After the material is selected, the heat treatment of threaded rod is the key to improving and ensuring the life of the drill rod.
1. foreign heat treatment process
Most of the threaded rod abroad choose 25CrNi3Mo steel and adopt the whole gas carburizing process. In this process, a deep well type gas carburizing furnace has a large investment. The thickness of the carburizing layer, the surface carbon concentration, the carburizing temperature, the carburizing time, the carburizing atmosphere in the furnace and the cooling process are all controlled by the microcomputer, so the ideal quality and performance can be guaranteed. After carburizing, the two threads are re quenched at medium frequency and tempered at low temperature to obtain high wear resistance of threaded parts. The hardness of the rod is controlled by cooling after carburizing.
Some foreign manufacturers also choose medium and low carbon CrNiMo series steel. After quenching and tempering, the two ends are subjected to high frequency quenching. The hardness of the rod is obtained by controlled rolling during the production of hollow steel. The hardened layer thickness of threaded parts is usually controlled below 2.5 mm.
The characteristics of the foreign heat treatment process are that both carburizing and high frequency quenching can make the thread surface of the drill rod get high hardness and wear resistance, and also obtain high surface compression stress. The surface compressive stress, the thin film like austenite structure in the lath martensite and the slab can improve the surface fatigue strength, the threshold value of fatigue fracture, and reduce the propagation rate of fatigue crack, which is very favorable for improving the life of the screw rod.
2. heat treatment process and performance characteristics of domestic threaded rod
In China, the 35SiMnMoV rod and gas carburizing process are used in most of the 32 - 1200 mm connecting rod. The domestic 105 kW gas carburizing furnace can handle 1200 mm long rods. Due to the high carbon content of 35SiMnMoV steel and the low temperature tempering of carburizing and quenching, the toughness and toughness of the drill rod are low and the life expectancy is not high. Carburizing and isothermal tempering can improve the toughness and toughness of the core, but the surface hardness of the infiltrating layer is not high, and the wear resistance of the thread is reduced. Therefore, only choose alloy hollow steel containing 0.23% carbon content, it is possible to improve the life of short threaded rod.
The screw rod of B28, B32, B35, D38, D45 and D51 is more than 1200 mm in length. If there is no deep well type gas carburizing furnace, gas carburizing and quenching low temperature tempering process can not be used. Some manufacturers have tried partial carburizing instead of carburizing, such as liquid carburizing. However, liquid carburization has the defects of poor mobility in salt bath and slagging on the surface of salt bath, which makes it difficult to carry out the process in mass production. Therefore, 35SiMnMoV, Z708 or 30CrNi3Mo steel are used most of the series of brazing rods made at present, and no large deep well carburizing furnace can only be quenched, middle and low temperature tempering or isothermal quenching. This kind of process is only quenched without carburizing because of the high carbon content of steel. The surface hardness is not high and the wear resistance of threaded parts is poor. So the life of the rod has not been high. Because of the poor equipment matching of the deep well gas carburizing furnace, the bending of the whole carburizing is difficult to straighten after the whole carburizing. Therefore, the heat treatment level of the screw rod in China still has a big gap compared with the foreign advanced level.