PSI - Issue 51

Alan Vaško et al. / Procedia Structural Integrity 51 (2023) 129–134 A.Vaško et al. / Structural Integrity Procedia 00 (2022) 000–000

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role in the formation of austenite by lowering the eutectoid decomposition temperature of austenite below room temperature. The term "Ni-Resist" is also used to group materials with nickel contents between 15 and 36% (Hasse 2022, Franke 2019, Stefanescu 2017, Morrison 1998). The primary alloying element of austenitic nodular cast irons is nickel. To keep austenite stable at low temperatures, a nickel content of roughly 20% is needed. This is necessary due to a lack of the supportive effect of high chromium content. In order to avoid excessive carbide formation, which would increase hardness and make machining more difficult, embrittlement, and deterioration of foundry characteristics, chromium can only be used in small amounts in graphitic cast iron. A part of the nickel may sometimes be substituted by other austenite-stabilizing elements, such as manganese or copper, primarily for economic reasons (Röhrig 2004). To ensure an austenitic matrix, the composition of cast iron (nickel, chromium, and manganese content) should correspond to the following equation for calculating the nickel equivalent: Ni eq = % Ni + % Cr + 2 ꞏ % Mn > 23.5 % (1) If a stable austenitic matrix below room temperature is required, the nickel equivalent of at least 23.5% is needed (Röhrig 2004, Kaňa 2017). The problem of the stability of austenitic structure in the cast state is dealt with in the literature (Davis 1999, Röhrig 2004, Berns 2008, Franke 2015).

Nomenclature A

elongation (%)

a 0 d 0

width of a test specimen (mm) diameter of a test specimen (mm)

F

load (N)

f frequency (Hz) HBW Brinell hardness (–) K0

absorbed energy (J) determined on the specimen without notch absorbed energy (J) determined on the specimen with V-notch

KV

l 0

length of a test specimen (mm) number of cycles to failure (cycles)

N f

Ni eq

nickel equivalent (%)

R

stress ratio (–)

R m

tensile strength (MPa) yield strength (MPa) shape factor (–) eutectic degree (%) temperature (°C) stress amplitude (MPa) fatigue limit (MPa)

R p0,2

S

S C

T

σ a σ c

Standardization for austenitic nodular cast irons is done in accordance with STN EN 13835. Ten grades of nodular cast iron are described in this European norm, including their chemical composition, required micro-structure, mechanical characteristics (such as yield strength, tensile strength, elongation, impact energy, modulus of elasticity, and Brinell hardness), and physical characteristics. The fatigue characteristics of these nodular cast irons are not specified by the norm, though (Věchet 2002). For the studies, austenitic nodular cast iron alloyed with nickel, chromium and manganese, designated as EN-GJSA XNiCr20-2, was chosen. This type of austenitic nodular cast iron has good corrosion and heat resistance, good bearing properties, a high coefficient of thermal expansion, and is non-magnetizable. It is used for pumps, valves, compressors, bushings, turbocharger housings, exhaust gas manifolds, and non-magnetizable castings. Some of these components may be exposed to fatigue loads, so it is important to know their fatigue characteristics.