When welded steel pipe is in the process of welding, there is a big potential threat which is called hot cracking that occurs immediately upon completion of the weld. Because of its larger grain structure, this material is particularly susceptible to the defect. Right filler metal, as well as proper heat input and post-weld cooling, is vital to cracking prevention and corrosion resistance during the welding process.
Welded steel pipe should be matched with right filler metals based on their chemical and mechanical features. Luckily, it is easier for this kind pipe to make this match than many other materials. The grade of welded steel pipe that will be used for application can be affected by the intended service condition, and the grade of material has a great influence on the filler metal selection. For example, it is better for welded steel pipe with 304L to choose filler metal with American Welding Society, but 316 welded steel pipes should be matched with a 316L filler metal. Sometimes, we can also have to weld steel pipe to carbon steel, so a 309 grade filler metal is proper.
The final weld of the welded steel pipe can be integrated because the filler metals contain alloy. In addition, we usually prevent the pipe from corrosion by chromium and molybdenum. Molybdenum can also be useful for good high temperature and if weld strength needs to be increased, the nickel may be useful. The addition is not only used to maintain weld poop fluidity, but also stabilize the amount of austenite in the weld, which will in turn be helpful for refining the grain structure of the weld and crack resistance.
One kind of stainless steels which is known as the ferrite number is popular among engineers and designers. The ferrite number can be applied to measure the amount of ferrite versus austenite in the structure of the material. The ability of welded steel pipe to resist cracking and corrosion can be indicated by the ferrite number. Generally, we use the chemistry of the material to determine the amount of ferrite present. When welding steel, it usually needs to reach a maximum ferrite number, but some special applications may require higher ferrite numbers.