Summary of welding properties of the hottest stain

Summary of welding properties of stainless steel

in the application of stainless steel, welding and cutting of stainless steel structure is inevitable. Due to the characteristics of stainless steel itself, compared with ordinary carbon steel, the welding and cutting of stainless steel has its particularity, and it is easier to produce various defects in its welded joints and heat affected zone (HAZ). Pay special attention to the physical properties of stainless steel during welding. For example, the coefficient of thermal expansion of austenitic stainless steel is 1.5 times that of low carbon steel and high chromium stainless steel ④ the trend of import replacement; The thermal conductivity is about 1/3 of low carbon steel, while the conductivity of high chromium stainless steel should be corrected according to the method given by the manufacturer, and the thermal conductivity is about 1/2 of low carbon steel; The specific resistance is more than 4 times that of low carbon steel, while that of high chromium stainless steel is 3 times that of low carbon steel. These conditions, together with metal density, surface tension, magnetism and other conditions, have an impact on welding conditions

martensitic stainless steel is generally represented by 13% Cr steel. When it is welded, there are problems such as low-temperature brittleness, deterioration of low-temperature toughness, and decrease of ductility with hardening due to (m) phase transformation in the heat affected zone that is heated to

above the phase transformation point. Therefore, preheating is required for the welding of general martensitic stainless steel, but preheating is not required for those with low carbon and nitrogen content and when using D-series welding materials. The structure of the welding heat affected zone is usually hard and brittle. For this problem, the toughness and ductility can be restored by post weld heat treatment. In addition, the brand with the lowest carbon and nitrogen content also has a certain toughness in the welding state

ferritic stainless steel is represented by 18% Cr steel. In the case of low carbon content, it has good welding performance and low sensitivity in welding cracks. However, due to the significant coarsening of the grains in the welding heat affected zone heated to above 900 ℃, it lacks ductility and toughness at room temperature and is prone to low-temperature cracks. That is to say, generally speaking, ferritic stainless steel has problems such as 475 ℃ embrittlement, phase embrittlement under long-term heating at 700 ℃ and 800 ℃, embrittlement caused by inclusion and grain coarsening, low-temperature embrittlement, corrosion resistance reduction caused by carbide precipitation, and delayed crack prone to occur in high alloy steel. Generally, preheating before welding and post welding heat treatment shall be carried out during welding, and welding shall be carried out in the temperature range with good toughness

austenitic stainless steel is represented by 18%cr 8%ni steel. In principle, pre welding preheating and post welding heat treatment are not required. Generally, it has good welding performance. However, high alloy stainless steel with high nickel and molybdenum content is easy to produce high-temperature cracks when welding. In addition, it is also prone to phase embrittlement. Ferrite generated under the action of ferrite generating elements causes low-temperature embrittlement, corrosion resistance decline and stress corrosion cracks. After welding, the mechanical properties of the welded joint are generally good, but when there are chromium carbides on the grain boundary in the heat affected zone, it is very easy to form a chromium poor layer, and the appearance of the chromium poor layer will easily produce intergranular corrosion during use. In order to avoid problems, low carbon (C 0.03%) grades or grades with titanium and niobium should be used. In order to prevent high-temperature cracks in welding metal, it is generally considered that it is effective to control ferrite in austenite. It is generally advocated to contain more than 5% ferrite at room temperature. For the steel with corrosion resistance as the main purpose, low-carbon and stable steel grades should be selected, and appropriate post weld heat treatment should be carried out; The steel with structural strength as the main purpose should not be subject to post weld heat treatment to prevent deformation and phase embrittlement due to precipitation of carbides

duplex stainless steel has low sensitivity to welding cracks. However, the increase of ferrite content in the heat affected zone will improve the susceptibility to intergranular corrosion, which can cause problems such as reduced corrosion resistance and deterioration of low temperature toughness

for precipitation hardening stainless steel, there are problems such as softening in the welding heat affected zone

to sum up, the welding performance of stainless steel is mainly shown in the following aspects:

(1) high temperature crack: the high temperature crack mentioned here refers to the crack related to welding. High temperature cracks can be roughly divided into solidification cracks, micro cracks, HAZ (heat affected zone) cracks and reheating cracks

(2) low temperature cracks: low temperature cracks sometimes occur in martensitic stainless steels and some ferritic stainless steels with martensitic structure. Because the main reasons are hydrogen diffusion, the restraint degree of welded joints and the hardened structure in them, the solution is mainly to reduce the diffusion of hydrogen during the welding process, properly preheat and post weld heat treatment, and reduce the restraint degree

(3) toughness of welded joints: in order to reduce high temperature crack sensitivity in austenitic stainless steel, 5% and 10% ferrite are usually left in the composition design. However, the existence of these ferrites leads to the decrease of low temperature toughness. When duplex stainless steel is welded, the austenite volume in the welded joint area decreases, which affects the toughness. In addition, with the increase of ferrite, the toughness value has a significant downward trend

it has been proved that the toughness of welded joints of high-purity ferritic stainless steel decreases significantly due to the mixing of carbon, nitrogen and oxygen. After the oxygen content in the welded joints of some steels increases, oxide type inclusions are formed, which become the source of cracks or the path of crack propagation, resulting in the decrease of toughness. Some steels are mixed with air in the shielding gas, and the nitrogen content increases, resulting in strip Cr2N on the cleavage surface {100} of the matrix, which hardens and reduces the toughness

(4) phase embrittlement: austenitic stainless steel, ferritic stainless steel and duplex steel are prone to phase embrittlement. As a few percent of the phase precipitated in the structure, the toughness decreased significantly. At the same time, it can also meet the performance requirements of low temperature resistance to about (4) 0 ℃, color difference △ e less than 0.5 C, and high dimensional stability. Generally, it is precipitated in the range of 600 ~ 900 ℃, especially at about 75 ℃. As a preventive measure to prevent phase formation, ferrite content in austenitic stainless steel should be minimized

(5) 475 is an important material in the household appliance industry. When it is kept at 475 ℃ for a long time (370540 ℃), the Fe Cr alloy is decomposed into solid solutions with low chromium concentration and high chromium concentration. When the chromium concentration in the solid solution is greater than 75%, the deformation changes from slip deformation to twin deformation, resulting in 475 ℃ embrittlement