These classic common sense of dissimilar metal welding

Some inherent problems of dissimilar metal welding hinder its development, such as the composition and performance of the dissimilar metal fusion zone. Most of the damage to the dissimilar metal welding structure occurs in the fusion zone. Due to the different crystalline characteristics of the welds near the fusion zone, It is easy to form a transition layer with poor performance and varying composition.

In addition, due to the long time at high temperature, the diffusion layer in this area will expand, which will further increase the unevenness of the metal. Moreover, when dissimilar metals are welded or after heat treatment or high temperature operation after welding, it is often found that the carbon on the low alloy side “migrates” through the weld boundary to the high alloy weld, forming a decarburized layer on both sides of the fusion line. And the carburization layer, the decarburization layer is formed on the low alloy side of the base material, and the carburization layer is formed on the high alloy weld side.

hinder and prevent the use and development of dissimilar metal structures are mainly manifested in the following aspects:

• At room temperature, the mechanical properties (such as tensile, impact, bending, etc.) of the welded joint area of ​​dissimilar metals are generally better than the performance of the base material to be welded, but the performance of the joint area is inferior to that of the parent metal after high temperature or long-term operation at high temperature. material.
• There is a martensite transition zone between the austenite weld and the pearlite base metal. This zone has low toughness and is a high-hardness brittle layer. It is also a weak zone that causes failure and destruction of components. It will reduce the welded structure. The reliability of use.
• Carbon migration during post-weld heat treatment or high-temperature operation will result in the formation of a carburized layer and a decarburized layer on both sides of the fusion line. It is generally believed that the reduction of carbon in the decarburized layer leads to a large change (generally deterioration) in the organization and performance of the area, which makes the area prone to early failure during service. Many of the high-temperature pipelines in service or the high-temperature pipelines in the test fail in the decarburized layer.
• Failure is related to conditions such as time, temperature and alternating stress.
• Post-weld heat treatment cannot eliminate the residual stress distribution in the joint area.
•  The non-uniformity of chemical composition.When dissimilar metals are welded, due to the obvious difference between the metal on both sides of the weld and the alloy composition of the weld, during the welding process, the base metal and the welding material will melt and mix with each other. The uniformity of the mixing varies with the welding process. Change, and the degree of mixing uniformity varies greatly in different positions of the welded joints, which causes the uneven chemical composition of the welded joints.
• The inhomogeneity of the metallographic structure.Due to the discontinuity of the chemical composition of the welded joint, after the welding thermal cycle, different structures appear in each area of ​​the welded joint, and very complex structures often appear in some areas.
• Discontinuity of performance.

The difference in the chemical composition and metallographic structure of the welded joint brings about the difference in the mechanical properties of the welded joint. The strength, hardness, plasticity, toughness, impact performance, high temperature creep, and durability of each area along the welded joint are very different. This significant inhomogeneity makes the behavior of different areas of the welded joint under the same conditions very different, and there are weakened areas and strengthened areas, especially under high temperature conditions, the dissimilar metal welded joints are in the service process Early failures often occur in the medium.

The characteristics of different welding methods when welding dissimilar metals

Most welding methods can be used to weld dissimilar metals, but when selecting welding methods and formulating process measures, the characteristics of dissimilar metals should still be considered. According to the different requirements of the base material and the welded joints, fusion welding, pressure welding and other welding methods are all used in the welding of dissimilar metals, but they all have their own advantages and disadvantages.

1. Welding

Fusion welding is the most commonly used method in dissimilar metal welding. Commonly used fusion welding methods include electrode arc welding, submerged arc welding, gas shielded arc welding, electroslag welding, plasma arc welding, electron beam welding, laser welding, etc. In order to reduce dilution, reduce the fusion ratio or control the amount of melting of different metal base materials, usually electron beam welding, laser welding, plasma arc welding and other methods with higher energy density of the heat source can be used.

In order to reduce the penetration depth, indirect arc, swing welding wire, ribbon electrode, additional non-current welding wire and other technological measures can be adopted. But in any case, as long as it is fusion welding, part of the base metal will always melt into the weld and cause dilution. In addition, intermetallic compounds, eutectic, etc. will also be formed. In order to alleviate such adverse effects, it is necessary to control and shorten the residence time of the metal in the liquid or high-temperature solid state.
However, despite the continuous improvement and perfection of fusion welding methods and technological measures, it is still difficult to solve all the problems in welding of dissimilar metals, because there are many kinds of metals, performance requirements are diverse, and joint forms are different, and in many cases, it is still necessary. Use pressure welding or other welding methods to solve the welding problem of specific dissimilar metal joints.

2. Pressure welding

Most pressure welding methods only heat the metal to be welded to a plastic state or even do not heat it, and apply a certain pressure as the basic feature. Compared with fusion welding, pressure welding has certain advantages when welding dissimilar metal joints. As long as the joint form allows and the welding quality can meet the requirements, pressure welding is often a more reasonable choice.

During pressure welding, the surface of dissimilar metals can be melted or not, but due to the effect of pressure, even if there is molten metal on the surface, it will be squeezed and discharged (such as flash welding and friction welding), only a few cases After pressure welding, the metal that was once melted (such as spot welding) remains.

pressure welding can reduce or avoid the adverse effect of thermal cycling on the properties of the base metal due to no heating or low heating temperature, and prevent brittle intermetallic compounds. Some forms of pressure welding can even squeeze the produced intermetallic compound out of the joint. In addition, there is no problem of weld metal properties change caused by dilution during pressure welding.

However, most pressure welding methods have certain requirements for the joint form. For example, spot welding, seam welding, and ultrasonic welding must use lap joints; in friction welding, at least one workpiece must have a cross-section of a rotating body; explosive welding is only suitable for Larger area connections, etc. Pressure welding equipment is not yet popular. These undoubtedly limit the application range of pressure welding.