Use of modelling to optimise homogenisation of high Mn heavy section ductile iron castings using partial melting

Abstract

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.

Keywords:

• PARTIAL MELTING HOMOGENISATION
• DUCTILE CAST IRON
• AUSTEMPERED DUCTILE IRONS
• SEGREGATION
• MODELLING