M.A. Islam

Tribological behaviour of quenched and tempered, and austempered ductile iron at the same hardness level

Abstract The tribological behaviour of ductile iron heat-treated by two different procedures viz. quenching and tempering, and austempering to an identical matrix hardness of 445 KHN is compared. Wear tests were carried out using a pin-on-disc type apparatus under dry sliding conditions at a linear speed of 1.18 m s−1. Applied load and sliding distance in the range of 7.5–30 N and 2×104–6×104 m, respectively, were used. It was observed that under all test conditions, austempered ductile iron exhibits a better wear resistance than quenched and tempered ductile iron, although both have an identical chemical composition and matrix hardness. The relative superiority of austempered ductile iron becomes even more pronounced at higher load and longer sliding distance. Microhardness measurement below the wear scar reveals that the hardness of austempered ductile iron increases while that of quenched and tempered iron decreases during the wear process. Metallographic study of the worn surfaces and X-ray investigation on wear debris indicate that oxidational wear is operative in both the samples. Stress-induced martensitic transformation of retained austenite as well as strain hardening of bainitic ferrite are thought to contribute to the improved wear performance of austempered ductile iron.

Study of wear of as-cast and heat-treated spheroidal graphite cast iron under dry sliding conditions

Abstract Wear behaviour of as-cast and heat-treated spheroidal graphite (SG) cast iron has been studied under dry sliding conditions using a pin-on-disc type apparatus. Wear tests were carried out at a linear sliding speed of 0.88 m s−1, under a constant load of 1.5 kg. All tests were performed in ambient air at room temperature. Extent of wear damage and wear mechanisms were investigated by means of weight loss measurement, optical microscopy, microhardness measurement and X-ray diffractometry on wear debris. The wear rate measured after 9500 m of sliding is found to be about three times higher in the as-cast sample than in the heat-treated material. In the case of the heat-treated material, abrasive wear is the main wear mechanism. A combination of adhesive wear, delamination and surface fatigue is believed to operate in the as-cast material.

Structure and properties of carburized and hardened niobium micro-alloyed steels

Abstract Low-carbon steels containing a small addition of niobium singly or in combination with nitrogen have been carburized in a natural Titas gas atmosphere at 950°C for different time periods such as 1, 2, and 4 h. After a pre-determined time, the specimens were pre-cooled to 860°C in the furnace and quenched in 10% brine. One set of the quenched specimens was tempered at a low temperature of 160°C and the other set was sub-zero treated at −195°C in liquid nitrogen followed by tempering at the same temperature of 160°C. The relationship between the structure and the mechanical properties of the carburized and heat-treated specimens was investigated by means of optical microscopy, surface hardness and microhardness measurements, X-ray diffractometry and impact tests. It was found that niobium refines the martensite and reduces the formation of retained austenite in the case of the carburized and hardened low-carbon steels. Niobium with nitrogen was found to be more effective than niobium alone in refining the martensite and in decreasing the retained austenite content. Niobium improves the surface hardness, case hardness and core hardness. Niobium with nitrogen is more effective in improving the surface hardness and case hardness, but is less effective for core hardness than niobium. It was also found that both the NbC and Nb(C, N) particles are detrimental to the toughness of the core of low-carbon steels, but Nb(C, N) precipitates are less harmful than NbC precipitates.