APPLICATION OF THE ULTRASONIC METHOD OF DIAGNOSTICS FOR 38KHN3MFA STEEL CRACK RESISTANCE ASSESSMENT

This paper describes a promising technique of nondestructive expert quantitative assessment of critical stress intensity factor values (K_1c) for 38KhN3MFA structural steel subjected to various modes of heat treatment. This method is based on ultrasonic scanning and mechanical tests of heat-strengthened samples of 38KhN3MFA steel after quenching from 850°C and tempering in a wide range of temperatures: 200, 400, 500, 580, 620°C. As a result, new correlations of considerable scientific interest were obtained between the velocity of longitudinal and transverse elastic waves and the mechanical properties of steel, including the values of critical stress intensity factor for structures. The proposed model, which explains the change in acoustic properties of 38KhN3MFA steel on the basis of the phase changes taking place in the structure during tempering, will allow similar studies to be carried out for other heat treatment modes and other steel grades. In this method, the coefficient K_1c is determined without breaking the integrity of structures. Deviation of the calculated critical stress intensity factors, derived using the acoustic measurement data, from the experimental values does not exceed 7.5%. The use of this method significantly reduces the amount of time and labor needed to determine mechanical characteristics of samples and finished products made of 38KhN3MFA steel since it will not be necessary to manufacture samples and carry out tests. The developed technique can be offered for introduction in manufacturing as the main or additional method of assessing mechanical parameters of materials subjected to various modes of heat treatment.

nondestructive control, acoustic parameters, crack resistance, heat treatment, martensite, temper, mechanical properties, phase transformations

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