Is Blue Super Steel Actually Super? The Facts about Tungsten-alloyed Steels

High carbon, tungsten-alloyed forging steels see consistent use in Japanese knives with steels like the Hitachi Blue series and Takefu V-Toku steels. These steels differentiate themselves from many other knife steels due to their use of tungsten alloying, but not for providing hot hardness like in high speed steels, but for wear resistance. Tungsten-alloyed steels are as old as tool steels themselves, as I covered in an earlier post: The First Tool Steel. However, simple tungsten-alloyed steels have been on their way out in the USA since at least the early 60’s [1]. The tungsten added to the steels leads to the formation of very hard tungsten carbides for steels that can be as wear resistant as air hardening steels like D2 or M2 but with the ability to normalize and anneal the steels without precise temperature control.

There are a variety of high carbon tungsten-alloyed steels, though many are difficult to obtain in the US. If we include O1 with its 0.5% W, here is a partial list:

Several of these steels are listed with chemistry ranges, so I selected mid-points to simplify further analysis.

Available Data on Tungsten-Alloyed High Carbon Steels

Information on these steels can be difficult to come by, with few micrographs or published toughness or wear resistance tests. However, we can piece together some scattered data. Toughness numbers are available in Tool Steels, as reportedly provided by Bethlehem Steel [2]; I have simplified comparison by showing O1, O7, and F2 all on one chart, and plotting them vs hardness rather than tempering temperature:

No wear resistance test results are provided in the book, though they do give a general rating for wear resistance and toughness of each steel on a scale of 1-9. They give a rating for O7 but do not show it on the convenient chart, so I added it:

So O1 gets a “4” for wear resistance and a “3” for toughness. Not too bad though not super exciting, as A2 is better in both categories. However, ease in forging and annealing, and a high working hardness are advantages that the tungsten steels have. O7 gets a bump up to “5” in wear resistance with little drop in toughness, and F2 gets a rating of “8” but a drop in toughness to “2” with its high tungsten content. A better understanding for where these properties come from can be seen in their micrographs; here are micrographs for O1 [3], O7 [3], and the similar to F2 though higher carbon 1.2562 [4]:

O1 Micrograph [3]

O7 Micrograph [3]

1.2562 Micrograph [4]

The first thing you notice is the vast different in carbide volume between 1.2562 and O1. O1 has a limited volume of very small carbides, while 1.2562 has a range of carbide size with some that are on the order of several microns. O7 is between the two but closer to O1 than to 1.2562, with generally small and evenly distributed carbides. Note also that while the O1 and O7 micrographs are at 1000x, the 1.2562 micrograph is at 500x, making the carbides look smaller than they are relatively. Another steel from Landes’ book with a micrograph at 500x is PM M3:2 (somewhat close to M4) where even it has a smaller carbide size and more even distribution of carbides [4]. Another example to show how much larger the carbides are than typical steels designed for forging is Cru Forge V, where its micrograph is also at 500x [5]:

PM M3:2 Micrograph [4]

Cru Forge V micrograph [5]

Through point counting, I calculated the carbide volumes from the micrographs as the following:

The 16.5% carbide volume puts it in the range of many air hardening steels, which helps explain why it has greater wear resistance than most other common forging knife steels.

Predicting Properties with Limited Information

Those carbide volumes fit relatively well with ThermoCalc estimated carbide contents:

Using the estimated carbide volumes along with the relative wear resistance and toughness values, these steels can be assigned estimated properties. I also included scores for “forgeability” and “hardenability” which I will explain later:

These toughness and wear resistance estimates can also be overlaid on that plot I showed from the Tool Steels book earlier (blue circles):

Toughness

That should give you some feel for the range of properties that are possible with the tungsten alloyed steels. So not stellar for toughness, as is common for low alloy high carbon steels, but a range of wear resistance can be found with the different options. The reduced toughness when compared with air hardening steels like A2 is likely due to the very high carbon in solution, which is known to reduce toughness, as I covered in an earlier article: Austenitizing Part 2. Grain size is not likely at work here because the tungsten alloying leads to a very fine grain size [2]. At equilibrium, these steels have very high carbon in solution, which is consistent with the very high hardness they achieve:

So the high carbon in solution is an important factor for these steels in controlling their toughness. However, when comparing between the different steels there is a relatively narrow range of carbon. When comparing between them the primary factor controlling the toughness is the carbide volume and size [6]. Therefore, general ratings can be given for each steel based on its calculated carbide volume, which is where the ratings came from in the chart earlier.

