Simultaneous surface engineering and bulk hardening of precipitation hardening stainless steel

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Abstract

This article addresses simultaneous bulk precipitation hardening and low temperature surface engineering of two commercial precipitation hardening stainless steels: Sandvik Nanoflex® and Uddeholm Corrax®. Surface engineering comprised gaseous nitriding or gaseous carburising. Microstructural characterisation of the cases developed included X-ray diffraction analysis, reflected light microscopy and micro-hardness testing. It was found that the incorporation of nitrogen or carbon resulted in a hardened case consisting of a combination of (tetragonal) martensite and expanded (cubic) austenite. The duration and temperature of the nitriding/carburising surface hardening treatment can be chosen in agreement with the thermal treatment for obtaining optimal bulk hardness in the precipitation hardening stainless steel.

Introduction

Martensitic stainless steels and precipitation hardening stainless steels are materials of choice in applications where a combination of wear resistance, high strength and corrosion resistance is required. Such applications include turbine blades, tools, bearings and medical and dental equipment [1]. In addition to chromium as the main alloying element, precipitation hardening stainless steels contain substitutional alloying elements as molybdenum, aluminium and titanium, all of which contribute to a strength increase by precipitation hardening [2]. The precipitation hardening process is generally carried out in the temperature range 673–773 K, for a controlled amount of time. Strengthening may derive from precipitates such as Laves phase, Fe2Ti and Fe2Mo, η-Ni3(Ti,Al) and Ni3Mo [2].

Although precipitation hardening stainless steels are characterised by high bulk hardness as compared to, say, austenitic stainless steel, it is desirable to improve the surface properties with respect to wear (and corrosion). The combination of relatively high bulk hardness combined with a hard and wear resistant surface layer would expand the application range for precipitation hardening stainless steel. Traditionally, thermochemical processes, e.g. nitriding and carburising are the processes of choice for surface hardening of steels. Nitriding and carburising of austenitic stainless steels by the use of plasma, ion implantation and combination processes have for many years attracted significant attention in the literature [3], [4], [5], [6], [7].

Recent research has resulted in the development of a method for low temperature gaseous nitriding and carburising of stainless steels [8]. Process-wise, the possibility of gaseous nitriding and carburising is highly desirable as compared to the plasma-based methods.

The interest in low temperature nitriding and carburising can be sought in the beneficial mechanical and chemical properties associated with the transformation of the surface zone into expanded austenite (also referred to as S phase). Expanded austenite (denoted as γN or γC, depending on whether this phase is “stabilised” by nitrogen or carbon, respectively) is a (metastable) solid solution of nitrogen and/or carbon in austenite which leads to an isotropic expansion of the f.c.c. lattice. High contents of interstitially dissolved nitrogen and/or carbon bring about high hardness, very favourable wear properties and increased corrosion resistance [6], [9], [10], [11]. Due to the incorporation of nitrogen/carbon high compressive residual stresses are introduced into the surface treated zone, which may contribute to an improvement of the fatigue properties of the steel [12], [13].

Several attempts have been made to nitride ferritic and martensitic stainless steels [14], [15], [16]. To the authors’ knowledge only one successful attempt of obtaining a zone of expanded austenite at the surface of precipitation hardening stainless steels can be found in literature [17]: A mixture of expanded austenite (γN) and different nitrides was obtained after treatment of AISI 17-4 PH stainless steel by a plasma-based process.

In the present study, low temperature gaseous nitriding and carburising of the commercially available precipitation hardening stainless steels Sandvik Nanoflex® and Uddeholm Corrax® were investigated. It is demonstrated that the processes of bulk precipitation hardening and surface treatment by nitriding/carburising can be combined in a single-step process. The durations and temperatures of the nitriding/carburising treatments correspond to the optimal time–temperature combination for bulk precipitation hardening [18].

Section snippets

Experimental

Sandvik Nanoflex® strip was delivered in cold-rolled condition: approximately 90% area reduction had resulted in a strip thickness of 0.5 mm. The microstructure consisted of strain induced martensite with only a negligible amount of retained austenite. Uddeholm Corrax® was delivered in the solution treated condition, i.e. a microstructure consisting of martensite with a minor amount of austenite. The hardness of the as-delivered materials was 400 HV and 330 HV for Nanoflex® and Corrax®,

Nitriding

Nitriding of the precipitation hardening stainless steels was carried out at various temperatures in order to evaluate the effect of temperature on the resulting microstructure. Additionally, the effect of different nitriding potentials was investigated. The nitriding potential is directly proportional to the nitrogen activity and, hence, determines the maximum possible nitrogen content, and governs which phases can be formed.

Discussion

On nitriding in NH3 / H2 gas mixtures nitrogen-expanded martensite, αN′, and possibly also nitrogen-expanded austenite, γN, develop, whilst nitriding in pure NH3 (KN = ∞) leads to the development of γN in coexistence with αN′ (Fig. 5). These results strongly suggest that the driving force for the development of γN increases with nitrogen activity and that there exists a minimum nitrogen activity below which γN does not develop. This hypothesis is corroborated by supplementary GD-OES nitrogen

Conclusions

Low temperature gaseous nitriding and carburising were investigated for the commercially available precipitation hardening stainless steels Corrax® and Nanoflex®. The materials were successfully surface hardened and proved very suitable for low temperature nitriding and carburising. Nitrogen-expanded austenite was obtained in both Corrax® and Nanoflex® for high nitriding potentials. Development of carbon/nitrogen-expanded martensite occurred for nitriding at intermediate to low nitriding

Acknowledgements

The authors gratefully acknowledge Palle Ranløv (Uddeholm) and Finn T. Petersen (Sandvik Materials Technology) for providing the test materials, as well as Sandvik Materials Technology for providing GD-OES nitrogen depth profiles. The present work was partly financially supported by the Danish Research Agency under grant 26-01-0079.

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