Steel Type Help

[Andrew Eliason]

Hello!

 

I am new to the knife making world and am trying my hand at some new steel.  I came across a plow wear blade I had laying around that I'd love to turn into a few knifes maybe a hatchet or two.  I reached out to the company that makes the blade (unfortunately they don't make the specific one anymore) but they told me that the steel used is 7 Gauge HRPO steel Grade 50. They didn't specify what type of steel other that what I already stated.  I feel dumb asking if this is potentially good steel to work with for a blade or not, but I cannot seem to find any answers online anywhere else. 

 

If this steel won't hold a good edge or won't harden as much as it should to hold an edge my plan is to make a set of tongs or two.

[Alan Longmire]

My best guess is it's either 1050 (also known as plow steel if used to make wire rope) or some alloy with a yield strength of 50kpsi, like 1045.  7 gauge is the thickness, 0.176 inches (4.5mm), slightly less than 3/16", and HRPO means Hot Rolled Pickled and Oiled.  Rolled hot, pickled in acid to remove the mill scale, and oiled to prevent rust.  

 

It should harden fine in warm water or brine.  Hot canola if it cracks in water.  Temper at 325 F and see what you've got.  It should come out around Rc 55, a bit softer than most small blade steels, but fine for choppers and other things than need toughness over edge retention.  Too many modern folks obsess over hardness, when for most of history Rc 50 -55 was considered perfect.  

[Andrew Eliason]

Thank you! 

Because it's potentially 1050, that means I could take a nice section to try a short sword? or full size? It's 42" long but I don't know if I want to try that big of a sword for my first one.

[Alan Longmire]

At 4.5mm thick it's not really enough for a full-sized sword, or even a big bowie knife.  Start small and see what you can do.  Think medium-sized hunting knife or skinner and give it a go.  Try the oil quench first, and only go to warm water if the oil won't fully harden it.

[Jerrod Miller]

47 minutes ago, Alan Longmire said:

Too many modern folks obsess over hardness, when for most of history Rc 50 -55 was considered perfect.  

To be fair, most things that were considered perfect for most of history aren't in fact great.  

[Alan Longmire]

True enough! 

[Doug Lester]

Love these discussions.

 

Doug

[Will Robertson]

In a case like this would folk recommend a quick spark test to get an initial idea of the composition of the steel? One guy recommended improving the accuracy and consistency of the spark test by having some samples of known steels to hand and comparing the sparks from these with the unknown steel (pasted the video below - I found it really helpful).

Another good tip that I learned on here was to test harden a sample of the unknown steel - that way the hardening could be tested and perfected without risking wrecking a whole piece.

How about the steels used for turning and milling parts where precision is more important than corrosion resistance? (I think "Tool Steel" in the USA but I think the same steel is sometimes called "Machine Steel" in Europe.)

The reason I ask this is that machine shops with large CNC machining centres over here routinely scrap pieces of new good quality stock steel which are too small to be handled efficiently by their large CNC machines but which are the perfect size for manual handling. The machines typically use flood cooling but work-hardening during machining can still be an issue so the work hardening properties of these steels are usually fairly well controlled by the manufacturers. Typically they're fairly low carbon steels so I'd guessed that they wouldn't be appropriate for knife making but I could be wrong about that.

(I'm trying to be very careful about how I describe these steel categories because direct translation of the names from Norwegian or German into English can give misleading results and I'm not even sure if the naming is consistent between US English and UK English.)
 

 

Edited April 10 by Will Robertson

[Jerrod Miller]

Spark testing is really good for simple steels, where carbon is the only real alloying element of consequence.  It becomes less useful (but not necessarily worthless) with more alloying elements.  

 

Testing heat treat on small sections of stock, even known alloy, is definitely something that should be done.  

 

Are you referencing the tooling or the stock being used in milling machines?  The tooling generally isn't good for forging at all, and the stock being machined can be absolutely anything, depending on the needs of the part being made.  

[Jaro Petrina]

Latelly some of my friends, probably because of some youtube videos wanted or asked me if the blade "will write on glass", I said, most of my choppers isnt that hard, now you cannot sharpen it now and always bring it to me when its completelly dull, why do you think you will sharpen it ,when it only responds to diamond.  The idea does not seem to take. 

[Will Robertson]

41 minutes ago, Jerrod Miller said:

Spark testing is really good for simple steels, where carbon is the only real alloying element of consequence.  It becomes less useful (but not necessarily worthless) with more alloying elements.  

 

Testing heat treat on small sections of stock, even known alloy, is definitely something that should be done.  

Thanks! That's very useful to know!

 

42 minutes ago, Jerrod Miller said:

Are you referencing the tooling or the stock being used in milling machines?  The tooling generally isn't good for forging at all, and the stock being machined can be absolutely anything, depending on the needs of the part being made.  

The stock being used. In this case it's a precision shop running large CNC machines so any down time or deviance from specifications would get very expensive and they'll only run good quality stock through their machines - usually a free cutting steel for precision work and 304 stainless if it's for food or pharmaceuticals.

Though the stock  and tools can change places - sometimes a piece of custom tooling can be made from special stock then hardened for use and sometimes a piece of custom tooling can be designed and made from stock and carbide tips mounted to give the hard cutting edge.

I've gotta be very careful about that because in German the term "Hochgeschwindigkeitsstahl" is sometimes used colloquially to refer to a steel which can be machined fast but the direct English translation "high speed steel" is a very hard steel used to make cutting tools before the days of indexable carbide tips (most folk still use it if they need a quick custom tip) - so there is risk of confusion between the two languages.

