Weldment to Casting Conversion
Casting designs allow the designer more freedom in the part design than a weldment. The design can be very fluid and conform to almost any shape. As well castings generally have equivalent fatigue properties when compared to a fabricated design.
One of the benefits of a cast design is the ability to have tapered sections, where as in a weldment you may have to use a certain thickness of plate that may only be needed for one high stress area in the design. Casting designs can have material placed where it is needed and removed from where it is not needed. This allows for a more efficient and cost effective design. Castings also exhibit unidirectional strength compared to differences in plate strengths due to rolling direction.
All weldment features can be cast into the design. Bosses can be cast into the part at bolting locations. In a weldment, these bosses would need to be fabricated and then welded on. Complicated sections can be easily recreated in a casting, in a weldment it is difficult and expensive to create a complex cross section. As well, if you are dealing with a part that flows fluid, a casting can be created that streamlines that fluid flow.
Because most castings undergo some sort of heat treatment, the internal stresses in a casting are typically lower than a weldment. Weldments also will have some distortion in the final part due to localized stresses at the weld areas.
Defining a Good Conversion Candidate
Any weldment can be converted to a casting but there are times where a weldment may be more economical. As you start looking at weldments and designing a cast equivalent, it is good to start with the weldments that take the most time to fabricate and may not make the best use of the plate material. As well, if there are any current issues with the weldment, such as fatigue failures, a casting may be a good candidate.
Designers who are considering the conversion of a weldment to a casting should first consider the molding process required to obtain the desired geometry. Steel investment castings yield excellent material properties and greater design flexibility but may be priced higher than a comparable ductile iron sand casting because of the material cost.
Steel is typically the choice if:
- Excellent material properties are required
- The casting may experience impact loading
- The casting needs to be weldable
A steel investment casting is the choice if:
- There are many undercuts and zero draft areas required.
- Good surface finish is required
- Good dimensional control is required
An iron casting is typically the choice if:
- Impact toughness of the material is not a concern
- Surface finish is not a concern
- The part is cost sensitive
Once the material and casting process has been determined it’s time to start looking at the weldment to see how a casting design would work within the design constraints.
Weldment Conversion Example