5160, tempered for 4 hours. . . Oops!

[JBaker]

So, I've been working on this knife for about a month now, and I just got through heat treating. I normalized x 3 at non-magnetic, quenched x 3 - ditto, and I put it in the oven for tempering, set the timer and forgot to pull it out at the end of an hour. So it ended up being there for about 4 hours. How much damage do you think I've done, and what's the best way to fix it?
While I'm at it, what's your guys' advice on heat treating a knife with a coal forge?
Thanks!

 

[chad2]

there will be no damage the only thing i would worry about is over heating the blade, temper your knife two more times at same temp but you only need to have it in there for 2 hours. when it comes to tempering there is no max just a minimum.

there will be nothing wrong with your knife.

 

[james terrio]

As long as there weren't any crazy fluctuations in temperature, the extra time won't hurt anything except your utility bill. Typical tempering cycles are 2 hours at a time, at least twice. The cool-down between temper cycles is important; 1 x 4 hours does not equal 2 x 2 hours.

While I'm at it, what's your guys' advice on heat treating a knife with a coal forge?

Don't.

Of course, many generations of knifemakers have gotten along pretty well with coal forges, but it's among the most difficult ways to achieve and maintain precise, consistent temperatures.

 

[JBaker]

I appreciate your guys' comments, and I'm happy to know that my blade won't be damaged, does anyone know exactly what the effects of an extra 2 hours added to the tempering cycle is?
Also, in regards to the coal forge, I would definitely switch to a gas forge if I had the resources, but as is, does anyone have advice on how to avoid the formation of scale, and ideas (other than a magnet) for measuring temperatures when using a coal forge? I did what Wayne Goddard suggested and made some coke with my first fire, then I started the HT fire with coke and charcoal mixed in the center. After it got burning the fire didn't seem to put out as much sulfur, so I figured that it was probably a fairly clean atmosphere. I'm still trying to figure out better ways of doing it, however.

 

[chad2]

I appreciate your guys' comments, and I'm happy to know that my blade won't be damaged, does anyone know exactly what the effects of an extra 2 hours added to the tempering cycle is?
Also, in regards to the coal forge, I would definitely switch to a gas forge if I had the resources, but as is, does anyone have advice on how to avoid the formation of scale, and ideas (other than a magnet) for measuring temperatures when using a coal forge? I did what Wayne Goddard suggested and made some coke with my first fire, then I started the HT fire with coke and charcoal mixed in the center. After it got burning the fire didn't seem to put out as much sulfur, so I figured that it was probably a fairly clean atmosphere. I'm still trying to figure out better ways of doing it, however.

there is no effect on the steel when left in for 4 hours. when tempering you must make sure you are at temp for at least 2 hours and also makes sure that before starting the next cycle the blade comes all the way down to room temp.

 

[Stacy E. Apelt - Bladesmith]

The martensite slowly changes structure as it tempers. This is driven by time and temperature. Temperature greatly affects the rate and degree of structural change. Time is much, much less important. In an extra two hours, I doubt even a good lab could tell any difference. It would take a day or more to make a significant change....as long as the temperature was consistent, you have no problem.

 

[Bo T]

"While I'm at it, what's your guys' advice on heat treating a knife with a coal forge?"

Get a color chart. Use the Curie point as a frame of reference. Note the closest color on your chart, the ambient lighting conditions, and the type of steel on that chart. This will give you a starting point that will be better than your unaided eye. Next, there are pens that I have heard of that will give you a line that changes color at a certain temperature range. Use them and note the best color on your color chart. This gives you a number of points to use that should be much better than the unaided eye. Also use a steel or iron pipe to heat your blade in. That will give you better temperature control.

 

[me2]

Stacey's got it. An exponential increase in time (from 1 hr to 2 hr to 4 hr) might result in the loss of a point or 2 of hardness, depending on the target hardness.

 

[Stacy E. Apelt - Bladesmith]

I started to put in the post that the temperature affects the change in a linear fashion, Meaning that every increase has a more-or-less equal increase in effect.- 10°, 20°, 30°, etc.
The time affects the change in an exponential fashion. Meaning that it is the exponent power of the time that is required to make an increase in effect. 1Hr, 2Hr, 4Hr, 8Hr, 16Hr, 32Hr, etc.

This isn't exactly how it works, but illustrates that temperature is far greater than time in reducing hardness.

A while back, someone did an accidental temper of a day or more. The blade was fine, with no measurable loss in hardness.

 

[chad2]

Stacey's got it. An exponential increase in time (from 1 hr to 2 hr to 4 hr) might result in the loss of a point or 2 of hardness, depending on the target hardness.

there will be no measureable difference from 2 hours - 4 hours at the same temperature

 

[JBaker]

Phew! I was really worried when I got back and found out I'd left it in there for that long, but thanks for clearing that up!
Bo, what would these pens cost, and where can I find them?

 

[Bo T]

Phew! I was really worried when I got back and found out I'd left it in there for that long, but thanks for clearing that up!
Bo, what would these pens cost, and where can I find them?

I misspoke. Check out tempilstik or similar at a welding supply or online.

 

[me2]

there will be no measureable difference from 2 hours - 4 hours at the same temperature

What are you basing this on?

Those tempil sticks are nice and quite accurate. Just be sure to read the directions and get a range of values if you go that route.

 

[Stacy E. Apelt - Bladesmith]

The difference between 2 and 4 hours at temper should make virtually no difference at the tempering range of knives .... 350-400F. IIRC, I read that 385F for two hours made about the same difference as 375F for twelve hours....probably 1 point max.....which would be hard for most of us to measure accurately. In 4 hours, I don't think there wild be even .25Rc points change......which is less than the accuracy range of most testers.

 

[chad2]

What are you basing this on?

Those tempil sticks are nice and quite accurate. Just be sure to read the directions and get a range of values if you go that route.

The difference between 2 and 4 hours at temper should make virtually no difference at the tempering range of knives .... 350-400F. IIRC, I read that 385F for two hours made about the same difference as 375F for twelve hours....probably 1 point max.....which would be hard for most of us to measure accurately. In 4 hours, I don't think there wild be even .25Rc points change......which is less than the accuracy range of most testers.

I love being the only one, out of many that is questioned. Where do you get your info?

 

[Stacy E. Apelt - Bladesmith]

I got that info from what I have learned, observed, been told, read, and deduced from those information sources.

My understanding is:
In tempering the brittle martensite starts down a path of degrading into ferrite and unstable carbides, and then into a stable cementite structure. Temperature drives this primarily. It happens in several steps...all primarily temperature related.
Between 300F and 400F (the first range), the carbon starts to relocate, changing into ε-carbon ( 24 iron atoms attached to one carbon atom). This makes the martensite more stable because the carbon sits in a less stressed place, but does not change the steel's structure.
This won't degrade much more until the heat is raised to 400-500F range ( second range). At that point the martensite and any retained austenite starts to degrade into lower bainite. Time is a factor here, but it takes a considerable amount of that time to make much transformation. At the low end of this range (around 400F) the change occurs very slowly....often taking days to happen. At the higher limits of this range ( around 500F), it still takes 6-10 hours to get much conversion.
The range from 500F to 900F should be avoided, as the degradation of the martensite causes the grain boundaries to become brittle ( temper embrittlement).
Between 1000F and 1200F (the third tempering range), the ε-carbon re-combines into cementite, creating coarse pearlite ( ferrite and cementite) and if held long enough sheroidite ( pure iron with rods and spheres of carbides) .

The above leads me to think that the difference in hardness change between 2 hours at 375F and four hours at 375F should be negligible. The caveat that you will see when I make these statements is IF THE TEMPERATURE WAS WELL REGULATED. If the tempering oven fluctuates from 350 to 400F and you call that an average of 375F, then all bets are off as to what the final hardness will, be.

If I have any of that wrong, let me know.
I don't recall seeing a chart or graph of the hardness vs time at specific temperatures, but there probably is one somewhere. If you have a link to one, I would like to add it to my references.

 

[Leethal Cutlery]

Deleted

 

[me2]

Stacey's got it. An exponential increase in time (from 1 hr to 2 hr to 4 hr) might result in the loss of a point or 2 of hardness, depending on the target hardness.

This is based on hardness measurements taken at 1 hr, 2 hr, and 4 hr intervals, with some qualifying statements for unanticipated circumstances. Most likely, no detectable loss will be found, however, the data does show some difference, about a point or less. To be clear, I don't think you're wrong. I am just trying to figure out what information you used to make these very definitive statements. I'm familiar with the information Stacey used, so I didn't question him.

In any case, the original poster's knife is fine and should serve well.

 

[chad2]

What are you basing this on?

Those tempil sticks are nice and quite accurate. Just be sure to read the directions and get a range of values if you go that route.

The difference between 2 and 4 hours at temper should make virtually no difference at the tempering range of knives .... 350-400F. IIRC, I read that 385F for two hours made about the same difference as 375F for twelve hours....probably 1 point max.....which would be hard for most of us to measure accurately. In 4 hours, I don't think there wild be even .25Rc points change......which is less than the accuracy range of most testers.

This is based on hardness measurements taken at 1 hr, 2 hr, and 4 hr intervals, with some qualifying statements for unanticipated circumstances. Most likely, no detectable loss will be found, however, the data does show some difference, about a point or less. To be clear, I don't think you're wrong. I am just trying to figure out what information you used to make these very definitive statements. I'm familiar with the information Stacey used, so I didn't question him.

In any case, the original poster's knife is fine and should serve well.

First you said 1 to 2 point lower.

no way will an extra 2 hours do that at the same temperature, as I from the begining and stacey and james all stated, IF the temperature stayed the same with no fluctuations in heat.

Yet I was the only one you questioned? Seems to me some one needs to read a little more.

 

[Bo T]

Heat Treater's Guide, Tempering Processes/Technology, ASM International, 1995, page 99

Quenched .83% carbon shows @ HRc of 62 to 61 going from 1 - 2 hour temper. 2 -4 hour temper would be very close to the same change. @ 205 C. Most steels show a similar logarithmic drop with increases tempering times, but the rate of change is different for different steels. The OP will have a slightly softer but tougher blade.