Forging and Investment Casting Explained

[scotjute]

Just a note to add to fletcher's writing, if my memory is still correct from college, the grains in the forged piece will align themselves along the lines as shown in fletcher's picture. The longer the piece is heated, the larger or coarser the grains will grow. Smaller grain size was generally equated with increased strength. Once optimum grain alignment and size is reached, the piece is quenched to "freeze" the desired grain structure into place. This induces stresses into the metal piece. These stresses were relieved somewhat by normalizing or further heat treatment.
Generally speaking, a good piece of plain carbon steel properly heat-treated is preferable to a high quality alloy steel with a lousy heat treatment.
Good post fletcher!

 

[fletcher]

^ Good way of putting it. Heat treatment can really make or break a high quality part.

I did a quick search on carburizing and I saw 2 hr times in one paper. So I guess carburizing takes hours.

Carburizing does take a while. It takes quite some time to diffuse a large amount of carbon atoms a few thousandths of an inch into a piece of metal. Diffusion is exponentially dependent on temperature, so raising the carburization temperature greatly reduces the time required (and we all know processing time = $). However, the higher the temperature, the greater the grain growth. As a result, longer times at lower temperatures are required. While grain growth can be corrected with additional heat treatments, it is undesirable since it would add time, cost, and risk decarburization of the surface.