EFFECTS OF MAGNESIUM ADDITIONS AND HEAT TREATMENT ON MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF AUSTEMPERED DUCTILE CAST IRON

Abstract

The effects of magnesium additions and austempering heat treatment on mechanical and microstructural properties of ductile iron produced using the rotary furnace were investigated. Two heats of eight tappings were made. Varied magnesium ferrosilicon was used as nodulizer to treat 8 kg mass of molten metal per ladle by sandwich process and poured into sand mould. Specimens were cut out using lathe machine for mechanical (tensile, hardness, fatigue, impact and wear) tests and microstuctural examinations. Samples D and E of 0.052 % and 0.061 % magnesium respectively were observed to have given improved mechanical and micostuctural properties. These two samples were further subjected to austempering heat treatment by heating to 850 ºC and soaking for one hour before quickly transferred into a salt bath of 50 % NaNO3 : 50 % NaCl maintained at 360 0C and soaking for an hour before finally air cooled. Mechanical tests and microstructural examinations were then carried out on them. Sample D had a tremendous increase in tensile strength, from 610.07 MPa to 891.22 MPa while specimen E increased from 611.16 MPa to 898.76 MPa. The result of abrasion test using sand of 800 grit size and 600 rpm in 300 seconds carried out on the samples indicated that samples D and E possessed abrasion resistance of 2.20 (m²) ×10¹¹ and 2.39 (m²) ×10¹¹ respectively. There were also relative increase in fatigue resistance and impact toughness for the two samples but with relative reduction in hardness