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Steel fabrication V Casting

28 March 2015

Steel fabrications and castings are often considered for the same project as they share very similar properties. Generally, castings provide better tolerances and superior mechanical properties.

Casting allow designers to be visually creative and explore exciting forms. It enables shape and form where fabrications cannot. Cast parts are specifically designed and engineered to suit, and only put material where it is wanted.  In contrast, the form of a fabrications is typically driven by the availability of material, and the ability to join the parts. It cannot be as creative in form as a casting.  Add this to the generally better strength characteristics, and castings come out lighter weight and cheaper.

The nature of the casting process means that the cast steel does not exhibit directionality. These parts can be stressed in any direction. Fabricated parts use steel that has directionality ie it is strong in one direction and not in another.

CastingsCastings are a single piece created by running molten metal alloys into cast, resulting in a homogenous single piece free from joints.

Contrary to common belief, steel parts are ductile and tough, not brittle and hard, lending themselves to  designs where longer life and improved performance are an advantage.

Fabrications are very relevant for small volume production, using readily available material to create the part.  No tooling cost allows for low set up costs and short timescales to production.   Castings require an investment upfront for th the creation of a mould or tool.  Sand casting tools are significantly cheaper than injection die cast tools, but all casting requires a tooling budget and timescale.

Fabrications allow you to make alterations to the design 'on the fly' evolving the iteratively. This can be positive if your start up budget is low.

However, fabrication design needs to be very  well considered. Joining parts together can lead to inaccuracy and additional/expensive fettling to correct the compound dimensional or alignment problems. For accuracy, castings will give you more precise tolerances than fabrication.

 

Fabrications are assemblies of components that have been joined together creating a larger part.

In addition, combining multiple elements together generates stress points, requiring additional strength to be designed back in.  Fabrication design needs to be sensitive to the limitations: welds can be hard to repair and are often are the cause of failure, whereas the strongest element in fabrications is the strength of a metal parts. Creating successful fabrications requires correctly recognized and allowed for in the final design.

Can you fabricate cast parts? Yes - again contrary to popular belief, cast parts are often better to weld than wrought, as the lack of directionality provides a ductile, not too hard, weld surface. You can also design the cast parts to move the 'weaker' weld line to an area of the part that is not exposed to high stress, therefore optimising the best of both processes. Assemblies can be fabricated using cast parts, reducing assembly time and complexity.

Fabrication may be cheaper and quicker setting up, but if your project is successful, fabrication will be more expensive and a lengthier procedure in the long run than a casting solution. 

 

Reference: Malcom Blair and Raymond Monroe, Steel Founders Society of America

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