High-speed thermo-cyclic nitriding carburized steels

Abstract

The task of developing new methods of chemical and heat treatment (CHT), aimed at increasing wear resistance, mechanical properties and reducing length of process operations seems to be quite vital. The objective of the work is development of a method of complex surface strengthening of structural steels with application of the mechanism of additional self-strengthening at wear. Suggested is application of high-speed thermo-cyclic nitriding of carburized structural steels for formation of gradient meta-stable phase-structural modifications in saturated layers. Specimens of 18Cr2Ni4Мo steel underwent preliminary carburization in solid carburetor at 930 °С temperature for 12 hours it leading to obtaining a carbonized layer 1.2…1.3 mm in thickness. Then high-speed thermocyclic nitriding (HSTCN) was carried out in carbamide medium with CHT heating, according to heating-cooling mode 1000↔20 °С, the number of thermo cycles being 2 to 12. The following investigation methods were applied: metallographic, scanning electronic microscopy, durometric, wear testing, according to dry friction-sliding metal against metal mode. As a result of combined CHT with thermo-cycling a thin nitrited layer 0.07…0.14 mm thick is formed on the surface (following 2 HSTCN cycles) and up to 0. thick (after 12 HSTCN cycles) with increased nitrogen and carbon content. Further structure along the depth of the carbonized layer consists of high-carbon martensite, carbonitrides and increased amount of residual austenite (Аres). Along the depth of the saturated layer the microstructure is changed in gradient: the amount of Аres and carbonitrides is reduced, while the amount of quenching martensite is increased. Depending upon the number of HSTCN cycles relative wear resistance is extremely maximally changed with maximum (ε=4,1) at 7 cycles. Аres is meta-stable and undergoes deformation induced martensite transformation γres→αʹ in the process of wear testing (DIMTWT) in the thin surface layer, it bringing some additional contribution into deformational self-strengthening and improvement of relative wear resistance. Thus, improvement of wear resistance ensures additional amount on nitrides and carbonitrides in the preliminarily carbonized and then nitrited steel layer, as well as high-carbon martensite and increased amount of meta-stable Аres (45…70 %), that undergoes γres→αʹ DIMTWT. Thermocyclic character of additional nitriding ensures grain refinement, it promoting improvement in steel impact strength.