Sandrin Carbide – What is it?

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Cemented Carbide

The company Sandrin produces knives out of cemented tungsten carbide. This is a common material for machining due to its high wear resistance, strength, stiffness, and hot hardness (resistance to softening at high temperatures and speeds). The material is sometimes called “tungsten carbide” or even just “carbide” as shorthand though it is in fact tungsten carbide plus some amount of cobalt “binder” that holds the carbide together. There are other types of binder and carbide used but cobalt and tungsten carbide is the most common and is also what Sandrin uses so I will not distract you with more details. An example micrograph of the WC-Co (tungsten carbide-cobalt) type is below:

Image from [1]

The properties of cemented tungsten carbide are greatly controlled by the amount of binder and the size of the carbide. The tungsten carbide itself is extremely high in hardness (which is why it’s so good at machining other metals) but the cobalt is relatively soft. Therefore the more binder is used the lower the hardness is of the cemented carbide. A finer carbide size for a given amount of binder also means higher hardness.

Image from [1]

Images adapted from [1]

In steels I am used to a finer carbide size meaning better toughness, but with cemented carbide it is the opposite. A coarser carbide size actually means higher toughness, as can be seen in the following figure:

Image from [1]

Sandrin Carbide

So with that very brief introduction to cemented carbide we can talk about what Sandrin is using in their knives. They advertise their cemented carbide as being 71 Rockwell C “while still remaining flexible and not brittle,” and as being “Polyhedral tungsten carbide.” They do not produce the knives with a laminate construction but the entire blade is produced from the cemented carbide, which is why they are advertising the flexibility and toughness of the carbide material. Sandrin’s parent company, Turmond, produces tungsten carbide so I was curious exactly what composition of carbide they decided to use with the knives. I was able to find the patent which took some searching since it seems to only be patented in Italy.

Design

The patent reports a preferred binder content of 20-25% cobalt, and a carbide size of 0.6-0.8 microns. That is a relatively high cobalt content in combination with a submicron carbide size. In my cursory search of cemented carbide grades I did not find one exactly like this. Generally, grades with high cobalt also have a relatively coarse carbide size, because they are produced for higher toughness. I don’t know enough about available carbide grades to say that it is necessarily unique, but it is an atypical combination. Certainly an expert in tungsten carbide would not be shocked to see such a grade but it appears that they didn’t simply take a common grade and claim that it was uniquely developed for knives.

Effect of Carbide Size on Edges

Sandrin compared the sharpened edges of a carbide grade which had a carbide size of greater than 3 microns, vs one which had a size of 0.8-2.0 microns (still greater than the final recommended size). They found that the edge was very irregular with the coarse carbide size which is why they determined a fine carbide size is necessary for knives.

Images from [2]

Cobalt Content, Hardness, and Wear Resistance

With the carbide size being necessarily fine for good sharpness and edge behavior, it was likely even more important that the cobalt content be relatively high to give the knives some toughness and resistance to fracture. With the relatively high cobalt content the resulting hardness was 1150 Hv or 71 Rc. This is lower than is possible with cemented carbide; grades approaching 2500 Hv can be obtained as shown in the previous chart of cobalt vs hardness. Pure tungsten carbide is about 2600 Hv. However, 71 Rc is still a high value. Only super hard high speed steels like M42, Z-Max, S290, Maxamet, HAP72, and Rex 121 are capable of achieving 70-72 Rc. However, the way steel and cemented carbide achieve that hardness is different. The tungsten carbide is very high in hardness and the cemented carbide material hardness is reduced with increasing amounts of relatively soft cobalt binder (330 Hv), in this case about 20%. With super high speed steel the iron matrix itself is hardened to 68-72 Rc but there is also some content of vanadium carbide (2800 Hv) and molybdenum/tungsten carbide (1400 Hv) which contribute to wear resistance. For example, Rex 121 has around 24% vanadium carbide and 7% of the molybdenum/tungsten carbide. The tungsten carbide that forms in high speed steels is a different type than cemented carbide which is why it is lower in hardness. So the wear resistance is not necessarily equal between cemented carbide and steel when at the same hardness, because their makeup in terms of carbide:matrix ratio and the hardness of the different phases are not the same.

Slicing Edge Retention

Slicing edge retention is largely a product of edge geometry and wear resistance, as described in this article. Therefore, when the edge geometry is constant the material with the highest wear resistance will typically win. In a test by Pete of the Cedric and Ada Youtube channel, a Sandrin knife performed more cuts in rope than any other tested knife (1540 cuts) [3], including a knife in Rex 121 (1300 cuts) which is the most wear resistant steel available. The edge on the Sandrin knife was at a more obtuse angle which makes the result even more impressive.

Flexibility

The “stiffness” of a material is measured by its “modulus,” where a higher modulus means that the material is less flexible. Cemented carbide has a very high stiffness but the modulus is reduced with increasing cobalt content. The modulus is largely unaffected by the carbide size, however.

Image from [4]

When Sandrin is advertising their carbide knives as being “flexible” I don’t know if they mean the stiffness is reduced or if they mean that the blade is capable of bending somewhat without breaking. The latter is perhaps more likely but I decided to write about stiffness anyway. How “flexible” a knife is is greatly controlled by its thickness. Thicker materials require more force to bend but also break at smaller amounts of deflection. You can read why in this article. The thin spine in the Sandrin knives would be expected to improve its flexibility.

