Rock bolt rods are usually made of high-quality alloy steel. The most commonly used materials are medium-carbon alloy structural steels such as 40Cr , 35CrMo and 42CrMo with good hardenability. To prevent corrosion, rock bolt body should be blue or blackened.
Technical Requirements for Heat Treatment of Rock Bolts
40Cr , 35CrMo , 42CrMo and other materials after quenching, hardness ≥ 50HRC , 95% of the core is medium-sized martensite structure; after tempering is 32 ~ 38HRC , tensile strength is 95kgf / mm2 (1kgf / mm2 = 9. 8MPa) , yield strength ≥ 80kgf / mm2 , elongation ≥ 10% , reduction of area ≥ 42% , impact toughness ≥ 6 ~ 7kgf / mm2 . The heart is a uniform tempered sorbite, allowing a small amount of free ferrite (1 to 3 qualified).
In order to ensure heat treatment quality of rock bolts, rock bolts after hot-punching hexagon should be spheroidized and annealed before final quenching. The annealing process specification is: 680 °C × 3h, then temperature is raised to 770 °C × 2h , and furnace is cooled to 550 ~ 580 °C × 2h, it is air-cooled and has a hardness of HB170 to 207 to obtain a spherical pearlite structure. Then prepare for subsequent quenching and tempering treatment.
Heat Treatment Process Details of Rock Bolts
In order to meet the above heat treatment technical requirements of rock bolt, heat treatment process should be in a salt bath furnace or in a continuous controlled atmosphere furnace (mesh belt), and heating temperature is selected from a high quenching temperature of 860 to 870 °C. This process is to make spherical pearlite completely transform into austenite structure, and fully exert beneficial effect of carbon and chromium to increase hardening. Holding time in a salt bath of 15 ~ 20min , then quench 20 is ~ 60 deg.] C to 10 ~ 30 for No. machine oil, quenching hardness should be ≥ 50HRC . The tempering is water-cooled in a pit furnace or a salt bath at 520 to 550 °C for 90 minutes , which can effectively avoid the occurrence of secondary temper brittleness, obtain tempered sorbite structure, and the hardness after quenching and tempering treatment is in HRC30. Within range of ~ 38 , the requirements for mechanical properties are ensured. If continuous mesh belt furnace is used, computer controls atmosphere, temperature and process parameters. With highly mechanized and automated equipment, and final product quality is stable. It is preferred heat treatment equipment for standard parts industry at home and abroad.
It should be noted that in order to avoid obtaining a coarse quenched martensite structure, quenching temperature should not exceed 870 °C, otherwise brittleness will increase, and even after high temperature tempering, it will cause fatal defects of fracture during installation operation. Quenching cooling medium uses oil instead of water, which is considered to prevent quenching of hexagonal head while still being hardened. Surface decarburization is one reason for short service life of connecting rock bolt. Therefore, atmosphere or deoxidation in heating furnace should be strictly controlled.
During quenching and tempering treatment of connecting rod bolt, defects of metallographic structure mainly include surface decarburization, excessive free ferrite or excessive undissolved carbide and overheating. These defects will affect rock bolt performance.