| The author studied, according to each heat-treatment structure, the effects which the amount of nickel gives to the toughness in the low-alloy steel of tough-hardening steel of Ni-Cr-Mo. For the comparison of toughness, each fracture toughness was compared with one another, using side notched test-pieces with pre-crack from fatigue and the fracture surfaces were observed. The important results obtained are as follows. 1. The amount of nickel has an effect to arrest the propagation of fracture-crack, independent upon the heat-treatment structure. 2. Intermediate structure that consists mainly of bainite is inferior to tempered martensitic structure in mechanical quality both of strength and toughness. 3. When low alloy steel of Ni-Cr-Mo undergoes isothermal transformation treatment for long hours, its toughness becomes inferior, however continuous cooling transformation structure becomes tough and its mechanical quality superior. |
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