Forged vs Stock Removal Knives

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Forging by the Steel Company vs Bladesmith

When the steel company makes the steel they produce a large melt of the steel with the desired composition and pour it into a mold which solidifies and produces an ingot. A typical ingot size for tool steel or high speed steel might be 10″ thick either round or square depending on the production process and the desired final shape. Smaller ingots are better for limiting the segregation of alloying elements and keeping the cast structure fine. At that point the steel is in “cast” form. The ingot is then heated up to high temperature and forged either with a hammer or press to a form ready for hot rolling. In some cases the ingot is ready for rolling as-produced. Next the steel is hot rolled to the final desired thickness. Hot rolling in an industrial setting is typically done with relatively few reheats, typically single digits. That depends on how much reduction in necessary, how difficult the steel is to work, and the capability of the rolling mill.

The steel as delivered to the knifemaker has been reduced by many times. With knife steel it has typically started with an ingot several inches in thickness (10″ or so) and has been reduced to 1/4″ or thinner. The knifemaker then forges it to shape, sometimes with some reduction in thickness as well. The vast majority of the forging reduction the steel has seen was not performed by the knifemaker.

Cast vs Forged Steel

Doing a general search online for the benefits of forging will likely bring up references to cast vs forged parts. In that context they will make claims about increased strength or toughness from using forged parts. However, even in that context some claims are misleading. In one article I found they were making comparisons between “forged steel” and “cast iron.” Cast iron is not the same as cast steel, “cast iron” refers to a specific type of iron which has a very high carbon content (greater than 2%) and usually high silicon additions. Comparing forged steel to cast iron is not the same as comparing forged vs cast steel. However, none of that matters because…

Knives are almost never produced from cast steel. One example is David Boye’s “Dendritic steel” which is cast 440C. You would know if you were using cast steel because you would have paid a company to cast the steel in a knife shape for you. Cast steel has a different microstructure from forged steel and usually has some porosity to it. The cast structure which is made up of dendrites, columnar grains, eutectic, etc. is eliminated by forging. I won’t go more in depth into the structures that form during casting in this article because knifemakers don’t usually need to worry about them and it would make this article too long. Maybe in another one. All steel available from knife steel suppliers has been forged.

Grain Orientation

The direction that steel has been hot rolled is called the “rolling direction.” In other words, the steel has been rolled to be longer in one direction than another. This elongates certain features of the steel, a common one is manganese sulfides. Iron sulfides have a low melting point and make forging and rolling operations difficult. Manganese sulfides (MnS) have a much higher melting point and are elongated during rolling. These sulfides still lead to lower ductility and toughness, though lower sulfur content means less MnS and better toughness. Below shows MnS in fractured steel (elongated gray features), and you can also see fractures which grew along the MnS in a few areas:

Image from [1]

Other microstructural features can be elongated during rolling, another common one is carbide. Especially in high alloy steels where the carbide is still present at forging temperatures. Below is M7 high speed steel with different levels of forging reduction, where you can see that the carbides are preferentially in bands along the rolling direction:

Image from [2]

The resulting elongated features are sometimes called the “grain” of the material. However, that is completely unrelated to “grains” or “grain size” in steel. Using the terms “grain” or “grain direction” is more an analogy than it is a reference to a specific microstructure feature.

The elongation of microstructural features during hot rolling means that the properties are somewhat different based on orientation, as cracks will more easily grow along MnS, carbides, or other brittle features. The most brittle direction is the “transverse” direction which has lower toughness than the “longitudinal” direction. This is shown schematically below with lines representing the elongated features relative to the charpy impact specimens that are used for toughness measurements. You can read more in my article about toughness of steel.

