There are some inherent problems in dissimilar metal welding that 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 crystallization characteristics of the welds in each section near the fusion zone, It is also easy to form a transition layer with poor performance and changes in 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 “migrate” through the weld boundary to the high-alloy weld, forming decarburization layers on both sides of the fusion line. And the carburization layer, the base metal forms a decarburization layer on the low alloy side, and the carburization layer forms on the high alloy weld side.
Obstacles and barriers to the use and development of dissimilar metal structures are mainly manifested in the following aspects:
1. At room temperature, the mechanical properties (such as tensile, impact, bending, etc.) of the welded joint area of dissimilar metals are generally better than those of the base metal to be welded. However, at high temperatures or after long-term operation at high temperatures, the performance of the joint area is inferior to that of the base metal. material.
2. 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 component failure and damage. It will reduce the welded structure. reliability of use.
3. Carbon migration during post-weld heat treatment or high-temperature operation will cause the formation of carburized layers and decarburized layers on both sides of the fusion line. It is generally believed that the reduction of carbon in the decarburized layer will lead to major changes (generally deterioration) in the structure and performance of the area, making this area prone to early failure during service. The failure parts of many high-temperature pipelines in service or under testing are concentrated in the decarburization layer.
4. Failure is related to conditions such as time, temperature and alternating stress.
5. Post-weld heat treatment cannot eliminate the residual stress distribution in the joint area.
6. Inhomogeneity of chemical composition.
When dissimilar metals are welded, since the metals on both sides of the weld and the alloy composition of the weld are obviously different, during the welding process, the base metal and the welding material will melt and mix with each other. The uniformity of the mixing will change with the change of the welding process. Changes, and the mixing uniformity is also very different at different positions of the welded joint, which results in the inhomogeneity of the chemical composition of the welded joint.
7. Inhomogeneity of metallographic structure.
Due to the discontinuity of the chemical composition of the welded joint, after experiencing the welding thermal cycle, different structures appear in each area of the welded joint, and extremely complex organizational structures often appear in some areas.
8. Discontinuity of performance.
The differences in chemical composition and metallographic structure of welded joints bring about different mechanical properties of welded joints. The strength, hardness, plasticity, toughness, impact properties, high temperature creep, and durability properties of various areas along the welded joint are very different. This significant inhomogeneity makes different areas of the welded joint behave very differently under the same conditions, with weakened areas and strengthened areas appearing. Especially under high temperature conditions, dissimilar metal welded joints are in service during the service process. Early failures often occur.
Characteristics of different welding methods when welding dissimilar metals
Most welding methods can be used for welding 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 metal and welded joints, fusion welding, pressure welding and other welding methods are all used in dissimilar metal welding, but each has its own advantages and disadvantages.
1. Welding
The most commonly used fusion welding method in dissimilar metal welding is 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, lower the fusion ratio or control the melting amount of different metal base materials, electron beam welding, laser welding, plasma arc welding and other methods with higher heat source energy density can usually be used.
In order to reduce the penetration depth, technological measures such as indirect arc, swing welding wire, strip electrode, and additional non-energized welding wire can be adopted. But no matter what, 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, eutectics, etc. will also be formed. In order to mitigate such adverse effects, the residence time of metals in the liquid or high-temperature solid state must be controlled and shortened.
However, despite the continuous improvement and improvement of welding methods and process measures, it is still difficult to solve all the problems when welding dissimilar metals, because there are many types of metals, various performance requirements, and different joint forms. In many cases, it is necessary to Pressure welding or other welding methods are used to solve the welding problems 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, but 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 interface surfaces of dissimilar metals may or may not melt. However, due to the effect of pressure, even if there is molten metal on the surface, it will be extruded and discharged (such as flash welding and friction welding). Only in a few cases Once molten metal remains after pressure welding (such as spot welding).
Since pressure welding does not heat or the heating temperature is low, it can reduce or avoid the adverse effects of thermal cycles on the metal properties of the base metal and prevent the generation of brittle intermetallic compounds. Some forms of pressure welding can even squeeze the intermetallic compounds that have been created out of the joint. In addition, there is no problem of changes in the properties of the weld metal 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; during friction welding, at least one workpiece must have a rotating body cross-section; explosion welding is only applicable to Larger area connections, etc. Pressure welding equipment is not yet popular. These undoubtedly limit the application scope of pressure welding.
3. Other methods
In addition to fusion welding and pressure welding, there are several methods that can be used to weld dissimilar metals. For example, brazing is a method of welding dissimilar metals between filler metal and base metal, but what is discussed here is a more special brazing method.
