Brief Introduction of Stainless Steel Classification and Welding Characteristics

  • By:Iris Liang
  • Date:2021/12/28

Stainless steel can be divided into four categories according to its steel structure, namely, austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and austenitic-ferritic duplex stainless steel. The following mainly analyzes the welding characteristics of austenitic stainless steel and two-way stainless steel.

(1) Welding of austenitic stainless steel

Austenitic stainless steel is easier to weld than other stainless steels. No phase change occurs at any temperature, and it is not sensitive to hydrogen embrittlement. Austenitic stainless steel joints also have better plasticity and toughness in the welded state. The main problems of welding are: welding hot cracking, embrittlement, intergranular corrosion and stress corrosion. In addition, due to poor thermal conductivity, large linear expansion coefficient, large welding stress and deformation. When welding, the welding heat input should be as small as possible, and should not be preheated, and the interlayer temperature should be lowered. The interlayer temperature should be controlled below 60℃, and the weld joints should be staggered. To reduce the heat input, the welding speed should not be excessively increased, but should be adapted to reduce the welding current.

(2) Welding of austenitic-ferritic two-phase stainless steel

Austenitic-ferritic bidirectional stainless steel is a duplex stainless steel composed of austenite and ferrite. It combines the advantages of austenitic steel and ferritic steel, so it has the characteristics of high strength, good corrosion resistance and easy welding. At present, there are mainly three types of duplex stainless steels: CR18, CR21, and CR25. The main characteristics of this type of steel welding are: compared with austenitic stainless steel, it has a lower thermal tendency; compared with pure ferritic stainless steel, it has a lower tendency to embrittlement after welding, and the degree of ferrite coarsening in the heat-affected zone of welding It is also lower, so the weldability is better.

Due to the good welding performance of this kind of steel, preheating and post-heating are not necessary during welding. Thin plates should be welded with TIG, and medium and thick plates can be welded with electrode arc welding. Special electrode with similar composition to the base metal or austenitic electrode with low carbon content should be selected for electrode arc welding. Nickel-based alloy electrodes can also be used for CR25 dual-phase steel.

Due to the existence of a large proportion of ferrite in dual-phase steels, the inherent embrittlement tendency of ferritic steels, such as brittleness at 475°C, σ phase precipitation embrittlement and coarse grains, still exist because of the presence of austenite The balance effect of the welding machine can be relieved to a certain extent, so you still need to pay attention to it when welding. When welding duplex stainless steels with no NI or low NI, there is a tendency of single-phase ferrite and grain coarsening in the heat-affected zone. At this time, attention should be paid to control the welding heat input, and try to use low current, high welding speed, and narrow pass welding. And multi-pass welding to prevent grain coarsening and single-phase ferrite in the heat-affected zone, the temperature between layers should not be too high, and it is best to weld the next pass after cold.

Both of the above are types that are easier to weld. However, there are also stainless steel varieties with poor weldability, such as ferrite. At this time, we recommend you to consider our patented welding auxiliary tool-electromagnetic control arc stabilization device. Hangao Tech (SEKO Machinery) summed up the experience and data in the welded pipe manufacturing equipment industry over the past 20 years, so that while the welding speed has been improved, it can also take into account the quality of the weld. The quality of the weld is guaranteed, and when the welded pipe enters the next process for processing, the scrap rate can be effectively reduced and the output can be increased.

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