Initial toughness Vs higher temper

[ColsonCustoms]

Assuming a rc of 58-59 is being sought for, would it be better to temper back 1095, or leave something like 1075 harder? My question mainly being, by tempering back 1095 quite a bit, will there be sizable toughness gains? or will the lower carbon steels inherently be tougher?

 

[james terrio]

...or will the lower carbon steels inherently be tougher?

The short answer is yes. With different carbon contents, you end up with different structures, independent of how far the (properly hardened) steel is tempered back. See the thread on hypo-euctectoid, eutectic, and hyper-eutectoid steels for more info.

 

[ColsonCustoms]

Awesome, will do. thanks.

 

[me2]

Generally best to choose a lower carbon steel than to keep tempering at a higher temperature. You can run into the temper embrittlement temperature ranges pretty quickly.

 

[Matt River]

If you really want toughness look at 5160 or 52100, they can have both, alloy steels have an advantage in toughness and strength, stress resilience.

 

[ColsonCustoms]

When is that usually? The highest I ever temper is 500 deg F (mostly 400-450). I always thought embrittlement happened at higher temps, 600+. May be completely wrong on that though. I'm sure it is different for each type of steel.

 

[me2]

When is that usually? The highest I ever temper is 500 deg F (mostly 400-450). I always thought embrittlement happened at higher temps, 600+. May be completely wrong on that though. I'm sure it is different for each type of steel.

It's surprisingly consistent across a large range of steels. I'd recommend not tempering between 450 and 650. Some steels have a toughness peak as low as 350 or even 325. For many steels, 500 is in the middle of the embrittlement range.

I will also say, since steels such as M2 and D2 are used in chopping knives, the requirements for toughness are frequently not that high, and embrittled 5160 is likely tougher than properly hardened and tempered D2.

There are embrittlement mechanisms that happen at higher temperatures as well. These are typically not a problem for low alloy steels, as they are not tempered in those ranges. The are also more subtle than TME, requiring some pretty in depth testing to detect. If you find your knives failing by fracture, tempering higher/lowering the hardness might not be the way to fix it.

 

[ColsonCustoms]

Yeah, I have a few large choppers made from 5160 that are tough as nails. Video of one in action. ( https://www.youtube.com/watch?v=A4LFZqoepRU ). Normally go with a 450 deg on those. This question was mainly for thin 3/32"-1/8" kitchen blades. I have been using 1095 and tempering the crap out of it. Hardness is secondary (to an extent) to toughness in my book. Rather have to sharpen a blade once a week than have it snap off and be worthless. I just picked up some 15n20 and will be giving that a shot here in a few days. May see how it performs at a 350ish temper, going from what I have gathered here.

 

[me2]

1095 and 5160 definitely have embrittlement zones, and I'd assume 15N20 also does. I am speaking specifically of impact toughness. Ductility also changes, as does strength. The relationship between toughness, ductility, and strength (as measured by hardness) is not linear. As tempering temperature goes up, hardness goes down. However toughness and ductility can do various things in relation to tempering temperature. This is because there are several reactions going on in the steel that happen because of the increases in temperature. The hardness is a byproduct of the changes, not the major effect. It's just MUCH easier to see changes in hardness than it is to see the changes in the steel due to these reactions. This requires microscope work, and some of it can get fairly sophisticated.

Now, these embrittlement zones happen and are in fairly well documented ranges, as long as standard heat treatments are followed, because that's what the research is based on. If one does some odd heat treatment, then the temperature ranges may change, even for the same steel.

 

[RX-79G]

I have been thinking more often that if you don't get the toughness you desire at 400 F, you need to change steels. Most of the best tough steels are inexpensive, so there isn't an excuse for making a chopper out of O1 or 1095.

 

[jdm61]

I have wondered of late if some of the 1095 knives that have the soft "super special proprietary heat treatment" are left that way to get them BELOW the embrittlement range? I can't figure out why you would ever use the stuff softer than 60 at the edge/on the surface since parts of the blade are going to be softer on a reasonably thick knife no matter what you do to it.

 

[Matt River]

Substantial interest on my part on regards to point of embrittlement, many blades are tempered right at the same range as first point of embrittlement, however my understanding of the process was that it could not occur without a substantial soak time, minimum one hour but full embrittlement taking longer. Want to know more, great thread.

