What are the properties and application characteristics of stainless steel materials for thermal spraying

       Stainless steel has a high chromium content, and the carbon content varies widely from ultra-low carbon with less than 0.03% to high carbon with more than 1%. Stainless steel has a wide range of uses, not only has excellent corrosion resistance, but also has excellent high-temperature oxidation and heat resistance properties. Therefore, it is often used as a heat-resistant steel coating in practice. It can also be used as a low-temperature, non-magnetic, wear-resistant material. All stainless steel wires currently used for welding can be used for spraying.

    1) Martensitic stainless steel

    The chromium content is 12%~18%, and the commonly used grades of wire are: 0Cr13~4Cr13, 7Cr13, etc. During the spraying process, the martensitic stainless steel particles have hardenability, high bonding strength between particles, high coating hardness, good wear resistance, and quite good corrosion resistance. It is often used for work under severe wear and moderate corrosion conditions. Surface strengthening of machine parts, especially suitable for spraying of shaft parts, the coating does not crack. Martensitic stainless steel coating has good wear resistance at medium temperature and can work stably at T<450 temperature.

However, due to the martensitic transformation and hardening during rapid cooling (quenching), accompanied by a 4% volume expansion rate, care should be taken to slow cooling after spraying. Martensitic stainless steel with high carbon content reduces the chromium content of the matrix structure due to the formation of chromium carbides. If the chromium content is less than 12%, the corrosion resistance will be reduced.

Martensitic stainless steel is generally used to prepare surfaces that require high hardness, and can be sprayed or melted.

    2) Austenitic stainless steel

    The austenitic stainless steel used in thermal spraying is mainly 18-8 type nickel-chromium stainless steel (also known as SUS304), which has a low carbon content, generally between 0.05% and 0.12%. A small amount of Mo prevents intergranular corrosion by adding a small amount of Ti element or Nb element (such as SUS321 or 347).

Austenitic stainless steel coating is austenitic structure at room temperature, it has excellent corrosion resistance, heat resistance and good wear resistance and good processing performance, non-magnetic. Mainly used for wear parts and good processing performance in most acid and alkali environments, non-magnetic. It is mainly used for the protection and repair of wear-resistant parts that work in most acid and alkali environments, such as spraying water pump shafts, papermaking dryers, etc. However, the coating shrinkage rate is large, and it is suitable for spraying thinner coatings, otherwise cracking and peeling are prone to occur. It should be noted that when applying austenitic stainless steel coating, the coating must be properly sealed, because the austenitic stainless steel coating is a cathode to the steel substrate, once the coating is damaged, the steel substrate will accelerate corrosion.

At present, there is no wire specially produced for arc spraying in China, and only suitable grades of wire can be selected from the existing welding materials for arc spraying. Commonly used grades of wire are 1Cr18Ni9Ti, 1Cr18Ni9, 0Cr18Ni9, 1Cr18Ni9Mo2, etc. These materials have good wear resistance and strength, as well as corrosion resistance, the shrinkage coefficient of the coating is large and the hardness is low. However, this type of stainless steel has a large coefficient of linear expansion and needs to be paid attention to when spraying.

    3) Ferritic stainless steel

    Ferritic stainless steel contains 15% to 30% chromium, and Cr8 is a typical steel grade. It is a ferrite structure at room temperature, and it has good corrosion resistance. In order to improve the corrosion resistance, pitting corrosion resistance and stress corrosion cracking properties, austenitic-ferritic complex phase gold steels were developed.