Causes and preventive measures of embrittlement of 50B steel parts after quenching by high-frequency induction hardening equipment

An agricultural machinery part is made of 50B steel. It is found in production that 50B steel workpieces often appear embrittlement during production and use, that is, boron embrittlement, which causes the workpiece to fail. According to technical requirements, after the workpiece is quenched by high-frequency quenching equipment, boron-brittle workpieces appear. The workpiece is very brittle, resulting in brittle fracture of semi-finished or finished workpieces, which not only affects the production of agricultural machinery, but also causes considerable economic losses.

Metallographic examination found that network-like precipitates of different grain sizes and shapes are distributed along the grain boundaries in the brittle workpiece structure. The more precipitates, the more serious the network, and the greater the brittleness of the workpiece. The workpiece is quenched by 1050°C high-frequency quenching equipment and then cooled slowly, or isothermally treated at 900-650°C. The 50B workpieces with a mass fraction of B greater than 0.003% all have boron-containing phase nuclei precipitated at grain boundaries, and the precipitates are isolated and intermittently distributed, mostly in the form of small balls or strips, and thick dendrites appear in the boron high-enrichment area. The number and size of grain boundary precipitates are closely related to the boron content of the workpiece material and the cooling rate. The high boron content leads to slow cooling rate, many precipitates and large size, and the performance of high boron steel decreases obviously. Where the embrittlement phase is precipitated, the morphology of the boron brittle fracture is presented, and the macroscopic fracture becomes thicker correspondingly, and there are many bright spots in the distribution. After quenching by high-frequency induction hardening equipment, in addition to quasi-cleavage and intergranular fracture morphology, the micro-fracture characteristics sometimes appear near-cleavage fractures, and no second phase particles are found on the fractures, and the fractures are flat and smooth, showing several cracks.

In order to prevent the embrittlement failure of 50B steel parts and eliminate the hazards of boron steel grain boundary precipitation embrittlement, measures must be taken to limit the boron mass fraction of 50B steel to within the range of 0.001%-0.002%, and the highest boron mass fraction is not more than 0.003% to avoid And put an end to the occurrence of boron brittle defect hazards.