Effect of different nitriding processes on the friction coefficient of 304 austenitic and 420 martensitic stainless steels

Abstract

Purpose - The purpose of this paper is to present the results of a study on friction characteristics of plasma, salt-bath and gas nitrided layers produced in AISI 304 type austenitic and AISI 420 type martensitic stainless steels.

Design/methodology/approach - Plasma nitriding processes were carried out with DC-pulsed plasma in 80% N-2 +/- 20% H-2 atmosphere at 450 degrees C and 520 degrees C for 8 h at a pressure of 2 mbar. Salt-bath nitriding was performed in a cyanide-cyanate salt-bath at 570 degrees C for 1.5 h. Gas nitriding was also conducted in NH3 and CO2 atmosphere at 570 degrees C for 13 h. Characterization of all nitrided samples has been carried out by means of microstructure, microhardness, surface roughness measurement and friction coefficient. The morphologies of the worn surfaces of the nitrided samples were also observed using a scanning electron microscope. Friction characteristics of the nitrided samples have been investigated using a ball-on-disc friction and wear tester with a WC-Co ball as the counterface under dry sliding conditions.

Findings - The plasma nitrided and salt-bath nitrided layers on the 420 steel surfaces were much thicker than on the 304 steel surfaces. However, there was no obvious and homogeneous nitrided layer on the gas nitrided samples' surface. The plasma and salt-bath nitriding techniques significantly increased the surface hardness of the 304 and 420 samples. The highest surface hardness of the 304 nitrided samples was obtained by the plasma nitrided technique at 520 degrees C. On the other hand, the highest surface hardness of the 420 nitrided layers was observed in the 450 degrees C plasma nitrided layer. Experimental friction test results showed that the salt-bath and 450 degrees C plasma nitrided layers were more effective in reducing the friction coefficient of the 304 and 420 stainless steels, respectively.

Originality/value - The relatively poor hardness and hence wear resistance of austenitic and martensitic stainless steels needs to be improved. Friction characteristic is a key property of performance for various applications of austenitic and martensitic stainless steels. This work has reported a comparison of friction characteristics of austenitic 304 and martensitic 420 stainless steels, modified using plasma, salt-bath and gas nitriding processes. The paper is of significances for improving friction characteristics, indirectly wear performances, of austenitic and nnartensitic stainless steels.