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Effect of molybdenum on the fracture characteristic of high‐speed steel quenched matrix
Author(s) -
Pacyna Jerzy,
Kędzierski Zbigniew,
Kusiński Kazimierz
Publication year - 1985
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.198500605
Subject(s) - tempering , materials science , molybdenum , metallurgy , tungsten , transgranular fracture , carbide , precipitation , alloy , intergranular fracture , quenching (fluorescence) , martensite , high speed steel , intergranular corrosion , fracture toughness , microstructure , meteorology , physics , quantum mechanics , fluorescence
The fractures of three model alloys, imitating by their chemical composition the matrixes of the quenched high‐speed steels of various Mo: W relations were analyzed. According to the measurements of the stress intensity factor K Ic and the differences in the precipitation processes of carbides it was found out that the higher fracture toughness of the matrix of the molybdenum high‐speed steels than on the tungsten ones is the results of the differences in the kinetics of precipitation from the martensite matrix of these steels during tempering. After tempering at 250 and 650°C the percentage of the intergranular fracture increases with the increase of the relation of Mo to W in the model alloys of the high‐speed steel matrix. This is probably the result of higher precipitation rate of the M 3 C carbide (at 250°C) and the MC and M 6 C carbides (at 650°C) in the privileged regions along the grain boundaries. The change of the character of the model alloy fractures after tempering at 450°C from the completely transgranular one in the tungsten alloy to the nearly completely intergranular one in the molybdenum alloy indicates that the coherent precipitation processes responsible for the secondary hardness effect in the tungsten matrix begin at a lower temperature than in the molybdenum matrix. After tempering for the maximum secondary hardness the matrix fractures of the high‐speed steels reveal a transgranular character regardless the relation of Mo to W. The higher fracture toughness of the Mo matrix can be the result of the start of the coherent precipitation processes at a higher temperature and their intensity which can, respectively, influence the size of these precipitations, their shape and the degree of dispersion. The transgranular character of the fractures of the S 6‐5‐2 type high‐speed steel in the whole range tempering temperatures results from the presence of the undissolved carbides which while cracking in the region of stress concentration can constitute flaws of critical size which form the path of easy cracking through the grains. The transgranular cracking of the matrix of the real high‐speed steels does not change the adventageous influence of molybdenum upon their fracture toughness. On the other hand, the carbides, undissolved during austenitizing, whose size distribution in the molybdenum steels from the point of view of cracking mechanics seems to be unsatisfactory, influence significantly the fracture toughness of these steels.