Effect of silicon content, austenitising temperature and variable austempering on mechanical properties of ductile iron

Abstract

In this study test specimens of unalloyed ductile iron containing three levels of silicon namely 1.9, 2,7 and 3.4% and about 0.28% manganese were subjected to a range of variable temperature and step austempering cycles. The effects of silicon, austenitising temperature and variable austempering temperature on the mechanical properties were studied. This study was divided in three parts, which will be summarised in the paragraphs below. 1- Primarily impact specimens containing 1.9, 2.7 and 3.4% silicon, were each austenitised at 850, 900 and 950°C for 2 hours and subjected to heat treatments 1 and 2, heat treatment 1 consists of austempering isothermally at 360°C and heat treatment 2 of steady cooling from 360 to 275°C for 1 hour. It was shown that for samples of increasing silicon levels optimum impact values were obtained when austenitised respectively at 850, 900 and 950°C. Decreasing the solution treatment temperature was found to increase the driving force controlling the transformation of parent austenite (y) to high carbon austenite (Yh.c) and acicular ferrite (a). For the low silicon iron, increasing the austenitising temperature beyond 850°C resulted in microstructures containing a continuous network of meta-stable low carbon austenite and martensite leading to a corresponding deterioration in impact energy. The formation of these phases was attributed to the segregation of manganese and a high austenitising temperature, both of which decreased the carbon diffusion rate and delayed ferrite nucleation and growth. Austenitising at 850°C increased the rate of austenite transformation and resulted a more uniform microstructure of stable high carbon austenite and acicular ferrite and a corresponding improvement in impact energy. Microstructures of medium silicon iron austenitised at 850°C contained pro-eutectoid ferrite zones, which led to relatively low impact energy values. Increasing the austenitising temperature to 900°C resulted in a homogenous structure and improvement in impact energy values. Further increase in austenitising temperature to 950°C resulted in a microstructure containing regions of low carbon austenite and martensite and a deterioration of impact energy. The high silicon iron austenitised at 850°C, contained pro-eutectoid ferrite zones which led to relatively lower impact energy values. The formation of this phase was attributed to the effect which silicon has on the Fe-C phase diagram and the formation of three phase regions of graphite I austenite and ferrite. Increasing the austenitising temperature to 950°C resulted in a more homogeneous microstructure and a corresponding improvement in impact energy values. This study showed that optimum impact energy values for the low, medium and high silicon irons are obtained following austenitising at 850, 900 and 950°C respectively. 2- In the second part of this study, impact and tensile test specimens with different silicon contents were austenitised at the temperature known from part 1 to yield optimum properties and subjected to various variable austempering cycles. Optimum tensile strength, percentage elongation and impact energy values were obtained with the medium and high silicon irons (2.7 and 3.4%). Samples containing 2.7 and 3.4% silicon content and subjected to heat treatments 2, 4 and 5 namely quenched at 360 and cooled steadily to 275°C, quenched at 275, heated rapidly to 360°C and held at that temperature, and quenched at 275, heated rapidly to 330 and steadily to 400°C respectively were shown to have a better combination of properties compared to isothermal treatment. For example samples of high silicon iron subjected to heat treatment 4 yield a tensile strength of 1353 N/mm2 , impact energy of 102 joule and a ductility of 6.89%. The corresponding microstructure consists of a mixture of upper and lower ausferrite. It was observed that as the silicon content increases, the ultimate tensile strength, 0.2% proof stress and hardness decreases marginally. 3- In the third part preliminary wear tests (pin-on-disk) were carried out on samples of medium silicon iron austempered isothermally at 360°C. The load and sliding speed were 5, 10 Kg and 0.5, 2 m/s respectively. It was found that a higher wear rate is obtained at the higher load and lower speed. We}lr tests were performed using a load of 5 kg and a speed of 0.5 m/s. The samples tested contained 2.7 and 3.4% silicon and were austenitised at 900 and 950°C respectively and austempered at 360°C for various duration. It was shown that austempering improved the wear resistance significantly compared to the as-cast ductile iron. The lower wear resistance was obtained at values of austempering time, which are either very short or larger than 2 hours. This can be attributed to the presence of martensite; decrease in the volume fraction of retained stable austenite and the presence of carbide. Other tribological tests were carried out on medium and high silicon iron austenitised at 900 and 950°C respectively and subjected to heat treatments 2 and 4. It was shown that variable austempering did not show any significant improvement in wear resistance compared to isothermally treated ductile iron.