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The regulation of solid-state structural transformations in high-chromium complex-alloyed cast irons with the object to improve their properties


Chabak Yu.G.


The objective of the work is the formation of the required microstructure state of high-chromium cast iron (HHCI) alloyed by in total 3-6% of the austenite-forming elements (Mn, Ni, Cu) by means of the development of new energy-saving heat treatment technologies in order to improve the complex of technological (machinability) and operational (hardness, wear resistance) properties. The kinetics of phase-structural transformations of primary, destabilized, retained austenite are studied, the corresponding TTT-diagrams are built. The effect of inhibition of pearlite transformation caused by precipitation of secondary carbides at destabilizing heat treatment is found and described for the first time. It is shown that destabilization of the austenite during softening annealing is not appropriate for above-mentioned cast irons. The new scientific grounded data on the impact of aging in the subcritical temperature range on the hardness of as-cast and as-quenched state are obtained. The kinetics of carbides formation during eutectoid transformation and martensite decomposition are studied and described.

It is shown that holding of 15%Cr-cast iron at 1000-1150 °C leads to an intensive surface decarburization, resulting in the dissolution of secondary and eutectic carbides. A model of chemical elements distribution in the austenite within the decarburized zone is developed. The regime of HCCIs quenching before the final machining, which forms the decarburized surface layer with improved machinability while maintaining high hardness of the base metal is proposed.

New technological schemes of HCCIs’ softening heat treatment involving high-temperature destabilization, followed by cooling in air and tempering (two-step or cycling) are developed. They result in optimal structure in HCCIs’ matrix ("granular ferrite + carbides"), decrease in hardness less than 40 HRC, increase of machinability by 35%,  reduction by 1.5-2 times the duration of heat treatment as compared with the known regimes.

The choice of parameters of HCCIs quenching which have been previously subjected to softening heat treatment and machining is substantiated. It is found that in contrast to the standard mode designated for the iron in as-cast state, the quenching should be fulfilled  from the lower on 50 °C temperature with shortened by 4-5 times holding. It allows to obtain improved by 25% abrasive wear resistance due to the formation of heterogeneous structure with metastable austenite. The results are tested in production at "South mechanical foundry" and implemented at JSC "Slavtyazhmash." The expected effect of energy saving while reducing the duration of the heat treatment and increase productivity of machining process is assumed as 1320 UAH per 1 ton of castings.

On the subject of scientific work 30 works are published (11 articles, 16 conference proceedings, 3 patents), including 6 articles published in journals referenced in international  data-bases (4 articles – in the journals of SCOPUS (with impact-factor), 2 articles - in the journals of Index Copernicus). The total number of applicant’s articles included in SCOPUS data-base is 6, the number of citations of articles in SCOPUS is 6.