CFE Levels in Wootz Ingots: The Basic Story

By Peter Burt 2023.

One of the most common questions in making wootz is ”How much CFE should I add?” The answer to this question depends a lot on your goals for the ingot, and hopefully this exploration of the topic will help guide your decisions.

First, what are CFEs? CFEs are the “Carbide Forming Elements”, and they range from Mn (manganese) at the weak end of the spectrum to Ti (titanium) as most likely the strongest of the CFEs. “Strength” in this case refers to how tightly the atoms bond with C (carbon), which in turn defines the temperature to which the resulting carbide is stable. Mn, for example, forms carbides only slightly more strongly than Fe (iron) itself, while Ti forms such strong carbides that they typically don’t dissolve until the steel itself begins to melt. The other CFEs fall in between these two, with the progression generally considered to go as such:

Fe, Mn, Cr, W, Mo, V, Nb, Ti

Something that is important to note here is the fact that Fe is on this list but is not one of the elements we are talking about when we use the term CFEs. This is important because every “excess” C atom that does not bond with one of the stronger CFEs will bond with Fe. The “excess” C atoms are those that exist beyond the eutectoid point; in simple steels the eutectoid point is roughly 0.8% C, so in a hypereutectoid steel of 1.5% C there is roughly 0.7% C that will become carbides one way or another. This portion of the carbon WILL BECOME CARBIDES regardless of whether it bonds with Fe or one of the stronger CFEs. There are obviously additional complicating factors, but this is a good place to start.

So, if we are going to get carbides anyway what is the big deal with CFEs? The CFEs have often been referred to as the “seeds of the pattern”, and this is a good way to think about them. In a steel that is truly just Fe and C there is no reason for a carbide to form in one location rather than another, and this is a recipe for no pattern at all. In such a steel the initial carbides to form will be scattered at grain boundaries in a random fashion, and there is no reason for a pattern to arise from these random beginnings. If we add even a very small amount of the CFEs, though, we have some sort of organization upon which a pattern can be based.

The reason that CFEs add some sort of organization to the pattern is that they segregate/concentrate during the solidification process. The CFEs don’t change how the steel solidifies, but since they are an “impurity” they do not fit nicely into the lattice of Fe atoms that is forming. Since they don’t fit nicely they are pushed ahead of the solidification front until they are trapped between the growing dendrites. We refer to these areas as the IDRs (interdendritic regions), whereas the dendrites themselves we refer to as the DRs (dendritic regions). Fe will solidify in a dendritic fashion regardless of the presence of CFEs or other impurities, but without those added elements that segregate to the IDRs there is nothing to outline the dendrites and highlight the solidification pattern.

The pattern of solidification won’t always be obvious in a test etch of the as-solidified ingot. This is because there are several different ways in which carbides might precipitate after solidification. The most common ways for carbides to precipitate are as follows:

1)  IDR or Dendritic: In this case the carbides precipitate predominantly in the IDRs, creating a grid-like pattern in a window etch. As a general rule the higher the CFE content and/or the higher the C content the more likely you are to see this pattern.
2)  GBC (grain boundary carbide): The carbides can also precipitate along the boundaries of the large grains formed during cooling, and this will create a spidery network of lines in a window etch.
3)  Acicular or Widmanstatten: These occur when the C is unable to diffuse to either the IDRs or the GBs and therefore carbides precipitate within the grains themselves. Acicular, meaning “needle-like”, carbides appear almost as small snowflakes or ice crystals scattered across a window etch, and they often appear in conjunction with GBC if the grain size is particularly large or the cooling rate is relatively fast.

It is likely that the ingot will actually contain a mixture of these, but in an etch only the dominant features will be visible to the naked eye.

If you proceeded by heating only below Acm for your roast and forging then it is likely that the pattern in your final blade will bear a substantial resemblance to the pattern in the raw ingot. It would certainly become elongated and distorted, but the underlying structures would still be discernible in the final blade. This is because, A) the initial pattern of carbides would never be fully diffused, and B) carbides tend to form near other carbides so existing patterns tend to be reinforced over multiple cycles.

What this tells us is that if we want a pattern different than what we see in the raw ingot then we need to substantially or completely erase that initial pattern before much, if any, forging has occurred. If we are happy with the pattern that we see in the ingot then it is not necessary to treat the steel in the same way before proceeding to forging. The treatment of the raw ingot prior to forging is called “roasting”, and that will be covered in a different conversation.

