Ductile iron properties and processes studied.

As competitive pressures mount for improved properties and process control of cast iron, metalcasters and researchers are studying methods of enhancing both.

Among the areas under close scrutiny are the properties of austempered ductile iron (ADI) and the use of computer simulation to determine the solidification patterns of ductile iron.

Because of its combination of toughness, strength, fatigue strength and wear resistance, ADI production has increased dramatically in recent years. While information regarding ADI's fatigue properties has been scarce, several researchers presented new papers on the subject at this year's Casting Congress. K.-P. Jen and J. Wu of Villanova University, Villanova, Pennsylvania, combined efforts with S. Kim, Drexel University, Philadelphia, to study the fracture and fatigue behavior of four grades of ADI.

Experimenting with one grade, they found the ductility of ADI austempered at 750F is unexpectedly lower than that at 676F. This indicates that austempering this ductile iron at 750F for 90 minutes isn't beneficial for its ductility and strength. To obtain the best combination of strength and ductility, this ductile iron should be austempered at 575-676F, or the time should be shortened at 750F. If the austempering temperature is lowered to 500F, however, both ductility and strength are decreased.

Tensile specimens indicated that microvoid coalescence of dimples is the main fracture mechanism for all four grades. For 90 minutes at 750F, shallow dimples develop. Austempering at 575-676F will produce conical equiaxed dimples, yet still provide the best ductility and strength.

Quality control on the original cast iron is the most important factor to provide dependable fatigue life.

D. Krishnaraj, H.N.L. Narasimhan and S. Seshan, Indian Institute of Science, Bangalore, India, concentrated on the structure and properties of ADI as affected by low alloy additions.

They concluded that additions of nickel, molybdenum and copper result in marked microstructural changes in bainitic ADI. All three of these alloy additions slow down the transformation to ADI, as well as change the bainite morphology.

In the thin test sections investigated, they found tensile strength generally decreases with alloying. When alloying is essential, a balanced addition of nickel and molybdenum should be used because molybdenum tends to neutralize nickel's adverse effects. Copper additions, on the other hand, do not bring about any appreciable change in tensile strength.

Additionally, alloying ADI increases elongation initially. A judicious combination of alloy contents and austempering conditions is essential to reach high toughness in ADI castings while avoiding adverse effects.

K.L. Hayrynen, D.J. Moore and K.B. Rundman, Michigan Technological University, Houghton, investigated the tensile and fatigue properties of relatively pure ADI. The team austenized this metal (ferritic as-cast matrix) at 1700F for two hours and then austempered at 770F, 700F and 601F.

They concluded that ultimate tensile strength, elongation and percent reduction in area change with position in the casting. Yield strength and fatigue strength, however, aren't sensitive to location.

Their results also showed fatigue strength wasn't affected by using three different austempering temperatures.

Their findings also showed that the relatively pure material exhibited significantly better tensile properties than ASTM specifications, but there's no significant gain in fatigue strength in using pure ductile iron for austempering.

Presenting information on the fatigue crack growth behavior of ADI were L. Bartosiewicz and A.R. Krause, Ford Motor Co., Dearborn, Michigan; B.V. Kovacs, Applied Process Co., Livonia, Michigan; and S.K. Putatunda, Wayne State University, Detroit.

The team discovered that fatigue threshold values in ADI increase with a decrease in volume fraction of ferrite. Fatigue threshold rises with an increase in volume fraction of austenite. Heat treated samples containing a fine grained microstructure and smaller grain size provide highest tensile strength and hardness.

Touching on factors affecting fatigue strength in commercial ductile iron castings were D. Venugopalan, University of Wisconsin-Milwaukee, and A. Alagarsamy, Grede Foundries, Milwaukee. They studied and compared the fatigue behavior of ductile iron under axial and rotary bending conditions.

From experiments on loading type effects, they concluded the fatigue endurance limit of ductile iron in rotary bending is higher than the endurance limit in axial loading. Ductile iron pouring practices such as gating design influence the amount of defects and, thus, the fatigue properties, and shot peening and shotblasting increase fatigue endurance limits.

Compiled into an AFS special report, seven papers were presented dealing with the numerical simulation of a set of standardized cast iron castings.

Each of the seven participants in the study was given identical casting blueprints of T-plate castings and encouraged to focus on the capability of the software to assess the shrinkage tendency of the casting. Each firm was also given specialized information as to chemistry weights, pouring temperatures, molding media and sprue geometry.

"Even the best castings sometimes go wrong," said C. Corbett, Micromet, Ltd., West Bromwich, England. "There must be a great deal of flexibility within the program for users to make their own judgments."