Converting steel weldments to ductile iron castings.

Working closely with its foundry, this OEM found cost savings by changing several products to ductile iron cast components.

Due to the competitive nature of the truck manufacturing industry, cost reduction - without sacrificing quality - is a continuous challenge, and one of primary importance.

One of the most effective ways to reduce cost with Oshkosh Truck Corp.'s (OTC) current designs is to convert steel weldments into ductile iron castings. With this in mind, OTC is examining its vehicles to determine potential areas for cost reduction through conversion.

Founded in 1917, OTC is a Fortune 500 company that manufactures a wide variety of specialized trucks and transport equipment. Products are engineered for specific market niches where a unique, innovative design will outperform general purpose equipment.

The company's major product categories include heavy-duty commercial and military trucks, trailers, proprietary drive components, service parts, and motorized chassis for the motor home, bus and walk-in delivery van markets. OTC fills a unique role in the transportation industry as the world's major manufacturer of specialized heavy-duty trucks.

Among the firm's different product lines are snow removal, construction, airport rescue and firefighting, refuse collection, chassis and Department of Defense (DoD) products. The drive line components include engineering advances in independent suspension, all wheel steer and central tire inflation systems.

OTC doesn't operate a cast iron foundry. Rather, it relies on the expertise of its cast iron suppliers in providing quality castings that meet its engineering requirements. Consequently, the conversion of steel weldments to ductile iron castings is a team effort among OTC's engineering and purchasing departments, and the foundry that produces the casting.

The methodology that goes into a weldment conversion is composed of three primary areas, respectively:

* engineering design and material property requirements;

* financial justification in convening the weldment to a casting;

* the castability of the geometry from the foundry's perspective.

Since each area plays an essential part in the process, the ability of these disciplines to work as a team determines the success or failure of a weldment conversion.

Design Methodology

In component design, the prototype is usually produced from a weldment. Weldments generally make good prototypes because of short lead times, ease of implementing changes and avoidance of pattern manufacturing costs.

After it is produced, the prototype is tested on a test stand or on the actual vehicle at a test course. Test courses provide accelerated durability information on the vehicle. A 2500-mile test course can be designed to represent 100,000 miles of actual service. As a result, the durability of a design for its projected life can be estimated on a test course.

Upon successful completion of the test, the weldment is taken into production, where it remains unchanged from its test configuration. While it may have been originally used based on its attractiveness as a prototype, it continues into production.

In addition, costs associated with the weldment are carried through each time the component is welded and absorbed into the overall vehicle cost. Fabrication costs (the number of different part numbers that make up the weldment) and the labor hours associated with the weldment increase the component's cost.

As an example, Fig. 1 shows a steering gear bracket weldment that consists of eight separate pieces that must be cut, fabricated and welded to produce the bracket. Generally, the costs - including labor - associated with producing a weldment are 10-20 times more than the parent material used to make it. This assumption appears reasonable in light of this example. The part number maintenance of this weldment itself can be very costly compared to the one part number of a cast bracket.

This example is representative of weldments that make good conversion candidates. These weldments are predominant in suspension applications such as beam hanger brackets, shock brackets and steering gear brackets.

They are good candidates not only because of the extent of welding or the number of different parts, but because there are usually at least two of these products on each vehicle. Hence, instead of converting one weldment per truck, two are changed and pattern costs can be recouped in half the time. These applications become ideal candidates for conversion into ductile iron castings based on piece-part cost savings and use.

The best opportunity for converting weldments to castings is at the beginning of a new vehicle's development. Ideally, the casting is produced during the prototype stage. This would enable it to be tested for durability and eliminates the conversion step. This is especially important with DoD contracts.

When a component fails during a durability test, a design change may be initiated to prevent future failures. The production vehicles must reflect the configuration after these changes have been made. Because all vehicles in the fleet must have a similar configuration, if a weldment is converted to a casting midway through production, any vehicles produced before that conversion must be retrofitted to maintain consistency.

The other problem arises in having to test the conversion to show that the conversion meets the form, fit and function requirements of the weldment. Both of these endeavors are costly and reflect the importance of producing a casting at the beginning of the initial development phase. Otherwise, the weldment will remain throughout the life of the contract.

Fortunately, technologies such as stereolithography, laminated object manufacturing (LOM) and others enable the prototyping of castings. These modeling techniques can produce models directly from computer-aided design (CAD) systems that can be used as patterns for the casting. Hence, rapid prototyping gives all the benefits of a prototype weldment while also allowing the casting design to be tested during development and used in production without a conversion step.

Although stereolithography and LOM techniques are becoming more common in the development stage, weldments exist and continue to be used in new vehicle designs that could be converted into ductile iron castings.

Conversion

The conversion process 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;

[TABULAR DATA FOR TABLE 1 OMITTED]

* Purchasing - someone who has knowledge of the weldment's quantities and cost;

* Foundry Official - who can identify the cast-ability of the weldment and what (if any) geometric changes must be performed to enable casting of the part's geometric envelope.

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), 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. The team walks around the vehicle and identifies candidate parts for castings. 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 weld-merit are only 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 between 96-140 Bhn, the conversion to a 6545-12 or 60-40-18 grade of ductile iron increases tensile strength [TABULAR DATA FOR TABLE 2 OMITTED] [TABULAR DATA FOR TABLE 3 OMITTED] 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 micro-structural transitions inherently present in the 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 reduced density and increased damping characteristics. In addition, the elimination of weldments provides the component with improved function through more uniform loading in the casting cross section.

Optimizing Design

Examination of a weldment for potential casting conversion also provides an opportunity to optimize the component's design. As part of the casting conversion process, a finite element analysis should be performed and confirmed with actual testing. During this activity, the design can be optimized for a particular application and load case.