 

[stezann]

I have been thinking more often that if you don't get the toughness you desire at 400 F, you need to change steels. Most of the best tough steels are inexpensive, so there isn't an excuse for making a chopper out of O1 or 1095.

QUOTE!! Also, don't understimate hardness. Edge stability is made of the high hardness you can use at the peak of toughness of the steel which happens before loosing significant hardness...for most steels it's at the tempering range 380-400 °F. Choose your alloy depending on the task.

 

[stezann]

Tempering means "squeezing" carbon out of the martensite, it makes carbides, at the expense of the martensite's carbon. Increasing temperatures have those carbide growing and merging together up to a point when they start creating preferential paths of crack propagation, hence embrittlement.
Under 400° F the carbides are so small and well dispersed that actually reinforce the matrix, hence the peak of toughness...Actually it is not a peak, it is the best strenght/toughness ratio.
After those temperatures the carbide driven brittleness is being balanced by the softness of the martensite, still the rise in toughness is slowed.
At the TME you still have martensite, with the worst carbide condition, it is the "walley" of the toughness function.
Then you are in the spheroidizing region...toughness rises again because you don't have martensite anymore, just ferrite and carbides

 

[RX-79G]

Substantial interest on my part on regards to point of embrittlement, many blades are tempered right at the same range as first point of embrittlement, however my understanding of the process was that it could not occur without a substantial soak time, minimum one hour but full embrittlement taking longer. Want to know more, great thread.

Standard temper times are 1 hour x2. That is enough time to make the steel less tough than it was at a higher hardness, so there is no point in using that temp range.

 

[Stacy E. Apelt - Bladesmith]

First, the steel in discussion is 1095 - which does not have a temper embrittlement range concern. It is .95% carbon, .40% manganese, and the bulk mainly iron. It is not a high alloy steel and contains no chromium or nickel, which along with high manganese are the drivers of embrittlement.

In my opinion, martensitic embrittlement is not an issue at any temperature we would use on a knife HT. Temps up to 500F are fine in all knife steels with two one hour temper cycles.

Temper embrittlement is a mis-applied subject that affects high alloy and high speed steels used in turbine engines and high performance rotary parts ... but not knives .

If you have any doubt, look at the charts and temperatures used to temper 1095 springs. Springs in general are made from the same groups of steels we use for knives. If embrittlement was a big issue, they would be failing left and right. Springs are pretty well known to not be brittle ... and to be quite tough.

In my understanding, temper embrittlement is a concern for high alloy steels at 700-1000F. Time is a factor, too. It is not an issue on most steels with less than .50% Mn, and the alloying ingredients have a lot to do with any risk. Chromium and nickel are the primary culprits if present in larger amounts. It is also time dependent, and longer times and slower cooling rates foster it. Tungsten and molybdenum are added to steel partly to decrease embrittlement. Another issue is where the embrittlement happens - in the grain boundaries. In steels with large grains, this can be a big issue. Knives are made with very fine grain ( hopefully). This makes many more places for the troublesome carbides to get tucked away with no strength problems. The larger the carbide size, the more the issue. Carbon-iron carbides are pretty small and won't effectively change the spacing between grains. Carbon-chromium carbides are big, and thus make a more likely dislocation point between the martensite grains.

BTW, One reason I recommend water quenching from black heat ( about 900F) in cycling and annealing steel is to stay out of any embrittlement range in slow cooling.

 

[Matt River]

Thank you for the great explanation Mr.Apelt. Knowing why I am water quenching from black during normalization is great, I like to know why and not just follow recipe type production. Understanding whar is behind the process is very helpful, thank you.

 

[me2]

Just to be clear, Stacy, you are talking about tempered martensite embrittlement, not temper embrittlement (2 different issues)? Temper embrittlement is likely no concern. Tempered martensite embrittlement can be a concern. 1095 shows a fairly dramatic drop in toughness above about 325-340 degrees and doesn't fully recover for a while. 4340 is a classic example for tempered martensite embrittlement, and is a low alloy steel, though it does contain Cr, Mo, and Ni. To be clear, for most knife use, it won't be seen. The knives are just not subjected to the impact necessary to cause a problem. Also, consider that just because it's been embrittled, the toughness isn't zero. Embrittled L6 is still likely considerably tougher than ATS-34 or S110V.

