The influence of welding condition on the microstructure of WC hardfacing coating on carbon steel substrate

Tungsten carbide (WC) hardfacing coating technique is widely used to improve the performance of carbon steel blade exposed to acidic and abrasive conditions during production. This paper deals with the influence of welding parameters on the microstructure and carbide distribution of WC. WC hardfacing was deposited onto carbon steel by shielded metal arc welding (SMAW). Welding parameters such as welding current, number of weld layers, electrode drying and base material preheat were the focus of this work. Coating hardness, microstructure and elemental composition were analysed in detail. The effects of the welding parameters on WC hardfacing coating microstructure and hardness value were characterized by scanning electron microscope (SEM) and micro-Vickers hardness tester respectively. The larger carbide growth in overall coating region is mainly dictated by high current (200 A), increased number of weld layers (3 layers) and presence of base material preheat due to sufficient heat energy initiating carbide growth. The investigation also revealed that high current affected the growth of smaller carbide particles in matrix region significantly. Meanwhile, number of weld layers and base material preheat influences were seen during hardfacing with lower welding current. The absence of electrode drying led to uniform smaller carbide distribution in matrix region. It was found that increased number of large carbides and uniformly distributed smaller carbides in WC hardfacing deposit increased the hardness value of the coating.     

The effect of localized dynamic surface preheating in laser cladding of Stellite 1

In laser cladding, high cooling rates create outcomes with superior mechanical and metallurgical properties. However, this characteristic along with the additive nature of the process significantly contributes to the formation of thermal stresses which are the main cause of any potential delamination and crack formation across the deposited layers. This drawback is more prominent for additive materials such as Stellite 1 which are by nature crack-sensitive during the hardfacing process. In this work, parallel to the experimental investigation, a numerical model is used to study the temperature distributions and thermal stresses throughout the deposition of Stellite 1 for hardfacing application. To manage the thermal stresses, the effect of preheating the substrate in a localized dynamic fashion is investigated. The numerical and experimental analyses are conducted by the deposition of Stellite 1 powder on the substrate of AISI-SAE 4340 alloy steel using a 1.1 kW fiber laser. Experimental results confirm that by preheating the substrate a crack-free coating layer of Stellite 1 well-bonded to the substrate with a uniform dendritic structure, well-distributed throughout the deposited layer, can be obtained contrary to non-uniform structures formed in the coating of the non-preheated substrate with several cracks.     

Corrosion Performance of Valve Seat Hardfacing Materials in Amine Environment of Boiler Feed Water Service

Protection of Metals and Physical Chemistry of Surfaces - As the valve seat hardfacing materials in boiler feed water service of coal power plants, the corrosion behavior of Stellite 6 alloy and...     

Evaluation of microstructure and electrochemical corrosion behavior of austenitic 316 stainless steel weld metals with varying chemical compositions

Austenitic stainless steel weld metals have, in general, inferior corrosion resistance compared with the base metals. This is due to the fact that the weld metal has an inhomogeneous and dendritic microstructure with microsegregation of major elements (i.e., Cr, Mo, and Ni) as well as minor elements (i.e., S and P) at the δ-γ interface boundaries. The nonuniform alloying element concentration around ferrite particles plays a major role in determining the electrochemical corrosion behavior of such weld metals. Although the presence of ferrite is considered to be detrimental as far as the localized corrosion is considered, its exact role in uniform corrosion is still not clear. The uniform corrosion behavior of an alloy is determined by the fundamental electrochemical parameters of the major alloying elements. In this study, an attempt has been made to correlate the microstructure and uniform corrosion behavior of type 316 stainless steel weld metals with varying concentrations of Cr and Mo, and different ferrite contents. From the empirical equations obtained during the analysis of the electrochemical corrosion data, an attempt has been made to understand the role of Cr, Mo, and ferrite in altering the electrochemical corrosion parameters of the weld metal. Based on the extensive microstructural characterization, a dissolution model for the weld metal in the moderately oxidizing medium has been proposed.     

火电站用T23/TP316H异种金属焊接接头服役损伤分析

通过试验研究分析了服役后的高温蒸汽管道中T23/TP316H异种金属焊接接头的微观组织、成分和氧化情况.结果表明,在540℃/17 MPa条件下服役10万h后,TP316H钢侧虽会被氧化,但未发现明显的会导致接头早期失效的潜在危险因素;T23钢侧会出现沿焊缝/热影响区界面的氧化,并出现了沿界面的开裂.T23钢与镍基焊缝...     

