Steel that Won't Harden Properly

[Stacy E. Apelt - Bladesmith]

Lately I have been getting a lot of emails from folks who have done HT on a carbon steel and had low Rc. There have been several threads about these problems, too.
I gave the standard advice about removing the decarb before panic, testing a sample piece in a water quench, oil temps, etc. I decided a thread discussing this problem would be a good thing for Shop Talk.

Reasons for a lower than expected hardness.
1) The hardness isn't being tested or read right - Tester not set up or operated right. testing non-parallel surfaces, decarb still on the blade. File used but not allowed to cut through decarb.
2) Steel isn't what it is thought to be. While this is the first thing to be thought of, it is the least likely to be true if the steel was bought from a reliable source.
3) HT was not right - Probably the most likely source of the problem. Temperatures, soak time, quenchant, etc. must all be right for the steel to achieve maximum hardness.
4) $#*! happens - Even the best of us has a failure now and then. Try again with close attention to all the details and see if it comes out right.

I'll let you guys post about these things and how you deal with and prevent them.


One thing that isn't regularly expected in a HT problem is the steel's condition before HT. Steels that are hyper-eutectoid, like 52100, 1095, W2, can be so well spheroidized that the carbon is all locked up. This will leave the rest of the matrix ferrite....plain iron. In HT you may release a bit of the carbon, but if it isn't well distributed and fully in solution, you will get a soft blade with hard balls of carbides in it. The Rockwell may be in the low 50's or worse even though the HT appeared to be done perfectly according to the charts. The solution is to release all the carbon back into the steel.
Heat it to 1600F and hold for 5-10 minutes and air cool. (this releases the carbon and puts everything back into solution)
Re-heat to 1500F and air cool or quench. ( this refines the grain and carbide size)
Heat to 1400F and quench. Cool to room temp. (this makes fine grain martensite)
HT as planned normally and the blade will likely hit Rc65-66 right out of the quench. ( you should get full martensite with fine grain and evenly distributed small carbides. )

Most of the time when I read that someone is tempering their 1095/W2/52100 drop point hunter blade at 350F and they like the edge, I suspect that it never got fully hard in the first place. It should be really chipping badly at such a low temper for any use but a delicate kitchen slicer. If they hit full hardness in the HT, it should be Rc 63-64 at a 350F temper.

 

[tryppyr]

It occurs to me to wonder also whether people are as careful about marking their billets when they arrive so the KNOW which steel they are working with. When I order bars from anyone, as soon as they come in I mark them about every 12 inches with the steel type. That way, as I cut a piece for use, the remainder of the bar remains marked no matter where I cut it. Zero confusion.

 

[Marko3]

Stacy would equipment come into play when people claim to like to temper at 350 ? And what effect does inconsistent tempering temperature say the oven is bouncing around at say (350 / 375 / 400) Last year I converted a old burnout kiln into a tempering oven and it holds a steady temp . I noticed a big improvement to hitting target hardness numbers . So I have been doing my first temper at 375 and I adjust from there. I mostly just use it for the first temper until my digital kiln cools down enough. But sometimes my RC numbers don't match up against the standard number from say Kevin Cashens site. I am almost always able to dial them in but they are never exactly 63 or 64 at 350 .

So couldn't equipment come into play when you heat treat . Also in another post I made in another thread . Thickness of steel has to be considered. If I have a 3/16 thick blade hitting 60 at the thickest point what should my expectations be at the edge ? As we all know we cant take a accurate reading there.

Thanks

 

[Stacy E. Apelt - Bladesmith]

The "numbers" tossed out in discussion and in "The Charts" are usually a mix of empirical and theoretical. They are based on testing and extrapolation of data in metallurgical labs with millions of dollars in equipment and decades of experience in the testers. The real world is a different place. With time, good equipment that you have become familiar with, and experience...you can get close to those numbers...but will probably never hit them dead on. Knifemakers aren't usually even in the real world - Home built equipment, mainly used or primitive testing equipment, occasional use of the equipment, experience levels ( or lack of it), HT methods posted on the internet by people with no experience, HT methods in magazines by "famous" knifemakers, quenchants ranging from Heatbath #50 to lard,....well you get my drift....nothing is exact in the ways we generally do HT. Even two makers with identical equipment may get different final hardness.

Testing is the way to determine if the knife blade is right. Edge chipping or rolling is what maters much more than what the Rc is. People put way too much stock in Rc this and Rc that. Rc 58 and Rc 62 cut the same. It is geometry, HT, and steel choice that determine a knife blade as good, better, or best.

