Influence of nonmetallic inclusions on the fracture resistance of steel under fatigue dynamic compression conditions

Impact machines are one of the most productive types of equipment for the rock destruction. They provide high drilling speed but generally have low durability and reliability. Modern researches show that the process of mining machines parts fracture often is initiated by nonmetallic inclusions and internal defects in the structure of steel. However, nowadays the question concerning the degree of the influence of the nonmetallic inclusions morphology, size, shape and distribution on the steel fatigue failure resistance is still not clear. The traditional approach to solving the problem of the inclusions influence on steel properties requires the preparation of a significant number of experimental samples, so mathematical modeling is a promising method for assessing the influence of nonmetallic inclusions. The purpose of the research was to estimate the stresses intensity field near nonmetallic inclusions under the influence of cyclic compressive stresses and to develop recommendations for the usage of steels with various impurities for the impact machines parts manufacture. The effect of nonmetallic inclusions under dynamic loading conditions was evaluated in several stages. At the initial stage, using the methods of metallographic and microspectral analysis, the size, shape and morphology of nonmetallic inclusions in ten melt of 45 steel were determined. The obtained results were used as input data for the software package ANSYS Workbench 15.0 for calculating the stress field intensity the near inclusions. The reliability of the obtained mathematical models was checked by measuring the internal stresses by the X-ray diffraction analysis with ARL X'TRA diffractometer. Metallographic studies have shown that the main types of inclusions in the steel are point oxides, whose size can reach up to 100 microns and plastic sulphides up to 1.mm in length. The results of numerical simulation show that under dynamic compression conditions an application of the approach in which the inclusion is assumed to be mechanically equivalent to the pore or crack is incorrect. Under the dynamic compression condition near the pores and low-strength inclusions in the base material regions with a high level of local stresses are formed. The maximum value of the equivalent stresses was revealed near the elongated defects in the case of their orientation perpendicular to the action of the external force. Stress distribution near hard and strong oxides inclusions is significantly different. The maximum stresses are localized in the inclusion, while the stress magnitude is insignificant in the boundary region between the inclusion and base material. To verify the reliability of the data of the numerical experiment, XRD analysis was used for zonal stresses measuring. The results of X-ray diffractometry confirm the assumption that under compression conditions the presence of pores in the steel and plastic low-strength inclusions leads to a 50 % increase of local stresses of the main material near the defects. The stress level near the solid high-strength inclusions in the base material is 20 % lower than the average level. The results of the research show that it is desirable to make the details of impact machines using the steels whose structure contains inclusions which hardness exceeds the hardness of the matrix or apply high-purity steels with a low content of nonmetallic inclusions.

steel, impact machines, dynamic compression, nonmetallic inclusions, X-ray diffraction