Medium carbon steel refers to carbon steel with carbon content of 0.25% – 0.60%, including 30, 35, 45, 50, 55 in high-quality carbon structure and zg230-450, zg270-500, ZG310-570 and zg340-640 in cast carbon steel. Because the carbon content of medium carbon steel is higher than that of low carbon steel, its weldability is worse than that of low carbon steel. When the mass fraction of carbon is close to 0.30% and the manganese content is not high, the weldability is good, but with the increase of carbon content, the weldability gradually becomes worse. When the mass fraction of carbon is about 0.50%, the weldability becomes worse.
The problems easily occurred in Carbon steel butt welded fitting are as follows:
1. Cold crack problem
Due to the high carbon content of steel, the HAZ is easy to produce hard and brittle martensite structure during welding, so it is easy to produce cold cracks. If the welding materials are not selected properly or the welding process is not formulated properly, the weld is easy to produce cold cracks.
2. Hot crack problem
When welding, the base metal with higher carbon content will also bring carbon into the weld after melting, which will increase the carbon content in welding. Carbon can aggravate the role of sulfur and phosphorus in the metal, which can lead to heat cracks, so it is easy to produce hot cracks when welding medium carbon steel. Especially when the content of sulfur and phosphorus in base metal or welding material is not strictly controlled, hot cracks are more likely to occur.
In addition, the high content of carbon in the steel also increases the tendency of CO porosity in the welding seam.
