Influence of initial heat treatment of 17-4 PH stainless steel on gas nitriding kinetics

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Abstract

Results of the investigation of nitrided layers on 17-4 PH type precipitation hardening stainless steel are presented in this paper. The layers have been produced in the process of gas nitriding in a partly dissociated ammonia at temperatures between 410 and 570 °C. Hydrogen chloride admixture to active atmosphere was used as a surface activator. Structure of the nitrided layers were examined using scanning and transmission electron microscopy, X-ray microanalysis (EDX and WDX), and X-ray diffraction. The influence of the initial steel heat treatment on the nitriding kinetics has been considered. 17-4 PH stainless steel was nitrided at various heat treatment conditions, i.e. after solution treatment or ageing at different temperatures. The influence of precipitation processes taking place during the heat treatment before nitriding on the diffusive process kinetics was proven. It was found that, that increasing of steel ageing temperature up to 600 °C before nitriding effects on an increasing of the nitriding kinetics.

Introduction

17-4 PH steel belongs to precipitation–hardening (PH) stainless steels group and it is more common than any other type of materials from this group due to balanced combination of good mechanical properties and excellent corrosion resistance. This alloy is a martensite stainless steel containing approximately 17 wt.% Cr, 4 wt.% Ni, 3 wt.% Cu and it is strengthened by the precipitation of submicroscopic copper rich particles in the martensite matrix. 17-4 PH steel has been used for a variety of applications including chemical process equipment, air craft fittings, fasteners, pump shafts, nuclear reactor components, gears, paper mill equipment, missile fittings, jet engine parts and rotors of centrifugal compressors. Mechanical properties of the steel depend on the heat treatment parameters. The heat treatment consists of solution treatment (1040 °C air or oil cooling) and aging at temperature between 480 °C and 780 °C for 1–4 h. Such a heat treatment enables obtaining the following mechanical properties: tensile strength 900–1400 N/mm2, yield point 590–1280 N/mm2, impact resistance 20–135 J, hardness 27–44 HRC. The highest hardness and strength correspond with the lowest aging temperatures due to the steel structure containing fine, coherent with matrix, bcc copper rich precipitates. Increasing of the aging temperature causes consequential loss of coherency between the steel matrix and the copper precipitates lattices, growth of the precipitates and transition of their lattice type to fcc. Precipitation processes in 17-4 PH steel were investigated by many researches [1], [2], [3], [4], [5], [6].

Unfortunately, precipitation hardening steels have insufficient wear resistance, therefore there is a necessity of increasing it by surface engineering methods. One of the most efficient method of increasing wear resistance is nitriding. In cases of complicated shape elements with many hard to reach surfaces, the most usable is gas nitriding contrary to plasma nitriding.

There are two aspects of PH stainless steels gas nitriding: first is a surface passivation due to high contents of chromium (surface activation is necessary), second is presence of the precipitation processes proceeding in the matrix during thermo-chemical treatment. Although limited studies have been conducted to investigate the plasma [7] and gas [8] nitriding response of PH stainless steels little work has been done concerning the influence of initial heat treatment state of the PH steel on nitriding kinetics and structure of obtained layers. Previous authors' works [9], [10], [11], [12], [13], [14] have suggested, that both heat treatment state, and precipitation processes have influence on the nitriding kinetics and structure of obtained layers. The purpose of the present work is to clarify this point by conducting a series of gas nitriding experiments on PH steel substrate in a various initial heat treatment states.

Section snippets

Experimental procedures

Nitriding layers were created on a 17-4 PH stainless steel of chemical composition: 0.03 wt.% C, 0.41 wt.% Si, 16.89 wt.% Cr, 4.39 wt.% Ni, 3.12 wt.% Cu, Fe-balance. With the purpose of determination of initial steel heat treatment on nitriding kinetics, before nitriding the steel was either solution heat treated (1040 °C, oil cooling) or solution heat treated and aged at temperatures between 200 °C and 800 °C for 4 h. All specimens were cut off along the bar rolling direction from bar of 70 mm

Results

Influence of initial heat treatment of 17-4 PH stainless steel on gas nitriding kinetics is the more distinct the lower is nitriding temperature. For nitriding at 470 °C and the atmosphere nitrogen potential from γ' nitride range, the biggest thickness is observed in case of 17-4 PH steel initially solutioned and aged at 600 °C, whereas the smallest thickness corresponds to the steel after solution treatment and solution treatment and aging at 200 °C, Fig. 1. Tendency like this is watched for

Conclusions

Influence of initial heat treatment of 17-4 PH stainless steel on gas nitriding kinetics has been confirmed. Increasing of the steel aging temperature to approximately 600 °C results in increasing of nitriding kinetics. Further increasing of aging temperature causes decreasing of the nitriding layer growth rate. It is possible to obtain three different types of the nitrided layers depending on technological nitriding parameters using chemical activated gas nitriding. Reasons of the nitriding

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