Effect of the quantity of nonmartensitic transformation products on the resistance to fracture of quenched and tempered structural steels
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After tempering to the same hardness (HV 300) the presence of nonmartensitic structures in the quenched steels has practically no effect on the standard mechanical properties (σb, σ0.2, δ, ψ) of quenched and tempered structural steels; there is a difference in the effect of these structures on the characteristics of resistance to fracture.
The even distribution of finely dispersed carbide phase in lower bainite after tempering increases the resistance to brittle fracture.
Upper bainite in quenched steels 18KhNMFA and 42KhMFA leads after tempering to the formation of large uneven branched carbide inclusions, which raises the ductile-brittle transition temperature and reduces the work of crack propagation.
The presence of austenite decomposition products in the upper range of critical temperatures, i.e., the ferritic-pearlitic component in structural steels quenched to martensite and bainite, leads to a reduction of the resistance to brittle fracture.
Structural steels hardened with continuous cooling may contain 50% lower bainite along with martensite, which does not impair the properties. This makes it possible to increase the critical section of machine parts, reduce the cooling capacity of the quenching medium, or use a less alloyed steel for the given application.
KeywordsCarbide Austenite Martensite Critical Temperature Bainite
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