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Austempered irons garner interest.

Because of their superior properties and potential to create new iron casting markets, austempered cast irons continue to be the center of much research and development work. ADI and ductile iron, in general, made up the lion's share of the cast iron program this year.

In his continuing work on austempering B. Kovacs, Kosmar Enterprises, studied the prediction of strength properties in austempered ductile iron through acoustical measurements. This testing method can be used to predict the strength properties of ADI, making it a useful tool for both quality and production control. Kovacs related the accoustical properties of ADI to yield strength, tensile strength, elongation and density as a function of the austenitizing and austempering temperatures. Resonant frequency, sonic velocity and internal damping were measured in ADI samples, which were austenitized at 1550, 1650 and 1750F, and austempered at 500, 600 and 700F.

Kovacs demonstrated a close relationship between accoustical and strength properties. Understanding this relationship, he said, makes it easy to determine what grade the ADI part is after heat treatment. Also, if the chemical composition is known, strength properties can be assessed.

In other research aimed at improving the understanding of ADI, Michigan Technological University, researchers presented their work on the tensile and fatigue properties of relatively pure ADI. They focused on hot isostatic processing (HIP) as well as comparisons of relatively pure material with heavily alloyed ductile irons.

They demonstrated that HIP prior to austempering resulted in a small increase in UTS while yield strength remained fairly constant. Ductility changed dramatically, however, with increases ranging from 21-100% from the bottom of the casting to the riser/casting interface.

The comparison of relatively pure and alloyed irons indicated properties improved with the relatively pure material. All improvements in tensile performance, however, couldn't be attributed to alloy purity due to the likely presence of bainite in the matrix of the heavily alloyed iron.

Carefully controlling the metal microstructure is essential to achieving the benefits of continuous cast ductile iron, said Y. Lerner, Grede Foundries, Inc. He noted the microstructure and service properties of continuous cast ductile iron are influenced by several key solidification factors--primarily a magnesium treatment method that preserves the structural and mechanical properties of the metal.

His investigations revealed one factor consists of changing the ratio of cooled to noncooled portions of the graphite casting dies. This, in turn, relies on a second important factor, the regulation of the water flow rate into the cooling jacket. His presentation detailed the development of an adjustable, jacketed die to facilitate the differential cooling rate. The die design provides accelerated, but uniform, heat withdrawal to regulate solidification for large bars. This assures a homogenous microstructure consisting of a large number of graphite nodules/|mm.sup.2~ will form in the surface zone of the cast bars.

A woven refractory cloth insert was described by R. Hetzler, Bellville Foundry, and C. Loper, University of Wisconsin-Madison, that, during shakeout, affects the breakup of large, complex gating systems and the self derisering of ductile and gray iron top risers.

The cloth inserts were tested over 50 different patterns of varying sizes and modified to accept the cloth inserts in the gating and risering systems. The inserts substantially eliminated the problem of gating system entanglement in shakeout and reduced the possibility of accidents occurring during degating and derisering.
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Title Annotation:CastExpo '93: 97th AFS Casting Congress, Chicago
Publication:Modern Casting
Date:Jun 1, 1993
Previous Article:New processes and materials reported.
Next Article:Superalloys, ceramic shells examined.

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