Optimization of mechanical properties of complex, two-stage heat treatment of Cu–Ni (Mn, Mo) austempered ductile iron

Abstract

The aim of the study was to design and optimize the complex, two-step heat treatment of Cu-Ni (Mn, Mo) ductile iron. A method for the formation and investigation of austempered ductile iron (ADI) by means of complex two-step and as comparative, standard one-step heat treatments has been developed, using quenching dilatometer. Investigations of proceeding phase transformations using differential dilatometric and DSC analysis supported by microstructural observations, hardness and austenite volume measurements have been carried out. An analysis of the temperature sequence of the ausferrite decomposition in the one-step and two-step ADI was performed, which allowed for the separation and identification of the effects responsible for the carbon-enriched austenite decomposition. A quantitative relationship was established between basic dimensional effects revealed on the differential dilatometric curve of ausferrite decomposition, which enables prognosis and optimization of the parameters of complex ADI heat treatment variants. Verification tests were performed on a stand equipped with salt furnaces enabling a quick transfer of samples from one bath to another without changing their initial temperature. Optimization of the two-step ADI heat treatment with the use of quantitative dilatometric analysis of the ausferrite decomposition, allowed to obtain for the temperature step-down heat treatment 390°C/15, 20 min ) 270°C/130 min the excellent mechanical properties, unattainable by means of standard 1-step ADI heat treatment.

Microscopic approach to micromechanism of damage in spheroidal cast iron

“…The third alloy was austempered ductile iron (ADI). The following specimens of ADI's were examined (Gazda et al, ): In the as‐cast state and after the isothermal heat treatment (ADI, elongation 2.7%). Two specimens after one‐step austempering (ADI 310, at 310°C/60 min, elongation 3.5%, and ADI 390, at 390°C/60 min, elongation 8.4%). After cyclic austempering (ADI cyclic, 600 cycles in a temperature range 270–390°C, elongation 6.9%). …”

“…However, the effect of morphology and size of the second phase particles, that is, graphite spheroids has been taken into account mainly in the context of material's macroscopic properties (Cooper, Elliott, & Young, ; Gaudig, Mellert, Weber, & Schmauder, ). Microscopic observations of fracture surface topography of cast irons were usually focused on the damage mechanism in the metal matrix, due to either applied loading mode or its microstructure characteristic (Fatahalla & Hussein, ; Gazda, Warmuzek, & Bitka, ; Martínez, ; Warmuzek & Polkowska, ).…”

Micromechanism of Damage of the Graphite Spheroid in the Nodular Cast Iron During Static Tensile Test

“…Alloy marked as ADI was examined as cast state and after the austempering heat treatment described in Ref. [4]. The graphite particles in the examined alloys were properly formed spheroids, ascribed mainly to group VI (according to EN ISO 945-1).…”

“…Tensile tests were carried out at room temperature, according to the PN EN ISO 6892-1 standard, by using EU20 universal testing machine. The SEM observations were carried out on the metallographic cross-sections, prepared according to general procedures, elaborated for spheroidal cast iron [4,32]. Samples were mounted in a conductive resin, grinded on abrasive papers of 250-1000 gradation, and polished with diamond pastes having a gradation of 9, 3, 1, 0.25 µm.…”

“…Usually, an optimization of properties of this material for specific applications was carried out by a selection of its chemical composition and by controlling the casting technology [1][2][3]. In the published works some attempts have been also presented to modify the matrix microstructure by tailoring heat treatment parameters, e.g., in the case of austempered ductile irons (ADIs) [1][2][3][4][5][6] or spheroidal cast irons (SCIs) in welded joints [7].…”