Wear Resistance

Tungsten carbide is much harder than cementite, as can be seen in this handy chart [7]:

Both the WC and W₂C carbides have hardness in the same range as other MC carbides such as vanadium (VC), titanium (TiC), and niobium (NbC). Interestingly, JMatPro predicts the formation of W6C carbides which have significantly lower hardness. However, M6C (M is either W or Mo) carbides are generally associated with high speed steels, and because the wear resistance is high for steels like F2 I think it is more likely that the ThermoCalc estimates are more accurate. Furthermore, ThermoCalc still accurately predicts M6C carbides for high speed steels. Because of the much higher hardness of tungsten carbide it contributes much more strongly to wear resistance, so the wear resistance of the tungsten alloyed steels is primarily controlled by the tungsten carbide fraction. Therefore, the wear resistance rating that I included is based on the tungsten carbide fraction of each steel.

Edge Retention

CATRA edge retention is primarily a function of wear resistance, so the edge retention ability of each steel will scale with the wear resistance rating that I predicted. An example of CATRA results can be seen from Bohler-Uddeholm reported values [8]. CATRA results can be predicted well with Thermodynamic calculations of carbide volume along with hardness [9]. I am only aware of one CATRA test of any of these steels, and that is for O1, where it measured 395 TCC at 64 Rc [10]. This is a decent score primarily because of its high hardness, the same set of tests showed that A2 got 522 at 62 Rc, M3 got 586 at 64 Rc, and T15 got 921 at 65 Rc. This is consistent with the relative score of 4 for wear resistance for O1 from Tool Steels that I included. Higher scores could be achieved for the higher tungsten steels, perhaps in the mid-700’s for F2, Blue Super, or 1.2562 at high hardness.

Ease in Sharpening

Ease in sharpening is the inverse of wear resistance. Higher wear resistance steels are more difficult to sharpen. For certain sharpening media, the tungsten carbides may be harder than the abrasive which may further add difficulty to sharpening.

Balance Between Toughness and Wear Resistance

To maximize the toughness-wear resistance combination, there should be the highest tungsten carbide to cementite ratio possible, as tungsten carbide offers much more wear resistance than cementite, while cementite likely still reduces toughness by a similar amount as tungsten carbide [6]. Therefore, the steels with the highest combination of toughness and wear resistance will be those that primarily have tungsten carbide. Blue Super and 1.2562 have a high carbon content, which primarily acts to increase its cementite fraction, which again, decreases toughness with little contribution to wear resistance. A lower carbon content for a given level of tungsten generally gives a better balance of properties, such as V-Toku 2 (1.05C-1.25W) and V-Toku 1 (1.15C-2.25W). Plotting the ratio of tungsten to carbon vs the predicted (toughness * wear resistance) shows this effect:

Edge Stability

Edge stability is a term used by Roman Landes [4] to refer to a steel that can take and hold a very fine edge. He reports that low carbide volume and high hardness contributes to edge stability. He reported the edge stability of 1.2562 among other steels, such as AEB-L which received the highest score of any steel shown with approximately 95/100. Interestingly, despite the relatively high carbide volume he reports the edge stability of 1.2562 is relatively high (70/100), even higher than the PM M3:2 (46/100) that I showed a micrograph for earlier, which appears to have a smaller volume of carbide. A clue to the higher score for 1.2562 than PM M3:2 may be due to hardness. He does not list a rockwell hardness but gives 1.2562 a maximum value for hardness but an intermediate value fo PM M3:2. Therefore, edge stability seems to be highly sensitive to hardness, but tungsten alloyed high carbon steels have a relatively high potential for edge stability. Lower carbide volume steels such as V-Toku 2 or O7 could likely achieve very high scores for edge stability.

