- Joined
- Jun 11, 2006
- Messages
- 8,651
I came across something the other day that made me think. It was a post on a forum by a guy that is more knowledgeable then I. His claim is forging does not do much for simple carbon steels because we can reduce grain size and devolve the aloys by simple heat treating methods. But he said air hardening high aloy steels (stainless) do benefit from forging quite a bit. This is becaus the aloys tend to band togather and don't devolve at the normal tempatures. He says forging breaks up these aloy bands and spreads them out making for a more uniform steel which is tougher.
Thy way he worded it made sense but I am not an expert. It did get me to start thinking about forging and the benefits of it besides just fun. I will quote his post below if that is ok.
Thy way he worded it made sense but I am not an expert. It did get me to start thinking about forging and the benefits of it besides just fun. I will quote his post below if that is ok.
Larrin:
I've heard or read from many established, respected forgers that they do not forge stainless steel because it "does not benefit from forging." However, the truth is, there is more benefit to forging stainless steels than there is to carbon steels! Gasp!I'm just going to break things down into good ol' metallurgy so we can understand the reasons why. There are several things you are trying to improve through forging, two chief ones are to decrease carbide size and decrease grain size. BTW, I can quote sources for much of this article straight from "Tool Steels" by ASM international, a highly respected book in the industry. I'm just putting together thoughts from a highly spread out group of information from several sources so that it's in one place.
We'll start with carbide size first. Carbides are very hard particles that form in steel from chemical bonds between carbon and various alloys, the most basic being iron, but most of the alloys form carbides: molybdenum, chromium, vanadium, etc. Carbides greatly add to wear resistance, more than the hardness of the steel. For the keenest possible edges, you want the smallest possible carbides. Carbides also greatly control toughness, again, you want as small as possible. When the original ingot is cast, the carbides are very large, and form in a tree pattern called "dendrites." There are two different methods to forge down that large ingot: rolling and forging. Though rolling is also a method of forging we will treat it as a different entity here. Forging is forging with a hammer or a press, the steel doesn't really go in any one direction. With rolling, the high alloy (chromium, molybdenum, vanadium) carbides form strings, they all collect and form in one direction, causing stress risers. Also, the carbides do not break up as easily when rolling, because they are all clumping together so that they do not break up. The thing with carbon steel is that cementite, or basic iron carbides, dissolve at about 1700F, so even with rolling, they do not form strings, and when they re-precipitate on cooling, they are uniform and evenly distributed. Steels with less than 3% chromium do not form chromium carbides, only high chromium cementite, or iron carbides, so even though 52100 has chromium, its carbides still dissolve at typical forging temperatures. Most steel mills roll the steel out, into round bar, flat bar, etc. So with air hardening (steels with more than 3% chromium), and stainless steels, a decrease in carbide size, and more uniformly distributed carbides, can be achieved if forged with a forging hammer instead of using the rolled bar stock from the mill. Some mills actually do use forging processes to prevent the carbide strings, but they are very few. The only ones I am aware of are Sandvik and Uddholm strip (makers of AEB-L and 13C26 stainless steel). With most air hardening or stainless grades, the carbide size will never be as small as carbon steels, but it can be greatly reduced, as I've said.
Carbides are broken up much more easily at higher temperatures, but luckily grain growth occurs at much higher temperatures with high alloy steels, so forging at low temperatures such as Ed does with his 52100 is not necessary. Some guys get in to trouble forging stainless, they think since it's stainless they have to get their forge as hot as they can get it. The truth is, after a certain temperature the steel actually gets harder to forge, 154CM is a good example in that above 2150F it becomes stiffer under the hammer. While high alloy steels are more difficult to forge, the rumor that they barely move under the hammer, or that they crack in forging, is perpetuated by guys who were trying to forge at ridiculously high temperatures, such as in the 2400F+ range.
Now we get to grain size. Like I said, grain growth occurs at much higher temperatures in higher alloy steels. The best thing to do would be to find information on your particular steel, to find at which temperature grain growth starts to occur, but this is not always possible. Luckily, the recommended anneal for stainless and high alloy steels typically reduces the grain size and makes it more uniform if there has been any grain growth in forging. It should not make it larger if you did succeed in making it smaller through forging.
Then where most of the grain refinement comes in: multiple quenching. The rules change with multiple quenching with air hardening and stainless steels. If you double quench at full austenitizing temperature, you will get a duplex grain rather than a decrease in grain size. A duplex grain is one that has very large grains mixed with small ones, which is more brittle, not tougher. For high alloy and stainless steels, you have to use prequenches. With D2, the recommended range is 1600-1800F for the first quench, followed by another at full austenitizing temperature. According to "Tool Steels", a grain size of 17 is possible for D2! This is brand new information to me, and I am going to study to find out what prequench range must be used with other air hardening and stainless steels.
IMG_0884