[Alan Longmire]

24 minutes ago, Will Robertson said:

free cutting steel

 

In the USA this is frequently 12L14, a leaded low-carbon steel designed for ease of machining. Useless for blades, as is 304 stainless.  There's another alloy whose name escapes me that is specifically designed to be case hardened or nitrided after machining, also not good for cutting tools.

[Will Robertson]

58 minutes ago, Jaro Petrina said:

Latelly some of my friends, probably because of some youtube videos wanted or asked me if the blade "will write on glass", I said, most of my choppers isnt that hard, now you cannot sharpen it now and always bring it to me when its completelly dull, why do you think you will sharpen it ,when it only responds to diamond.  The idea does not seem to take. 

We had that with chainsaw chains - they made chains with carbide tipped cutting teeth that stayed sharper longer - but they were also impossible to sharpen using normal tools so only ever got used for specialist applications.

[Will Robertson]

1 minute ago, Alan Longmire said:

In the USA this is frequently 12L14, a leaded low-carbon steel designed for ease of machining. Useless for blades, as is 304 stainless.  There's another alloy whose name escapes me that is specifically designed to be case hardened or nitrided after machining, also not good for cutting tools.

Thanks!

Good to know that's a bad idea and can be abandoned!

There's been pressure from environmental regulators to phase out lead resulting in research into low-lead free cutting steels and some lead-free free cutting steels coming into commercial use but ROHS exemption No. 6a allows up to 0.35 % lead in machine steels so leaded free cutting steel still seems to be with us in Europe for now. I'm not sure if the steel used here is leaded or lead-free but I could test it if it would make any difference?

What's the limitation in steels that require nitriding in knife making? Is it that the properties of the steel in general aren't suitable for knife making or is it just that most folk don't have the equipment or potentially dangerous chemicals needed for nitriding?

[Jerrod Miller]

For blades you want to have at least 0.60% C in the steel (lower is possible with certain alloying combinations or specialized applications, but this is not overly common).  There aren't any "nice to machine" alloys that make great blades.  The more carbon you have in the matrix, the harder the martensite that is formed will be.  At a certain point you start forming carbides and retained austenite, rather than harder martensite; so there is a bit of a tradeoff to look out for.  

 

Steels meant to be carburized and/or nitrided are low in carbon and generally fairly tough with a chemistry that responds well to heat treatment.  This machines well, then you carburize/nitride which modifies the chemistry near the surface so the surface gets much harder in heat treat than it otherwise would have.  This is still generally less than blade alloy carbon levels and is only for a very thin skin.  

[Will Robertson]

On 4/10/2023 at 9:49 PM, Jerrod Miller said:

For blades you want to have at least 0.60% C in the steel (lower is possible with certain alloying combinations or specialized applications, but this is not overly common).  There aren't any "nice to machine" alloys that make great blades.  The more carbon you have in the matrix, the harder the martensite that is formed will be.  At a certain point you start forming carbides and retained austenite, rather than harder martensite; so there is a bit of a tradeoff to look out for.  

 

Fundamentally, if a steel is optimized to be nice to machine, nice to weld or to minimize cost (e.g. structural...) then it's not likely to make great blades?
 

Thanks for explaining the equilibrium between martensite and an austenite / carbide mix - can see how this would lead to there being an optimum carbon content for knife making with declining performance at either side.

 

On 4/10/2023 at 9:49 PM, Jerrod Miller said:

Steels meant to be carburized and/or nitrided are low in carbon and generally fairly tough with a chemistry that responds well to heat treatment.  This machines well, then you carburize/nitride which modifies the chemistry near the surface so the surface gets much harder in heat treat than it otherwise would have.  This is still generally less than blade alloy carbon levels and is only for a very thin skin.  

For work we use Silky pruning saws which are made of a fairly flexible steel that then has tungsten and carbon implanted into the surface via a process that sounds very similar to plasma nitriding. Great blades but they can't be sharpened.

I did a bit of hunting on the history of patents for plasma nitriding, carbiding and boriding processes and on the constituents of the old-time carbiding / nitriding powders - I should write that up and put it in a separate thread.

The snag with all of that would be that once the blade is blunt it would have to be returned to someone with carbiding / nitriding facilities to re-harden it after sharpening?
 

[Jerrod Miller]

2 hours ago, Will Robertson said:

Fundamentally, if a steel is optimized to be nice to machine, nice to weld or to minimize cost (e.g. structural...) then it's not likely to make great blades?

Correct.  

 

2 hours ago, Will Robertson said:

can see how this would lead to there being an optimum carbon content for knife making with declining performance at either side.

Not necessarily a decline in performance.  There is not perfect alloy/heat treat/property for every job.  Sometimes carbides are nice to have, sometimes not.  

 

3 hours ago, Will Robertson said:

The snag with all of that would be that once the blade is blunt it would have to be returned to someone with carbiding / nitriding facilities to re-harden it after sharpening?

This is a great example of different uses requiring different solutions.  Some people would rather have a softer blade that loses it's edge quickly but is very easy to sharpen.  Others would rather have a harder blade that keeps an edge longer but needs diamond stones and a lot of time to sharpen.  Then there are things like saw blades and indexing cutters that don't need to have the same geometry as a knife edge to do their cutting well and can use other materials/processes.