Knife Steels-Material Science
 

I wanted to start a thread about knife steels, kind of a materials engineering topic that would talk about steels used to make knives. This will range from car springs to particle metal steels. We can see if I've learned anything teachable in the last twenty five years of working this stuff.

My hope is to dispel some myths and aid in the maintenance of whatever knife you have or might have to make useful.

The biggest difference between knives is any knife over no knife at all. I'm not going to judge or promote, just trying to help clear up some common misconceptions, if there are any.

More to follow and starting questions welcomed.


In a recent conversation with a QP I learned that field welding can be important. When we get to carbon contents in various steels, I have some useful stuff there too.

There will be as much Redneck Engineering as possible here.

Finally, something I know a little about.

If we have not beaten it to death elsewhere, for a starter, I would like to see a little about the older carbon steels, tool steels, stainless and near stainless steels, heat treating, hardening, blade profiles, thicknesses, grinds, bevels, etc.

Just a few thoughts.

TR

Mr. Harsey,
When you temper your blades, do you use oil or nitrate salt baths?

Easy question first, All the steels I use here are air hardening and no oil or salt baths are used or needed.

Reapers question might take volumes but I will use redneck engineering to narrow down the answers.

That was not a single question, but was a variety of topics that I would like to know more about.

Hope that at least some of it is in the direction you were looking.

TR

Yes, you asked a very good series of questions and my response referring to that as a single question was an attempt at understatement.
Yes this is the direction I hoped to go. Well done.

First basic, steel in it's most basic form is iron with carbon. The higher the carbon content, the more the steel can be hardened by heat treating which involves heating a given steel to it's "transformation point" and a rapid, controlled cooling to lock the carbides into a particular type of structure. The more carbon a steel has, the more potential it has to be hardened.
An even simpler definition of heat treating steel is "changing the physical characteristics without changing the chemical composition".

The three most basic categories of steel are Mild Steel, Medium Carbon Steel and high carbon steel. There are many alloys of steels within each of these groups.
Here are some simple examples of the carbon steels:
Low carbon steels: structural steels, car bodies, common steel purchased for welding AKA "mild steel", etc.
Medium carbon steels: Firearm barrels, hammers, gears, axles, spring steels, etc.
High carbon steels: Knives, machine tooling, punches, forming dies, drill bits, milling cutters, saws, spring steels etc.

more to follow.

note: I find metals fascinating, and as a mechanical engineer I need to know all about their charactersitics, hence the questions, but I have little working experience with high carbon steels like Mr. Harsey uses every day. So...

What is the typical AISI number of steel you use Mr. Harsey?

Maytime,
I have just reviewed my material data sheets for the steels I use here and cannot find any AISI designation and to the best of memory, I've never seen one for either the 154CM I've used so much of in the past or the CPM S-30V we use now.
Both steels are made by Crucible Materials Corporation in Syracuse New York.

Edited to add: Maytime, so you can jump ahead: www.crucibleservice.com

The topic of alloys is very interesting. Historically some steels were much better than others because the naturally occurring ore dug from the ground contained alloys unknown to the steel makers but beneficial to the performance of the finished blade.
The Indian Wootz steel (think fine Persian blades) of old is a good example of this. When the ancient mine, the original source, of this iron ore played out, the quality of the swords and daggers diminished but no one knew why.

The first alloy of Iron to make steel is carbon. Both high carbon steels and tool steels may contain similar amounts of carbon but not used for the same jobs.
The difference is in the degree of refinement during steel making.

Many truck springs contain .90 carbon content but would not be acceptable for some tool and die jobs needing a tool steel with the same carbon content. This is because the truck springs contain some impurities and inclusions like slag that still make great springs but do not have the grain refinement and purity of manufacturing to hold an extremely fine edge in a hard use production tool.

I have seen some decent knives made from truck leaf springs.

Is this an alloy which has adequate potential to be a good (economical) knife steel, if hardened and heat treated properly?

TR

Good question and the answer is YES, spring steels, like 1095 can make good knives. They are very forgable and with just a little care can be field hardened and tempered using only the simplest of tools and fire.

I've seen video of full sized jeep springs made into big blades by heating the entire spring, hanger ends and all, over an open fire to orange heat and hammered over a set chisel make the rough cuts then forged to a finished blade. All this with a fire, hammer and crude anvil.

Be careful where you park your jeeps.

If your trying this yourself and in doubt of exactly what steel you have, make a test piece from the steel, heat it up so a magnet no longer sticks to the surface (good indicator of transformation point in simple hardenable steels) and try quenching in warm (100-120 F) 20 weight oil rather than water to minimize the chance of cracking. If it gets hard enough to become difficult to file then the oil quench works. If not try water quench next. If neither of these work, do not proceed with this steel.
Always temper simple tool steels for at least an hour or two at 300-400 F after hardening.

A knife blade can be forged to usable net shape without grinding and made excellent by some simple draw filing.
Yes I've done this before with good results. No real shop needed only some simple tools.

I just received a hard use brush and wood cutting knife from Taiwan, made by an indigenous traditional knifemaker there from leaf spring steel. The knife is very well made and a nice example of direct craft with the file marks part of the design.

My dad had a book, Knights Modern Seamanship. It was written in the 30's. It was a great resource book. It had directions on making a shief knife using a worn file.

Bill do you think old files would be a good source for a knife blank?

Hollis, A file will make a knife but it's a tough way to go.
Most files are designed to cut steel and have a very high carbon content for maximum hardness. This makes them hard to work (make into a knife) and they tend to be brittle in hard use.

I'd rather go with the spring.

Edited to add: Hollis, by a good source of knife steel, around here that means a steel I can work with in a timely and resource effective manner. Sometimes "free' can cost you a lot of time and money.

A better answer is yes they will make a good knife for some applications but the difficulty of working the file into a usable knife blade would put it down on my list of materials to use.

And then there is passivation in the Stainless Steels!!!:D

Bill, thank you for the reply.

H.