NON-DESTRUCTIVE MAGNETIC TESTING OF THE PHYSICAL AND MECHANICAL PROPERTIES OF MISSION-CRITICAL MOUNTING COMPONENTS MADE OF 30HGSA STEEL

Abstract

The mounting products with mission-critical functions (bolts, studs, rods) used in aviation technology are subject to increased strength, hardness, and wear resistance requirements. They are made of specially developed medium-carbon steel 30KhGSA alloyed with chromium, manganese, and silicon. According to international classifications, the steel 30KhGSA is steel 42CrMo4-T (EN), 1.7225 (DIN, W.NR), 4140H (AISI). The balance between the strength and plastic properties of the products is achieved by the choice of the mode of their heat treatment – quenching and subsequent medium or high temperature tempering. Possible deviations in the chemical composition of the product materials and the modes of their heat treatment from the prescribed ones lead to unacceptable deviations of the properties of the products. This requires monitoring all products. The physical basis of magnetic structural analysis relies on the fact that mechanical and magnetic properties of steels are sensitive to the structural transformations occurring in them during thermal treatments. It has been proven that the coercive force Нc and the remanent magnetization Мr of many steels are related to their structure. When controlling mass batches of products, the best results in reliability and productivity are achieved by magnetization of products when they fall through a coil with the direct current and measuring the remanent magnetic flux Fd in the product. In this case, Fd in the product is not proportional to Mr, but to Hc of the product material. A characteristic feature of alloyed steels with a carbon content greater than 0.3 %, including steel 30KhGSA (steels 42CrMo4-T, 1.7225, 4140H, 37Cr4, 41Cr4, 46Cr2 and others), is a non-monotonic change in Hc with an increase in the tempering temperature Tо of products. Therefore, magnetic quality control of products from such steels requires a specific procedure. To test the tempering mode of such products, we have proposed to expose the magnetized products to a graded demagnetizing field with the strength Нp, before measuring Fd. The developed method allows us to choose the value of Hp, at which the optimum sensitivity to changes in Tо is achieved while monitoring products of specific sizes. In this report we show that such testing is based on the sensitivity of Fd in the product after its reverse magnetization in the field Hp to the remanent magnetization of the material Mr. We also article the technical means ensuring application of the developed methodology in industrial settings with a control output of up to 2 products per second. Examples of application of the method for controlling the hardness of small bolts made of steel 30HGSA, intended for use in aircraft construction, are given.