Gray Iron Property Measurements Using Ultrasonic Techniques

Correlations between ultrasonic velocity measurements and the gray iron microstructure, machinability, and selected mechanical properties were developed based on samples removed from laboratory and commercial castings. Some castings were heat treated to produce some ferrite in the microstructures. At the same ferrite content, irons with higher graphite content have lower ultrasonic velocity values. Ferrite was found to have a slightly higher modulus than pearlite. At the same graphite volume fraction, irons with higher pearlite content exhibited lower ultrasonic velocities. At the same carbon equivalent, gray irons with higher ferrite contents exhibited lower ultrasonic velocities, because the higher ferrite contents were associated with higher free carbon values. Brinell hardness was higher at lower free carbon and ferrite contents, and higher hardness and strengths were associated with higher velocities. Ultrasonic velocity was found to be a good indication of graphite volume fraction, Brinell hardness, and strength in fully pearlitic gray irons. Drilling experiments were conducted to evaluate machinability of commercially cast gray irons. It was found that increasing the free graphite and decreasing ferrite volumes reduced the ultrasonic velocity at a constant carbon equivalent. Irons with higher ultrasonic velocities had higher tool wear rates (worse machinability). Good correlations were found between velocity measurements and the properties of the castings. INTRODUCTION The microstructure of cast iron consists primarily of ferrite, pearlite or a combination of the two phases, and graphite. These constituents largely determine the mechanical properties and machinability. Suitable NDE procedures must reflect the volume of the major phases present. Minor amounts of harder phases including carbides and nitrides may be present, but, because of their small volumes, are more difficult to detect than the major phases. Minor phases such as titanium carbides and nitrides, phosphorous carbides, and molybdenum-chromium intercellular carbides may be important because they are abrasive to the tool tip but may be present in such small quantities that they are not detectable using NDE. Most of the prior work on NDE of cast iron has used ultrasonic and resonance techniques and focused on the relationships between the acoustic response and the tensile yield and ultimate strengths (Emerson and Simmons, 1976; Fuller, 1977 and 1980; Kovacs and Cole, 1975; Kovacs, 1977 and 1993; Lerner, 1995; Lerner and Vorobiev, 1998; Patterson and Bates, 1981). The current investigation focused on correlations between ultrasonic velocity, microstructure, certain mechanical properties, and machinability of gray iron. The ultrasonic velocity is a function of the modulus, and density of the material. The porosity size and distribution also affects ultrasonic velocity due to their effects on density and modulus. The velocity of a longitudinal ultrasonic wave through a material can be expressed as (Krautkramer, 1983): ( ) ( )( ) ν μ μ μ ρ l E = − + − 1 1 1 2 Equation 1 where νl is the longitudinal ultrasonic velocity, E is Young’s modulus, ρ is density, and μ is Poisson’s ratio. Young’s modulus increases as the square of the density, i. e., as porosity decreases (Klima and Baaklini, 1984): E = E0 exp(-bP) Equation 2 Where E0 is Young’s modulus for the nonporous material, P is volume fraction porosity, and b is a factor related to the pore size, shape and location. Poisson’s ratio generally increases with increasing density, but the effect is usually small. Paper 05-122(05).pdf, Page 1 of 11 AFS Transactions 2005 © American Foundry Society, Schaumburg, IL USA