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The Chemical Composition And Heat Treatment of Vermicular Graphite Cast Iron

Vermicular graphite cast iron is a new type of cast iron material that has developed rapidly in recent years. Because most of its graphite is worm-like with a small amount of spheroids in between, its structure and performance are between spheroidal graphite cast iron and gray cast iron, and it has good comprehensive properties. In addition, the casting performance of vermicular graphite cast iron is better than that of nodular cast iron, close to gray cast iron, and has better heat resistance. Therefore, castings with complex shapes or parts that work at high temperatures can be made of vermicular graphite cast iron.

Vermicular graphite cast iron is produced by adding a certain amount of vermicularizing agent to molten iron and incubating. There are many types of creeping agents, and rare earth ferrosilicon alloys such as SiFeRE21 and SiFeRE27 are widely used in my country.

In the production of vermicular graphite cast iron, the composition near the eutectic is used to help improve the casting performance, generally containing wc=3.0%~4.0%, the upper limit is taken for thin parts to avoid white mouth, and the lower limit is taken for thick parts to avoid production Graphite floats. Containing WSi=1. 4%~2.4%, mainly used to prevent white mouth and control the matrix. With the increase of silicon content, the amount of pearlite in the matrix is relatively reduced, and the amount of ferrite increases. At the same time, silicon has the effect of strengthening ferrite. Manganese plays a role in stabilizing pearlite in vermicular graphite cast iron. The production of mixed matrix vermicular graphite cast iron can adjust manganese. If it is required to obtain ferrite matrix vermicular graphite cast iron with good toughness, then manganese WMn<0.4%; To obtain pearlite vermicular graphite cast iron with higher strength and hardness, manganese must be increased to WMn=2.5%. Phosphorus is generally controlled at Wp=0.08%, and phosphorus can be increased to wp=0.2%~0.35% for wear-resistant parts. Sulfur has a strong affinity with the creeping element, which will weaken the effect of the creeping agent. Therefore, the sulfur content WS<0.03% is required.

The heat treatment of vermicular graphite cast iron is mainly to adjust its matrix structure to obtain different mechanical properties.

(1) Normalizing of vermicular graphite cast iron. Vermicular graphite cast iron can increase the amount of pearlite in the as-cast state, and its matrix contains a large amount of ferrite. Through normalizing, the amount of pearlite can be increased to improve strength and wear resistance.

(2) Annealing of vermicular graphite cast iron. The purpose of annealing is to obtain more than 85% of the ferrite matrix, or to eliminate the free cementite at the thin wall.