1. Common steel types and basic requirements for B22 and B25 hexagonal drill rods B22 and B25 hexagonal drill rods include tapered connections and integral drill rods. Most users of drill rods in my country use tapered connection drill rods. my country's steel for tapered connection drill rods has been proven through nearly 40 years of extensive testing, research, production and use that 55SiMnMo steel is a relatively excellent type of drill steel that is unique and in line with my country's resources. 55SiMnMo steel is a bainite steel. In the normalized state or hot rolling state (controlled rolling and controlled cooling), the rod body hardness is HRC35~38, and it has high elasticity and fatigue strength. The metallographic structure is granular bainite or lath. Bainite (both upper bainite) and carbon-rich retained austenite.
The normalized structure (or hot-rolled state) of 55SiMnMo steel has a great relationship with the chemical composition and cooling rate of the steel. The hardness of the rod in the normalized state (or hot-rolled state) is also closely related to the chemical composition and cooling rate of the steel. The amount of retained austenite in the metallographic structure and whether acicular bainite or martensite appears or the cooling rate is too slow to produce pearlite or even ferrite have a significant impact on the service life of the drill rod. When the rod hardness is controlled at HRC35~38, the microhardness of bainite in the microstructure is around HV420, while the microhardness of high-carbon retained austenite is as high as around HV750. Practice has proved that: when martensite, lower bainite, pearlite, and ferrite do not appear in the microstructure, the amount of upper bainite + retained austenite is in the range of 25% to 35%, and the drill rod Has high rock drilling life.
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