The role of tool steels in outdoor knife production

 The role of tool steels in outdoor knife production*

*meaning Not for custom knife production, as this area it is impossible to know who is using what steels/experimenting etc..

A new kid on the block appears to be S600 aka AISI M2. We now have the well known Italian manufacturer Extrema Ratio, producing several of their proven models which are made with stainless HSS N690 but now also using  Böhler's S600 High speed tool steel for the same models.

If you are familiar with M2, S600 can be hardened to over HRC 65. Much research has been done about M2 and S600 over the years, specifically in regards to optimising the heat treatment parameters

Optimisation of such parameters is of course not only beneficial to the machining industry but can also benefit that of outdoor knife manufacturers. 

Here's a list of some common tool steels.

A2 (similar to D2 both subject to corrosion)

O1 An "Old fashioned" tool steel for knife manufacture.

M43? Usage unknown.

A7 (high vanadium content version of A2) the use in outdoor knives unknown-little data.

D7  (high vanadium content version of D2) the use in outdoor knives unknown-little data. Some custom knife production?

SK85 (HSS) Low chromium content  Used by WorkTuffGear for some super hardcore knives. Obviously not a stainless steel.

M2 AISI (HSS) similar to S600 both are non stainless with only 4% chromium and this possibly affords some resistance to organic acids (Plant juices!), although a patina can be quick to develop however, this has zero practical effect upon the edge retention.

S600 (HSS) As commonly (in industry) used high speed tool steels: high Tungsten, high Vanadium and Molybdenum tool steel containing 4% Chromium. Capable of being hardened above HRC65. 

Consequently S600 requires exacting heat treatment parameters to be adhered to in order to maximise it's desired end qualities. M2 and S600 heat treatment regimes have been studied for many years and there is clearly still much to learn about this steel alloy.

D2 A tried and trusted low Cr (12%) semi-stainless tool steel whose migration into edged outdoor tools is well received and other variants such as K340 & K110 and different configurations of production eg ISODUR ie electro slag production producing a more homogenous end product.

N690 (a high speed stainless tool steel and like all High Speed Steels (HSS) can operate at high speeds without loss of temper and retain stainless properties (harness dependent)

3V a Powder metallurgical produced steel. Therefore elemental constituents more accurately controlled for a better homogeneity   

C-0.8% Cr 7.5% V 2.75% Mo 1.3%

A brief note about PM steels.

Powder Metallurgical Steels are quite in vogue because they purportedly provide excellent desirable edge holding characteristics in terms of long wear. As to date however (and I do own several PM knives) I'm yet to see any serious scientific evaluation of these knives. That is comparing exact edge and blade profiles of knives (more than an "n" number of one) to those of a non PM steel construction. This is the only non biased way to say for sure if PM steels are better or noticeably better than non-PM steels in outdoor knives. 

Certainly, it is far easier to compare data from machining tools as they can be made exactly identical, a group of non PM steel Vs PM steel cutting the same objects and the cutting is of course identical because it is done by a machine and not the human hand. I have to put on my scientist's cap here because saying otherwise about outdoor knife evaluation is simply not statistically valid, nor is it scientifically correct; it is only hearsay with a heavy bias from manufacturer's marketing literature.  In the machining world the object is to be able to use for as long as possible tools which show little failure and produce the best possible products without having to be constantly exchanged and hence break production runs These requirements are not necessary for outdoor knife usage. So many of the benefits of PM steel are more relevant to that in the machining world than that of the knife industry, exceptions being for the production of industrial cutting blades.

Knives made with S600 High Speed tool steel

 Knives made with S600 tool steel

Example Knife:

Extrema Ratio ONTOS II S600

S600 (Böhler) or generic M2 tool steels are high Tungsten, High Mo and High Vanadium content high speed tool steels with a carbon content around 0.9% and a chromium content around 4%. These are industry proven high speed steels synonymous in the machining world. 

S600 has been formulated to be hardened right up to  HRC 67 (Possibly harder?) and as such much research has been conducted upon not only this version but also that of M2 to enable metallurgists to optimise the grain structure of such alloys. There is a current great interest in the use of various cryogenic treatments from spraying liquid nitrogen to deep cryogenic processes of greater than 24 hour treatments in order to maximise the transition of austenite to martensite. As such, there is a much opportunity to exploit the physical properties of S600 (as shown in the table below- from Böhler): 

for the use of this alloy in the manufacture of long lasting sharp blades for outdoor usage. S600 unlike many other tool steels can be optimised for hardening at very high Rockwell values. 