Corrosion Resistance

The corrosion resistance of cemented carbide materials is sometimes improved by using nickel as a binder material rather than cobalt. It is common to add small amounts of chromium carbide (Cr₃C₂) to cemented carbide materials to somewhat improve the corrosion resistance with either binder material. In the Sandrin material they added a relatively standard amount of 2%. The chromium carbide addition also helps to refine the tungsten carbide size [5]. It is hard to compare corrosion resistance of cemented carbide to steel since the concern with steel knives is typically rusting, which does not occur in cemented carbide since there is no iron. However, the cobalt can corrode leading to the cobalt “leaching” out leaving unsupported tungsten carbide [6]. In the Sandrin patent they said they tested corrosion by exposing a knife to pork for 2 days and reported that “no spots or other permanent alterations were found.”

Toughness

Since no toughness numbers are reported for the Sandrin cemented carbide grade it is challenging to compare toughness to common knife steels. Crucible reported that a 12% binder cemented carbide grade was measured at 2 ft-lbs in their charpy c-notch test [7]. This has a lower cobalt content and higher hardness than the Sandrin grade but may provide some reference point. The toughness may be slightly better for the Sandrin grade, though even 3 ft-lbs may be too high to hope for. A comparison with other knife steels is below [8][9][10][11]:

Sharpening

The very high hardness of the tungsten carbide means that sharpening of the carbide material is expected to require CBN or diamond sharpening stones/plates. Unlike steel where the matrix will wear from softer abrasives like aluminum oxide, abrading away the cobalt matrix just leads to tungsten carbides falling out. I have not sharpened any of the knives myself to see what sharpening of the material is like. I have heard from one or two people that they were having a difficult time matching the sharpness of the knives as they came from the factory. Which means that the factory sharpening is good but matching it may be challenging due to the material’s high wear resistance and low toughness.

Density

The density of a 20% cobalt cemented carbide grade is about 13.6 g/cm³ [12], which is much higher than a typical tool steel which is around 7.7 g/cm³ [13]. Even T1 high speed steel with its 18% tungsten is still only 8.7 g/cm³ [14]. This means that a cemented carbide knife of the same volume is significantly heavier than one made in steel. This weight difference may be counteracted somewhat by modifying the blade design or handle material/design. The extent the difference in weight matters will depend on user preferences, of course.

Polyhedral Tungsten Carbide

The marketing material calls Sandrin’s carbide “polyhedral tungsten carbide.” This makes it sound as if it is some special type of carbide that makes it “polyhedral.” However, I have found no reference to “polyhedral” carbide anywhere other than Sandrin marketing materials. The word “polyhedral” or any other similar term is not found in the patent and the patent describes a relatively standard cemented carbide, apart from the somewhat atypical combination of high cobalt along with submicron grain size. Therefore I think we can conclude that “polyhedral tungsten carbide” is simply a marketing term.

Summary and Conclusions

With the carbide knives being limited to Sandrin at this point, whether or not you want one may depend on whether you like the designs of the available knives. However, I think we know enough about the properties of cemented tungsten carbide to talk generally about the properties of the material. It is high in hardness, wear resistance, and therefore slicing edge retention. The toughness is low when compared to typical steel, and probably lower than even many of the steels capable of similar hardness. Sharpenability is likely poor. So it offers maximal edge retention at the cost of many other properties. If that is a quality you desire and you like the knives then give it a try and see what you think.

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[1] https://www.ceratizit.com/uploads/tx_extproduct//files/GD_KT_PRO-0689-0517-1_SEN_ABS_V1.pdf

[2] https://worldwide.espacenet.com/beta/search/family/056940185/publication/ITUA20163471A1?q=UA20163471

[3] https://www.youtube.com/watch?v=yPYjRTJmjTg

[4] Okamoto, S., Y. Nakazono, K. Otsuka, Y. Shimoitani, and J. Takada. “Mechanical properties of WC/Co cemented carbide with larger WC grain size.” Materials Characterization 55, no. 4-5 (2005): 281-287.

[5] Banerjee, D., G. K. Lai, and G. S. Upadhyaya. “Effect of binder-phase modification and Cr 3 C 2 addition on properties of WC-IOC0 cemented carbide.” Journal of materials engineering and performance 4, no. 5 (1995): 563-572.

[6] https://www.federalcarbide.com/corrosion_resistant_tungsten_carbide_grades.html

[7] G. Steven and J.R. Handyside. “C-Notch Impact Test for Steels at High Hardnesses.” ASTM Proceedings 63 (1963): 1122-1146.

[8] http://www.crucible.com/PDFs/DataSheets2010/Data%20Sheet%204V.pdf

[9] Wojcieszynski, Andrzej L., and William Stasko. “High-speed steel article.” U.S. Patent 6,057,045, issued May 2, 2000.

[10] http://www.crucible.com/PDFs/DataSheets2010/dsM4v1%202010.pdf

[11] http://www.crucible.com/PDFs/DataSheets2010/ds76rev1%202010.pdf

[12] https://www.federalcarbide.com/tungsten_carbide_grade_chart.html

[13] https://www.alphaknifesupply.com/Pictures/Info/Steel/D2-DS-Crucible.pdf

[14] http://www.astmsteel.com/product/t1-tool-steel-aisi/