Hollow hexagonal tapered drill rod belongs to the category of small-section drill rods. The more common ones internationally are B19, B22, B25, and B28, of which 22mm is the most widely used and 25mm is the second. Therefore, this article mainly shares their common steel grades, basic requirements, working conditions, and usage characteristics with B22 and B25 hexagonal drill rods.

Common steel grades of B22 and B25 hexagonal drill rods

B22 and B25 hexagonal drill rods include tapered and integral drill rods. After a lot of tests, research, production, and use, 55SiMnMo steel has been proven to be better steel.

55SiMnMo steel is a bainite steel. When the rod hardness is HRC35~38 in normalized or hot-rolled conditions, and the metallographic organization is granular bainite or slatted bainite and carbon-rich residual austenite. 55SiMnMo steel’s normalized organization (or hot-rolled condition) has a great relationship with the chemical composition and cooling rate of the steel. The hardness of the rod in the normalizing state (or hot rolling state) is also closely related to the chemical composition and cooling rate of the steel. The amount of residual austenite in the metallographic organization and whether there is needle-like bainite or martensite or too slow cooling speed pearlite, or even ferrite have a significant impact on the service life of the drill rod. The 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 a high rock drilling service life.

Basic requirements for B22 and B25 hexagonal drill rods

1. High fatigue strength and good toughness.

2. Low notch sensitivity and low fatigue crack growth rate.

3. The rod must have good elasticity, and the shank adapter will not pile up or explode under the impact of the rock drill piston.

Working conditions and failure characteristics of B22 and B25 hexagonal drill rods

B22 hexagonal drill rods work under the condition of longitudinal stress and corrosion under high-frequency impact. Under such working conditions, the normal failure of the drill rod is fatigue fracture failure. When there are defects on the outer surface of the drill rod, under high-frequency stress, some defects cause stress concentration, fatigue cracking, and expansion, and then external fatigue fracture occurs. Internal fatigue fracture occurs when there are no defects on the outer surface of the drill rod or when it has been surface strengthened.