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3 key points & notes of martensitic stainless steel welding

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After the austenitic stainless steel pressure vessel is formed by the plate rolling machine, it will be welded. What should we pay attention to?

What is Martensitic Stainless Steel

 3 key points & notes of martensitic stainless steel welding

Martensitic stainless steel can be divided into Cr13 type martensitic stainless steel, low carbon martensitic stainless steel and super martensitic stainless steel. Cr13 type has general corrosion resistance. From Cr12-based martensitic stainless steel, due to the addition of nickel, molybdenum, tungsten, vanadium and other alloying elements, in addition to certain corrosion resistance, it also has high high temperature strength and high temperature resistance. Oxidation properties.

Martensitic stainless steel welding characteristics

The quenching tendency of Cr13 martensitic stainless steel weld and heat-affected zone is particularly large, and the welded joint can obtain hard and brittle martensite under the condition of air cooling. Under the action of welding restraint stress and diffusible hydrogen, welding easily occurs. Cold cracks. When the cooling rate is small, coarse ferrite and carbides will be formed in the near seam area and the weld metal, which will significantly reduce the plasticity and toughness of the joint.

After the weld and heat affected zone of low carbon and super martensitic stainless steel are cooled, all of them are transformed into low carbon martensite, but there is no obvious hardening phenomenon, and they have good welding performance.

How to choose martensitic stainless steel welding consumables?

Martensitic stainless steel has a relatively high tendency to cold crack, so it is necessary to strictly maintain low hydrogen, even ultra-low hydrogen, which must be paid attention to when selecting welding materials.

Austenitic welding consumables

Among stainless steels, martensitic stainless steels can be heat treated to adjust their properties. Therefore, in order to ensure the performance requirements, especially for heat-resistant martensitic stainless steels, the composition of the weld should be as close as possible to the composition of the base metal. In order to prevent cold cracks, austenitic welding consumables can also be used, and the weld strength at this time must be lower than that of the base metal.

When the composition of the weld is similar to that of the base metal, the weld and the heat-affected zone will be hardened and brittle at the same time, and a tempering softening zone will appear in the heat-affected zone. In order to prevent cold cracking, components with a thickness of more than 3 mm are often preheated, and heat treatment is often required after welding to improve the joint performance. Since the thermal expansion coefficient of the weld metal and the base metal is basically the same, it is possible to completely eliminate welding after heat treatment. stress.

Austenitic Welds

When the workpiece cannot be preheated or heat treated, austenitic welds can be selected. Because the welds have high plasticity and toughness, they can relax the welding stress, and can dissolve more hydrogen, which can reduce the joint damage. Cold cracking tendency, but this kind of joint with uneven material, due to the different coefficient of thermal expansion, under the working environment of cyclic temperature, shear stress may be generated in the fusion zone, resulting in joint failure.

For simple Cr13 type martensitic steel, when the weld with austenite structure is not used, there is little room for adjustment of the weld composition, which is generally the same as the base metal matrix, but the harmful impurities S, P and Si must be limited. Si can promote the formation of coarse martensite in the weld of Cr13 martensitic steel. Reducing the C content is beneficial to reduce the hardenability. The presence of a small amount of elements such as Ti, N or Al in the weld can also refine the grains and reduce the hardenability.

Three main points of martensitic stainless steel welding process

For Cr13 martensitic stainless steel, when welding electrodes of the same material are used, in order to reduce the sensitivity of cold cracks and ensure the plasticity and toughness of the welded joints, low hydrogen electrodes should be selected and the following measures should be taken at the same time:

  1. Warm up. The preheating temperature increases with the increase of the carbon content of the steel, generally in the range of 100℃ ~ 350℃.
  2. After the heat. For welded joints with high carbon content or high restraint, post-heating measures should be taken after welding to prevent welding hydrogen-induced cracks.
  3. Post-weld heat treatment. In order to improve the plasticity, toughness and corrosion resistance of welded joints, the post-weld heat treatment temperature is generally 650°C ~ 750°C, and the holding time is calculated as 1h / 25mm.

For super and low carbon martensitic stainless steel, preheating measures are generally not required. When the degree of restraint is large or the hydrogen content in the weld is high, preheating and post-heating measures are taken. The preheating temperature is generally 100 °C ~ 150 °C , The post-weld heat treatment temperature is 590 ~ 620 ℃.

Notes

For martensitic steels with higher carbon content. Or when preheating before welding and post-welding heat treatment are difficult to implement, and the joint is more restrained, austenitic welding consumables can also be used in the project to improve the plasticity and toughness of the welded joint and prevent cracks. However, at this time, when the weld metal is austenite or austenite-dominated structure, it is actually a low-strength match compared with the strength of the base metal, and the weld metal and the base metal are in chemical composition, metallographic structure, thermal The physical properties and mechanical properties are very different, and the welding residual stress is unavoidable, which can easily cause stress corrosion or high temperature creep damage.