View Full Version : Rookie Temper Question
07-01-2011, 10:18 AM
I had a 5160 blade that I was tempering in the kitchen oven. I was going to turn the oven off but got in a hurry to go somewhere and forgot to turn the oven off. The blade was left in the oven at aprox 350 for 4 hours and this was the 3rd temper cycle.
Being new at doing this I asked some folks and they said the extra time was no problem. The blade is now a light gold color and running a file over the edge still seem hard. Maybe a little too hard.
My question: I know tempering in simple terms makes the blade a little softer. What is really going on with the steel when you temper it and why or do the time spent in temper temp make a difference?
Thanks for your help
07-01-2011, 12:48 PM
What's going on when you temper a blade is that you allow some of the carbon trapped in the distorted body centered "cube" of martensite to migrate out of the crystal and distort the bonds between the iron atoms less. It is this stress on the atomic bonds between the iron atoms that causes hardness. You can sort of equate tempering with reducing the stress on a spring. At maximum stress the spring is close to breaking and hard to move. Reduce the stress and it's more flexable or, in a way, softer. Carbon migration is much more dependant on temperature than it is on time. Theoretically you could over temper a blade by leaving it in the oven for too long but that's going to take a very long time. If anything, the extended time in the tempering oven/oil bath/molten salt bath is going to do is allow the cementite and ferrite created by tempering to convert to bainite, which would be a pluss in my book.
Another thing that tempering does it trigger retained austinite, iron crystals in a face centered cube, to convert to other conversion products, including untempered martensite. The reason that this is important is that austinite is not stable below around 1425 degrees. It will convert to untempered martinsite in use over time as with shock through the steel caused by chopping, for example. If there is too much retained austinite in the steel the blade will become more brittle with use. Putting the blade though multiple tempering cycles shocks the retained austinite into converting to untempered martensite, along with other conversion products, and then temperes the untempered martensite.
I'm not surprised that you're finding the blade to be a little hard; 350 degrees is a bit low for tempering 5160. It might be a little low for 1060. I would take it up to 375 for a cycle and then retest. You might even want to go up to 400 degrees depending on your tempering set up and whether or not the blade is to be a chopper, slicer, or more general purpose.
07-01-2011, 02:01 PM
Thanks for your help and explanation. That makes perfect sense. You may be right about the temperature. The reason I say 350 is I am usning the household over right now for tempering and I monitor the temp with a seperate thermometer that has a probe that goes in the oven and the display sits on the counter. What I noticed is when i set my oven above 350 the temp will go over 400 when the oven is doing one of its cycles. That seemed to hot to me for 5160.
The variables: does the steel actually get 400, is the extra thermomter right, is it 400 for very long? I don't know, I guess I may have to play around and do some flex and cutting tests and see what works. I will be stuck with using the house oven for a while.
07-01-2011, 02:11 PM
do you have any way of heating just the spine to about 400? something like a propane torch. lite gold is about the perfect color for the edge, if its a small knife it might be alright. post some pics if you can. id love to see it
07-01-2011, 11:42 PM
If your wife will allow it you might put your knives in a tray of sand to buffer the temperature swings. If not, you might have to invest in a toaster oven for tempering or you can use an electric roaster and do your tempering in oil. Roasters max out at around 430-450 degrees and take a while to heat any volume of oil, I'm thinking that I keep over two gallons in mine but I'm not sure, but once up to heat the temperature in mine is very constant. I put an rack from an old toaster over in the bottom with wire loops that stick above the oil level to keep the blades off the bottom of the roaster and to allow me to more easily remove the blades from the bath. Those temperatures are usually above thermometers that you can get at the grocery store for measuring the temp of liquids. MSC Direct sells dial thermometers that will handle that temperature range.
07-02-2011, 09:10 AM
Doug: sand tray to buffer temp swings - I literally slapped myself in the forehead. Thanks!!!
07-02-2011, 04:10 PM
Ya, that's what I did when it was suggested to me too.
07-07-2011, 11:20 AM
Thanks for the help guys.
Does the toaster oven hold a good constant heat. I have heard a lot of guys using them. The sand also sounds good. What the wife doesn't know won't hurt. The oil bath sounds good as well. What type oil do you use. i have a good supply of peanut oil.
07-07-2011, 06:22 PM
No, due to the small volume toaster ovens probably hold a constant temperature less well than a kitchen oven. That's what I used the sand in. Being that I don't have to ask anyone for permission, I now use my kitchen oven which seems to be more constant according to an internal thermometer. I use peanut oil in my roaster to austemper in. The down side is that they may take a couple of hours to get up to the temperature range that you want. I'm trying to work up the nerve to open one up to see if I can over ride the stop and get the temperature up to about 510 degrees so that I can upquench to them temperature from 470 degrees and to a two stage austempering.
For safety keep an oil fire rated extinguisher on hand to put out any fires. There shouldn't be a problem from flash over but, if it did occure, a water hose will make things much worse very fast instead of putting out the fire. Always keep the lid handy to clamp down over the roaster. Actually, I drilled a hole through the lid that I stick the probe from my dial thermometer through.
Another solution would be to build a low temperature salt pot if you know someone who can weld stainless and you can hock up a PID controler to a burner. I'm thinking about getting a second hand kiln that runs on 110v and rigging a salt tube in it. Less effecient than a gas burner but it is something that I can rig.
07-10-2011, 04:27 PM
Just thought I would give an update on this blade. I got it finished to a point where I could sharpen and test it. I got 110 cuts on 3/4" hemp rope and it was slowing down but still cutting. It was about 100 degrees out side so I quit. I did an edge flex on a 90 dgree corener of a table saw table. First side did very well, second side seemed to bend over or get a burr. I have finished it a bit more and did some more rope cutting and easily chopped through a pine 2x2. I am going to consider this a personal victory. I have been shooting for results like this and better, but this is the closest I got. It is only my fourth blade ever though.
I now have a crude guard soldered on and a handle. This knive will not be finished to a high degree as I am going to test it to disctruction.
Just wanted to give an update and say Thanks Again for your help.
07-10-2011, 10:19 PM
Sounds good. I would go by the rope cut more the the flex test, which I feel is less than reliable. What I do is sharpen the blade to a rough edge and cut some soft iron wire to see if the blade chips out or curls over when I strike the spine with a mallet to drive it through. Others cut thin brass rod. When you get around to breaking the blade what you will want to see is a very fine broken surface. It should have to texture of very fine sand. If it looks grainy you had too much grain growth somewhere along the line that was not corrected.
07-19-2011, 09:03 AM
One more question. I am still playing with this knife and haven't done the bend test yet. Guess I'm not in a hurry to ruin my first knife that performs fairly well.
I sharpened it and then chopped thorugh a pine 2x4. No knots in it but was resonable hard for pine. The cutting edge is only 4 1/2" and has about 1 1/2" ricasso. It took a bit but still shaved after I was done. Pulled a bit but did shave, not dings or chips in the edge.
Where does this fall in the performance level roughly.
07-19-2011, 05:23 PM
Looks like you nailed the heat treatment with that blade.
07-19-2011, 05:27 PM
You hear all kinds of stuff, I just wanted to know if I was getting there. I don't know any makers in my area and this and other forums are about all I have.
I like your signature.
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