The Knife

The knife has long been an essential tool for the outdoorsman and man kind in general. The outdoors enthusiast would never be caught without it. When you step out the door, it is the first tool in your pack or pocket, it is a sort of companion on all your great adventures. At Hook Knives we believe that companion aught to be a little part of you, a part of who you are. So let us make your dream knife to fit you.

Wednesday, November 2, 2016

demystifying blade steel

Demystifying Blade Steels:
Understanding just enough metallurgy to see through a plethora of knife maker myths. 

     Strolling through a local gun show I paused to pick up an oddly shaped leather sheath. "Best blade steel there is, actually ground from old files!" bellowed the elderly salesman from behind his booth as I twirled the small skinning knife in my hand. I ran my finger over the small patch of still present file teeth in the back of the blade. I am newly exploring the world of knife making but I've learned enough to be wary of the "magical steels".  I place the blade back in it's sheath on the table and give my best smile to the salesman as I move on.  I have heard of several different objects being turned into the "best" blades: files, chisels, lawnmower blades, truck springs and all manner of scavenged tools find new lives in the hands of knife makers, each claiming fantastic feats of edge retention and toughness. It seems that every one of the old time knife makers has their own test which their blades must endure, such as thirty strikes to a deer antler or slicing a penny in two. The thought occurred to me that, if every knife has to endure these tests, then some must fail, so why can't modern knife makers find better consistency in blade qualities?

     Knife blades were traditionally forged, meaning that scavenged sources of tool steel were heated until they were soft, hammered into the desired shape, heat treated to harden, re-ground and finished into a blade. Because the sources of steel are so vastly varied with this method, the maker himself rarely knew what alloy of steel he really had. This inconsistency led to an air of mysticism behind good blade steel. When a blade turned out with proper qualities for a knife it was a phenomena, and if a maker could produce several of these phenomena, he just might slip into legend.

     Unfortunately prospective knife buyers are easily lost in a flurry of myth, legend, and fact; unsure of how to know that a knife will have a good, useful blade.

     A second method of knife making is stock removal. Stock removal involves utilizing a billet cut from an ingot of a known alloy, thus removing the uncertainty of alloy and greatly improving consistency. The question yet remains, which alloy should be used to attain the qualities most desired in a blade? To answer that we must first discuss just what qualities we look for in a good blade steel. In brief, they are as follows:

• Hardness: Hardness is most pivotal in the blades edge retention capabilities, that is; how often the blade will have to be re-sharpened.

• Corrosion resistance: Corrosion can be as mild as motley discoloration to as damaging as rust that destroys the blade completely. The steels resistance to corrosion is an important factor in the maintenance requirements of the blade.

• Abrasion resistance: Closely correlated with hardness, abrasion resistance refers the the steels ability to resist being scratched. This will also play a role in edge retention.

The chiefest amongst these qualities to be considered in general applications of a knife is hardness. 
     Carbon content of the alloy plays a direct roll in steel harness, the greater the concentration of Carbon, the greater ability the steel has to harden. I say ability to harden because steel can exist in several forms of the same alloy, exhibiting different qualities, namely hardness. Which brings us to the manipulation of these forms through controlled temperature changes.

     Steel can separate into phases which are areas of homogenous molecular content. Three phases occur in carbon alloy steels, Ferrite, cementite, and austenite. Ferrite is simply Iron, cementite is a compound formed by carbon and steel (Fe3C) and austenite is formed when cementite dissolves into the iron, in much the same way as salt dissolves in water. A non heat treated steel at room temperature will form a grain with microscopic areas of ferrite and cementite. Because ferrite and cementite are somewhat separated from each other the carbon cannot contribute its full hardness to the steel as a whole and the metal is overall soft and malleable. If this same steel is heated above a given point the cementite can dissolve and all of the steel enters the phase of austenite. To prevent the carbon from re-forming cementite when cooled to room temperature, the steel is rapidly cooled to freeze the carbon molecules in their homogenous state. This is called quenching and forms martensite, a form of the steel alloy in which carbon is even dispersed amongst the iron molecules, changing the crystal lattice and dramatically increasing the hardness of the steel while decreasing its malleability.

     Ok, that's the best I can simplify of what I understand of the process, probably not as much detail as what you metallurgists would give and more than most of us care to hear about! Bear with me though, I'll bring it back down to real life here. As a side note however, if any of you out there do have a deeper understanding of these processes and would like to correct anything I say, I would be delighted to hear from you. So please do shoot me an e-mail.

     The temperature to which the steel must be brought and the rate at which it must be cooled to create martensite are greatly dependent upon the contents of the alloy, thus heat treating processes are as varied as the types of steel in existence. Understanding the specific requirements of heating, quenching and tempering for an individual steels heat treat is essential in attaining the proper hardness for a finished blade. This is why working with a known alloy is essential for consistent quality.

