| Along with the development of new equipment and new technologies,the dissimilar metal welding parts is widely used in area of mechanical engineering.Welding is the main means of composite bonding of dissimilar metal materials.This kind of joints not only can fully utilize the excellent properties of different materials and meet the actual usage,but also can save costs and make great economic profits.Therefore,how to improve the performance of welded joints of dissimilar materials composite parts has become a research hotspot.Austenitic stainless steel(ASS)has been widely used in the hydropower,petrochemical,transportation and food industries because of high strength,good toughness,excellent corrosion resistance and fatigue resistance.With the rapid development of new energy industries,the ASS is increasingly used as structural materials in the fields of nuclear power and wind power.Meanwhile,austenitic stainless steel/low carbon steel dissimilar materials have attracted more attention recently,with the increasing quality requirements of the joints.The traditional melting welding or brazing technologies have been unable to meet the quality requirements of austenitic stainless steel/low carbon steel welded structural components.Vacuum diffusion welding technology originated from the US,and a series of dissimilar materials have been used for the Boeing F series fighters,including the joints of super alloy turbine blades and leaves with long life expectancy of 9600 h.Vacuum diffusion welding is usually carried out in vacuum or protective atmosphere to avoid excessive oxidation.The dissimilar materials are fit together at a certain temperature and pressure,ensuring the atoms between the contact surfacesdiffuse each other.This welding method has advantages such as large contact area and high joint strength.This study proposes a novel welding process,i.e.vacuum solid phase diffusion welding technology,for the austenitic stainless steel/low carbon steel with aiming to enlarge the application of dissimilar material and improve the quality of their welded joint.Aseries of experiments have been carried out to improve the properties of the welded joint such as high strength,high toughness and high fatigue resistance.Meanwhile,the microstructural evolution,elements diffusion,and the growth behavior of compounds and its effects on the performance of the welded joints have also been investigated.The present results are expected to provide guidance for development of dissimilar welding between austenitic stainless steel/mild steel.In this paper,the welding processing parameters of austenitic stainless steel/low carbon steel were optimized.The results of diffusion welding process show that the welding temperature and welding time are the main parameters that affect the microstructure and properties of dissimilar materials.As the welding temperature is below 800?,and welding time less than 60min,the interface has relatively lowerjoining strength and worse toughness.This is mainly due to the formation of carbides(Cr23C6)at the interfaces.As the welding temperature reaches 850?,the carbides begin to decompose.As the welding temperature is above 900?,the secondary carbides(Cr23C6)and the intermetallic compounds(FeCr)form near the interface,resulting in a decrease of toughness of the joints.Therefore,it can be concluded that 850? is an optimal welding temperature range for the present austenitic stainless steel/low carbon steel.The tensile strength of the joints can reach 441 MPa,the impact toughness at room temperature is 138.8J/cm2,and the fatigue strength reaches 178.2MPa.The joints exhibit superior performance to the low carbon steel base metal.According to the microstructural analysis,the mechanism of vacuum diffusion welding of austenitic stainless steel/low carbon steel has been revealed.Diffusion welding makethe grain boundaries of two different regions adjacent to each other on both sides of the interface of austenitic stainless steel/low carbon steel,i.e.,the fcc and fcc grains on both sides of the interface are completely joint by grain boundaries,guaranteeing the strength of the joints.Typical compounds at the interface are carbide Cr23C6 and intermetallic compounds FeCr.The size of Cr23C6 has a great influence on the overall performance of the joints:as the carbides with a size less than 200nm are beneficial to improve the strength and toughness of the joints;as the size is larger than 200 nm,the crack source easily occurs,reducing the properties of the welded joints.On the other hand,the intermetallic compound FeCr with high hardness and brittleness mainly appears at the interface of diffusion weldinghas a negative impact on the strength of the welded joints.Considering that carbide and intermetallic compounds are rich in Cr,which leads to the local lean Cr region of austenite stainless steel,thereby reducing the corrosion resistance of the stainless steel base metal.The element diffusion mechanism of the welded austenitic stainless steel/low carbon steel has also been studied by the distribution of elements at the interface of dissimilar materials.The interstitial diffusion of C and substitution diffusion of Cr and Fe are the important diffusion mode.The diffusion number and the diffusion distance of elements increase with increasing the welding temperature and the welding time.The diffusion rate of C in low carbon steel increases through the interface to the stainless steel matrix,and meanwhile,the ferrite strength on the side of low carbon steel layer is enhanced by the solid solution strengthening of Cr.However,excessive diffusion of Cr can lead to the formation of intermetallic compounds in the ferrite layer and reduce the ductility and toughness of the welded joint.Therefore,it is required to control the welding temperature and welding time to ensure the overall performance of the welded joints. |
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