Nickel-based alloys have many unique properties and are also ideal for welding dissimilar steels. The major difficulty in welding dissimilar steels with nickel-based filler metals lies in the weldability and metallurgical adaptability of nickel-based filler metals, as well as the physical properties and service conditions of dissimilar steel joints. If all of these limitations are adequately addressed, dissimilar steel joints can be obtained with excellent mechanical properties and long service life.
1. Selection of welding method
According to the weldability characteristics of nickel and nickel alloys, the welding process method is the key to whether nickel and nickel alloy materials can be welded well. Production practice has proved that there are many ways to weld these materials, and different welding methods can be selected according to different production conditions and structural performance requirements. For example: electrode arc welding, submerged arc automatic welding, TIG, MIG, resistance spot welding, etc.; methods such as plasma arc welding, electron beam welding and brazing can also be used. However, tungsten argon arc welding (TIG) and electrode arc welding are more widely used in production.
2. Preparation before welding
The first step is to clean the joint zone. Nickel alloys are more susceptible to cracking caused by sulfur and other non-metallic elements. Therefore, the welding area must be kept clean and free from dust, grease and other impurities. Special tools such as stainless steel wire brushes and special grinding wheels work very well in practice. Nickel oxides are more stubborn than iron oxides and must be removed by grinding, pickling, etc. Although the stainless steel wire brush is very effective in removing welding slag and other residues, it can only polish the nickel oxide and clean the surface.
Compared with welding carbon steel, welding nickel-based alloy has the characteristics of low penetration, small molten pool, and poor fluidity of cladding metal. From the perspective of welding performance, it is not advisable to use large linear energy to increase penetration, so as to prevent overheating of weld pieces, excessive burning of deoxidizing elements and poor weld formation caused by excessive agitation of welding pool. In order to ensure enough penetration, the joint form with a large groove angle and a small blunt edge should be selected.
3. Preheating and post-weld heat treatment
Rolled nickel-based alloys generally do not need to be preheated, but when the temperature of the base metal is lower than 15°C, the 250-300mm wide area on both sides of the joint should be heated to 15-20°C to prevent moisture condensation from causing weld porosity.
The interlayer temperature should be strictly controlled, and in production practice, it is mostly controlled below 100°C to reduce overheating.
Although stabilization treatment is sometimes used to avoid intergranular corrosion or stress corrosion, post-weld heat treatment is generally not recommended.
4. Welding process of argon tungsten arc welding
Tungsten argon arc welding is the most widely used welding method in the production of nickel-based alloys. Generally, DC positive polarity, high-frequency arc ignition, and delayed gas cut-off welding technology are used.
(1) Argon is used as protective gas, which must be dry and of high purity. Generally, 99.999% argon is used, and the back side should be protected by argon too.(2) Cerium tungsten electrode, usually used in nickle-based alloys weldinged, should be ground sharp with a tip diameter of 0.4mm and an included angle of 30-60 degrees, which can ensure arc stability and sufficient penetration. Care should be taken to avoid contact between the tungsten electrode and the molten pool, and contaminated rod tip must be ground off.
(3) The selection of welding wire is the key to the quality and performance of welded joints. Most of the welding wire used for TIG welding has the same composition as the base metal.
(4) Process characteristics
- Short arc and fast welding should be adopted during welding.
- It can be slightly oscillated during operation, but the angle of the welding torch and welding wire should be mastered.
During multi-layer welding, the temperature between layers should be controlled and not exceed 100°C. Take care to fill the craters.
The line energy should be as small as possible under the premise of enough penetration. The fluidity of the molten nickle-based alloys is poor, and the penetration depth is shallow. Pay attention to the observation of the molten pool during welding to prevent defects such as pores and incomplete penetration. Rapid cooling measures should be taken after welding.