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5160
5160 is a low alloy steel known for its excellent toughness. It has been used by many forging bladesmiths due to its good properties and also wide availability, especially in the form of leaf springs. However, information on how to maximize the steel’s properties with heat treating is scant. So along with my father, Devin Thomas, we did a small study on the toughness of 5160.
Prior processing
The steel had to be forged down to the size suitable for toughness testing so that adds in several steps, and has a significant effect on the response of the steel to the heat treatment given. While the heat treatments we performed may apply to a broader range of 5160 given a different prior forging and annealing, the heat treatment is most likely to give the same result when forged, normalized, and annealed in the same way.
Forging temperature = 1800°F
Normalization = 1600°F for 20 minutes followed by a plate quench
Annealing = 1250°F for 2 hours
Austenitizing = 15 minute hold at the indicated temperature. Temperatures of 1475, 1500, 1525, 1550, and 1575°F were tested.
Quench = Parks 50 oil, followed by 15 minutes in liquid nitrogen
Tempering = 1 hour performed twice (cooled to room temperature between tempers). Temperatures of 350, 375, 400, and 450°F were used.
Toughness Testing
We used the same test procedure as has been used in all of our other toughness testing, with full specifications for the test samples on this page. They are 2.5 x 10 x 55 mm specimens without any notch (unnotched) and tested with a charpy impact tester. The toughness is measured by the energy required to fracture the specimen. You can lean about how charpy works in this article on toughness. Using this standard specimens allows us to compare the toughness of 5160 to all of the other knife steels we have tested.
Hardness
The lowest hardness was measured with the lowest austenitizing temperature (1475°F), as expected. However, hardness was relatively flat from 1500-1575°F, indicating that more carbide was dissolved by increasing from 1475 to 1500°F austenitizing temperature, but that there was not a significant change above that temperature.
For increasing tempering temperature the hardness was reduced, as expected. Just over 60 Rc was reached with a relatively low tempering temperature of 350°F, and only 57.3 Rc with 450°F.
Toughness
A peak in toughness was reached with an austenitizing temperature of 1525°F, though the toughness is relatively flat between 1475-1525°F. The toughness drops above 1525°F, either because of excess carbon in solution or because of grain growth. Because the hardness was flat above 1500°F, grain growth is more likely. 1525°F is the recommended temperature of heat treatment in the ASM Heat Treater’s Guide.
With tempering there was a peak in toughness with a temperature of 400°F, with slightly less using a 375°F temper. There is a surprisingly large drop in toughness when reducing the tempering temperature from 375 to 350°F. This large drop is surprising because in testing of CruForgeV and AEB-L we did not see similarly large drops in toughness even when tempering as low as 300°F. There was also a drop in toughness when increasing the tempering temperature from 400 to 450°F, despite the reduction in hardness. The drop in toughness when tempering at 450°F is because of “tempered martensite embrittlement” which is described in this article.
Hardness-Toughness Balance
In terms of tempering, the hardness-toughness balance is highest with 375 or 400°F. Using 350°F led to an increase in hardness of only 0.7 Rc, but the toughness was drastically reduced, so the hardness-toughness balance is much poorer. Increasing the tempering temperature from 400 to 450°F led to a decrease in both hardness and toughness. Therefore tempering above 400 or below 375°F is not recommended.
Increasing the austenitizing temperature between 1475 and 1525°F led to increases in both hardness and toughness, for a peak in the hardness-toughness balance at 1525°F. Austenitizing higher than 1525 led to a small decrease in hardness and a significant reduction in toughness so those austenitizing temperatures are not recommended.
Because of the drops in toughness with low or high tempering temperatures, and the relatively flat hardness with different austenitizing temperatures, 5160 does not appear to be very flexible in terms of its possible property combinations. Austenitizing in the range of 1500-1525°F and tempering in the range of 375-400°F provides the best combination of properties, and adjusting from there provides either a drop in toughness, hardness, or both. Perhaps removing the cryo step would lead to an improvement in toughness along with a corresponding drop in hardness. Otherwise, don’t get too creative in heat treating 5160.