Welding equipment for reinforcing the edges of corrugated walls of the casings of transformers of a new generation of TMG series

The application of aluminium alloy, which is a typical lightweight material, has been expected in the construction of transportation vehicles to achieve energy saving by reduction of weight. However, structures made of whole aluminium alloy have problems with low strength and high cost. Thus, hybrid structures made of Al alloy and steel are useful because of light weight and higher strength. To construct the hybrid structure, it is necessary to weld aluminium alloy and steel. However, conventional welding methods, like brazing and mechanical fastening, have problems such as low mechanical strength and low productivity. Also, it is difficult to weld Al alloy and steel by conventional fusion welding.

In this study, spot welding between aluminium alloy and low carbon steel by friction stirring was carried out. Especially, optimization in welding conditions was carried out. Moreover, the effect of welding conditions on the joint strength and weld interface was studied. As a result, relatively higher tensile shear strength of the weld, which was achieved at optimum welding conditions, was obtained. Temperature near weld interface measured by K-type thermocouple during welding was found to be lower than the melting point of A5052. From the observation results on microstructure of the weld interface, it was found that a Fe/Al intermetallic compound layer was formed.     

电渣堆焊高铬铸铁界面温度场及组织性能

用电渣堆焊的方法在D32低合金钢表面堆焊高铬铸铁硬面层,测量了堆焊过程中热影响区的温度场,研究了热影响区、复合界面及硬面层的微观组织和力学性能.结果表明,电渣堆焊加热和冷却速度较慢,稳定阶段时低合金钢基材温度分布均匀,在堆焊方向最大温度梯度为-21.25℃/mm;低合金钢基板内最大热应力为53.4 MPa,低于低合金钢的抗拉强度,有效避免了裂纹的产生;复合界面平整清晰,存在宽度约50μm的奥氏体带状区;热影响区晶粒有所长大,为铁素体加珠光体组织;高铬铸铁硬面层由奥氏体、碳化物和少量马氏体组成,M₇C₃型碳化物细小且均匀分布于奥氏体晶界;复合界面结合强度为96 MPa;试样熔合区的冲击吸收能量(53 J)较硬面层冲击吸收能量(10.7 J)明显提高;亚共晶高铬铸铁硬面层在较大磨损载荷下发生马氏体相变,硬度提高,耐磨粒磨损性能优良.     

Cracking of a Cobalt-Based Hardfacing of a Gate Valve Disk in a Desalination Power Plant

Cobalt-based alloys of the Stellite family are used as hardfacing for sealing surfaces of valves operating in desalination and power plants because of their excellent low friction and anti-galling properties in high-load sliding contact under the prevailing conditions. However, insufficient control of pressure and temperature during operation can degrade the integrity of the hardfaced material thus leading to its premature failure. This article presents a failure investigation carried out on the disk of a main stop gate valve that was used in a desalination plant. The disk was manufactured from X20 as a substrate material and a cobalt-based alloy for hardfacing. The cobalt-based hardfacing suffered from many surface and subsurface cracks that degraded its integrity. It was concluded that high-pressure steam flowing against the disk had tilted it and, thus, disturbed the alignment between the surfaces of the disk and the seat, leading to wear and large frictional heat that resulted in the degradation of the microstructure of the hardfacing layer and formation of the observed cracks.     

Effects of substrate preheating during direct energy deposition on microstructure,hardness, tensile strength,and notch toughness

This study examined the effects of substrate preheating for the hardfacing of cold-press dies using the high-speed tool steel AISI M4. The preheating of the substrate is a widely used technique for reducing the degree of thermal deformation and preventing crack formation. We investigated the changes in the metallurgical and mechanical properties of the high-speed tool steel M4 deposited on an AISI D2 substrate with changes in the substrate preheating temperature. Five preheating temperatures (100-500 °C; interval of 100 °C) were selected, and the changes in the temperature of the substrate during deposition were observed. As the preheating temperature of the substrate was increased, the temperature gradient between the melting layer and the substrate decreased; this prevented the formation of internal cracks, owing to thermal stress relief. Field-emission scanning electron microscopy showed that a dendritic structure was formed at the interface between the deposited layer and the substrate while a cellular microstructure was formed in the deposited layer. As the preheating temperature was increased, the sizes of the cells and precipitated carbides also increased. Furthermore, the hardness increased slightly while the strength and toughness decreased. Moreover, the tensile and impact properties deteriorated rapidly at excessively high preheating temperatures (greater than 500 °C). The results of this study can be used as preheating criteria for achieving the desired mechanical properties during the hardfacing of dies and molds.     

The effect of upsetting conditions on flash weld toughness. Study on toughness improvement of flash welded joints in high strength steel

An investigation was carried out of the factors influencing the toughness of flash welded high strength steel plate.¹ It was explained that the major factors for toughness degradation can be summarised by the following: there is an inclusion factor, that is oxides which remain at the weld interface and act as the starting point for fracture and the inclusions within steel which tend to be parallel to the weld interface due to metal flow; there is a microstructural factor, that is degradation of the microstructure due to coarsening of former g grains, a localised softened layer at the weld zone centre and the texture formed where there has been heavy rolling. Consequently, in order to secure toughness, a reduction of oxides at the weld interface and an improvement of steel material purity are required for the former case and the post weld heat treatment is required for the latter.     

