Research on welding technology of nickel-based corrosion-resistant alloy
1. Introduction
Alloys with a nickel mass fraction of more than 30% are generally called nickel-based corrosion-resistant alloys, which are similar to austenitic stainless steel. The microstructure of nickel-based corrosion-resistant alloys is single-phase austenite, and there is no phase change in the solid state. The grains of the parent material and weld metal cannot be refined by heat treatment.
It has unique physical, chemical and corrosion resistance properties. It can resist corrosion from various corrosive media in the range of 200-1090℃. At the same time, it also has good high and low temperature mechanical properties. It is widely used in petrochemical, nuclear energy, aerospace and other industries.
Commonly used nickel-based corrosion-resistant alloys can be divided into four major series:
① Ni-Cu system is composed of Ni and Cu as the main elements, called Monel alloy, represented by 4000 series numbers;
② NiCrFe system is mainly composed of Ni, Cr is more than Fe, called Inconel alloy, represented by 6000 series numbers;
③ Nickel-iron-chromium system, Ni content is less than 50%, Fe is more than Cr, called Incoloy alloy, represented by 8000 series numbers;
④ Ni Mo or Ni Mo Cr system is called Hastelloy alloy, with Ni as the main element and high Mo content.
2. Common problems in the welding process of nickel-based alloys
2.1 Hot cracks
In the welding process of nickel-based alloys, the weld is prone to macro cracks (solidification cracks) and micro cracks (polygonal cracks), or both.
Intergranular liquid film is the main metallurgical factor causing solidification cracks in single-phase austenitic welds of nickel-based alloys. When welding nickel-based alloys, impurities such as S and Si segregate in the weld metal to form a low-melting eutectic (NiNiS, 645°C).
During the crystallization of the weld metal, the low-melting eutectic liquid film remains in the grain boundary area. Due to the large linear expansion coefficient of nickel-based alloys, significant tensile stress is generated during welding, and the liquid film is prone to cracking under the action of shrinkage stress during crystallization.
The microstructure of the nickel-based alloy weld is single-phase austenite. The formation and development of polygonal grain boundaries between pure metal and single-phase alloy welds are the main reasons for the generation of polygonal cracks in nickel-based alloy welds.
In order to improve the ability of single-phase austenitic alloy weld metal to resist thermal cracking, Mo, W, Mn, Ta, Cr, Nb and other solid solution strengthening elements are added to the welding material, which can effectively inhibit the formation and development of hot cracks and polygonal grain boundaries in nickel-based alloy welds.
2.2 Porosity
The melting range of nickel-based alloys is between 1287 and 1446°C, and the solid-liquid temperature difference is very small. The deposited metal is viscous and has poor fluidity. Under the conditions of rapid cooling and crystallization, the gas does not have time to escape and forms pores in the weld.
2.3 Slag inclusion
The oxides of pure nickel and nickel alloys are different from those of steel. For example, the melting point of pure iron is 1538℃, FeO is 1420℃, and Fe3O2 is 1565℃. Therefore, during steel welding, the oxides and the parent metal are almost melted.
However, there are significant differences in the melting points of nickel and nickel oxide, such as the melting point of pure nickel is 1446℃, while the melting point of NiO is 2090℃. It can be seen that the oxides of nickel alloys are retained in solid form during welding, resulting in incomplete fusion and discontinuous oxide inclusions.
2.4 Undercut
Due to the poor fluidity of the weld metal of nickel-based alloys, the required penetration depth can be achieved by using a slight swinging technique. However, when swinging to the extreme position of each side, if the holding time is too short and there is not enough time to fill the molten weld metal, it will cause undercutting.
3. Key points of welding process
3.1 Selection of welding method
For welding of nickel-based alloys, stick arc welding, gas tungsten arc welding and gas metal arc welding are preferred. Automatic submerged arc welding can also be used for butt welding of thick plates.
Many different welding processes are suitable for different materials and applications. These include:
Shielded metal arc welding (SMAW): Also known as stick welding, SMAW uses a consumable electrode coated with flux for welding.
Gas metal arc welding (GMAW/MIG): It uses a continuous solid wire fed through the welding gun and a shielding gas to protect the weld pool from contamination.
Gas tungsten arc welding (GTAW/TIG): This process uses a non-consumable tungsten electrode and a shielding gas. If required, the filler metal can be fed separately.
Flux-cored arc welding (FCAW): Similar to MIG welding, the core of the wire is filled with flux.
Submerged Arc Welding (SAW): This process involves forming an arc between a continuously fed welding wire and the workpiece, submerging the welding area under a layer of flux.
