Structure of high-speed alloy after plasma surfacing in nitrogen and heat treatment

Abstract

The authors have studied the effect of plasma surfacing and s sequent high-temperature tempering on surface morphology and elemental composition of deposited coatings of high hardness chrome tungsten steel, such as R18, additionally alloyed with aluminum and nitrogen (0.86 % C; 4.84 % Cr; 17.0 % W; 5.40 % Mo; 0.50 % V; 0.65 % Al; 0.06 % N). 30KhGSA steel, which has a complex of high mechanical properties, is used as the base metal. Specific feature of the surfacing was low-temperature preliminary and concurrent heating, as well as thermal deposition cycle, consisting of three main stages. At the first stage with limited heating time and increased cooling rate at high temperatures, grain growth and decomposition of austenite was prevented with formation of equilibrium low-strength structures. At the second stage deposited metal stayed in austenitic state. At the third one, the deposited metal with low tendency to crack has been obtained. Using the method of scanning electron microscopy and X-ray spectral microanalysis, surface morphology and elemental composition of the coatings were studied in two states: immediately after surfacing and in state after surfacing and high-temperature tempering. It has been established that in the first case, the main part of material surface is pearlite grains. At the joints and along grain boundaries, cementite of complex composition and compounds based on iron, tungsten and molybdenum, of variable composition (Fe 4 W 2 N, FeWN 2 and Fe 4 W 2 C) are located. Solid solutions based on aluminum and, possibly, the AlN phase also present. High temperature tempering leads to hardness increase, change in shape and size of grains, quantitative change in elemental composition, and uniform distribution of alloying elements over the volume of material. The proposed method of plasma surfacing with high temperature tempering provides all the basic requirements for the surface of working rolls of cold rolling, which is confirmed by test results of the batch of deposited rolls.