Work is devoted to hard alloy breaking by thermal shock for following crushing. This scientific work presents calculation of stress which has appeared on hard alloy WC-8%Co surface during quenching in liquid is presented in this work. There is the histogram of hard alloy WC-8%Co sample flexural strength distribution before and after hardening in a temperature of 1400 °C in 10% NaCl solution water. Microstructure and hardness of destroyed and embrittled samples during the heat treatment are researched. Portion of samples which has been broken by again heat treatment is determined.