Refining the size of the carbides helps with improving transverse toughness so that carbide banding is not present to the same extent. This is most evident in powder metallurgy steels where the conventionally produced steel has large carbides and lots of carbide banding. Read about powder metallurgy in this article. Below compares longitudinal and transverse toughness for D2 and M2 steel with either conventional production or powder metallurgy (PM):

Data adapted from [3]

However, even in low alloy steels that do not have carbide banding to the same extent there is still directionality to the properties. We compared longitudinal and transverse toughness of CruForgeV steel in a study presented in this article. There was a significant difference in toughness between the two directions despite carbide banding in CruForgeV being greatly reduced relative to a high alloy steel like D2. Therefore the directionality is probably controlled by other features like MnS to a great extent.

We have done a limited number of other comparisons between longitudinal and transverse toughness. Z-Tuff was 45.7 ft-lbs longitudinal and 29.2 ft-lbs transverse, both at 61.3 Rc. Vanadis 4 Extra was 13 ft-lbs longitudinal and 9 ft-lbs transverse when at 64.6 Rc. No appreciable difference in toughness was seen between direction in Vanax or M390, which may be because there is less difference between orientations at lower toughness values.

Forging to Shape

All of this information about orientation is important because a common argument for forging of knives is that the grain orientation is modified through forging for improved properties. Perhaps an image such as the following is used to show the improvement:

Image from [4]

However to understand if the forging to shape provides benefits we have to look at actual knives, not the idealized shapes shown above. There are two major orientations used with knives:

Generally the weakest parts of a knife are the tip and the edge. Using the transverse direction shown above leads to weaker tips because the tip can break along the elongated features like MnS and carbides:

Therefore using the longitudinal direction is preferred so that tips are stronger. Whether the knife is forged or stock removal won’t affect this difference. Forging bladesmiths start with material that has already been hot rolled and has a “grain” to it already. Forging to shape provides a minor change to the orientation of the “grain” but there would still be reduced toughness in the direction where the tip can more easily break. Whether forging or using stock removal it is better to produce knives along the rolling direction instead to make the tips less prone to breaking.

In terms of edges, the weakest direction is when torquing the edge to the side as chipping can occur along the carbide bands and/or MnS. For more information you can read my article about chipping of edges. In extreme cases chipping can look like this:

Image from [5]

Forging typically leads to maintaining the orientation of the “grain” along the edge rather than modifying it, such as in this simple schematic, where stock removal is on top and forged is on bottom. Therefore I wouldn’t expect a drastic difference in edge behavior.

Other weak points in knives exist at “stress risers,” areas that are more prone to breaking based on geometry. You can read about stress risers in this article. Perhaps if the shapes that create stress risers were forged into the knife they would be somewhat less prone to breaking, it is hard to predict. However, it would be a better idea to eliminate the stress risers instead.

There is probably a knife design or application where forging a knife can lead to improved behavior based on modified grain orientation. However, those examples are not obvious. And in some cases forging bladesmiths will cut in a tip or handle before forging it to final shape, which would also reduce the extent to which the “grain” orientation is even modified relative to stock removal.

Grain Size

Another area where forging bladesmiths hope to improve their knives is through reduction in grain size. Finer grain usually means better properties. At high temperature grains grow, here is a video showing the process:

Grains can be refined during forging; the grains area “pancaked” and strain in the material is increased until the point where new small grains form, called recrystallization. I first introduced this topic in this article on cold forging of steel. With cold forging, recyrstallization does not occur until the steel is heated up to higher temperatures. When forging, the steel is already at high enough temperature for recrystallization to regularly occur, called “dynamic resytallization.” The grains are continuously deformed and then recrystallized:

Image from [6]

However, with forging of knives the amount of reduction is very small. The steel is reheated to high forging temperatures multiple times while reducing its thickness by only small amounts. So grain growth is more likely than grain refinement.

