Q: We have a customer that states on every order, “Forgings must include at least a 3-to-1 forging reduction ratio from starting billet stock.” How do I figure this and prove it mathematically?
A: Forging reduction is generally considered to be the amount of cross-sectional reduction taking place during drawing out of a bar or billet. The original cross-section divided by the final cross-section is the forging ratio (say 3:1).
There is an equivalent reduction on upsetting for forgings being upset during forging (gear blanks, for example). In this case, the upset ratio of beginning billet length over final height is the upset ratio. This is similar in total reduction to the bar reduction. However, the uniformity of deformation from center to edge may not be as easy to estimate as the drawing out reduction.
The point is that the key reasons for getting a good amount of deformation in forging is to eliminate any porosity in the material and to refine the grain size, as well as to break up inclusions in the starting steel or nonferrous materials.
Because the deformations occurring in a closed die are so variable, it is not possible to provide a specific number to the reductions. This is why the forger is aiming to get the higher level of deformations in the pre-forming stages.
Consider a cast billet that is forged in a closed die. In such an example, there would be areas that are heavily worked and others that are hardly deformed at all, depending on the shape of the dies. This could result in some areas that contain voids and unhealed porosity as well as widely varying grain size. This is why the forger puts so much emphasis on the bar reductions and pre-forming reduction stages.
There is another point that I should make about the importance of reductions in forging: if the temperature for forging steel is above about 2,200ºF, then reductions of at least 15-20% are needed to ensure that the grain size from heating is refined again.
Heating to above 2,200°F (closer to 2,300°F) can cause an overheating effect if no deformation occurs during subsequent forging. This is an important metallurgical distinction. When I was specifying forging temperatures for 4140 steel, I would specify this way.
- For hot forging with large reductions: 2,325ºF max.
- For closed-die forging with wide ranging reductions but with areas that are not heavily worked: 2,250ºF max.
- For reheating for strikeovers that receive very little work: 2,150°F max.
Similar adjustments are made for other grades.
The exception to these guidelines is when forging microalloyed steels. In this case, the need for significant deformations during final working is essential to develop the properties and grain size for maximum response to the air-cooling processes that follow. This is because there is no normalizing or heat-treating cycle to refine the grain size. In this case, the final grain sizes are a function of the amounts of deformation and the cooling rates that follow forging.