Effects of Cooling Parameter and Cryogenic Treatment on Microstructure and Fracture Toughness of AISI D2 Tool Steel

The application of cryogenic treatments is gradually increasing in tool steels. In this sense, these heat treatments can modify the mechanical properties of high chromium alloy steels. However, the influence of the cooling parameter (during quenching) is often neglected. This investigation aims at evaluating the effects of cooling parameter and cryogenic treatment on microstructure and fracture toughness of AISI D2 tool steel. The cooling parameter (lambda) was defined as the cooling time from 800 to 500 degrees C divided by 100. During quenching, different cooling parameters (lambda 1 = 2.8 and lambda 2 = 1.8) were employed, and the samples were tempered three times or, besides, cryogenically treated by a single tempering. Later, the samples were evaluated through phase analyses and their proportions, sizes, and distribution of carbides by optical and scanning electronic microscopy, hardness measurements, x-ray diffraction, and fracture toughness tests. For the lambda 2 cooling parameter followed by cryogenic treatment, the results presented an increase of 51% over the initial condition of secondary carbides. However, the higher hardness values were found in the lambda 1 cooling parameter followed by cryogenic treatment. In the fracture toughness evaluation, a reduction of 8% in K-IC was observed for samples with cryogenic cycles compared to the samples without it.