情况复杂多变,裂纹及强韧性不匹配、熔合区及热影响区性能恶化等问题突出,成为限制高强钢在焊接结构中扩大应用和提高生产效率的关键。本文针对煤矿机械液压支架用低合金高强钢Q550和Q690展开研究,在不预热条件下,采用熔化极气体保护焊(GMAW)方法对Q550和Q690钢进行焊接,并对焊接接头的微观组织、精细结构、裂纹形态以及接头力学性能进行研究,建立焊接参数、焊接材料、接头组织结构、接头区裂纹扩展及接头力学性能之间的关系。采用高强钢对接接头裂纹试验研究不同强度匹配的焊丝和焊接热输入对Q550、Q550+Q690以及Q690钢接头裂纹敏感性的影响,随着焊丝强度级别的升高,接头断面裂纹率增加；随着焊接热输入的增加,接头的断面裂纹率也逐渐升高。对Q550和Q550+Q690钢接头力学性能进行测定,采用ER50-6焊丝时,拉伸试样从焊缝中断裂；采用ER60-G焊丝,断裂发生在熔合区或热影响区,接头抗拉强度与母材抗拉强度相当。冲击试验结果表明,Q550和Q690钢焊接接头热影响区的冲击吸收功最好；采用ER60-G焊丝的接头焊缝和熔合区的韧性均优于ER50-6焊丝焊缝和熔合区。采用金相显微镜、扫描电镜、透射电镜及能谱分析仪等研究了Q550和Q690钢焊缝区的显微组织、精细结构及夹杂物的成分等,分析了焊接热输入、合金元素及焊缝中夹杂物对焊缝组织的影响。随着焊丝中合金成分的增加,焊缝组织中的先共析铁素体含量逐渐减少；针状铁素体的含量先升高,然后逐渐降低。对于ER50-6焊丝焊缝,因奥氏体晶界被先共析铁素体完全覆盖,热输入对针状铁素体的含量的影响相对不敏感；而对ER60-G焊丝焊缝,热输入较小(14kJ/cm)时会产生以贝氏体为主的组织,降低焊缝韧性。焊缝中夹杂物组成的不均匀性及夹杂物的尺寸是影响夹杂物形核的重要因素。当夹杂物含有多种相组成,并且夹杂物尺寸为0.5～0.8μm时,有利于针状铁素体的多维形核。透射电镜分析显示针状铁素体板条内部有高密度的位错；焊缝中铁素体基体上分布有条状残余奥氏体,衍射结果表明焊缝组织中α与γ相存在K-S位相关系。研究了焊接热输入对Q690钢热影响区的组织和冲击韧性的影响,当焊接热输入从14kJ/cm提高的20kJ/cm时,热影响区冲击吸收功先升高再降低；控制焊接热输入在约16kJ/cm可获得较高的热影响区冲击韧性。提高焊接热输入虽使奥氏体晶粒尺寸增大,但下贝氏体的先期形成有效的细化了马氏体板条,形成的大角度晶界对提高热影响区韧性起到了重要作用。中等热输入时产生下贝氏体时,碳化物分布在贝氏体铁素体内并与铁素体主轴方向呈一定角度；较大热输入条件下在上贝氏体铁素体板条侧产生的Fe3C条对接头韧性有不利的影响。在组织结构分析的基础上,采用扫描电镜对Q550+Q690接头区裂纹及冲击试样断口形貌进行了研究。接头断面裂纹起源于Q690钢侧根部熔合区,沿熔合区穿晶扩展；部分裂纹在奥氏体晶界处沿先共析铁素体或贝氏体板条边界转向焊缝扩展；焊缝中裂纹扩展模式为微裂纹形核→尖端钝化→发生撕裂→重新形核；当裂纹遇到夹杂物时会导致萌发空洞,形成更多的不同方向的裂纹。焊缝金属断口纤维区为穿晶断裂,断口韧窝产生的机理是微孔聚集型,针状铁素体区对应的韧窝较大,晶界铁素体对应的韧窝较小。采用ER50-6焊丝,焊缝断口放射区呈河流花样,断口平滑；采用ER60-G焊丝,焊缝断口放射区为准解理断口,解理面层次不平。马氏体+下贝氏体组织的Q690钢热影响区纤维区具有韧窝特征,塑韧性好；放射区有较大的撕裂台阶。大热输入条件下的热影响区断口中,纤维区断裂具有塑性变形的滑移特征,上贝氏体中粗化的碳化物阻止了韧窝形成；放射区中碳化物作为裂纹源形成小的撕裂刻面。本文系统研究了不预热条件下液压支架用高强钢Q550和Q690焊接接头的显微组织、精细结构以及接头力学性能,并研究了接头区裂纹扩展形态及断裂机制,为高强钢的不预热焊接提供了试验依据和理论基础,有利于提高高强钢焊接效率和促进高强钢应用范围的扩大。更多还原

【Abstract】 High strength low alloy (HSLA) steels have been widely used in engineering machinery, energy, bridges, vehicles and vessels industries etc., due to their excellent strength and toughness, reasonable economy and high allowable design stresses. But the welding of high strength steel is complicated. Major problems may be encountered in joint during welding, including cold cracking, mismatching of toughness and strength, poor performances of fusion zone and heat affected zone (HAZ), and so on, which become limitations in extending the application of HSLA steel and improving production efficiency. In this paper, HSLA steels Q550 and Q690 used in hydraulic support of coal mine were welded by gas metal arc welding (GMAW) processes without pre-heating. The microstructure, fine structure, cracks and mechanical properties of welded joints were investigated to reveal the relation of welding parameters, alloying elements, microstructure and fine structure of joints, propagation of cracks and performance of joints.Oblique y-groove and straight Y-groove cracking tests of butt joints were conducted to research the influences of welding wire and welding heat input on the cracking ratio of Q550, Q550+Q690 and Q690 joints. Results indicated that with the increase of strength of welding wires and welding heat input, the cracking sensitivities of joints increased. The mechanical properties of Q550 and Q550+Q690 welded joints were studied via tensile test and Charpy V-notch (CVN) impact test. Tensile samples ruptured at the weld metal in the joints fabricated using ER50-6 welding wire, stating that weld metal was the weakest zone in the joints. The tensile strength of joints produced with ER60-G welding wire was close to that of base metal with the failure location at the fusion zone or HAZ. CVN impact test results showed the toughness was the best in the HAZ for Q550 and Q690 steel joints. Weld metal and fusion zone in joints produced with ER60-G welding wire exhibited better toughness than that fabricated using ER50-6 welding wire.The microstructure, fine structure and composition of inclusions in weld metal were investigated by means of optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). The effects of welding heat input, alloying elements and inclusions on the microstructure of weld metal were analyzed. With the increase of concentration of alloying elements in welding wires, the content of proeutectoid ferrite (PF) decreased, and that of acicular ferrite (AF) increase firstly, then decreased gradually. When the content of alloying elements became higher, the formation of bainite was promoted. For the weld metal with small alloying elements (ER50-6 welding wire), the content of AF in weld metal was not sensitive to welding heat input since the prior austenite grain boundaries were almost fully covered by PF. With the increase of