     Different alloys of steel have the ability to attain varying degrees of hardness through the heat treating process. The harder a material is the greater its abrasion resistance and thus the longer it will maintain a sharp edge.  So then, the answer is that the hardest steel wins right? This is not completely true.
     A softer material will be less brittle and easier to resharpen. Meaning that if a very hard steel is subjected to torque or prying, it is likely to chip or crack, rather then bending like would be expected of a softer steel (eg. spring steel). Knives are usually made of harder steels in favor of edge retention, which is why a knife should never be used in prying applications, such as opening a paint can. 

     This trade off of abrasion resistance for malleability also comes in to play in edge retention.
Figure A
A knife can dull in one of two ways, the edge can be abraded, or “rolled”. A sharp edge is formed when two bevels of similar or identical angles intersect. (fig. A) An abraded edge has rounded or blunted the tip of the edge by grinding, usually against tiny particles of dirt or grit in whatever is being cut. (Fig. B) An edge is rolled when it comes in contact with a solid surface and the thin tip of the edge is bent, or rolled, to either side. (Fig. C) It is important to note that when an edge is rolled, the metal is merely bent and no steel has been removed, in contrast to dulling by abrasion in which small amounts of steel are actually removed.
Figure B

Edges are re-sharpened in the same way they were dulled, that is a rolled edge can be bent straight again, and an abraded edge must be re-ground. Re-grinding an edge requires the removal of metal by abrasion, with harder steels this is primarily accomplished with diamond hones or aluminum-oxide abrasives. Although it is possible to grind a rolled edge sharp, it is more efficient to draw the blade across a rod which is harder than the blade, a tool referred to as a hone steel. A hone steel is non-abrasive and does not remove any metal, rather it presses the bent edge back into its original shape, restoring it to its original sharpness. A rolled edge is more easily restored, with less detriment to the knife, than an abraded edge. 

Figure D

Figure C

     The harder the steel the more resistant it will be to both rolling and abrasion. Because a very hard steel is unlikely to roll its edge, it will generally have to be sharpened by abrasive means when it does dull. When a very hard steel edge comes in contact with a hard surface the edge will begin to roll, however, because malleability has been sacrificed in favor of hardness the edge will often crack or chip rather then roll. Figures D and E both show very hard blades which have chipped edges. The knife in figure D has fine chips in the edge; the entire bevel will have to be ground as far back as the deepest chip to fully restore the edge. Had this knife been a more malleable steel those points would likely have rolled and been easily corrected by a steel. Figure E shows a very hard, thin blade which not only met a hard surface but also torque tension from its user, to an obvious detriment of the whole blade. This is something no knife should endure, but given a more malleable steel the damage may have been more reparable.
Figure E
      The solution is not to create the hardest blade steel, or the softest, but rather to strike a balance between the two. Bob Crowder is the president of the Montana knife makers association and a well renowned, successful knife maker for over 30 years. Crowder, along with most knife makers, considers this balance to be between 58HRc and 60HRc. As long as a steel is capable of attaining this range of hardness and is properly heat treated, it can have excellent qualities of edge retention and durability.

     Corrosion resistance is determined by elements other than carbon and iron in the alloy. It is also influenced by the degree to which the blade is polished, this is why mirror polishing was considered a standard before the development of stainless steels. Non stainless steels, such as basic carbon/iron steels can still make great knives given that they achieve proper hardness, but they will require a greater degree of care in maintenance to avoid corrosion. Blades of these alloys should always be oiled and clean. Stainless steels live up to their names by the addition of metals such as chromium and nickle which prevent the oxidation of the steel. Stainless qualities can be of great advantage in extended outdoor use or exposure to sea water, such as scuba knives. 

     Overall, there is no magic steel. Files have made great knives because they are made from a high carbon steel which can form martensite, having a hardness in the neighborhood of 60HRc. The same is true of multitudinous similar steel sources. This is nothing mystical and can be repeated with a number of commercially available tool steels such as O-1 and D-2, furthermore these qualities can be repeated with greatly improved corrosion resistance in stainless steels like 440C ATS34 or 9Cr14MOV. When selecting a blade it is important that the maker has properly heat treated to the appropriate hardness, and corrosion resistance should be considered in light of the blades intended purpose. There are a great deal of steel alloys available which can fulfill the requirements of a good knife.  Buy your knife from a quality maker and talk to them about how you want to use the knife and what an optimal steel might be for your application. 
     I hope this article has been helpful to you in clearing up some fog surrounding this topic and of course, I'm always willing to grind you a knife from whatever steel most peaks your interest. Continue to seek understanding on this subject as there is always far more to learn.  I look forward to hearing from you all. Happy collecting! 

Primos, Terry. ""Forging or Stock Removal?" by Terry Primos." KnifeArt. Web. 1 Dec. 2015.
"Fundamentals of the Heat Treating of Steel." Practical Heat Treating. Second ed. Materials ParkASM International, 2006. Print.
"Handbook-Structure of Steel." Handbook-Structure of Steel. Web. 1 Dec. 2015.

"All About Stainless Steel." All About Stainless Steel. Web. 30 Nov. 2015.

No comments:

Post a Comment