Differences between Casting and Forging Steels

There are several different methods and techniques that can be used in manufacturing a variety of different metal components. Based on the purpose and ultimate use of the metal part, different procedures are used to create the highest quality piece possible. While determining the best process, some arrive at the question of whether they need castings or forgings. Which would be more effective for their metal component and most cost effective? Keep reading to find out the differences and find out which process would be more beneficial to your business!

What Are Forgings?

Forging is a technique where metal is heated up and then hammered and manipulated into the final desired shape. Forgings are generally stronger than castings, but there are some limitations to manufacturing capabilities of forging. Components can not be very intricate or complex when forging. Usually if a component needs to be forged and also needs to meet certain size requirements, the component needs to be machined after it has been produced.

What Are Castings?

The casting process is where the metal is heated until molten and then poured into a die to form a specific shape. This process is beneficial when the shape of the component is intricate or when the component needs to be a very large size. Another advantage of castings is that machining costs can sometimes be cut down because the component can be made to certain specifications.

Are Castings or Forgings Better?

Which metal component production method is better? This will completely depend on what the component will ultimately be used for. Both castings and forgings have their advantages, so depending on what the component will be used in will determine which production method will serve you better.

Non-Destructive Testing

Large, heavily loaded parts are often non-destructively tested (NDT) in order to verify internal part integrity. The most common methods are ultrasonic (UT) and radiographic testing (RT).

Common specification pitfalls are to discount the effects of surface finish and machining when specifying NDT methods. For example, since UT functions by measuring reflected sounds waves, it works best on a part that is machined and has two parallel surfaces. Using UT on an un-machined surface compromises the sensitivity of the test. RT indications will change appearance before and after machining since the section thickness is reduced.

The main benefit of RT is that a permanent record is created. The acceptance criteria are based upon a comparison against ASTM reference radiographs, which are rated 1 through 5 (best to worst). The SFSA (Steel Founder's Society of America) sponsored a research project to determine the applicability of the ASTM referenced radiographs. In essence, the study had experienced ASNT Level III radiographers evaluate the reference radiographs in a blind test. This group was able to agree on the best and worst conditions (levels 1 and 5). However, this expert group could not agree on which reference radiographs represented the middle levels of 2, 3, and 4.

Both of these examples demonstrate that each method has its limitations, and the purchaser and the producer need to understand these limitations. Application of a stringent NDT requirement does not necessarily result in a high-quality part.

Summary

The main difference between a steel casting and a forging is that the forging is mechanically worked after solidification. This mechanical working imparts directionality, or anisotropy, to the forging. Castings and forgings are both susceptible to manufacturing problems and misapplication by the buyer.

In general, a forging is best suited to simple configurations that can be easily worked in a die or other tooling. It is also suited to applications in which the principal applied stresses are the same as the direction of mechanical working. A casting is best suited to complex shapes, custom or tailored chemistries, and to applications that are subject to multi-axial stresses.

Casting buyers need to work closely with foundries at the design stage in order to insure that the design is able to take advantage of directional solidification. The poor quality image of castings is often the result of the buyer not understanding this process. The casting buyer must also understand that there are limitations to relying solely on NDT to verify quality. Quality is best enhanced by using tools such as solidification modeling at the design stage to insure the production of a high-quality product.

Reference(s) .. www.ferralloy.com