 

[RX-79G]

First, the steel in discussion is 1095 - which does not have a temper embrittlement range concern. It is .95% carbon, .40% manganese, and the bulk mainly iron. It is not a high alloy steel and contains no chromium or nickel, which along with high manganese are the drivers of embrittlement.

In my opinion, martensitic embrittlement is not an issue at any temperature we would use on a knife HT. Temps up to 500F are fine in all knife steels with two one hour temper cycles.

Temper embrittlement is a mis-applied subject that affects high alloy and high speed steels used in turbine engines and high performance rotary parts ... but not knives .

If you have any doubt, look at the charts and temperatures used to temper 1095 springs. Springs in general are made from the same groups of steels we use for knives. If embrittlement was a big issue, they would be failing left and right. Springs are pretty well known to not be brittle ... and to be quite tough.

In my understanding, temper embrittlement is a concern for high alloy steels at 700-1000F. Time is a factor, too. It is not an issue on most steels with less than .50% Mn, and the alloying ingredients have a lot to do with any risk. Chromium and nickel are the primary culprits if present in larger amounts. It is also time dependent, and longer times and slower cooling rates foster it. Tungsten and molybdenum are added to steel partly to decrease embrittlement. Another issue is where the embrittlement happens - in the grain boundaries. In steels with large grains, this can be a big issue. Knives are made with very fine grain ( hopefully). This makes many more places for the troublesome carbides to get tucked away with no strength problems. The larger the carbide size, the more the issue. Carbon-iron carbides are pretty small and won't effectively change the spacing between grains. Carbon-chromium carbides are big, and thus make a more likely dislocation point between the martensite grains.

BTW, One reason I recommend water quenching from black heat ( about 900F) in cycling and annealing steel is to stay out of any embrittlement range in slow cooling.

I don't understand your post.

TME is also known as "500° F embrittlement". It occurs primarily in low alloy steels, since those are the steels that are tempered in that range.

This thread had a 1095 temper chart that the people who could see it noted the TME toughness dip:
http://www.bladeforums.com/forums/showthread.php/679044-1095-tempering-temp-hardness-toughness-Chart

I don't know if the missing image is this one, but you can see the same standard toughness dip in this 1095 temper chart:

One-step embrittlement usually occurs in carbon steel at temperatures between 230 °C (446 °F) and 290 °C (554 °F), and was historically referred to as "500 degree [Fahrenheit] embrittlement." This embritttlement occurs due to the precipitation of Widmanstatten needles or plates, made of cementite, in the interlath boundaries of the martensite. Impurities such as phosphorus, or alloying agents like manganese, may increase the embrittlement, or alter the temperature at which it occurs. This type of embrittlement is permanent, and can only be relieved by heating above the upper critical temperature and then quenching again. However, these microstructures usually require an hour or more to form, so are usually not a problem in the blacksmith-method of tempering.

Here's an article about TME in Low Alloy steels:
http://www2.lbl.gov/ritchie/Library/PDF/1978_Horn_MetTransA_MechanismsOfTemperedMartensite.pdf


It doesn't sound like the information you're putting out is accurate. TME occurs in low alloy steels at around 500°F due to factors invovling cemetite, is made worse by manganese (which 1095 has), and the temper charts depict it.


I believe you are thinking of "Two Step Embrittlement" which occurs over 700° F:

Two-step embrittlement typically occurs by aging the metal within a critical temperature range, or by slowly cooling it through that range, For carbon steel, this is typically between 370 °C (698 °F) and 560 °C (1,040 °F), although impurities like phosphorus and sulfur increase the effect dramatically. This generally occurs because the impurities are able to migrate to the grain boundaries, creating weak spots in the structure. The embrittlement can often be avoided by quickly cooling the metal after tempering. Two-step embrittlement, however, is reversible. The embrittlement can be eliminated by heating the steel above 600 °C (1,112 °F) and then quickly cooling.

Due to the charts and mechanism of TME, I don't think there could be any doubt that it occurs in 1095.

 

[me2]

Great. More reading to do...