Assessment of the nitriding process paramaters to improve the wear resistance of selected steels

Abstract

Liquid nitriding is one of the most popular heat treatment methods due to its application temperature which is generally in the ferritic region. This results in a good dimensional control after nitriding. Still, it is possible to apply nitriding in austenitic region to increase compound and nitriding case thickness. Nitriding increases surface hardness and wear resistance of specimens sharply. In this thesis, to see these effects of nitriding, three different alloys have been chosen and nitrided in two different salt baths for four different nitriding durations at four different process temperatures according to a predetermined nitriding test matrix. This matrix has been designed to see the effects of initial material microstructure and composition, nitriding temperature, nitriding duration, and nitriding bath composition. Afterwards, the specimens have been tested on a pin-on-disc wear setup, built according to ASTM G99-95a specification. Weights and lengths of all specimens have been measured consistent with given directions in ASTM G99-95a specifications to compare their wear resistance. Furthermore, microstructural characterizations have been completed by using OM, SEM, EDS, XRD, microhardness, and surface roughness analyses after nitriding to identify the properties of compound and diffusion layer. The evaluation of the data gathered gives an insight in to the nitriding. According to the results, time and temperature have similar effects on nitriding. The increase in nitriding time and bath temperature increases the compound layer and nitriding case thickness. However, the comparison of these two parameters shows that the effect of temperature on nitriding case is more than that of nitriding duration in consistent with Fick’s law of diffusion. Analysis of bath composition has shown that higher cyanate content, which is responsible for nitriding, make low temperature nitriding applications possible since nitriding in a bath consisting of high amount of cyanate results in a thick compound layer and nitriding case at low nitriding temperatures. Further data analysis shows that initial chemical composition of specimens has affected nitriding case thickness and microhardness directly. Low carbon steels have thicker nitriding case due to easy diffusion of nitrogen. On the contrary, alloys with high chromium and aluminum reduce nitrogen diffusivity. Nitriding case thickness of these specimens has been found to be relatively thin, although they have high surface hardnesses. Pin-on-disc wear results have shown that wear resistance of low alloy steels have increased after nitriding. Wear test results of specimens have clarified that wear on disc which tested with pins having thicker compound layer brings about more volumetric loss than those having thin compound layer on pin specimens. According to results, it is concluded that increase in the compound layer thickness results in a higher wear resistance. However, very thick compound layer has triggered abrasive wear on the disc and pin due to its brittle structure.