Steels with carbide present at the forging temperature see much less grain growth. The carbides pin the grain boundaries so they are unable to grow:

However, the most common steels used for forging: 52100, 1095, 5160, W1, etc. the carbides are fully dissolved at forging temperatures and grain growth occurs relatively rapidly. CruForgeV and W2 have some vanadium carbide which is stable to higher temperature to help suppress grain growth. Some O1 also has a vanadium addition. High alloy steels which have stable carbide up to high temperatures like M2, D2, and most stainless steels and powder metallurgy steels, grain growth is suppressed at common forging temperatures. Here is a chart showing several different steels, higher number fracture grain size means a finer grain (lower on the chart):

Image from [2]

It is not impossible to refine the final grain size of heat treated steel through various forging processes [7], but steel simply being “forged” does not guarantee grain refinement, and the opposite is perhaps more likely. Without a designed forging process that is tested for grain size or toughness improvement, the grain size has probably not been reduced.

Thermal Cycling

Normalizing and grain refinement treatments can help reduce the grain size that is blown up during forging. Some forging bladesmiths talk about grain refinement from forging because of the grain refinement treatments that they perform. However, grain refinement heat treatments are not limited to forging. It is not a forging process, it is a heat treatment process. Therefore I don’t see “thermal cycling” as part of the forging vs stock removal discussion. Learn more about grain refinement in heat treatment here and here.

Carbide Size

With high alloy steels there are carbides that are present even at forging temperatures. With more reduction from the cast ingot, the carbides are refined to a greater degree for better toughness and uniformity in properties. Below I have examples for T1 high speed steel and D2 tool steel, both starting from a 10″ round ingot. The size it was forged down to is indicated in the micrograph:

D2 Tool Steel with different degrees of reduction. Image from [8]

T1 High Speed Steel with different degrees of reduction. Image from [8]

You can see that with more reduction there is less banding and finer carbides. However, the temperature that the steel is forged and the way it is forged matters. Power hammer forging is better at breaking up carbides than rolling. Too low temperature means that carbides are not broken up and segregation can occur. Too high temperature leads to growth of carbides due to “Ostwald ripening” which is described in this article. That article also has more information about the optimal selection of forging temperature for ease in forging as well as the dangers of forging too hot or too cold. Because hammer forging is better at breaking up carbides, it may be possible to get somewhat finer carbides by forging from relatively large stock with a power hammer down to the thickness needed for a knife. However, I don’t know of any forging bladesmiths doing this regularly with the high alloy steels that would benefit. Below shows carbide size with different forging temperatures with sprayform Vanadis 4 steel (non-extra). The finest carbides were achieved with a forging temperature of 1050°C (1925°F); carbides were larger with either higher or lower temperature:

Image from [9]

With low alloy steels like O1, 1095, 52100, W1, W2, 5160, etc. their carbides dissolve at forging temperatures and those carbides will form again at lower temperature. Therefore forging is not really necessary for refining the carbide size. Below I have micrographs for 1095 steel with different levels of reduction; much less reduction is necessary to have a consistent microstructure that doesn’t change much with further forging/rolling. I can’t tell much difference between the “3 in.” and the “1/2 in.” structures.

1095 simple carbon steel with different degrees of reduction. Image from [8]

Stainless vs Carbon Steel

Low alloy steels are easier to forge, as described in this article, and are by far the most commonly used by forging bladesmiths. When it comes to forging to shape, the properties of high alloy steels are more sensitive to direction because of carbide banding. Therefore they have more potential benefit for any scenario where adjusting the “grain orientation” would improve toughness of the blade. In terms of forging to refine the microstructure, high alloy steels are less sensitive to grain growth, and have the potential for carbide refinement with hammer forging from appropriate temperatures if given enough reduction. It is sometimes claimed that only low-alloy steels should be forged for knives because other steels do not “benefit” in the same way. That is a myth perpetuated by forging bladesmiths to justify their steel choices. I am not saying that low alloy steels are poor choices, but they are not chosen because of more benefit derived from forging.

Edge Packing

There is an old forging myth about forging the edge to increase the density to make it stronger or better in some way. I think it is pretty widely known as a myth now so I won’t waste much time on it. Forging steel doesn’t increase its density.