Time may not have been available during the prototype stage to perform this type of in-depth analysis. Instead, reliance was placed on experience and a liberal safety factor. Hence, examining it for a casting conversion provides a unique opportunity to reinvestigate the part. A redesign may improve its function or merely improve its cosmetic appearance.

Casting the weldment will also result in fewer internal stresses, depending on how it is cooled, which results in less distortion during subsequent machining operations. This will aid in more accurate dimensional conformance.

Weldments inherently contain residual tensile stresses that can approach the materials yield strength. Removing material during machining operations has a tendency to relieve these residual stresses and generate distortion. Only if the weldment is stress-relieved before machining can the distortions be eliminated. These benefits lend justification to converting the weldment into a casting.

Austempered Ductile Iron

In addition to the 65-45-12 and 60-40-18 ductile iron grades available, the onset of austempered ductile iron (ADI) provides a conversion material that can accommodate weldments produced from high-strength, low-alloy, quench and tempered steel plate, such as those grades referenced in ASTM A514.

Table 2 provides a summary of material characteristics available. Keep in mind that the base metal's strength has little effect on the as-welded fatigue strength. Although ADI is lower in ductility and toughness, the tensile properties of a 150-100-07 grade are comparable to the ASTM A514 material. The lower hardness ADI grades are commonly used in structural applications, while the higher hardness ADIs are excellent for wear applications.

It is essential to thoroughly understand the weldment's specific application, and the critical material properties prior to converting it to a casting. Each team member must be involved in the decision process. Some weldments may be ideally suited for conversion to ADI, such as those designed specifically to yield strength characteristics; while others - such as those designed specifically to accommodate shock loading at low temperatures - may not.

Cost Justification

While there are many benefits and options to consider before a conversion can be made, the bottom line is cost justification. Questions such as "How much will the casting tooling cost?" and "How much will be saved on the piece parts per annum?" are the team's primary concerns.

After candidate weldments are selected during the "walk around," a formal cost study is done to assure that the casting offers an annual savings in comparison with the weldment's cost.

Table 3 contains six generic weldments that OTC determined to be good candidates for conversion. The weldments consist of various steering gear and beam hanger brackets.

Good conversion candidates need enough use in production to accommodate the tooling cost associated with the casting pattern. Amortizing the pattern cost into the casting cost produces a paperwork nightmare for both the foundry and the original equipment manufacturer (OEM). A much simpler, more cost-effective approach is to maintain a tooling budget to purchase the patterns up-front.

After evaluation, the six parts in Table 3 result in a potential first year savings of about $66,246 - after the cost of the patterns are subtracted. Hence, a financial justification exists in all cases to convert the candidate weldments to castings.

Figures 2-3 show additional conversions to weldments on OTC's product line. (See sidebar below for more complex examples of successful casting conversions.)

OEM-Foundry Partnership

It is essential that the foundry be involved during the early stages of the conversion process and participate in the "walk around." The foundry understands its own capabilities and can offer valuable insight when choosing conversion candidates.

As a member of the conversion team, it is important that the OEM works with its foundry on the project. The foundry understands its casting processes and the methods and geometries that work best in its facility, and can provide insight into any changes that can be made to increase the part's castability.

The foundry can predict how much shrinkage can be expected, the tolerances available, the minimum section thickness that can be poured, how much machine stock should be left, the draft angles, and how to orientate the pattern so the parting line doesn't reside in a high-stress area. Prediction of these variables is critical to the weldment conversion's success.

Casting the component also offers the ability of casting in logos and product numbers that aren't available with weldments.

However, because most standard steels provide better weldability when compared with ductile iron, when the conversion candidate is subsequently welded onto another structure, retaining the weldment may be more desirable.

Communication between the foundry and OEM should be open and honest to optimize the benefits and recognize the drawbacks. Since all foundries vary, what works well for one may not for another.

The process of converting weldments into castings is a partnership effort between the customer and the foundry.

Design, Manufacturing & Cost

Conversions are advantageous to the product engineer and the manufacturing plant. Conversion candidates must offer improved value, lower piece price or improved installed cost by solving other manufacturing problems.

Converting weldments to ductile iron castings:

* eliminates weld zone defects;

* provides more uniform loading throughout the part:

* improves strength to weight ratio (especially with ADI);

* reduces internal residual stresses;

* improves machinability, damping characteristics and appearance through redesign;

* provides parts consolidation.

Welded brackets are costly in both the number of pieces required to make the weldment and the labor hours associated with the joining process.

This article was written following a presentation given by Robert Hathaway and Tim Fleming, Oshkosh Truck Corp., and John Young, Ohio Cast Products, Canton, Ohio, at ASM/TMS 1994 Materials Week.

References

J. Doering and R. Wilson, "Nodular Iron Conversions at John Deere," Metal Progress, (July 1974), pp 83-86.

J.R. Laub, "Austempered Ductile Iron Comes of Age," Metalworking News, April 23, 1990, p 21.

"Cost Saving Design Ideas," Designs in Ductile Iron, The Ductile Iron Group.

J.M. Barsom, S.T. Rolfe, "Fracture and Fatigue Control in Structures," Applications of Fracture Mechanics, Second Edition, (1987), pp. 93.

RELATED ARTICLE: Poor Candidates for Casting Conversion

* parts with low annual use (unable to accommodate the pattern costs);

* parts that rely on specific materials characteristics unavailable with ductile iron;

* parts whose geometries aren't readily castable.

RELATED ARTICLE: Good Candidates for Casting Conversion

* complex weldments that consist of several individual pieces;

* parts that have a castable geometry;

* parts welded from a material whose engineering material properties can be approximated by the foundry.