Investigation of abrasive wear after combined hardening by deformation wave and heat treatment

. The article describes a new method of combined hardening, using sequential processing by wave strain hardening (WSH) and heat treatment (HT). The possibility of increasing the hardness of the surface layer by the proposed method while maintaining the impact strength of the material at an acceptable level has been established using the example of steel 65G. The results of studies of operational properties (abrasive wear resistance) of samples after hardening by combined technology are presented. Technological recommendations are given for the choice of hardening modes, which makes it possible to increase the mechanical properties of the material and reduce the abrasive wear of samples made of 65G steel. The proposed technology can be applied to the processing of heavily loaded machine parts operating in aggressive conditions and significant shock loads.


Introduction
One of the most common types of wear, due to which the failure of machines and mechanisms is abrasive.It is typical for parts of mining equipment, transport, road construction, agricultural machines that work in dusty conditions.
An increase in the abrasive wear resistance of parts made of metallic materials is directly related to an increase in their hardness, which is achieved by various methods of hardening treatment.The most common way to increase the hardness of metallic materials is hardening by heat treatment (HT).Recently, combined methods of hardening treatment have proved to be quite effective, especially combining the different nature of physical impact on the metal material being strengthened [1][2][3][4][5].Thus, the use of preliminary hardening by surface plastic deformation (SPD), in particular, wave strain hardening (WSH), before HT, makes it possible to further increase the hardness of the metal material being hardened with a slight loss of impact strength [6].
The use of WSH in combined hardening is due to the possibility of creating a deep hardened surface layer with a riveted structure.Technological features of the WSH are the impact on the hardened surface by controlled deformation waves formed in the acoustic system of the striker-waveguide.During hardening, the striker hits the waveguide statically pressed against the hardened surface, as a result, plane acoustic waves are generated in the shock striker-waveguide system, which are characterized by the law of force change (strain wave amplitude) in time, the maximum value of forces, the time of action of forces (strain wave duration) and the strain wave energy.The period of the deformation wave is the shock impulse.During hardening, the shape of shock impulses is maximally adapted to the properties of the material and loading conditions, which increases the efficiency of the process, making it possible to create a hardened layer up to 6-10 mm [7].The hardened surface after the WSH consists of a set of plastic prints, which are formed from shock impulses applied at a given frequency and displacement.
An important feature of the WSH technology is that it allows one to accurately control the uniformity of hardening, creating both a uniformly and heterogeneously hardened structure that combines high hardness and plasticity at the same time [8].Uniformity regulation is achieved due to the overlap of plastic prints obtained as a result of the action of shock impulses on the material, which is estimated through the overlap coefficient where δ -size of the plastic print, mm, s -feed of hardening material, mm/min; ffrequency of shock impulses, Hz.
If K = 0, then the edge of one print borders on the edge of another; if 0 < K < 1, then the prints overlap; at K = 1, the tool is repeatedly pressed into the same place.
The technological parameters of the WSH are: -impact energy; -static force of compression of the waveguide with the tool to the hardened surface; -coefficient of plastic imprints overlapping K; -number of passes.

Methods
The technology of combined hardening WSH + HT is as follows: the pre-hardened surface of the workpiece is subjected to WSH, forming a work-hardened surface layer.Then the workpiece is subjected to quenching followed by tempering.Since it has been established that the combined hardening of WSH + HT of metal materials makes it possible to increase the hardness relative to hardening of HT only, it is necessary to establish the possibilities of such treatment to increase abrasive wear resistance.
The studies were carried out on samples made of steel 65G.Steel 65G is used for the production of spring washers and springs for non-critical purposes, critical parts of material handling equipment, as well as blade knives for bulldozers and motor graders.Since the impact strength of 65G hardened steel is low after hardening, products made from it, especially those experiencing abrasive wear and shock loads at the same time, must undergo high tempering at temperatures of 550-600 ° C, with cooling in still air.
As a result of the research, flat samples were made from steel 65G, and then they were subjected to hardening TO and combined treatment of WSH + HT.After that, Vickers hardness, impact strength and abrasive wear were measured on them.
Preliminary studies have established the technological parameters of WSH + HT, which provide an increase in the degree of hardening of the surface layer.The degree of hardening after combined treatment with WSH + HT was determined as: where HV is the hardness after hardening by combined treatment with WSH + HT, MPa; HV0 -hardness after hardening only HT, MPa.
For steel 65G, when using a preliminary WSH before HT, the highest average degree of hardening was ΔHVav = 8% and was obtained under the following modes: WSH -K = 0.4 and the number of passes 2; HT -tempering 550°С (Fig. 1).Under these modes, there is a decrease in impact strength by 14% compared to HT alone.By analogy with the installation described in [9][10], tests of abrasive wear were carried out on the basis of a planing machine, where the samples performed reciprocating motion on the sanding paper.For this, samples of a prismatic shape were made with an area of the working part of 10 x 10 mm, which were fixed in a special holder installed in the tool holder of the machine.Grinding paper was fixed on the machine table.The friction path was 36.8 m.The normal load acting on the sample was 63 N. In order to minimize the friction of the test surface of the sample on the trace left from the previous stroke, which can introduce a significant error in the test results, the amount of transverse displacement of the grinding paper in one double stroke sample was maximum for this machine and was 5 mm.Wear was determined by weighing the samples before and after testing.
With combined hardening, the WSH was carried out in the following modes: -impact energy 150 J; -static force of compression of the waveguide with the tool to the hardened surface 40 kN; -coefficient of plastic imprints overlapping 0.4; 0.6; 0.8; -number of passes 2.
A rod roller with a diameter of 10 mm and a width of 40 mm was used as a tool.
HT was carried out under the following conditions: quenching 810°С (oil), tempering 550°С (air).As a result, with hardening only HT, the hardness of the samples was 3680 MPa, the impact strength was 56.8 J/cm2, and the wear was 0.59 grams (Fig. 2).With combined hardening of WSH + HT, the hardness of the samples was increased to 3770-3960 MPa, but at the same time, the impact strength decreased to 46.6-48.7 J/cm2, and the wear decreased to 0.50-0.54grams, which is 8.5-15.3%less relative to specimens hardened only by HT.The highest hardness was obtained in the modes of combined hardening WSH + HT, when the WSH was carried out with an overlap coefficient of 0.4, and the highest wear resistance was obtained in modes with an overlap coefficient of 0.8.

Conclusion
It has been established that the development of combined hardening technologies combining the impact on the hardened material of different physical nature, such as strain hardening and hardening by heat treatment, is promising.The use of combined hardening WSH + HT makes it possible to increase the strength and performance characteristics of metallic materials, in particular, the wear resistance of specimens made of 65G steel up to 15.3% relative to specimens hardened only by HT.

Fig. 1 .
Fig. 1.The distribution of the degree of hardening in depth of samples from steel 65G after combined treatment with WSH + HT (tempering 550°С).

Fig. 2 .
Fig. 2. Hardness and wear of samples from steel 65G after hardening by HT and combined treatment with WSH + HT at different coefficients of plastic indentation overlap K.