alloying elements in welding wire (ER60-G), welding heat input had important influences on the microstructure of weld metal. Small heat input (14kJ/cm) generated bainite dominated microstructure in weld metal, resulting in poor impact toughness.Both the composition and size of inclusions are the deciding factors for the nucleation of AF. Relatively large inclusions with the diameter of about 0.5-0.8μm were much more effective in promoting the nucleation of AF and refining microstructure within austenite grain than other small inclusions. Compared with single phase inclusions, those comprised of several different phases were easy to stimulate several AF plates with different directions leading to higher nucleation frequency of AF. The result of TEM analysis revealed high density dislocations inside of AF laths. The lath like bainitic ferrite and retained austenite had been observed by TEM. The crystallographic orientation between a and y conformed to K-S relationship.We also investigated the influence of welding heat input on microstructure and impact toughness of the heat affected zone (HAZ) in Q690 steel joints. With the increase of welding heat input from 14kJ/cm to 20kJ/cm, the impact toughness in the HAZ had the tendency to increase at first and then decrease. The optimum impact properties of HAZ were obtained with about 16kJ/cm heat input. When the welding heat input was raised to a suitable value, the formation of lower bainite ahead of the transformation from austenite to martensite could refine microstructure within austenite, providing more high angle lath boundaries, which played a very important role in enhancing impact toughness in HAZ of Q690 steel joints. Lower bainite was obtained with carbides adopting a single crystallographic variant in lower bainitic ferrite at about 16kJ/cm heat input. When higher heat input was employed, carbide particles paralleled to the habit planes of the bainitic ferrite were formed, which were detrimental to impact toughness of Q690 steel joints.Based on the investigation of microstructure and fine structure in Q550+Q690 steel joints, the propagation of cracks and fracture surface of Charpy impact specimens were studied via SEM. Cracks initiated from the root fusion zone of Q690 steel side and propagated along the fusion zone. Some cracks would turn from fusion zone to weld metal along the laths of PF or bainite. The mode of crack propagation was:nucleation of micro-crack→crack tip blunting→crack tearing→nucleation of new micro-crack. When the tip of crack encountered inclusion, more crack paths would be initiated from the interface between inclusion and acicular ferrite plates.The fibrous zone in fracture surface of weld metal fabricated with ER50-6 and ER60-G welding wires performed dimpled transgranular type of failures with the mechanism of micro-void coalescence. The analysis had proved that acicular ferrite structure region corresponded to relatively large dimples while grain boundary ferrite structure corresponded to small dimples. Radical region in the fractured surface of weld metal from ER50-6 joint was flat with river patterns, while that of weld metal produced using ER60-G welding wire exhibited tortuous crack propagation path and rough cleavage planes. The propagation of cracks in fusion zone and weld metal exhibited excellent cracking resistance of AF. Fibrous zone in fractured surfaces of impact specimens in Q690 steel HAZ with mixed microstructure of lath martensite and lower bainite was characterized by dominant elongated dimples. Complex river patterns and cleavage steps were observed within cleavage facets in the radical zone. But slip bands were shown in the fibrous zone in the HAZ fabricated at higher heat input. Coarsen carbides in upper bainite held back the formation of dimples. The radical zone was composed of small cleavage facets due to more crack initiation sites provided by carbides.This paper systematically researches the microstructure characteristics and mechanical properties of welded joints without preheating of HSLA steels Q550 and Q690 used in hydraulic support for coal mine. The propagation of cracks and fracture mechanisms in Q550+Q690 steel joints were also studied. The relations of welding parameters, alloying elements, microstructure and mechanical properties including tensile strength and impact toughness were established. The results provides experimental and theoretical basis for improving the application of high strength low alloy steel and promoting the efficiency of welding.更多还原