Low-temperature carburizing/nitriding of austenitic stainless steels - Influence of alloy composition on microstructure and properties
Doktorsavhandling, 2017

Austenitic stainless steels are among the most used materials in applications where corrosion resistance is important, as the food, pharmaceutical, chemical, oil and gas industries. However, low hardness and poor tribological properties is often an obstacle for their applicability. Conventional surface hardening techniques, such as high-temperature carburizing (T > 850°C) and nitriding (T > 550°C) are not suitable for these alloys. Rapid precipitation of chromium-rich carbides/nitrides at the grain boundaries would in such cases induce chromium depletion in the alloy and compromise the corrosion resistance. Since the middle of the ‘80s, low-temperature thermochemical treatments have been developed for surface hardening of austenitic stainless steels, including gas carburizing and plasma nitriding. These processes can induce formation of a precipitate-free interstitially supersaturated metastable expanded austenite, also known as S-phase, having superior hardness and improved wear resistance, while maintaining corrosion resistance. The aim of this thesis is to increase the understanding of the microstructure-properties relationship of the surface modifications obtained by treating austenitic stainless steels with low-temperature carburizing and nitriding processes. In particular, the research focus is on the influence of alloy composition and surface finish on the microstructural evolution, phase constituents, thermal stability, strain, hardening, wear and corrosion resistance of the expanded austenite layers. The investigations were carried out by means of different analytical techniques, such as XPS, AES, XRD, SEM, GDOES and EBSD among others. It has been found that alloy composition and surface finish have a paramount influence on the microstructural characteristics and properties of the expanded austenite layers. High molybdenum content and plastic deformation enhances interstitial diffusion and supersaturation, while nickel decreases both. On the other hand, nickel prevents the formation of detrimental nitride/carbide-containing compound layers, enhances the thermal stability of the alloys and directs the expanded austenite decomposition towards a discontinuous route. During thermochemical treatments, precipitates tend to form preferentially at the surface, in correspondence of slip-planes or ferritic/martensitic regions. A hard, expanded austenite layer prevents severe adhesive and abrasive wear during dry sliding tests, improving wear resistance at low load. N-stabilised expanded austenite potentially possesses stronger hardening effects, but is more brittle. The strong interaction between chromium and carbon in expanded austenite alters the iron/chromium ratio within the passive film, making the surface more noble and conductive. When expanded austenite is thermally decomposed, hardening and corrosion resistance are at least partially compromised.

GDOES

tribology

EBSD

corrosion

XPS

thermal stability

surface engineering

Austenitic stainless steel

S-phase

surface analysis

expanded austenite

SEM

low-temperature carburizing

XRD

plasma nitriding

Virtual Development Laboratory, Hörsalsvägen 7A, Chalmers
Opponent: Thierry Czerwiec, Université de Lorraine, Nancy, France

Författare

Giulio Maistro

Chalmers, Material- och tillverkningsteknik, Yt- och mikrostrukturteknik

Multi-technique characterization of low-temperature plasma nitrided austenitic AISI 304L and AISI 904L stainless steel

Surface and Interface Analysis,; Vol. 46(2014)p. 856-860

Artikel i vetenskaplig tidskrift

THERMAL DECOMPOSITION of N-EXPANDED AUSTENITE IN 304l AND 904l STEELS

Surface Engineering,; Vol. 33(2017)p. 319-326

Artikel i vetenskaplig tidskrift

Microstructural characterization and layer stability of low-temperature carburized AISI 304L and AISI 904L austenitic stainless steel

European Conference on Heat Treatment 2015 and 22nd IFHTSE Congress - Heat Treatment and Surface Engineering from Tradition to Innovation,; (2015)

Paper i proceeding

G. Maistro, S. Kante, L. Nyborg and Y. Cao Low-temperature carburized high-alloyed austenitic stainless steels in PEMFC cathodic environment

G. Maistro, M. Esneider, S.A. Pérez-García, L. Nyborg and Y. Cao Tribological behaviour of low-temperature carburised austenitic stainless steels

G. Maistro, S. Kante, Y. Yao, U. Klement, L. Nyborg, Y. Cao Surface carbides in low-temperature carburized austenitic stainless steels

Every day, we are surrounded by materials made of austenitic stainless steels. Kitchenware, utensils, food containers, jewelry… just to name a few applications that we cannot imagine our daily life without. Austenitic stainless steels are also necessary for industries, such as chemical, biomedical, pharmaceutical and petrochemical. One of the most attractive features of austenitic stainless steels is high corrosion resistance, especially important in acidic and marine environments. The chromium present within the metal, which gives rise to a chemically and mechanically stable chromium-oxide layer when exposed to oxygen, protects the material from corrosion. Additionally, nickel is added to give the material an “austenitic” structure, further improving the corrosion resistance and enhancing the formability. Unfortunately, one of the most significant drawbacks of austenitic stainless steels is the inherently low hardness. Moreover, austenitic stainless steels cannot be heat-treated like conventional steels, which limits their applicability to non-load bearing applications, otherwise high wear and mechanical failures may occur.

Conventional steels can be treated with nitrogen or carbon at high-temperatures to increase hardness. Unfortunately, this is not a viable option for austenitic stainless steels. High-temperature nitriding/carburizing causes the formation of chromium-compounds, which drastically decreases the corrosion resistance. In contrast, low-temperature treatments, that introduce nitrogen or carbon in the austenitic crystal structure without removing chromium from the steel, can be used. The austenitic structure expands, creating a hard but still corrosion resistant layer.

In this thesis, both plasma- and gas-based carburizing/nitriding techniques were used to form expanded austenite layers on different austenitic stainless steels. The influence of alloying elements on the microstructure and properties of the expanded austenite structure was investigated. The microstructural characteristics, such as thickness and phase constituents, as well as hardness, thermal stability and corrosion resistance were studied by a variety of microscopic and spectroscopic analysis techniques.

Ämneskategorier

Materialteknik

Styrkeområden

Materialvetenskap

ISBN

978-91-7597-679-2

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4360

Utgivare

Chalmers tekniska högskola

Virtual Development Laboratory, Hörsalsvägen 7A, Chalmers

Opponent: Thierry Czerwiec, Université de Lorraine, Nancy, France