There is a method called fusion welding-brazing, that is, the low-melting-point base metal side of the dissimilar metal joint is fusion-welded, and the high-melting-point base metal side is brazed. And usually the same metal as the low melting point base material is used as the solder. Therefore, the welding process between the brazing filler metal and the low melting point base metal is the same metal, and there are no special difficulties.
The brazing process is between the filler metal and the high melting point base metal. The base metal does not melt or crystallize, which can avoid many weldability problems, but the filler metal is required to be able to wet the base metal well.
Another method is called eutectic brazing or eutectic diffusion brazing. This is to heat the contact surface of dissimilar metals to a certain temperature, so that the two metals form a low-melting-point eutectic at the contact surface. The low-melting-point eutectic is liquid at this temperature, essentially becoming a kind of solder without the need for external solder. Brazing method.
Of course, this requires the formation of a low-melting-point eutectic between the two metals. During diffusion welding of dissimilar metals, an intermediate layer material is added, and the intermediate layer material is heated under very low pressure to melt, or form a low melting point eutectic in contact with the metal to be welded. The thin layer of liquid formed at this time, after a certain period of heat preservation process, makes the intermediate layer material melt. When all the intermediate layer materials are diffused into the base material and homogenized, a dissimilar metal joint without intermediate materials can be formed.
This type of method will produce a small amount of liquid metal during the welding process. Therefore, it is also called liquid phase transition welding. Their common feature is that there is no casting structure in the joint.
Things to note when welding dissimilar metals
1. Consider the physical, mechanical properties and chemical composition of the weldment
(1) From the perspective of equal strength, select welding rods that meet the mechanical properties of the base metal, or combine the weldability of the base metal with welding rods with non-equal strength and good weldability, but consider the structural form of the weld to meet the equal strength. Strength and other stiffness requirements.
(2) Make its alloy composition consistent with or close to the base material.
(3) When the base metal contains high levels of C, S, and P harmful impurities, welding rods with better crack resistance and porosity resistance should be selected. It is recommended to use calcium titanium oxide electrode. If it still cannot be solved, low hydrogen sodium type welding rod can be used.
2. Consider the working conditions and performance of the weldment
(1) Under the condition of bearing dynamic load and impact load, in addition to ensuring strength, there are high requirements for impact toughness and elongation. Low hydrogen type, calcium titanium type and iron oxide type electrodes should be selected at one time.
(2) If in contact with corrosive media, appropriate stainless steel welding rods must be selected based on the type, concentration, working temperature of the media, and whether it is general clothing or intergranular corrosion.
(3) When working under wear conditions, it should be distinguished whether it is normal or impact wear, and whether it is wear at normal temperature or high temperature.
(4) When working under non-temperature conditions, corresponding welding rods that ensure low or high temperature mechanical properties should be selected.
3. Consider the complexity of the collective shape of the weldment, the stiffness, the preparation of the welding fracture and the welding position.
(1) For weldments with complex shapes or large thicknesses, the shrinkage stress of the weld metal during cooling is large and cracks are prone to occur. Welding rods with strong crack resistance must be selected, such as low-hydrogen welding rods, high-toughness welding rods or iron oxide welding rods.
(2) For weldments that cannot be turned over due to conditions, welding rods that can be welded in all positions must be selected.
(3) For welding parts that are difficult to clean, use acidic welding rods that are highly oxidizing and insensitive to scale and oil to avoid defects such as pores.
4. Consider welding site equipment
In places where there is no DC welding machine, it is not advisable to use welding rods with limited DC power supply. Instead, welding rods with AC and DC power supply should be used. Some steels (such as pearlitic heat-resistant steel) need to eliminate thermal stress after welding, but cannot be heat treated due to equipment conditions (or structural limitations). Welding rods made of non-base metal materials (such as austenitic stainless steel) should be used instead, and post-weld heat treatment is not necessary.
5. Consider improving welding processes and protecting workers’ health
Where both acidic and alkaline electrodes can meet the requirements, acidic electrodes should be used as much as possible.
6. Consider labor productivity and economic rationality
In the case of the same performance, we should try to use lower-priced acidic welding rods instead of alkaline welding rods. Among acidic welding rods, titanium type and titanium-calcium type are the most expensive. According to the situation of my country’s mineral resources, titanium iron should be vigorously promoted. Coated welding rod.
Post time: Oct-27-2023