A. A. Lebedev and V. P. ShvetsUDC 620.178.15/179.119The authors discuss new experimental data on hardness of structural steels 25, 45, and 0N9, which were obtained for flat specimens subjected simultaneously to uniaxial tension, including the cases of intermittent unloading at various levels of residual strain. Based on numerous measurements (up to 30), mean values of hardness and their Gumbel scatter parameters have been determined for specimens in the loaded and unloaded states. The paper substantiates the procedure of determination of indices of sensitivity of measured hardness values and their scatter to loading conditions, which make it possible to refine the hardness characteristics as measured during technical diagnostics of equipment in operation.Keywords: structural steels, flat specimen, hardness, scatter of properties, homogeneity coefficient, stress, strain, unloading, sensitivity to stress level in material.Introduction. Mechanical properties of any material (substance) depend on its nature and structural state. Almost all of the currently available methods for their characterization involve testing of specimens under specified thermomechanical actions.Hardness is one of the comprehensive material characteristics, which can be found by fairly simple and easy-to-use test methods, including procedures for preparing special specimens.Classical hardness test methods amount to assessing the resistance of a material to penetration of a ball-, cone-, or pyramid-shaped indenter. Hardness is judged from the indentation dimensions or the indenter penetration depth [1, 2].Hardness of a material does not belong to strictly physically substantiated parameters of the material properties, such as characteristics of elasticity. Nevertheless, there have been numerous publications demonstrating a correlation between hardness and some mechanical characteristics of structural materials [3,4], confirming the possibility to plot tensile strain diagrams based on hardness measurements, and establishing a relation between the scatter of hardness values and the fracture toughness of metallic materials [5].The instability of the proposed correlations, which is observed in some case, is usually attributable to inadequate reliability of resultant hardness characteristics because of the lack of any strict specification of test conditions, including the mechanical stress level in a specimen to be tested. Improving reliability of assessment of the influence of intensity and type of stressed state in a material on the hardness measurements as well as the developing substantiated recommendations of how to monitor this influence for individual classes of materials have acquired great importance recently in the context of the ever increasing scope of hardness measurements performed on functional equipment, where operation-induced stresses may be rather high.The applicable regulations that govern the hardness measurement methods say nothing about any possible effect of the above factors, which in some case may lead to inconsistency of ...