Boronizing Prosess And Performance Of Boride Layers On Cr12 Cold-work Tool Steel

Cold-work tool steels were commonly subjected to the harsh working condition which was higher stresses and longer friction. That led to failure earlier than service lives. The traditional heat treatment had been unable to satisfy the practical needs. Boronizing surface treatment had the advantages of low cost, simple devices and easy operation. So it was preferred to applied on Cr12 by power pack method. The objective was to obtain the lower brittleness, thicker and more uniform borided layers. Dies finally extended the service lives.At first, an optimum formula was researched. The most suitable boriding process was determined by orthogonal experimental method. And then the formula was generalized to other cold-work tool steels, for instance, Cr12MoV. Combined with Cr12, the performances of borided layers were investigated, such as Vickers microhardness, fracture toughness and wear resistance. The growth kinetics of the borided layers were estimated to take aim at controlling and predicting the layers thickness of Cr12 and Crl2MoV. It showed the diffusion rate constants of boron atoms, average diffusion activation energies and the iso-thickness diagrams of borided layers helping for actual production.The optimum formula was 5%B4C+20%Na2B4O7+65%Na3AlF6+10%C. The most suitable boriding process required according to actual production and Crl2MoV needed higher boriding temperature. The major phase in borided layer of Crl2 was Fe2B and the surface micro-hardness was 1714HV/0.2. The borided layer of Crl2MoV gave priority to FeB and it was 1400HV/0.2. The crevice structure among borided layer and the boundary of boride with shape of finger in transition zone were detected Cr7C3 phase, large carbon and a little borons. With the temperature increasing and holding time extending, fracture toughness decreased gradually. The fracture toughness of borided layers of Crl2MoV ranged from 2.97～6.43 MPa·m1/2. Under the same conditions, it was smaller than Cr12. Moreover, it had better wear resistance than Cr12 at room temperature. The coefficient of friction reduced as the testing temperature increasing. The average diffusion activation energies were calculated, QCr12=224.68kJ/mol, QCr12Mov=236.87kJ/mol.