This paper presents the results of equal channel angular pressing (ECAP) and subsequent heat treatment (HT) as a method to improve the wear resistance of metallic materials in friction sliding. The effect of ECAP and HT on the microstructure and mechanical properties of low carbon steel is investigated in this work. The mechanisms of wear resistance of steel with ultrafine and nanostructures produced by equal-channel angular pressing is analyzed. The results show that ECAP at room temperature and annealing at 350° C and 450° C can be used as a technology of reducing wear in friction sliding.
Severe plastic deformation (SPD) by equal channel angular pressing (ECAP) significantly affects the structure and properties of the material [
Objective is to study the tribological properties of low carbon steel with ultra and nanostructures formed by ECAP and low-temperature annealing.
The studies were conducted on widely used in Russia steel 09G2S. Chemical composition of 09G2S is: Fe-0, 09C-0,64Si-1,26Mn-0,007P-0,003S-0,08Cr-0,1Ni-0,02Al-0,14Cu-0,002V-0,01Nb-0,013Ti. Samples of 09G2S with the diameter of 20 mm and the length of 100 mm were pressed for ECAP at 20˚C. ECAP was held by two press cycles on the route Bc (90˚ turn after each compression) with the angle of intersection of channels Φ = 90˚; after ECAP samples were subjected to heat treatment (HT): short-term low temperature annealing at 350˚C and 450˚C delayed for 1 hour [
Microstructure studies were conducted using a scanning electron microscope JEOL JSM-6480LV. Tribological tests were performed on a friction machine with the contact area 5 × 5 mm. Tests of dry sliding friction was carried out on the steel plate with the hardness of 50 - 52 HRC, friction path―560 m, load―150, 225, 300 and 375 N [
The microstructure of the starting material was ferritic-pearlitic, ferrite grain size ranges between 4.35 µm (the middle diameter −10 µm). The previous studies [
Mechanical properties of tensile specimens are shown in
Tribological properties of ferritic-pearlitic steel is determined by a complex of physical and mechanical charac-
Processing Options | Mechanical properties | ||
---|---|---|---|
σT, MPa | σB, MPa | δ, % | |
Initial state | 325 | 470 | 21 |
ECAP at 20˚C, without HT | 930 | 1300 | 4 |
ECAP at 20˚C, annealing at 350˚C | 985 | 1400 | 3 |
ECAP at 20˚C, annealing at 450˚C | 850 | 1195 | 4 |
teristics depending on the number and dispersion of structurally free ferrite, the size and shape of the carbide particles, the deformation behavior, resistance emergence of micro cracks. To improve wear resistance, we need to apply chemical, thermal and mechanical processing, which cause growth of hardness, optimal distribution of dispersed particles of the carbide phase, microalloying, etc.
At a load of 375 N massive deterioration in air samples is reduced as a consequence of changes in the regime of friction, which increases the ability of a material to resist cracking and the formation of wear particles, but in the sample after ECAP and annealing at 450˚C there was a single increase in depreciation to the value of 260 mg, which corresponds to changing the nature of friction from microcutting to adhesive seizure. During the tests in the areas of actual contact under the load, softer pattern is strengthened by increase of density of dislocations and the formation of secondary fragmented structure of the surface layer. Further, the plastic deformation causes mutual reinforcement of the friction pair materials and change in roughness of contact surfaces. This continues until the friction operating voltages decreasing with the growth of real contact area, will not be comparable to the yield strength of the materials of the friction pair. At this point, there comes a stage of steady deterioration, characterized by the equilibrium roughness and stabilization structures: there takes place dynamic equilibrium
The test load, N | Mass loss of samples, Δm, mg | |||
---|---|---|---|---|
Initial state | ECAP | ECAP HT at 350˚С | ECAP HT at 450˚С | |
150 | 7 | 7 | 10 | 6 |
225 | 8 | 5 | 6 | 5 |
300 | 360 | 140 | 42 | 3 |
375 | 260 | 2 | 20 | 2 |
between the formation and destruction of secondary structures of mechanic and chemical origin, i.e. uneven over time cyclic wear of friction surface can be observed due to the specifics of elastic deformation of the surface layers of bodies.
Better wear resistance was shown by the steel, treated under the regime of ECAP and HT. High wear resistance steel with UFG and nanostructure formed by ECAP HT was due to the peculiarities of volumetrically strained material tribological destruction. After ECAP can be observed texturing of ferrite grains along the axis of the billet, which coincides with the axis of the channel matrix, which should affect the structure of the surface formed by friction, depending on the direction of tribological pair. Under the tribological treatment of the deformed steel in the surface layer, the following processes occur: the destruction (fragmentation) of ferrite grains, texturing of ferritic bands and redistribution of carbides along the slip line; there can also be observed greater effect of such things as grain-boundary sliding and rotation of large-grains in the UFG and nanostructured materials. Effect of tribological pair heating partially causes recrystallization of grains in the surface layer of the material. Smaller grain size should provide more uniform removal of wear particles from the friction surface, compared with the initial coarse-grained material. Further movement of the wear particles along the friction surface should cause less destruction of the friction surface, i.e. effect of abrasion wear is reduced as well as “gouging” the friction surface by wear particles.
The results obtained show that during the low-temperature annealing at 350˚C and 450˚C, ECAP can be used as a technology to reduce wear in friction sliding unless the critical load is not exceeded. Then the load is greater than the tensile strength of the material.