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Selection factors of raw materials for stainless steel welded pipe fittings 2021/01/31 12:35:15

Selection factors of raw materials for stainless steel welded pipe fittings

 The most commonly used materials for welding pipe fittings are carbon steel and stainless steel. The most common stainless steel materials are 304&304L, 316&316L, and some special stainless steels, such as duplex stainless steel. The selection of materials is described below. If you have any questions, please contact EATHU sales department. 

1.Stainless steel: stainless steel refers to the general term for alloy steel with high chemical stability that can withstand corrosion by air, water, acid, alkali, salt and its solutions and other corrosive media. The main corrosion forms of stainless steel are uniform corrosion, pitting corrosion, crevice corrosion and stress corrosion. Uniform corrosion refers to the phenomenon that all metal surfaces in contact with corrosive media are corroded; pitting corrosion refers to the partial corrosion that occurs on the surface of metal materials without corrosion or slight corrosion; crevice corrosion, in the electrolyte, such as oxygen In an ionic environment, when there are gaps between stainless steels or between surfaces that are in contact with foreign objects, the solution flow in the gaps will be delayed, which will lead to the phenomenon that the passive film of stainless steel in the gaps is locally destroyed; intergranular corrosion occurs near the grain boundaries. Selective corrosion phenomenon; stress corrosion refers to the phenomenon of brittle cracking of stainless steel that is lower than the extremely strong under the action of specific corrosive media and tensile stress.

2.Measures to prevent pitting corrosion: 

1) Reduce chloride ion content and oxygen ion content 

2) Add chromium, nickel, molybdenum, silicon, copper and other alloying elements to stainless steel 

3) Try not to perform cold processing to reduce the possibility of pitting corrosion at the dislocation outcrop 

4) Reduce the carbon content in steel. 4. High-temperature performance of stainless steel and heat-resistant steel: brittleness at 475°C, mainly in ferrite with Cr>13%, long-term heating and slow cooling between 430-480°C, resulting in an increase in strength at room temperature or negative temperature High and decreased toughness. 

3. Corrosion resistance of austenitic stainless steel welded joints: 

1) intergranular corrosion, 

2) intergranular corrosion in the sensitized zone of the heat-affected zone, and knife-like corrosion. 

4. Measures to prevent intergranular corrosion in welds: 

1) Through the welding material, the weld metal is either ultra-low carbon or contains sufficient stabilizing element Nb. 

2) Adjust the weld composition to obtain a certain δ phase. The theory of intergranular corrosion is essentially the cause of chromium depletion. 

5. Intergranular corrosion in the sensitized zone of the heat-affected zone: refers to the intergranular corrosion that occurs in the part of the heat-affected zone where the heating peak temperature is in the sensitized heating zone. 

6. Knife corrosion: The intergranular corrosion produced in the fusion zone is like a knife cut, so it is called "knife corrosion". 

7. Measures to prevent knife-shaped corrosion: 

①Select low-carbon base metal and welding materials 

②Using the stainless steel with different structure 

③Using low current welding to reduce the degree of overheating and width of the coarse-grained area 

④The welding seam in contact with the corrosive medium is finally welded 

⑤Cross welding 

⑥Increase the content of Ti and Tb in the steel, so that the grain boundary of the welded coarse-grain zone has enough Ti, Tb and carbon to combine. 

8. Why is stainless steel welded with low current? In order to reduce the temperature of the welding heat-affected zone, prevent the generation of intergranular corrosion of the weld, prevent the welding rod, the welding wire from overheating, welding deformation, welding stress, and reduce heat input. 

9. Three conditions that cause stress corrosion cracking: environment, selective corrosive medium, and tensile stress. 

10. Measures to prevent stress corrosion cracking: 

1) Adjusting the chemical composition, ultra-low carbon is beneficial to improve the ability to resist stress corrosion, and the problem of the matching of the composition and the medium. 

2) Remove welding residual stress 

3) Electrochemical corrosion, regular inspection and timely repair, etc. 

11. To improve pitting resistance: On the one hand, the segregation of Cr and Mo must be reduced. On the other hand, the so-called "superalloyed" welding materials with higher Cr and Mo content than the base metal must be used. 

12. Austenitic stainless steel will produce hot cracks, stress corrosion cracks, welding deformation and intergranular corrosion during welding. 

13. Reasons for welding hot cracks of austenitic steel: 

1) Austenitic steel has low thermal conductivity, large linear expansion coefficient, and high tensile stress. 

2) Austenitic steel is easy to co-crystallize to form a weld structure with strong columnar crystals, which is conducive to segregation of harmful impurities 

3) The composition of austenitic steel alloy is more complex and easy to dissolve eutectic. 

14. Measures to prevent thermal cracks: 

① Strictly limit the content of P and S in the base metal and welding materials. 

②Try to make the weld form a duplex structure. 

③Control the chemical composition of the weld. 

④Small current welding. 

15. Attention should be paid to the selection of austenitic stainless steel: 

① Adhere to the "applicability principle". 

② Determine whether it is applicable according to the specific composition of each selected welding consumable. 

③ Consider the fusion ratio that may be caused by the welding method and process parameters of the specific application. 

④ Determine the degree of alloying according to the overall weldability requirements specified in the technical conditions. 

⑤ Pay attention to the weld metal alloy system, the role of specific alloy components in the alloy system, and consider the performance requirements and process weldability requirements. 

16. Analysis of weldability of ferritic stainless steel: 

1) Intergranular corrosion of welded joints. 

2) Embrittlement of welded joints, high temperature embrittlement, σ phase embrittlement, embrittlement at 475°C.

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