Identification of phases and precipitates by AFM in duplex stainless steel unions

ABSTRACT

Atomic Force Microscopy (AFM), a characterization technique that generates topographic images of surfaces at very high resolutions, works by recording the surface relief details of the material with a cantilever which moves over the sample, while a piezoelectric sensor monitors changes in height. This technique is useful for identifying phases, particles and precipitates on a nanoscale and is therefore proposed for identifying them in the fusion zone of welded components and for early identification of faults due to cracking. This is a technique that does not require conductive samples, or special metallographic preparation of specimens as is the case with electron microscopy. The material studied is a duplex 2205 stainless steel, welded by means of gas tungsten arc welding (GTAW). The welded material was subjected to stress tests. The test specimens were characterized using optical microscopy, scanning electron microscopy, atomic force microscopy and nanoindentation tests. The results focus on the identification of the specific magnetic, topographic and hardness characteristics of each of the phases, particles and areas present in the base metal, the fusion zone and the zone deformed by the fracture.     

Crack resistance of austenitic stainless steel pipe and pipe welds with a circumferential crack under monotonic loading

This paper describes the experimental studies carried out on cracked austenitic stainless steel pipe and pipe welds under bending loads. Pipe welds were produced by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW). Fracture resistance curves for pipe and pipe welds were compared. Results indicate that the fracture resistance of pipe and pipe weld (GTAW) is comparable but that of pipe weld (GTAW+SMAW) is inferior. Cracks do not deviate from their original plane during propagation as observed in the cases of carbon steel pipe and pipe welds. The fracture resistance of pipe welds does not depend on the loading histories to which it has been subjected prior to fracture test. Initiation and crack propagation were observed prior to the maximum moment. An existing limit load expression is applicable for the pipe base material but gives non‐conservative results for the pipe welds. Multiplication factors have been suggested for the pipe welds for evaluation of limit loads using the existing expression. Fracture resistance for the pipe and compact tension specimens have also been compared for base material and welds.     

迭代学习控制在GTAW焊接过程中的应用研究

针对相同工件的批量焊接,并且焊接轨迹相同的情况下,焊接过程具有极高的重复性。提出了基于迭代学习控制(ILC)的脉冲气体钨极氩弧焊(GTAW)焊接过程跟踪控制方法。根据GTAW焊接的动态过程模型,设计了GTAW焊接过程控制的ILC算法,并对算法的收敛性进行了证明。研究结果表明,ILC可以有效地利用焊接过程中的重复信息,经过60次迭代学习后,焊接系统输出可以较好的达到期望轨迹,并获得较高的控制精度,验证了方法的有效性。与PID控制相比ILC控制器不但可以获得较好的跟踪效果,而且还能有效抑制外部扰动的作用,具有较强的鲁棒性。     

氩弧熔敷原位合成Ti（C,N）-TiB_2/Ni60A基复合涂层的组织与耐磨性

在Q235D钢表面采用氩弧熔敷技术制备了Ti（C,N）-TiB2增强Ni60A基复合涂层。利用SEM对复合涂层的显微组织进行了分析,Ti（C,N）颗粒呈花瓣状和不规则的球状,TiB2颗粒呈短棒状和六面体,并对其组织结构进行表征。利用显微硬度计和摩擦磨损试验机,对复合涂层的性能进行了测试和分析。涂层表面平均硬度达到了1250 HV。摩擦磨损实验表明,涂层的磨损机制主要为磨粒磨损,伴随着粘着磨损。     

高压环境下管道焊接技术研究

分析了高压环境下钨极氩弧焊(GTAW)的特点.针对渤海湾海底管道修复,自行设计了一套高压模拟试验装置,该装置主要包括高压焊接试验舱和焊接机器人.进行了高压模拟焊接试验,研究表明,随着环境压力的增加,焊接电流减小,氩气流量增大.在0.1 MPa,0.3 MPa,0.5 MPa和0.7 MPa压力下,选用合理的焊接工艺参数,X56级管线钢均获得了单面焊双面成形的良好效果.     

