A magnetic method for determining the concentration of residual austenite in maraging steels

Abstract

810Maraging steels are widely used in shipbuilding, the aviation industry, and rocket production. Their advantages are associated with high strength combined with sufficient plasticity and high corrosion resis tance, good weldability, and the property of hot and cold plastic deformation. The drawbacks of these steels are related to the fact that variations in the chemical composition within the steel grade (e.g., in C and Ti) can substantially change the mechanical properties in the aged state. In addition, the mechanical properties substantially depend on the concentration of residual austenite, its stability, and alloying [1,2]. Thus, the technological process is developed depending on the purpose of the items that are made of these steels, as a result of which the concentration of residual austenite in their microstructure is regulated within a rather narrow range; thus, the optimal combination of mechanical properties is reached. As the result, it becomes necessary to develop nondestructive methods for evaluating the concentrations of resid ual austenite in the microstructures of ready made items.As shown by Apaev [3], the most suitable parameter for controlling the concentration of residual aus tenite is saturation magnetization. For example, specialists of the Institute of Metal Physics (IMP) devel oped a differential magnetic device that was designed for determining the concentration of residual austenite based on the measurements of the magnetization close to saturation in hardened and tempered items from ShKh15 steel [4]. However, when using this method for evaluating the concentration of residual austenite in martensite aging steels, they found a substantial spread in the obtained magnetization values [5].The saturation magnetization M s is known to be a characteristic that is sensitive to phase and chemical steel composition; thus, the obtained spread in the obtained saturation magnetization values can be explained by two factors. First, depending on the type of thermal treatment, the time of exposure at the specified temperatures, and heating and cooling rates, the martensite and the residual austenite in steels of such grade will differ in the degree of their alloying. For example, the concentration of nickel in residual austenite may substantially exceed that in the chemical composition of steel. Second, precipitation of par ticles of the second phase (intermetallic compounds, etc.) is accompanied by depletion of martensite with iron and nickel atoms, which should yield a decrease in magnetization. The testing reliability in this case can be increased by selecting a corresponding reference. This is a very complicated problem [3] since at the assigned concentration of residual austenite, the saturation magnetization of the martensite phase of the reference and tested objects should have similar values.Kifer [6] proposed using the internal demagnetization coefficient N i for studying the porosities of pow dered materials. Since the particles of residual paramagnetic austenite can be considered as pore...