 

[Stacy E. Apelt - Bladesmith]

The "numbers" tossed out in discussion and in "The Charts" are usually a mix of empirical and theoretical. They are based on testing and extrapolation of data in metallurgical labs with millions of dollars in equipment and decades of experience in the testers. The real world is a different place. With time, good equipment that you have become familiar with, and experience...you can get close to those numbers...but will probably never hit them dead on. Knifemakers aren't usually even in the real world - Home built equipment, mainly used or primitive testing equipment, occasional use of the equipment, experience levels ( or lack of it), HT methods posted on the internet by people with no experience, HT methods in magazines by "famous" knifemakers, quenchants ranging from Heatbath #50 to lard,....well you get my drift....nothing is exact in the ways we generally do HT. Even two makers with identical equipment may get different final hardness.

Testing is the way to determine if the knife blade is right. Edge chipping or rolling is what maters much more than what the Rc is. People put way too much stock in Rc this and Rc that. Rc 58 and Rc 62 cut the same. It is geometry, HT, and steel choice that determine a knife blade as good, better, or best.

 

[Willie71]

Stacy, your comments on the condition of the steel is a great one. I had a bar of 1/4" 1084 that was very inconsistent in heat treat. I found this quite puzzling as it was supposed to be the easiest steel to heat treat. Even using brine, the steel would not consistently harden. It turns out the steel was highly spheroidized, and needed the normalizing top get everything into solution. I used the three step normalizing now on EVERY carbon steel I have, except the Hitachi steels that I use very occasionally, and the 15N20 I got from Aldo. I tried it both ways, as from the mill, and after normalizing, and found no difference. I might need to check this on the next batch I get though.

For stock removal knives, I mark off the blades I want to make, and cut the lengths to rough size. I normalize in the kiln with a programmed run. It would be better to do each blade after the blank is rough ground, in case it was overheated in grinding, but I grind without gloves, and dip frequently. I use this method to minimize decarb when closer to final size. I did a few blades with stanite washes, or foil protection, but found for my shop, and my process, normalizing first works best for me. Others will have their own preferences. As a side note, with 52100, I pull the blades from the kiln to air cool to magnetic after each heat, as cooling in the kiln is too slow. For water quenching steels, this isn't an issue.

I think this is the most likely problem after the obvious concerns like rudimentary equipment that has little control, or improper quenching. If you have eqiupment such as a PID kiln, and commercial quench oil, I think steel condition is the key.

 

[Willie71]

Stacy would equipment come into play when people claim to like to temper at 350 ? And what effect does inconsistent tempering temperature say the oven is bouncing around at say (350 / 375 / 400) Last year I converted a old burnout kiln into a tempering oven and it holds a steady temp . I noticed a big improvement to hitting target hardness numbers . So I have been doing my first temper at 375 and I adjust from there. I mostly just use it for the first temper until my digital kiln cools down enough. But sometimes my RC numbers don't match up against the standard number from say Kevin Cashens site. I am almost always able to dial them in but they are never exactly 63 or 64 at 350 .

So couldn't equipment come into play when you heat treat . Also in another post I made in another thread . Thickness of steel has to be considered. If I have a 3/16 thick blade hitting 60 at the thickest point what should my expectations be at the edge ? As we all know we cant take a accurate reading there.

Thanks

Kevin Cashen has commented a few times on the process of tempering, and hardness testing. This was demonstrated at the Southern Alberta Hammer in this year. After the first temper, the Rc numbers varied a bit, after the second temper, the variance decreased. After a third temper, there was basically no variance beyond the accuracy of the tester. Now most people say the third temper has no real world improvement in performance, and that is probably correct. Since I temper low initially, and walk up to the desired hardness, I find I typically use 3 to 4 tempers to get where I want to be. I make no claims about performance increases, but it does address the testing number issue.

 

[Willie71]

The "numbers" tossed out in discussion and in "The Charts" are usually a mix of empirical and theoretical. They are based on testing and extrapolation of data in metallurgical labs with millions of dollars in equipment and decades of experience in the testers. The real world is a different place. With time, good equipment that you have become familiar with, and experience...you can get close to those numbers...but will probably never hit them dead on. Knifemakers aren't usually even in the real world - Home built equipment, mainly used or primitive testing equipment, occasional use of the equipment, experience levels ( or lack of it), HT methods posted on the internet by people with no experience, HT methods in magazines by "famous" knifemakers, quenchants ranging from Heatbath #50 to lard,....well you get my drift....nothing is exact in the ways we generally do HT. Even two makers with identical equipment may get different final hardness.

