CASE STUDY: Converting a Part for Oshkosh Truck

The process of converting weldments to castings at Oshkosh Truck begins by identifying candidate weldments. A “walk around” is performed on a vehicle with representatives from:

• Engineering—those who understand the form, fit and function of the weldment;
• Purchasing—someone who has knowledge of the weldment’s quantities and cost;
• Metalcasting official—who can identify the castability of the weldment and what (if any) geometric changes must be performed to enable casting of the part’s geometric envelope, from both a castability and structural standpoint.

Weldment

Casting

Forming a team, these individuals work together to understand components that are candidates for conversion. Not all weldments are good conversion candidates. Poor candidates are weldments that have a low annual use (unable to accommodate the pattern costs of casting), rely on specific materials characteristics that are unavailable with ductile iron and whose geometries aren’t readily castable.

Therefore, the team must work together and look at all aspects of the part to determine if the weldment is an appropriate conversion candidate. These candidates are typically complex weldments that consist of several pieces, have a castable geometry and are welded from a material whose engineering material properties can be approximated by the foundry.

The tensile properties of the weldment only are a fraction of the possible material characteristics that must be considered when evaluating conversion candidates. Table 1 shows a comparison of material characteristics that may need to be considered for any application. For a typical 1020 steel weldment with a minimum tensile strength of 48 ksi, 27 ksi yield strength and hardness ranging from 96-140 Bhn, the conversion to a 65-45-12 or 60-40-18 grade of ductile iron increases tensile properties.

Normally, components are designed to yield strength characteristics with an added safety factor. But, because most failures occur from repeated application of loads (fatigue) that generate stresses below the material’s yield strength, notch geometries and microstructural transitions inherently present in a weldment become the critical factor in the survivability of the component. In most cases, the casting offers increased durability and resistance to fatigue failure by reducing or eliminating these discontinuities.

Although lower in ductility and toughness than steel, ductile iron offers significant toughness, a slight reduction in density and some damping characteristics. In addition, the elimination of weldments provides the component with improved function through more uniform loading in the casting cross section.