S31000不锈钢表面激光熔覆Stellite12合金层的组织和性能

针对高温阀门密封副易发生擦伤、碰伤等问题,采用激光熔覆技术在S31000不锈钢密封副基体表面制备了Stellite12合金层。通过光学显微镜(OM)、扫描电镜(SEM)、X射线衍射仪、显微硬度计及均匀腐蚀全浸试验,研究了熔覆层微观组织、显微硬度和均匀腐蚀性能。结果表明,熔覆层与基体形成了良好的冶金结合,微观组织主要由平面晶、柱状晶和等轴晶的枝晶结构组成。熔覆层显微硬度较高,平均显微硬度为600.68 HV0.3;基体的显微硬度最低,平均显微硬度为204.57 HV0.3。相比于S31000不锈钢的腐蚀速率,表面激光熔覆Stellite12合金后,腐蚀速率显著降低。同时,S31000不锈钢表现出尺寸不均的腐蚀坑现象,Stellite12合金层表现出较均匀的腐蚀行为,但在晶界处的腐蚀现象比晶内更为明显。     

修边模具堆焊工艺的研究

修边模具堆焊工艺是以42CrMo为基体,通过在刃口部位堆焊高硬度合金钢制造修边模,代替传统的修边模制造工艺。分析了工艺参数对堆焊层金相组织、堆焊层硬度及耐磨性的影响,选取了适当的工艺参数并给出了堆焊法制造冷冲模的工艺要点。堆焊层金属及过渡区无气孔、裂纹等缺陷,堆焊的刃口平均硬度在57.5HRC以上,焊缝成形良好并具有很高的耐磨性能。并实际堆焊制造了修边模具,现场应用表明,堆焊层不脱落,寿命达到或超过传统工艺制造的修边模。该工艺对于修复其它冷作模具也具有很好的应用前景。     

Tensile resistance,microstructures of intermetallic compounds,and fracture modes of welded steel/aluminum joints produced using laser lap welding

The joining of DP780 steel to Al5052 was conducted by laser lap welding, in which the metal vapor and spatters were monitored by a high-speed camera. A universal testing machine was used to test the mechanical properties of the welded joints, and the changing law of lap tensile resistance with the laser welding parameters was analyzed. Optical microscope and scanning electron microscope were used to observe the macro-structure and micro-structure, respectively. Three different intermetallic compounds (IMCs) phases, i.e. banded Fe₂Al₅, FeAl₂ and needle-like FeAl₃ were generated at the steel/Al interface on microscopic observation. The aim of this research is to investigate the relationship among the lap tensile resistance, the welding parameters and the failure mode under different energy densities. Experimental results showed that the steel/Al joints have two different fracture modes at low heat input and high heat input. The failures happened along the heat-affected zone of the weld and along the steel/Al joint interface, respectively. And both of the two failure modes are brittle fractures. Additionally, cracks appeared at the fracture interface, and needle-like particle clusters were found in the fracture microstructure.     

Weld growth mechanisms during resistance spot welding of two and three thickness lap joints

Abstract

Understanding the mechanisms of weld growth during resistance spot welding is a prerequisite for the development of optimum welding conditions which ensure high levels of joint quality in autobody manufacture. This study compared the heat development/weld growth of double and triple sheet lap joints as a means of understanding the role of welding conditions, sheet thickness configuration, and physical material properties on joint microstructure and performance. High temperature heat patterns have been determined using metallographic and thermographic techniques for double and triple sheet lap joints in uncoated low carbon steel. In the case of double sheet joints, initial heat generation and weld nugget formation was observed to occur where the resistance to the flow of current was the greatest, i.e. at the sheet/sheet interface. This was also the case for heat generation in triple sheet joints, but in this more complex case weld nugget formation was dependent on sheet thickness and joint configuration. Furthermore, the incubation period to weld nugget formation was dependent on total joint thickness with a longer weld time required for joints of greater thickness. For non-uniform triple sheet joints, heating was observed to be concentrated around the thinnest sheet as a result of the way heat was dissipated by the steel substrate.     