CFEs and the total CFE content play key roles in these early stages of the process. First, as noted previously, the higher the CFE content the greater the likelihood that the carbide pattern in the raw ingot will be an IDR pattern. This is because the CFEs are in the IDRs and form stable carbides there that can survive all the way down to Acm and then grow larger and brighter as the ingot cools between Acm and A1. Second, the addition of CFEs, especially in higher volumes, raises the Acm temperature of the steel so it becomes more difficult to erase the raw pattern during the roasting treatment. This aspect is amplified by the fact that the CFE carbides are more stable and take longer to dissolve even once the steel is heated above Acm. Remember here that some CFEs, such as Ti, form such stable carbides that they are unlikely to be dissolved at any temperature.

A complicating aspect of this has to do with how much C is present in the steel. If we look at the Fe-C Phase Diagram we can see that the Acm temperature increases as the C level increases. In some ways adding C has a very similar effect as adding CFEs, in that the Acm temperature is increased by additions of either. This means that we need to look at both aspects when deciding on the formulation of our steel. For example, W1 and 52100 are similar steels in terms of C content, but very different in terms of CFEs and this renders 52100 a VERY different steel than W1. CFEs define the location of the Acm line, while C content dictates where our steel crosses that line.

This brings us back to the original question: How much CFE should I add? Unfortunately there is not a simple answer to this question. We can, however, look at some general ranges of CFEs to get things started. I am breaking this down into three basic categories: Historical, W2, and High.

CFE Level Historical <0.1%: In historical wootz we typically see the total CFE content as being below 0.1% by weight. There are some outliers, but as a general rule the CFE levels were so low that it has only been in modern times that metallurgists realized they were having an effect. Historical wootz also tended to have traces of a variety of different CFEs along with small amounts of other elements such as P, Ni, Cu, etc. Thanks to the published work of Pendray and Verhoeven most modern smiths focus heavily on V (vanadium), but this was far from the only element present in the historical blades. If you are looking for watered patterns this is probably the best place to start. The full range of patterns should be achievable with these CFE levels and C content in the 1.3-1.8% range, but close temperature controls are needed to achieve the desired result.

CFE Level Medium 0.1%-0.5%: This range includes modern steels such as W2, which contains roughly 0.25% V, although sometimes this is replaced by a larger proportion of W (tungsten). Ingots made in this range will typically show pattern easily, but it can already be difficult to achieve anything other than IDR patterns. Reducing the C content makes it easier to achieve other pattern structures but will reduce the vibrancy of the result. The difficulty of forging will depend greatly on the C content and the specific CFEs used. This range is typically the easiest to make from the available materials and will forge and heat treat more like a standard blade steel.

CFE Level High 0.5%-2%: In this range it becomes essentially impossible to achieve anything other than IDR patterns, even with lower C contents. Some commercial steels in this range, most notably A2, can show an apparently watered pattern; it is likely, however, that this is simply due to an extreme elongation of the IDR pattern seen in the very large, original ingot, and current information indicates that the same effect cannot be achieved when forging small ingots of this chemistry. Forging tends to be quite difficult even with lower C contents; C contents of 1.5% and above can render the ingot essentially unworkable due to excessive carbide content in the standard forging range. Longer soak times become necessary to dissolve enough carbides for each forging cycle.

CFE Very High +2%: Beyond 2% CFEs it will become extremely difficult to forge the ingot, and both the appearance and performance of the steel is likely to suffer. In all of these cases the exact CFEs in question will make a substantial difference in how the steel behaves. For example, a steel with 2% Cr (chromium) would probably still be quite workable at a 1.5% C content, but swap in 2% V and the steel will become impossible.

Because of the complexity of the interactions between different alloying elements within the steel it is a good idea to start from a relatively basic formulation and work up from there. Not everyone has the materials available to make an ingot that fits into the Historical range, but staying in or below the W2 range is good as a starting point.

One misconception is that adding more CFEs automatically makes the steel better, but this is often not true at all; most often the exact opposite is true. There are two very good, basic reasons why adding more CFEs can actually make it harder to achieve your goals.

1)  Higher CFE levels, especially once you go past the W2 range, tend to result in larger carbides, and larger carbides are generally bad for performance. Large carbides also appear as very bright spots in the etch and can lead to poor pattern definition.
2)  Higher CFE levels often lead to lower carbide volume overall. This may seem counterintuitive, but it is caused by the shift away from Fe3C and toward MC carbides. Stronger CFEs create carbides that are 1 metal + 1 carbon, whereas Fe forms a carbide that is 3 Fe + 1 carbon. This means that the volume of carbide created is much higher when the C is being bonded by Fe rather than by something like V.

There are obviously a lot more complexities to this subject, but this basic outline will hopefully help you choose the starting point that aligns with your goals.