外加磁场对AZ31镁合金焊接接头组织性能的影响

在对5mm厚的AZ31镁合金板进行GTAW焊接的过程中,外加纵向交流磁场,研究了磁场对焊接接头的力学性能和显微组织的影响,并探讨了磁场的作用机理。结果表明:外加纵向磁场通过旋转电弧对熔池进行搅拌,不仅可以细化晶粒,使焊接接头的抗拉强度、硬度得到改善,还可以净化熔池中液态镁合金(使杂质球化并弥散分布),此外磁场的引入还可降低镁合金焊接热裂纹的敏感性,抑制热裂纹的产生。     

Evaluation of the occurrence of mechanical constriction of the Arc in A-TIG welding of austenitic stainless steels

This paper evaluates indications of arc constriction by the thermal and electrical insulation caused by oxides of a single component in A-TIG welding of austenitic stainless steels. Changes in arc dimensions, in its electric voltage and in weld bead morphology caused by three oxide fluxes (SiO₂, Cr₂O₃ e Al₂O₃) applied with two surface densities (30 e 60 g/m2), and with and without a flux-free central strip (of 1, 2 and 4 mm) were studied. Results showed no significant change in the width of the electric arc for the experimental conditions used, therefore not supporting a possible mechanical constriction in the electric arc by oxide electrical and thermal insulation. Lateral filming indicated that the arc is delayed by the fluxes with silica causing the strongest effect. The presence of a clean central strip in the flux layer decreased weld penetration and weld bead cross section, besides the reduction of the width of the bead. Therefore, the results of the present work seem to support Marangoni convection as the main mechanism responsible for increasing penetration in A-TIG welding of stainless austenitic steels.     

焊接过程的神经网络建模及控制研究

采用人工神经网络方法对焊接过程进行建模及控制，并以ＧＴＡＷ作为对象加以实施。用误差反传网络（ＢＰ网）建立了静态过程ＧＴＡＷ熔透情况下表征其质量的正面熔宽、背面熔宽与施焊参数之间的关系，即静态模型，并从质和量两个方面验证了该模型的可行性。提出了采用两个网络对ＧＴＡＷ动态过程建模及控制的方法，并通过实验验证了所提方案的正确性。文中所提的方法从根本上不同于以往的以固定结构数学模型为基础的建模及控制的方法，无需对过程作任何假设，所建模型与实际过程符合较好。实验证明，该方法具有容错性好、通用性强和抗干扰能力强等优点，为焊接过程的建模及智能化控制找出了一条新途径。     

哈氏B-3合金大型压力容器的研制

在简要介绍哈氏合金的发展、分类与使用的基础上，重点介绍了哈氏B-3合金的发展、性能与各项合格指标。并结合世界首台哈氏B-3合金大型压力容器制造的实例，详细介绍了该类材料在焊接制造大型压力容器时在施工环境、冷热加工、耐蚀试验与质量控制等方面的具体要求。     

钛合金薄板带热沉GTAW焊的应力场

采用数值模拟和试验相结合的方法，比较钛合金常规钨极氩弧焊(CTAW)及带热沉的钨极氩弧焊，即动态控制低应力无变形(DC-LSND，dynamically controlled low stress no-distortion)GTAW焊接过程中应力场的形态与发展历史。DC-LSND焊接过程中，热沉的急冷收缩对热沉作用部位与熔池之间已凝固但仍处于高温状态的金属产生很强的拉伸作用，使焊缝中拉伸塑性变形增大，近缝区压缩塑性变形相应减小，从而导致焊缝与近缝区不协调应变减小，残余应力降低。与常规焊最大残余拉应力位于焊缝中心不同，在所选用的焊接条件下，DC-LSND焊最大残余拉应力位于近缝区，残余应力分布形态发生改变。