Effect of PTA Hardfaced Interlayer Thickness on Ballistic Performance of Shielded Metal Arc Welded Armor Steel Welds

Ballistic performance of armor steel welds is very poor due to the usage of low strength and low hardness austenitic stainless steel fillers, which are traditionally used to avoid hydrogen induced cracking. In the present investigation, an attempt has been made to study the effect of plasma transferred arc hardfaced interlayer thickness on ballistic performance of shielded metal arc welded armor steel weldments. The usefulness of austenitic stainless steel buttering layer on the armor grade quenched and tempered steel base metal was also considered in this study. Joints were fabricated using three different thickness (4, 5.5, and 7 mm) hardfaced middle layer by plasma transferred arc hardfacing process between the top and bottom layers of austenitic stainless steel using shielded metal arc welding process. Sandwiched joint, in addition with the buttering layer served the dual purpose of weld integrity and ballistic immunity due to the high hardness of hardfacing alloy and the energy absorbing capacity of soft backing weld deposits. This paper will provide some insight into the usefulness of austenitic stainless steel buttering layer on the weld integrity and plasma transferred arc hardfacing layer on ballistic performance enhancement of armor steel welds.     

Effect of microstructure and chemical composition of hardfacing alloy on abrasive wear behavior

Hardfacing, a surface modification technique, is used to rebuild the surface of a workpiece. The economic success of the process depends on selective application of hardfacing material and its chemical composition for a particular application. In this context, three hardfacing electrodes having different chemical compositions have been selected and their abrasive wear responses was compared with that of mild steel. The emphasis has been made to realize the effect of microstructure and chemical composition on the wear response of the hardfacing material with respect to mild steel. It has been observed that the wear rate of hardfacing alloys is lower than that of mild steel. The hardfacing alloy having the highest chromium content exhibits the lowest wear rate.     

Assessing the kinetics and mechanisms of corrosion of cast and HIPed Stellite 6 in aqueous saline environments

Cobalt-base (Stellite) alloys have seen extensive use in wear environments mainly due to their high strength, corrosion resistance and hardness. Co-base superalloys rely primarily on carbides, formed in the Co matrix and at grain boundaries, for their strength and the distribution, size and shape of carbides depends on processing conditions. Currently use of Stellite alloys has extended into various industrial sectors (e.g. pulp and paper processing, oil and gas processing, pharmaceuticals, chemical processing) and the need for improved information regarding corrosion (and often tribo-corrosion) of Stellite alloys has increased. It has been recognised that processing changes, which affect the microstructure of Stellite alloys, will most probably affect the corrosion performance.In this paper the corrosion behaviour of Stellite 6 in the as-cast and the HIP consolidated forms has been compared and contrasted using DC electrochemical techniques in static saline conditions. It has been shown that there is a significant difference in the corrosion performance of HIP consolidated Stellite 6 and it is possible to link the corrosion mechanisms to the microstructure. The benefits of using HIPing as a manufacturing process for the corrosion performance of Stellite 6 are discussed.     

Brazing zirconium alloys with an amorphous rapidly quenched strip brazing alloy

Application of aluminum alloy, which is a typical lightweight material, has been expected to achieve energy saving and prevention of pollution in many kinds of transportation vehicles. While the structure made of whole aluminum alloy, however, is lightweight, it still has problems, such as low mechanical strength and high cost. Hence, a hybrid or joining structure made of aluminum alloy and steel seems to be reasonable because of its light weight and higher strength.

To make a hybrid structure for transportation vehicles, we examined welding by friction stirring between aluminum alloy and low-carbon steel, which could be welded without melted weld materials. As a result, welding between aluminum alloy and low-carbon steel that had a thin intermetallic compound at the weld interface was obtained.

In recent automobile manufacturing, zinc-coated steel has been used for structural parts in general. On welding between zinc-coated steel and other materials such as aluminum alloy, existence of a Zn layer between aluminum and steel has to be taken into account to get a high-quality joint between the materials.

In this study, spot joining between aluminum alloy and several kinds of zinc-coated steels by friction stirring was carried out, and the effect of the coated layer both on the weld strength and weld interface microstructure was investigated. As a result, the joint between aluminum alloy and zinc-coated steel was stronger than that between aluminum alloy and non-coated steel, when the coated layer was removed at the weld interface by plastic flow of aluminum alloy.     

Laser-Welded Steel Foils with Sapphire Substrates

A nanosecond pulsed laser was used to weld stainless steel foils of 10 μm thickness to the sapphire substrates.The microstructure of the bonded interface was studied.Both materials were partially ablated under the influence of the laser beam.An inhomogeneous distribution of the steel and sapphire along the bonded interface is observed.The electrical resistance of the steel foil was measured before and after laser welding,showing that the weld slightly increases the electrical resistance but still keeps the values acceptable for electrical contacts.     

低频磁场对堆焊层组织及耐磨性的影响

通过对低频纵向磁场作用下埋弧堆焊焊缝成形、硬度、耐磨性及微观组织分析,研究了低频磁场对熔敷金属的组织和力学性能的影响规律,探讨了低频磁场改善熔敷金属组织形态、提高堆焊层耐磨性的机理.试验发现,加低频纵向磁场后焊缝的熔宽增加,熔深略有减少;晶粒尺寸由25.6μm降低到10.6 μn,硬度由50.5HRC提高到55.0HR...