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The samples of austempered ductile irons (ADI) with nanosized particles of TiC-N+ TiN, TiN and cBN are investigated. The austempering mode consists of heating at 900°С, 1 h and isothermal retention at 380°С, 2h. The obtained structure of the upper bainite is examined by an optical metallographic microscope GX41 OLIMPUS. SEM and EDX analyses of cast samples with nanosized particles are performed. Wear test is carried out of ADI samples on thumb-disc circuit during friction on the fixed abrasive. The influence of nanoadditives on the tribological characteristics, hardness and microstructure of ADI is established. [ABSTRACT FROM AUTHOR]

The tribological characteristics of spheroidal graphite cast irons with and without nanosized additives are investigated. The tests are performed as in cast iron condition as well after austempering. The spheroidal graphite irons are undergone to austempering in the bainite field, including heating at 900 °C for an hour, after that isothermal retention at 280 °C, 2 h and at 380 °C, 2 h. The lower bainitic and upper bainitic structures are formed during the process. Nanosized additives of titanium carbonitride and titanium nitride TiCN+TiN influence on the graphite phase characteristics and on the microstructure of the cast and austempered spheroidal graphite irons. The changes in the micro structure the irons with nanoadditives lead to an abrasive wear resistance increase. The formation of the strain induced martensite from the retained austenite in the friction contact area during wear is determined in the austempered irons. This is the reason for the wear resistance increase of the irons. The experimental testing of the wear is carried out by cinematic scheme tapper-discunder friction on the fixed abrasive. The microstructure of the patterns is observed by optical and quantitative metallography, X-Ray analysis, SEM and EDX analysis. The hardness testing is performed by Brinnel and Vickers methods. [ABSTRACT FROM AUTHOR]

Partial melting homogenisation (PMH) of heavy section castings produced from a high Mn ductile iron was studied. Homogenisation treatment for ∼8 h with an isothermal holding period of 4–4·3 h was found to be effective in reducing segregation of manganese and silicon. Heavily segregated regions only were melted and the liquefied region shrank during the PMH treatment and eventually disappeared. The fraction of liquid phase formed showed good agreement with Thermo-Calc predictions. Scanning electron microscopy–energy dispersive X-ray spectroscopy analysis showed that heating to below the eutectic temperature (1116°C) reduced segregation extensively, whereas at higher treatment temperatures (up to 1125°C) the intensity of segregation increased. A comparison of experimental results for Mn and Si segregation with calculated values obtained from a previously developed model also showed acceptable agreement. Austempering of as cast and PHM specimens at 370–390°C for 0·25–4 h showed that PHM gave a significant reduction in untransformed austenite volume following austempering. Hardness of the homogenised specimens initially decreased with austempering time owing to tempering of martensite then increased as austenite decomposed to ferrite and carbides. [ABSTRACT FROM AUTHOR]

A nonisothermal annealing was applied to austempered Ni-Cu-Mo alloyed and unalloyed ductile irons to determine the thermal stability of the ausferritic structure. Differential thermal analysis (DTA) results were used to build the corresponding stability diagrams. The transformation starting temperature of the high carbon austenite was found to be strongly dependent on the austempering temperature, the heating rate, and the chemical composition of the iron. The Ni-Cu-Mo alloying elements and high austempering temperature increased the stability. The transformation of the austenite to ferrite and cementite is achieved via the precipitation of transition carbides identified as silico-carbides of triclinic structure. [ABSTRACT FROM AUTHOR]

In this research, different ductile irons and austempered ductile irons were successfully developed using several alloying contents of nickel, copper and microalloying with niobium. Additionally, special nanocarbon powder was added to the molten iron to enhance the nucleation tendency of spheroidal graphite and compensate for the possible negative effect of Nb addition on the nodule morphology. Metallographic analysis showed that increasing the niobium content in the alloy to 0.1 wt % raises the number of graphite eutectic cells and refines the final structure of the graphite. Moreover, the nodule count of graphite slightly increased, but it concurrently decreased the nodularity when the Nb amount reached 0.1 wt %. SEM micrographs illustrated that nano- to microsized niobium carbides (NbC) particles were dispersed in the matrix of the Nb microalloyed ductile irons. Both optical and SEM micrographs clearly showed that alloying of ductile irons with nickel, copper and microalloying with niobium had a significant effect on defining the final pearlite structure. Coarse, fine, broken and spheroidized pearlite structures were simultaneously observed in all investigated alloys. Dilatometry studies demonstrated that the nano NbC particles acted as nucleation sites for graphite and ferrite needles. Therefore, Nb addition accelerated the formation of ausferrite during the austempering stage. Finally, alloying with Cu, Ni and microalloying with Nb led to developing novel grades of ADI with excellent strength/ductility property combination. [ABSTRACT FROM AUTHOR]