Structure formation in the joining zone of steels upon contact fusion welding

A mechanism of the structural and phase transformations in the contact zone upon the production of permanent joints during fusion welding of ferritic-pearlitic steels, which lead to the formation of a light band (LB) with a “silvery” stitched line in the middle, is proposed. It is shown that this effect is based on the phenomenon of local contact melting of cementite that had no time to be dissolved during the rapid heating (10⁴ K/s) at the boundary with the adjacent high-carbon inhomogeneous austenite. This mechanism is proposed as an alternative to those suggested earlier in the last more than fifty years by some domestic and foreign researchers.     

基于数学模型的焊接过程中热影响区晶粒长大行为分析

在能量和曲率的基础上,建立了晶粒长大的能量-曲率驱动的元胞自动机模型。该模型能够准确建立晶粒生长过程中晶粒尺寸和时间、生长速率和温度、生长速度和曲率之间的关系,还能反映晶粒尺寸分布的时间不变性规律。建立了元胞自动机和实际时间的转变公式,实现了元胞自动机对现实时间中焊接热影响区的晶粒长大进行模拟。结果表明,模拟结果与热影响区晶粒的理论分布规律十分吻合。     

碳化钒析出对X80管线钢焊接热影响区韧性的影响

通过Gleeble-3500热模拟机对钒(V)含量分别为0.06%和0.02%的X80管线钢进行了峰值温度为1350℃和750℃的焊接热模拟试验,利用硬度计和光学显微镜(OM)分别分析了焊接热模拟试样硬度和试样中马氏体-奥氏体岛状组织(M-A)的数量、尺寸和形貌。利用透射电镜(TEM)对母材、粗晶区和部分相变区中粒子析出进行了表征。结果表明,与0.02%含V量X80管线钢相比,0.06%含V量的X80管线钢由于V含量较高,在受到多道次焊接热循环影响时,部分相变区有较大量碳化钒(VC)粒子析出,明显阻碍位错运动,使其强度明显提高的同时脆性增大、塑性变形难以进行,导致其焊接热影响区韧性下降。     

微量Ca对低合金钢中夹杂物及焊接热影响区的影响

研究了添加微量Ca和未加Ca两种类型的低合金高强钢中夹杂物的特征及微量Ca对模拟焊接热影响区组织与性能的影响.结果表明,试验钢中添加0.006%Ca能够有效提高分布在钢中的含钙类夹杂物数量;使焊接热影响区形成针状铁素体,细化热影响区奥氏体晶粒,从而可提高热影响区的力学性能.     

热输入对X70管线钢焊接热影响区组织和韧性的影响

管线钢焊接突出的问题是热影响区晶粒粗化会导致接头韧性下降。利用热模拟技术和超窄间隙焊接方法对X70管线钢进行试验,得到了不同热输入下焊接热影响区粗晶区的组织,并测试了其韧性。结果表明,在室温下,当热输入低于8 k J/cm时,以板条贝氏体和少量针状铁素体组织为主,原奥氏体晶粒尺寸小于48μm,韧性与母材相当;高于8 k J/cm时,随着热输入增加,板条贝氏体和针状铁素体逐渐减少,原奥氏体晶粒逐渐长大,韧性开始急剧下降。分析认为,对于这类C含量低的管线钢,板条贝氏体的形成并不会导致韧性降低,原奥氏体晶粒不断长大才是造成韧性下降的主要原因。采用超窄间隙焊接方法,可以有效缩短高温停留时间,防止原奥氏体晶粒长大,从而避免焊接热影响区粗晶区的韧性下降。     

Hydrogen embrittlement properties of heat affected zone of high strength steel in shielded metal arc welding

Corrosion resistance, mechanical properties and hydrogen embrittlement were investigated from an electrochemical view, with the slow strain rate test (SSRT) method with applied constant cathodic potential. Fracture surface was analyzed by SEM. Corrosion resistance and mechanical properties were increased by post-weld heat treatment (PWHT) compared to those in the as-welded condition. Elongation and time-to-fracture were decreased with shifting cathodic polarization potential to the low potential direction. On analysis of SEM fractography, the quasi-cleavage (Q.C) fracture mode was also observed with an increase of susceptibility to hydrogen embrittlement. At the applied cathodic potential between −770 mV and −875 mV (SCE; saturated calomel electrode), the fracture morphology was of the dimple pattern with ductile fracture, while it changed to the transgranular pattern at under −900 mV (SCE). Eventually it is suggested that an optimum cathodic protection potential range was from −770 mV to −875 mV (SCE) without regard to PWHT condition.     

