Journal of Al Azhar University Engineering Sector Vol. 5, No. 14, January 2010, 49- 58 ENHANCED MECHANICAL PROPERTIES OF AUSTEMPERED SG-CAST IRON BY TWO-STEP AUSTEMPERING HEAT TREATMENT Mahmoud Hafiz Al-Azhar University, Faculty of Engineering, Department of Mechanical Engineering, Nasr City, Cairo, Egypt, Postal No. 11371 . ) .( ٣ ٢ ٠ ° ٤٠٠ ° . . ٤ ٠ ٠ ° ٣ ٢ ٠ ° . ) ٤٠٠ ° ٨ ٣٢٠ ° ٥ ٢ ( " - " . ﻋﻠ ـ ﻰ . ABSTRACT Study of a single step austempering and two-step high–low temperature regimes has been carried out. The results have shown that with increasing austempering temperature from 320 to 400°C, the morphology of ausferritic ferrite changes from acicular or lath to feathery. This microstructural evolution leads to an increase in ductility and a decrease in strength and hardness. Two-step “high–low”, (400– 320°C) temperature austempering produced a mixture of upper and lower ausferrite matrix microstructure. An improvement in the elongation to fracture and impact properties was observed after 400–320°C two-step processing. KEYWORDS : SG-Cast Iron, Two-Step Austempering, Tensile Properties, Impact Properties 1. INTRODUCTION Austempered spheroidal graphite (SG) cast iron gains wider acceptance in many engineering sectors owing to its excellent combination of mechanical properties, weight savings and low manufacturing cost [1-3]. These properties are provided by the unique matrix structure of ausferrite, which is produced and controlled by the chemical composition and the isothermal holding at the austempering temperature in the range of 250-400°C [4-9]. 49