Surface Nanocrystalline Structures Improve the Wear Resistance of Steel Through Mechanical-Pulse Treatment

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Medium-high carbon steel alloys are some of most widely used materials for wear resistant applications. Steel is known to contain many nanocrystalline structures and can be produced by many methods. One of interest is that of nanocrystalline surface structures. A team of researchers from Ukraine has tested a variety of steel compositions, produced through mechanical-pulse treatments (MPT), to produce alloys with nanocrystalline surface structures and improve their properties for use as wear resistance materials.

The choice of chemical composition and the heat treatment of a metal are known to be the driving factors for wear resistance in steel. Due to their wide use in many outdoor-based applications and environments, special attention has been paid to steel in terms of high wear resistance- especially in crack resistance.

The presence of nanocrystalline structures presents unique and complex mechanical properties, where the ability to combine high characteristics of strength, plasticity and brittle fracture resistance is possible. The complexity of the property combinations has opened nanocrystalline structures to be studied in-depth, especially as the properties are considered to be the main factor of improving the wear resistance in metallic materials. Compared to microcrystalline structures, surface nanocrystalline structures also contain a lower friction coefficient.

Traditionally, one of the most widely-used processes to produce surface nanocrystalline structures is through severe plastic deformation (SPD), which generally involves vibrational balls cold-work hardening, sand-blasting, wire brush hardening, and ultrasonic impact treatment.

An alternative way to produce nanocrystalline surface structures is by mechanical-pulse treatment (MPT). MPT is based around grinding principles, but modified machines are required. The general process involves the heating of the surface layers, with high speed friction using a rotating cylindrical tool. The process occurs above the phase transformation temperature where rapid cooling, thermoplastic deformation and heat transfer processes occur simultaneously, with the aid of a coolant. Throughout the process, the coolant is responsible for not only the rapid cooling process, but also causes saturation of the surface layers.

Out of the many steels products produced in industry, medium-high carbon alloys are the most widely used as wear resistance materials. The researchers have tested a variety of steel compositions that contain nanocrystalline surface structures through MPT treatments. The researchers also compared the various steel compositions after various treatments, quenching and low-temperature tempering processes. The pulse treatments were carried out using a CuKα-radiation method and were analysed using the JCPDS-ASTM index.

From their results and comparisons, the researchers found five main conclusions.

Firstly, they found that the influence of MPT on the steels is considered to be responsible for an increase in the wear resistance of the material. Secondly, the grain boundaries were in the range of 14-40 nm and showed a presence of strongly distorted grain boundaries and dislocation clews. Thirdly, the strengthening of the steel was due to the surface nanostructurization, structural-phase transformations, carbon surface saturation. Fourthly, the MPT technique was found to improve the wear resistance of the steel alloys more than any other treatment method (quenching and low temperature tempering etc). And Finally, the decrease in the friction coefficient led to a significant increase in the wear resistance of the alloys.

The researchers also found that higher increments of carbon in the alloy produced a stronger material. The microhardness was found to be between 7 and 12 GPa for the steels with a carbon content between 0.35 and 1%. The process was also found to increase the surface layer of the steel by 4-6 times than untreated steels alloys and a depth of up to 200 µm was achieved.

All in all, the main principle to take away from the research is that by utilising an MPT technique to introduce nanocrystalline surface structures, the researchers were able to increase the strength of the steel alloys, which could be used in wear resistance applications.

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This information has been sourced, reviewed and adapted from materials provided by SpingerOpen.