Q345C耐候钢多层多道焊焊接热影响区热模拟性能

焊接热影响区是焊接接头的薄弱部位,其微区力学性能不能通过常规性能测试获得。采用Gleeble 3500热模拟试验机模拟Q345C耐候钢四层四道焊热影响区,研究每道热影响区的微观组织和力学性能,结果表明:四层四道焊第二道热影响区的组织与母材相同,为铁素体和条带状珠光体,其余三道热影响区的金相组织均发生变化,为贝氏体和铁素体组织。第二道热影响区的硬度和强度最低,冲击韧性最高。     

2.25Cr-1Mo-0.25V耐热钢焊接热影响区热模拟试验研究

采用Gleeble-3500热模拟试验机和工业模拟热处理炉对2.25Cr-1Mo-0.25V钢进行了焊接热循环和焊后热处理试验,并对其组织和-30℃冲击韧性进行了分析和测试.结果表明:经一次和二次焊接热循环试样的组织均发生粗化,其中一次热循环产生的粗大组织在二次热循环中得到继承,上述组织特点导致该钢焊后热影响区低温冲击韧性恶化.经700℃焊后热处理后低温韧性得到显著提高,且随回火时间的延长韧性进一步提高.断口由处理前的准解理断裂转为韧窝为主的韧性断裂特征.一次和二次焊接热循环后,析出相粒子的形状变得很不规则,且在二次循环后有部分小粒子出现,这可能与焊接快速加热及循环后的冷却过程有关.经焊后热处理后,粒子形貌变的规则并发生粗化.     

Nb含量和热输入量对X80管线钢焊接粗晶区的影响

以工业含铌X80管线钢为对象,研究了不同Nb含量条件下,焊接热影响区粗晶区中的原奥氏体晶粒和析出的状态;通过对焊接热影响区中粗晶区的热模拟实验,研究了不同热输入下,高Nb管线钢焊接粗晶区的晶粒粗化、显微组织演变、及大角晶界分布等情况。结果表明,高Nb钢的粗晶区范围更窄,粗晶区内的晶粒尺寸更小;经过双道焊的热循环后,高Nb钢粗晶区的析出尺寸更小,没有对韧性有害的大尺寸析出。此外,原奥氏体晶粒的粗化,以及显微组织中大角晶界密度的下降,明显降低了高Nb管线钢焊接粗晶区的韧性。     

高速列车转向架SMA490BW耐候钢焊接热影响区模拟试验

实验分析高速列车转向架构架用钢焊接接头热影响区的组织性能.采用Gleeble-3500热模拟试验机对SMA490BW耐候结构钢进行焊接时温度循环的模拟,研究了三种焊接线能量状态下两个不同特征热影响区的组织和性能,为焊接工艺的优化提供理论依据.SMA490BW耐候钢的过热温度区间热影响区组织多为贝氏体,并且组织性能根据采...     

On the mechanism of porosity formation during welding of titanium alloys

The mechanism of porosity formation during the fusion welding of titanium and its alloys is studied. Porosity formed during the electron beam welding of titanium is characterized using high-resolution X-ray tomography, residual gas analysis and metallographic sectioning; the results confirm that porosity formation is associated with evolution of gas, especially hydrogen. A model for hydrogen diffusion-controlled bubble growth is proposed, to aid in the interpretation of these findings. To investigate further the effect of hydrogen on porosity formation, hydrogen charging is used to achieve different hydrogen levels prior to welding. The results confirm that porosity can be suppressed even at every high hydrogen levels, when welding is carried out with optimized welding parameters and perfect joint alignment; on the other hand, porosity is exacerbated when a small beam offset is employed. This is because any beam offset alters the size of the liquid zone at the melting front, where the joint edges first become melted. It is proposed that the thickness of the liquid film at the melting front is crucial for bubble nucleation and bubble survival in the weld pool; bubbles can escape through the keyhole by breaking through this liquid film, when it is too thin. This challenges the common assumption of bubble escape by flotation to the weld pool surface. Thus the nucleation rate in the liquid zone at the melting front determines the likelihood of porosity occurring. This suggests that the beam offset is likely to be one factor influencing porosity formation in these circumstances. The paper provides fundamental insights into the mechanism of porosity formation during the welding of titanium alloys and guidance to aid in its elimination.     

Effects of ultrasonic pulsation of the welding current on the geometry of the fusion zone and heat-affected zone in submerged arc welding

This paper proposes investigation of an arc ultrasonic excitation technique in the SAW process in order to evaluate the main parameters’ influence associated with pulsed current on weld beads, such as the frequency and amplitude of ultrasonic excitation current. Therefore, an experimental bench was assembled, which consists of two power sources: a conventional welding source and another able to provide pulse current at ultrasonic frequencies. During the tests two ultrasonic excitation current amplitudes were employed, 25 A and 50 A, each one in three different pulse frequencies, 20, 50 and 80 kHz. Macrographs of these welds are observed, and width, penetration, molten area and heat-affected zone area (HAZ) are measured. As a result, contrary to information available in the literature, there were no significant changes in weld bead geometry, as well as in the HAZ, that could be attributed to the current excitation in studied frequencies.     

高强度桥梁钢焊接粗晶热影响区中粒状贝氏体对韧性的影响

采用热模拟试验机模拟高强度桥梁钢Q460q不同焊接工艺参数t8/5条件下的CGHAZ（粗晶热影响区）,并分析了CGHAZ的显微组织及组织中的粒状贝氏体对韧性的影响。结果表明,当t8/5〈60 s时,组织中板条贝氏体和粒状贝氏体相互交错,且粒状贝氏体内的脆性长条状M-A组元较少,使韧性保持较高的水平。当t8/5〉100 s后,晶粒粗大,组织中先共析铁素体和粒状贝氏体量不断增加,M-A组元尺寸变大从而使裂纹扩展中的有效晶粒尺寸粗大,导致韧性显著下降。