Testing is the way to determine if the knife blade is right. Edge chipping or rolling is what maters much more than what the Rc is. People put way too much stock in Rc this and Rc that. Rc 58 and Rc 62 cut the same. It is geometry, HT, and steel choice that determine a knife blade as good, better, or best.

I agree with everything you say here Stacy, but have a slightly differing perspective on testing, and how I use it. I have found heat treat recipes that give me consistent results in performance, that give me the max performance my equipment can generate. If I test at different stages, such as after quench, after tempers, I can verify there wasn't a steel or equipment problem at that stage. I know what type of abuse my 15n20 can take at Rc60, 61, 62, and 63, so I can provide a blade at the performance level the client wants. I have good information on 52100 at different hardnesses (Rc60-62, but I need more info in higher hardness numbers), but not as much as my 15n20, and am gathering this information on W2 as I experiment more (and rely on Don Hansen III, who has worlds of experience with this steel.) To me its about verifying what has happened in the process, and what previous blades hardened this way were capable of.

Its important to not equate Rc#s as being equal. My Rc62 on my 52100 could perform significantly differently than someone else's, depending on how the steel was set up going into austentizing. For me, its a comparison of my work against my other work, not a generalization to all blades with the same steel.

 

[Stacy E. Apelt - Bladesmith]

Good point about more than two tempers. On most steels it gains very little beyond two tempers...but if you are zeroing in a HT to get a desired edge durability, it can take four or five tempers. In subsequent tempering of that same batch of steel ( once you have a well established HT and Tempering parameter), you can probably get away with two tempers...but three won't hurt anything.

Now, here is how to get one free temper:
When quenching a carbon steel blade, pull it from the quenchant after about eight seconds. With gloved hands, check for straightness and adjust as needed until you feel it start to firm up in your gloved hands). It is now about 400F. You can stick it back in the oil when straight if you want, but by letting it cool slowly in air, you get the advantage of auto-tempering. I think many people who test their "as-quenched" hardness and aren't getting mid 60's numbers may have had this effect on the blade. It can easily drop the blade several Rc points, and even more on a very thick blade that cools more slowly. This mini-temper is a good thing, not bad. This isn't a full temper in any way, but it will affect the degree of subsequent tempering. Just another reason why working up form a lower temper is a good method. I usually temper at 380F or 390F and then set the oven to 400F while I am cooling the blades in running water. I put them back in and within a few minutes the oven is at 400F. If testing an edge, I wait to set the oven up until I run out to the shop and grind a quick edge and flex test it.

 

[Jason Fry]

If 1084 from a certain reputable supplier were in fact highly spheroidized, would that result in low hardness when quenched in 10 second oil from 1500 degrees with no normalization?

I've used maybe 150 ft of 1084 bar stock without a problem, using the 10 sec oil and a PID controlled kiln. Typically 62.5 out of the quench, and 60-60.5 after two 450 tempers. I also have 3 bars labeled 1084 that I haven't been able to get even into the mid 50's. Said supplier is currently having these three bars tested to make sure there wasn't some kind of switch-up.

 

[Stacy E. Apelt - Bladesmith]

eutectoid and hypo-eutectoid steels are less a problem than hyper-eutectoids.

 

[Willie71]

If 1084 from a certain reputable supplier were in fact highly spheroidized, would that result in low hardness when quenched in 10 second oil from 1500 degrees with no normalization?

I've used maybe 150 ft of 1084 bar stock without a problem, using the 10 sec oil and a PID controlled kiln. Typically 62.5 out of the quench, and 60-60.5 after two 450 tempers. I also have 3 bars labeled 1084 that I haven't been able to get even into the mid 50's. Said supplier is currently having these three bars tested to make sure there wasn't some kind of switch-up.

It sounds like you might have had a bar of the same stock I did. After normalizing, I got Rc 65 out of quench in DT48, with 1/4" stock (1485f austentize.) Thinner stock I got full Rc66. Prior to normalizing, I got low 60's to Rc37 IIRC. Another possible explanation is that the carbon and manganese were not evenly distributed from the mill. I spoke with my supplier, who tested his remaining stock, and his hardened fine. It may have just been that that bar was an oddity, but about 1-1.5 years ago, several people had the same problem with the 1/4" stock. It was the exception, not the norm definitely.