5160 vs Other Knife Steels
For our optimized heat treatment, 5160 toughness is very high compared to other steels. This is in line with what we would expect based on the very low carbide fraction in heat treated 5160. 5160 was better than any other steel apart from 8670. However, the 8670 steel was heat treated without a cryo treatment so I am not sure if the 8670 has inherently better toughness, perhaps due to its nickel addition, or if higher retained austenite from lack of cryo improved toughness.
Summary and Conclusions
5160 has very good toughness with a heat treatment using an austenitizing temperature of 1500-1525°F and tempered at 375-400°F. That results in 58.5-59.5 Rc and very high toughness when heat treated with a cryo step. Skipping cryo may improve toughness slightly with a drop in hardness but this was not tested. The prior processing used in this study (forging temperature, normalizing, and annealing) is recommended to ensure the same results, but the heat treatment may still work with annealed barstock used for stock removal.
Thanks for optimizing another HT regimen! 8670 basically replaces .4% Cr & Mn with .9% Ni & 1.1% more C. Would you make any adjustments to its HT? Do you think either really needs differential HT for a heavy wood chopper?
Edit: 0.11% C
It’s hard to predict the optimal heat treatment of one steel based on the results of another. The datapoint for 8670 toughness on the chart in the article came from 1525°F for 10 minutes, quench in DT-48 oil, then tempered at 400°F. There was no cryo treatment. That heat treatment led to nearly 60 Rc and high toughness.
I would have liked to see how 5160 compared to other steels such as 1095, 1070, and 1050, which are commonly used for machete.
One test at a time. If you want things to go faster you can contribute your own specimens for me to test.
My apologies, I had forgotten you were doing your own physical research rather than literature research.
No problem 🙂
How long would you hold an as-cast 5160 to fully austenitize it before quenching?
Also, how did you account for decarburization through forging?
An as-cast part would be a completely different ball game. The performance would not be as good. The pieces were ground from oversize so decarb is not an issue.
What if we’re after something softer and more flexible than 48ft-lb and 58-59rc?
I’ve seed some recommendations for tempering at 250c, which looks like it would put it into the martensite embrittlement area based on your other article. Seem like you would have to go up to around 400c before you start seeing more toughness.
Presumably you haven’t tested it, but does that sound right? What’s your impression?
It’s better to use lower carbon than to temper higher. But lower hardness does not equal better flexibility: https://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/
So would you consider it considerably beneficial to perform a cryo treatment on the 5160? And if so does it have to be immediately after the knife has cooled. Perhaps you have a good source on how to perform a good cryo treatment?
A cryo treatment increases hardness somewhat with a corresponding decrease in toughness. It also expands the range of possible austenitizing temperatures for some steels. To perform cryo you quench and then go into liquid nitrogen. Dry ice works nearly as well especially if the delay between quenching and cold treatment is short. I have three cryo articles on this website you can read, starting with part 1: https://knifesteelnerds.com/2018/12/03/cryogenic-part1/
I’m making a throwing knife from 5160 so I think I’m after high toughness. Based on the above it sounds like I want to hold at 1525, quench then temper at 400 with no cryo for max toughness. Am I understanding this right, or would tempering higher for less hardness be better for this application even at the cost of toughness?
Either 400F below TME or way higher to get above it. But we haven’t tested in that range.
How high would you have to temper to get above TME? I am wondering about the eye of a something like a Council Tool camp carver axe?
That’s a good question
Having designed, perhaps foolishly, a 10 inch blade bowie knife with some khukuri elements ( in order to try to have a single knife that meets many needs), I have learned that 5160 is an excellent steel, especially if you intend to beat the knife’s spine with a wooden mallet often to split wood, and if you intend to chop wood with it. I do not think that the factories in Nepal are going to optimize the characteristics of this steel because the videos that I have watched on their manufacturing process shows a very primitive and uncontrolled ( temperature-wise) fabrication. Are my suspicions founded ?