For example, high chromium alloys for which Rockwell values exceed HRC 58, resistance to corrosion is lost as the HRC exceeds 58 and even at 58, for example N690 a high cobalt stainless steel will begin to show signs of corrosion if the blade is left unattended after exposure to saltwater and or water either vapour or in liquid form (not to mention corrosive organic substances; eg saps, etc. Hence for N690 to be of any benefit for outdoor knife production HRC 58 is probably the practical limit to such an alloy for outdoor knives, in order to still capitalise upon good stainless properties.

Since S600 (and M2) have little chromium present, it serves less for providing resistance to corrosive chemical action. 

The Practical Tests Conducted*

We conducted practical tests, using an S600 made knife for which the entire blade was Teflon coated with exception of the cutting edge. This product had been heat treated and cryogenically treated for a HRC value of 64. The specific cryogenic treatment was not disclosed. The knife (meaning the entire steel structure) was exposed to high humidity for a period of almost 3 weeks; exposed to heavy rain (the main blade); water rinsing and or wiping the blade clean (via inspection by eye only) after cutting various organic substances containing saps and or resins, whether food based or not.

Since the Teflon coating during this period remained undamaged, it provided ample exclusion of air and or any liquids coming in contact with untreated steel. The only untreated steel was that of the actual blade cutting surface, which we will refer to as "the edge". Despite such treatment, which by the way, was exactly the same as we conduct upon any stainless steel alloys (with the exception of seawater exposure), examination of the edge of this S600 knife showed negligible effects of corrosion. In fact in a few small places (less than 5mm from one entire side of the edge) there were a few dark patina staining marks, which were not removed by wiping the edge against a paper hand towel. 

Surface staining/Patina more than likely induced via plant saps/resins.

Closer examination at > X 4 magnification, surface rusting in the form of orange brown stains were also observed and also noted to be staining rather than any kind of corrosive pitting. Previously with similar testing to an M2 knife (Work Tuff Gear) which had not been coated in Teflon, this blade too showed some patina marks which were also negligible in surface area. 

What kinds of materials were cut/encountered?

Entire green coconuts (with exposure to the coconut water)

Green rainforest timber-entire trunks**

Green cypress timber

Green palm fronds

Wild chives

Living bracket fungi (Sheep's foot fungus)

Raw beef

Preserved salted dry meats

Cabbage

Green grass-specifically Paspalum sp.

Dry birch bark

Fat wood from Pinus sp.

Seasoned Eucalyptus (hardwoods)

Seasoned Australian native species (hardwoods)

Termite material

Ficus species (living branches)

Cutting against HDPE commercial chopping boards

Cutting against Bamboo cutting boards

Cutting against dry palm bracts

Images of blade surface

Edge Chipping.

Zero evidence of chipping!

Summary

Was the blade still deadly sharp after approximately 3 weeks of serious outdoor treatment?

Answer:

YES- deadly sharp. 

Did the Teflon coating: breakdown, peel, show signs of damage after a 3 week intense period of usage?

Answer:

NO

Conclusions:

With nothing but basic care this S600 blade (Extrema Ratio ONTOS II S600) performed flawlessly. The extremely minor amount of patina was to be expected from a non stainless steel blade after exposure to the range of materials listed above. It is clear that with the appropriated heat treatment a knife blade can indeed be hardened to HRC 64 and retain sufficient toughness and durability of the edge after encountering a broad range of materials thus enabling such an alloy to be used with great success for high Rockwell rated outdoor blades.

It should of course be noted that to allow such a high Rockwell blade to actually go blunt would be a big mistake and require some effort to resharpen the edge. It is recommended that a maintenance "strop" be conducted based upon usage. As we found it still extremely sharp after all of this work, we can not say what sort of time period would be expected to strop, since it was still so super sharp. In reality, I doubt anyone would use a knife at such a high frequency as we did for these tests and therefore, the edge longevity would be very long for S600 at HRC 64.

BCT will keep you posted here as to the "when" we have to strop the edge of this blade!

* for the full practical tests, see the upcoming video field review of this knife made from S600 Tungsten tool steel on @BushCampingTools

So Stay Tuned!

** all cutting or felling of trees was conducted upon our own farmland property.

Here is the video you've been waiting for: