Hopefully Kevin Cashen will pop in here and give you a better explanation, but based on the testing I've had done, I don't think you're ever going to eliminate ALL the retained austenite.......at least not with the environment, and with the equipment that is usually found in a Bladesmith's shop. My goal has always been to minimize retained austenite, realizing that I'll never totally eliminate it.

Based on my experience, putting the blade in a typical freezer won't to anything....except make the blade cold. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' /> The temp needs to be much colder, such a liquid nitrogen.

Personally, 61.5 Rc is harder then I would want my 5160 blades to be.... I prefer my 5160 blades to be in the 57-59Rc range. It increases the overall toughness of the edge, and more important to my clients, it makes the blade easier to resharpen.

Over the years I've learned to come at thing like this with a mixture of the "Science" tempered with "practicality"......meaning that I've come to understand that scientific testing is only part of the equation...... practicality is the other portion. Knowing how things happen with the steel, and what goes on inside it, should be blended with testing and using to find what parameters are acceptable/desired to you as the individual Bladesmith.

Chun, I have to ask just be sure it's covered. Did you draw the blade back twice?

Does anyone else think that 61.5 RC is kinda high for 5160 tempered at 400*F? 5160 comes out of the quench around 61-62 RC as I remember. Seems to me that it should have come out softer at that tempering. (Which would explain the edge loss) I just checked my notes (I haven't used 5160 in a while) and they say temper at 275*-300*F for right around what Ed says is desirable. A quick run over to Cashen Blades finds this info:

http://www.cashenblades.com/steel/5160.html

to Ed：

Maybe I should drop the hardness to 58.....I always tempered my knife 400℉ in oven but never measure hardness before,I thought the thermometer probably is not accurate,the temperature in the oven is lower than the thermometer shows. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//sad.gi f' class='bbc_emoticon' alt=':(' />

to Lin：

I draw back 3 times~ <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

to Joshua：

Maybe the hardness is a problem.....it seems too hard for 5160. Maybe the thermometer in my oven is not accurate,because I do tempered it to 400℉ <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//unsure.gi f' class='bbc_emoticon' alt=':unsure:' />

here are some pictures I took form 200x microscope

This is the edge without damage.

This is white line on the edge .

This is side view of white line on the edge.

I am not too certain of the conclusions reached by the lab. Retained austenite is a problem associated with heavy alloying or carbon contents in excess of the eutectoid (.8%), really odd things done during the quench, or all of the above. 5160 has minimal alloying and .6% carbon or less, one would really have to work on intentionally stabilizing the austenite to get it to reach any significant levels in 5160. The micrographs are not very detailed and reveal very little more than an overall view of martensitic type structure. One can find retained austenite with a light microscope if the magnification is very high, and the retained austenite is at very high levels in which case it will look something like this:

Even here the carbon content is almost 3 times that of 5160, the sample was oversoaked to produce lager martensite plates and excessive retained austenite, and I had to stain the martensitic plates to get the contrast with the retained austenite between the needle like plates. Retained austenite levels really cannot be accurately ascertained with light microscopy, the best tool for that job is X-ray diffraction crystallography, which will measure the amount of FCC (face centered cubic) structure compared to BCC (body centered cubic, or body centered tetragonal in this case), and if you didn't specifically ask the lab to do the crystallography I doubt they would.

There is always some levels of retained austenite but it is mostly in percentages less than 5%, it is rather rare in steels like 5160 which only produces tightly arranged lath martensite. I suppose that if you really overheated it and then held it above M50%, around 204C(400F)or so, for a very long time it might give greater levels of retained austenite, but then you probably wouldn't get a 61 HRC. I would suspect that your issue may be more a matter of decarb or some other localized softening mechanism. It is hard to tell from the edge photos if the deformation is brittle or plastic in nature, but I have some doubt that 5160 would display brittle edge failure due to its chromium supported lath martensitic structure.

I have squeezed 63HRC out of 5160 as-quenched when I really nailed it, but that is about as good as it gets. This is why it is really best for larger chopping type blades that will tolerate softer HRC levels when tempered.

Hello Kevin：

It's so good to hear from you~~I were using English name "Frank", the student who got appendicitis and had to undergo a surgery in your class in 2012.

That's what I'm confused about,I know 5160 don't have to do sub zero heat treat, because its MF is not as low as other high carbon or high alloy steel,

I thought cool to room temperature after quenching might be sufficient .And I didn't do any interrupt quenching on this blade,I quenched and sliced it in oil.

Maybe there are some soft area I created without noticed when I was grinding.

I also asked Jimmy Chin,he also think 61 HRC is too hard for 5160,he advised me that I should check my oven thermometer.

So I examined my oven thermometer by using my digital thermometer which I used for heat treating,

found out that when the oven thermometer reached 205c(400F),the digital thermometer shows it is only 365F in the oven.

I think that's probably why I got 61 HRC on 5160. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//rolleyes.gi f' class='bbc_emoticon' alt=':rolleyes:' />

Thanks for your advices,I think I'll do it better next one~~~~ <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

Hello Frank, it is good to talk with you again. The appendicitis was a first for one of my classes but I would have remembered you anyhow, the China Airlines jet model you gave me sets on top my computer desk as I type this very reply. It sounds like you have the 5160 pretty much under control.

|quoted:

Hello Frank, it is good to talk with you again. The appendicitis was a first for one of my classes but I would have remembered you anyhow, the China Airlines jet model you gave me sets on top my computer desk as I type this very reply. It sounds like you have the 5160 pretty much under control.

The steel I use mostly is 1084(sk5).Because it can make crazy hamon very easy and easy to heat treat.

I haven't try any hyper eutectoid steel, I thought it might be difficult to do spheroidizing annealing in gas forge.

I got some 5160 from Jimmy ,so I 'm working on it~~~