Achieving Higher Strength and Ductility in Heavy-Section Ductile Iron Castings

Air-cooling from an intercritical (IC) temperature produced an acicular pearlite in ferrite structure and was effective in increasing the strength and ductility of ductile iron in sections up to 10 inches (250 mm). Five alloys were melted and poured as 3-inch Y-blocks, and IC heat treatment produced a fine-grained acicular pearlite + ferrite structure in all five alloys. The compositions ranged from 3.39–3.59% C, 2.34–2.45% Si, 0.29–0.44% Mn, 0.59–0.61% Cu and 0.24–1.78 % Ni, with CE ranging from 4.34 to 4.40. Samples were removed from the bottoms of the Y-blocks to obtain test samples for tensile and Charpy V-Notch (CVN) impact testing. The test samples were heat-treated at intercritical temperatures near the upper critical temperature (UCT) and cooled at various rates to simulate air-cooling of castings up to 10 inches (250 mm). Prior to intercritical heat treatment, experiments were conducted to determine the optimum heat treatment parameters. It was found that the path of the thermal cycle on heating to the IC temperature can affect the quality of the heat-treated structure. Among the things learned are the following: The lamellar austenite in ferrite structure persisted at temperatures as high as 60° F above the predicted UCT. Heat treatment at UCT + 20 °F produced the most dense, acicular pearlite + ferrite structure. Ferritizing prior to IC heat treatment minimizes the pearlite network in the cell boundaries. A minimum soak time of 2 h at the IC temperature is recommended. The lamellar structure is stable for at least 8 h. Copper appears to delay the time to reach equilibrium at the UCT. Nickel (at 1.8%) counteracts the Cu effect. The IC heat-treated samples displayed significantly higher ferrite contents over the as-cast materials. The high strength and elongation were attributed to grain refinement and increased ferrite contents, respectively. Furthermore, the material appears to be forgiving of marginal graphite structures. Despite the coarse graphite inclusions with poor shapes, 7–9% elongation was attained in test bars exceeding 60 ksi (414 MPa) yield strength when cooled as a 10-inch (250 mm) section.