PSI - Issue 58

Alan Vaško et al. / Procedia Structural Integrity 58 (2024) 48–53

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A. Vaško et al. / Structural Integrity Procedia 00 (2019) 000–000

1. Introduction Austenitic cast irons, often referred to as Ni-Resist, are highly alloyed cast irons containing a sufficient amount of nickel (or other elements) to obtain an austenitic matrix that has unique and excellent properties, especially high corrosion resistance, wear and erosion resistance, high heat resistance, toughness at low temperatures, non magnetizability etc. Austenitic cast irons can have lamellar or nodular graphite. Austenitic nodular cast irons are becoming more important because of their higher strength, ductility and elevated temperature properties. However, many foundries still produce austenitic lamellar cast irons due to lower costs, fewer foundry problems and better machinability and thermal conductivity (Fischer 2022, Franke 2019, Röhrig 2004). Austenitic cast irons are suitable for applications that are in contact with salt solutions, sea water, mild acids, alkalies and oil field liquids, both sweet and sour. Their corrosion resistance is much better than that of common and low-alloyed cast irons. They are characterised by uniform corrosion rather than localised deterioration (Inco 2022, Kaňa 2017). Each of the alloying elements in austenitic cast iron affects the microstructure and properties in a different way. Nickel gives austenitic cast irons their defining properties. It is primarily responsible for the stable austenitic matrix and contributes significantly to corrosion and oxidation resistance and to mechanical properties over the entire usable temperature range. Chromium contributes to improvement in strength and corrosion resistance at elevated temperatures. It also causes increased hardness, which improves wear resistance, corrosion and erosion resistance. Chromium decreases ductility by forming hard carbides. Manganese does not provide an improvement in corrosion resistance, high-temperature resistance or mechanical properties. However, it is an austenite stabiliser that contributes significantly to the properties of cast iron at low temperatures and to non-magnetic alloys (Fischer 2022, Otáhal 2009, Skočovský 2005, Röhrig 2004). Austenitic cast irons have a corrosion resistance between unalloyed or low-alloyed cast iron and stainless steel. They corrode in a similar manner to unalloyed cast irons, but because of their chemical composition and austenitic matrix, they form denser, more adherent corrosion product films that suppress further corrosion. Stainless steel often corrodes in a localised ways. This means it suffers from pitting, crevice corrosion and sometimes stress corrosion cracking. Austenitic cast irons rarely have these forms of attack. Their corrosion is usually uniform at relatively low rates (Fischer 2022, Stefanescu 2017, Spence 2005). The shape of the graphite in austenitic cast irons has relatively little influence on corrosion resistance; therefore, knowledge from lamellar cast irons can also be applied to nodular cast irons (Otáhal 2009). In this paper, only nodular cast irons were used for the experiments.

Nomenclature A

elongation (%)

C E

carbon equivalent (%) corrosion potential (V)

E corr

HBW Brinell hardness (–) i corr

corrosion current density (μA cm -2 )

K0

absorbed energy (J) determined on the specimen without notch

R

stress ratio (–)

R m

tensile strength (MPa) yield strength (MPa) temperature (°C) exposure time (weeks) corrosion rate (mm year fatigue limit (MPa)

R p0,2

T

t

v

corrosion rate (g m -2 day -1 ) determined by the exposure immersion test

v corr

-1 ) determined by the potentiodynamic polarisation test

σ c