Forgeability

The hot workability or hot ductility of these steels likely varies. Tool steels often have poor hot ductility because they have carbides present even at forging temperatures [11]. Hot ductility is also affected by other factors such as overall alloy content [11] and grain size [12]. Therefore, even O1 which has no carbides at typical forging temperatures has poorer hot ductility than pure iron or a simple carbon steel [11]. To estimate the effect of overall alloy content and carbide volume at forging temperature, I used ThermoCalc to find the temperature at which all of the tungsten carbide is dissolved:

You can see that F2 and 1.2562 are predicted to have carbides present even at the relatively high temperature of ~2100°F, which is higher than the suggested forging temperature [2]. Those carbides reduce the forgeability of the steel. Therefore I set arbitrary scores of “8” for forgeability of O1, and “4” for forgeability of 1.2562, and then used the prediction of WC dissolution temperature to set ratings for the others. They would still be easier to forge than steels like D2 with high fractions of carbide at forging temperatures, but the lower tungsten steels would move more easily under the hammer and be less prone to cracking.

Hardenability

Tungsten adds little to hardenability [1], so the primary contributors to hardenability are the Cr and Mn additions to these steels. Hardenability controls how rapidly the steel must be quenched to form full martensite, and O1 as an oil hardening steel has relatively high hardenability relative to water hardening steels. However, higher hardenability also increases the difficulty of processes like normalizing and annealing as slower cooling rates are required. Therefore, a balance can be achieved for both ease in processing but also ease in quenching to achieve full hardness. I used values for the relative effect on hardenability of 3.67 for Mn and 2.73 for Cr for estimating hardenability for each steel [1]. As expected, O1 received the highest score. O7 received an intermediate score which is expected since it is known to be the lowest hardenability of the oil-hardening tool steels and is sometimes recommended to be water quenched [2]. Several of the other steels have relatively low hardenability scores, which is consistent with the recommendation for water quenching them. Therefore either water, brine, or a fast oil such as Parks 50 would be recommended for those steels.

Difficulty in Working in the Small Shop

While the low hardenability of these steels means that they can be normalized and annealed with simple processing, they are not necessarily classified as “easy” to work and heat treat. As covered earlier, high tungsten content can lead to more difficulties in forging. Furthermore, the carbide dissolution during hardening/austenitizing is described as “sluggish” [1] and requires higher temperatures, longer soak times, or both. Graphitization, or the formation of graphite within the steel, is possible with the higher carbon and tungsten steels, if annealing is prolonged, though additions of Cr help to mitigate that [2].

Summary and Use Cases

Tungsten-alloyed high carbon steels fill a relatively small niche – higher wear resistance for forging bladesmiths. Where air hardening is preferred, which is most industry applications, high tungsten steels were replaced by other steels long ago. And perhaps because of the relatively small number of use cases these steels can be somewhat difficult to obtain in the USA. A range of wear resistance properties can be obtained through the use of different tungsten contents allowing the use of these steel for many applications. Most of these steels are generally recommended for high hardness (64 Rc+), which means they are primarily intended for applications that require fine edges combined with good wear resistance. With their unique set of properties: good forgeability, wear resistance, and hardness, they offer many opportunities for the forging knife maker.

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[1] Roberts, G.A., et al. Tool steels. American Society for Metals, 1962.

[2] Gill, James Presley, et al. Tool steels. American Society for Metals, 1944.

[3] Chandler, Harry. “Heat Treater’s Guide.” ASM International, Geauga County (1995).

[4] Landes, R. “Messerklingen und Stahl.” Aufl. Bad Aibling: Wieland Verlag (2006).

[5] https://www.alphaknifesupply.com/Pictures/Info/Steel/CruForgeV-DS.pdf

[6] https://www.bladeforums.com/threads/predicting-toughness-with-steel-composition.1534942/

[7] Theisen, W. “Hartphasen in Hartlegierungen und Hartverbundstoffe.” (1998).

[8] http://www.bucorp.com/media/CATRA_Test2.pdf

[9] https://www.bladeforums.com/threads/how-good-is-aeb-l-edge-retention.1542343/

[10] https://jeffpeachey.com/2009/01/18/results-of-testing-steel-types-for-leather-paring-knives/

[11] Kriaj, Abe, Monika Jenko MatevFazarinc, and Peter Fajfar. “Hot workability of 95MnWCr5 tool steel.” Materiali in tehnologije 45.4 (2011): 351-355.

[12] Imbert, C. A. C., and H. J. McQueen. “Dynamic recrystallization of A2 and M2 tool steels.” Materials Science and Engineering: A 313.1-2 (2001): 104-116.