Scale Formation and Decarburization

At high temperature steel tends to form scale and to lose carbon to the surrounding atmosphere. The decarburized steel needs to be removed, particularly at the edge. In some cases with poor practices the steel may be fully decarburized. Forging bladesmiths need to understand the changes to steel and how to limit those reactions. Stock removal makers also need to avoid decarburization when heat treating, particularly with high alloy steels that need high temperatures. Usually stainless steel foil or specialized liquid coatings are recommended.

Reasons to Forge

There are many reasons to forge that aren’t performance-related. Forging close to shape can mean less steel is lost to grinding compared to stock removal. One obvious reason to forge is for producing forge welded Damascus steel. Producing those steels provides another artistic avenue when it comes to knifemaking and the possibilities are nearly endless. On the flip side, there are commercially produced Damascus steels for stock removal makers, though the maker is limited to the Damascus options that are available. Some knives are more feasible to produce when forged, including curved blades or knives with integral bolsters. It is possible to produce those knives with stock removal but there is more material loss. For some customers and makers they like the “rustic” look from having some scale on the unground portion of the blade. That look is easiest to produce by forging. Another reason to forge is for continuing the tradition of producing knives in that way. It is certainly possible to enjoy producing knives with a hot forge, hammer, and anvil. The best reason to forge a knife is because you want to.

Summary

Virtually all steel used in knives has been forged, so it doesn’t necessarily make sense to differentiate between knife steel as “forged” or “not forged.” The steel has been reduced by a great amount before the knifemaker ever sees it, whether the final knife is produced by forging or stock removal. Forged and hot rolled steel has elongated features such as sulfides and carbide bands which gives steel directional properties, sometimes called the “grain” of the steel, which is not the same as “grains” in steel. Because of the grain direction there may be some cases where forging to shape can lead to better resistance to fracture of the knife. However, in most cases forging of the blade does not affect the tip or edge toughness. The literal grain structure of the steel, however, is affected by forging, and the grain size is easily increased at the high temperatures used for forging, which is undesirable. Grain refinement heat treatment cycles are necessary to return the steel to an appropriate grain size. High alloy steels, not typically used by forging bladesmiths, have many carbides present at forging temperatures which help prevent grain growth. High alloy steels also have more potential benefit from forging because 1) carbide structures can be refined by forging with a hammer at appropriate temperatures, and 2) the increased carbide banding means that forging to shape to modify grain orientation has more effect. There are other reasons to forge, however, including the production of Damascus steel, curved blades, integral bolsters, and enjoyment from forging knives.

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[1] Maciejewski, Joseph. “The effects of sulfide inclusions on mechanical properties and failures of steel components.” Journal of Failure Analysis and Prevention 15, no. 2 (2015): 169-178.

[2] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.

[3] Schneider, R., A. Schulz, C. Bertrand, Alfred Kulmburg, A. Oldewurtel, V. Uhlenwinkel, and D. Viale. “The performance of spray-formed tool steels in comparison to conventional route material.” In Proc. 6th Int. Tooling Conf, pp. 1111-1124. 2002.

[4] http://www.cblade.it/why-forging-is-better.html

[5] Wang, Xinchen. Fatigue behavior and microstructure examination of AISI D2 trim dies. Wayne State University, 2013.

[6] Biglou, Jajal, and John G. Lenard. “A study of dynamic recrystallization during hot rolling of microalloyed steels.” CIRP annals 45, no. 1 (1996): 227-230.

[7] McNelley, T. R., M. R. Edwards, A. Doig, D. H. Boone, and C. W. Schultz. “The effect of prior heat treatments on the structure and properties of warm-rolled AlSl 52100 steel.” Metallurgical Transactions A 14, no. 7 (1983): 1427-1433.

[8] Roberts, G A, and Robert A. Cary. Tool Steels. Beachwood, Ohio: American Society for Metals, 1980.

[9] Yan, Fei, Haisheng Shi, Bingzhong Jin, Junfei Fan, and Zhou Xu. “Microstructure evolution during hot rolling and heat treatment of the spray formed Vanadis 4 cold work steel.” Materials Characterization 59, no. 8 (2008): 1007-1014.