If so, who in the USA would pay attention to the proper processing of this steel to optimize the toughness that I seek ? I ask you because you are most likely to be impartial in this regard. I can not pay a fortune for a knife but I am willing to pay more than what the Nepal factories are asking.
In your book, you list the high end forging temperature for 5160 as 2200. Here you list it as 1800. I’m assuming you are forging at 1800 to limit grain growth? Can you explain the thinking behind 1800 vs say 2000?
The book lists the Heat Treater’s Guide maximum recommended forging temperature. Listed in this article is the forging temperature we used for a specific study. 1800F was selected to minimize grain growth.
I know that this is an older thread, but I will give it a try. Council Tool is selling their newer Wood Craft models at 55Rc. . I read a customer review that said that he had some chipping and rolling issues with his little 2 lbs Wood Craft axe. Could that be because they fell for the “softer is tougher” argument and have run into some TME issues? This article seems to indicate that if you don’t want 5160 in that 58-59.5 range, you probably need to find another steel.
Another possibility is that it is all rolling. Chipping/rolling usually means deformation that is being misinterpreted as chipping. So the steel had insufficient strength/geometry for how it was used.
I saw that review and it got me to wondering. Back around 2009, I made a W-2 camp knife for a marine officer who was replacing a 5160 Bowie made by a relatively well-known company. He said he had chipping issues when trying to hack up a cooked goat. He bought the knife for me when I got back to the states and went out and beat up on an old dead tree in his backyard and was happy. But I’ve wondered since then if that might’ve been a TME issue on the 5160 knife?
I’ve encountered some chipping and rolling issues on blades due to poor heat control after HT eg power sharpening etc. The edge rolls for a portion until it hits the harder steel underneath and then can chip behind that point.
Hey there Larrin!
Thanks very much for the continued excellent work as well as making these resources available for the broader community.
I’m just wondering if you could help me understand your reasoning for plate quenching after normalisation here.
My guesses are to reduce grain size (increased toughness with slightly reduced quench response) and to limit the time that chromium carbides have to grow (increased toughness). Is there an interaction with chromium carbide size and austenitizing that I’m missing?
Take care,
Andrew and the team at Kailash
My father did those heat treatments so I can’t explain all of the reasons he might have chosen to plate quench following normalizing. The most common concern with air cooling after normalizing is the formation of grain boundary carbide. That is most common with very high carbon steels like 26C3 and is less likely in a steel like 5160. Plate quenching could also help with maintaining flatness. A reason not to plate quench is because higher hardenability steel may form some martensite. But if an anneal is performed afterward it shouldn’t be a big deal.
Thanks very much for your reply Larrin- it did seem a little out of place.
It struck me that potentially just to speed up cooling time- the kind of habit formed by a full time knifemaker rather than researcher.
Do you think there’s any further performance you could get out of it with your current knowledge? Gotta say that your new 15n20 results make it look pretty dusty
Given that our 5160 results are quite good it seems unlikely we could find any major improvements.
Hi, Larrin
Firstly I have to thank for all your work at esting and explaining all this. I can’t overestimate how important is for me and my knifemaking colleages down here in Argentina. We discuss your posts regularly.
But i have a question. Ive noticed that TME isnt portrayed as very relevant in most worksheets about 5160. The charts show smaller differences between tempering in the peak TME ( say 300C) and the peak toughness at 200C. That puzzles me a little. I suspect your HT with cryo may be gives a better toughness at 200c and hence we see a bigger difference.
Also i have measured several Weyesberg Kirchbaum sabre of late xix century and they are consistently at 48/50 rc.
Did they decided such a low hardness to avoid TME? Is toughness so much better there than at 200c?
To answer these questions im grinding two sabers tempered one at before and one past TME range. I m thinking in 200C and 370C.
Question is Would be 370C be enough to have a higher toughness than at 200C?
Thanks in advance
370°C would likely be enough to have higher toughness than 200°C, yes.
This study https://repository.unilibre.edu.co/bitstream/handle/10901/10578/DOCUMENTO%20FINAL%20TESIS%202017.pdf?sequence=1
Is a good example of values I see in the web.