Drill rod is a key component in rock drilling and blasting. Usually, its actual service life is only half an hour to more than ten hours. Prolonging the service life of drill rods has important practical significance for improving the efficiency of rock drilling operations and reducing production costs. There are many factors that affect the service life of drill rods. This paper shares its influence on the service life of drill rods through the surface decarburization of 55SiMnMo steel.

55SiMnMo steel has a high elastic limit and fatigue strength, and the strength and toughness of drill rod produced have a good match.

Decarburization is a phenomenon in which carbon atoms from the surface layer of steel diffuse and migrate to the steel surface under high-temperature conditions, resulting in the dispersion of carbon atoms within a certain range of the steel surface.

Decarburization of steel surface belongs to the oxidation diffusion process, that is, the carbon in the steel reacts with the oxygen in the medium to generate carbon monoxide and burn it or generate metal carbides. Due to the small diameter of carbon atoms, it is easy to carry out interstitial diffusion in metals, and the activation energy of carbon diffusion in iron is relatively small, which makes this interstitial diffusion proceed faster. Therefore, the decarburization susceptibility is stronger for medium-carbon steel, which has a high content of both carbon and silicon elements.

The fracture crack source region of drill rod is a low strength and low plasticity region with porcelain or mud-like fracture; there is a higher plastic area near its inner side, and the fracture SEM image is a dimpled fracture, and the proportion of dimples is much higher than the inner and crack growth zone of drill rod. Combined with the surface decarburization of drill rod, decarburization will reduce the carbon content of the surface layer of the material, resulting in a significant decrease in the strength of the material in the decarburization layer area, so that the stress generated in the cyclical and alternating stress service environment can easily exceed the strength of the material cracking due to the limit, this part of the cracking shows porcelain or mud-like fracture under the scanning electron microscope. The decarburization of steel surface, that is, the dissipation of carbon atoms gradually decreases with the increase of depth from the surface layer. There must be some areas with a carbon content higher than the surface full decarburization and lower than that of the matrix, and its strength will be lower than the matrix. Plasticity will be higher than the matrix, a higher proportion of dimples appear on the scanning electron microscope image of the artificial fracture, and this high plasticity zone will cause cracks to further expand into the drill rod matrix due to the increase in the use time of drill rod, resulting in drill rod failure.

In summary, the existence of decarburization on the surface of drill rods is not conducive to improving the service life of drill rods and should be avoided as much as possible.