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Compare Casting vs. Forging

Comparison of casting and forging, including what is casting, and what is forging? Why use castings, and why use forgings? The advantages and disadvantages of castings and forgings. How to choose between casting and forging?

What is casting, and what is forging?

Casting is the process where metal is heated until molten. While in the molten or liquid state it is poured into a mold or vessel to create a desired shape.

Forging is the application of thermal and mechanical energy to steel billets or ingots to cause the material to change shape while in a solid state.

Forging and casting are two very different manufacturing methods. When something is cast the material is heated above its melting temperature and poured into a mold where it solidifies. When something is forged it is physically forced into shape while remaining in a solid state – although it is frequently heated.
As an engineer, I have always known that forgings normally have less surface porosity, finer grain structure, higher tensile strength, better fatigue life/strength, and greater ductility than castings.
In other words, forgings are generally better for shackles. The basics of why are pretty simple. When you melt metal to cast it, the grain size is free to expand. When it cools back to a solid, the grain structure is courser and more random, decreasing its strength.

Why use castings?

The advantages of casting include:

  •  No real upper size limit in casting weight

  •  Large range of alloy choices

  •  As forgings remain solid, custom alloys are far more difficult to get into production whereas with casting, alloys including Chrome, Nickel and Mo can be added at the molten stage

  •  Tooling is often less expensive than forge dies

  •  Smaller production “runs” required

  •  Complicated/complex parts are no problem

Casting advantages and disadvantages

Advantages of Die Casting,
Investment Casting and Sand Casting

Disadvantages of Casting

Higher Tolerances – the metal molding process offers an opportunity for a higher level of detail and meeting of exact specifications.

Structural Integrity – products formed by casting are more porous than forged products.

Few Secondary Operations – casting provides a primary product that will require very few secondary machining operations.

Costs – tooling costs are generally higher for die casting than for forging. Tooling costs for other casting methods may vary.

Production Rate – once a casting mold is created, the production process can allow for a high production rate.

Process Control – the casting process requires frequent monitoring and inspection to maintain quality and prevent defects.

For general parts as well as large and complex components - casting is a fantastic method of manufacture.

Why use forgings?

Forging offers uniformity of composition and structure. Forging results in metallurgical recrystalisation and grain refinement as a result of the thermal cycle and deformation process. This strengthens the resulting steel product particularly in terms of impact and shear strength.
Forged steel is generally stronger and more reliable than castings and plate steel due to the fact that the grain flows of the steel are altered, conforming to the shape of the part.

The advantages of forging include:

  •  Generally tougher than alternatives

  •  Will handle impact better than castings

  •  The nature of forging excludes the occurrence of porosity, shrinkage, cavities and cold pour issues.

  •  The tight grain structure of forgings making it mechanically strong. There is less need for expensive alloys to attain high strength components.

  • The tight grain structure offers great wear resistance without the need to make products “super hard” We have found that, on a blank HRC 38-42 forged grinder insert wear/wash is about the same as a high alloy HRC 46-50 cast grinder insert. The difference being a HRC 46-50 casting does not have the ductility to handle high impact grinding.

Based on one paper written by the members of the Industrial & Manufacturing Engineering Department at the University of Toledo, shared by the Forging Industry Association.

  • Forged parts had a 26% higher tensile strength than the cast parts. This means you can have stronger shackles at a lower part weight.

  • Forged parts have a 37% higher fatigue strength resulting in a factor of six longer fatigue life. This means that a forged shackle is going to last longer.

  • Cast iron only has 66% of the yield strength of forged steel. Yield strength is an indicator of what load a shackle will hold before starting to deform.

  • The forged parts had a 58% reduction in area when pulled to failure. The cast parts only had a 6% reduction in area. That means there would be much greater deformation before failure in a forged part.

Forging advantages and disadvantages

Advantages of Forging

Disadvantages of Forging

Structural Integrity – development of grain flow through beating of the metal leads to increased product strength and a high level of material predictability.

Tolerance Levels – products formed through forging may not meet requirements for high tolerances.

Costs – materials are generally less expensive than the materials required for casting. Limited scrap and rework. Reduced labor costs and lower tooling equipment expenses than casting.

Secondary Operations – many forged products require secondary processes to refine and finish to exact specifications.

Reliability – consistent ductility, known yields, and increased strength due to grain development.

Limitations & Defects - forged products may be limited in shape and may include defects from die failure.

How to choose between casting and forging?

There are many important factors to consider when selecting the best process to use for the manufacturing of your product, including:

  1. Suitable for process. Forging has great restriction for the complexity of shapes. If the part is too complex for forging, then the casting will be the better choice.

  2. Cost consideration. Castings are cheaper than forgings normally, so if castings could meet the requirements, then castings will be the better choice.

  3. Machining consideration. Normally, the forging parts will be very simple; it can not produce complex shapes, so the further machining works will be needed. If you want to reduce the machining costs, then castings will be the choice.

  4. Special mechanical properties. If you required the special mechanical properties, such as high hardness, abrasive resistance, anti-rust ability etc., then castings will be the easier choice.

All in all, if casting could meet the requirements of mechanical properties and stable quality, then it will be the better choice, however, if it can not meet your requirements, then you will have to consider forging or fabrication. This is from Dandong foundry in China.

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