overheating before annealing

[canada goose]

I am having trouble over heating when I try to heat treat my projects, its hard to heat evenly

[Steve Sells]

When steel over heats it allows grain growth. This growth into larger grain size is exponential, so a little over heating goes a long way. The larger grain in those areas cause your steel to become weaker.

The best solution is to learn how to heat evenly. Until then: if you over heat, you should normalize the entire unit again, once or twice to cycle it, reducing grain size, and re start until you get it right.

[canada goose]

I understand that grain size is larger at higher temps but does it not shrink again as it cools?

[Steve Sells]

The grains of steel grow larger and they stay large unless we do something to change them. Such as impact from hammering and thermal cycling. Just cooling from high heat wont reduce them alot. The normalizing helps reduce stress and evens out the transitions from a failed attempt at hardening, I dont understand everything about heat treating, just basics for hardening,tempering and grain size control

[Rich Hale]

I had heard and read about grain growth long ago and being I cannot just take that as it was given to me I had to see for myself. I took an old file, not a real large one and heated it too much..that is I watched the color of it when a magnet would not stick to it and then heated it again past that point but not to melting. I quenched in water. I then stuck the file in a vice and left about an inch of it above the vise. With proper protection I smacked it with a hammer and examined the broken edges. They looked just like the pictures I had seen of abnormal large grain growth. Then I took the rest of the file and heated it to non magnetic and let it darken to black,,,twice. Then I heated it up again to non magnetic and quenched in water. Then again broke the end off of it. The grain was smooth and tiny. looked nothing like the first over heated piece.
I did not take this test any farther like making a blade from each test piece and doing cutting and edge holding tests along with bending tests.
But from this rather brief test I found out a couple of things that I use in all of my work. Some of it was not new at all like you need small grain structure after heat treat to make good knives. I cannot depend on the color of the steel to tell me when to quench or anneal. Ambient light is a big deal when looking at colors. So I rely on a magnet. I also now believe that when I forge a knife I must each and every time thermal cycle it to relieve stresses. In addition to the above tests I have followed my lead and base my work on testing that I can record and count on whenever I want to repeat a process that has worked for me in the past. For instance; If I know wot always works for me when heat treating say a piece of 1084 steel I have to do those same initial tests each time I try a steel new to me. I am only one of a lot of smiths that use this type of method to guide their work. For anyone new a good plan is to stick with basics. and many of them are here on ther site for you to follow. And as you wish get into a plan of seeing just how wot you are doing with steel affects the later life of that steel. Sorry to be so long in this responce just wanted to share some insite on wot you can do if you choose.

[MattBower]

I am having trouble over heating when I try to heat treat my projects, its hard to heat evenly

Yes, it can be difficult. But if you want to achieve decent results in heat treating, you have to learn to do it. What sort of forge are you using?

[thingmaker3]

Heat it more slowly. If using solid fuel, place it higher in the fire & go slower on the blower. If using gas, throttle back on the BTUs. Pay attention. Be patient. It may not be rocket science or brain surgery, but it IS art & craft. Be involved.

[canada goose]

hey,thanks for all your help.Learning to control and use my coal fire is definitely one of the challenging parts of this fascinating craft.

[Nakedanvil - Grant Sarver]

I understand that grain size is larger at higher temps but does it not shrink again as it cools?

The grain will reflect the highest temperature the piece was last heated to.

[thingmaker3]

Another trick using a coal-fire is called a "muffle." Find a pipe or tube just big enough to accommodate your work & build your fire around and over the pipe. This helps diffuse the heat as well as slowing down the heating rate.

[MattBower]

Heat treating with coal can be tricky. Here are a few suggestions:

(1) Use charcoal for heat treating. It's a little easier to control.

(2) Build a firebrick "cave" on top of your fire to create a sort of oven. That way you don't have to actually insert the work down into the fuel. You can see what you're doing, and the steel won't heat as fast -- which in this case is a good thing.

(3) Heat work by pumping or sawing it through the fire. This lets you handle longer work in a shorter fire, it lets you see the work as it heats and, again, it slows down the heating -- which in this case is a good thing.

(4) Slow way down on the blower. You don't need (or want) your fire much hotter than the temperature to which you're trying to heat the steel. Coal forges easily get far hotter than that, so you have to make a conscious effort to back off. (Again, using charcoal as your fuel helps with this.)

I second the muffle suggestion. In fact muffles are good for all kinds of forges, not just coal.

Good luck.

[canada goose]

The grain will reflect the highest temperature the piece was last heated to.

So if the piece is heated to a yellow heat for ease of working the grain size would be large. But if it was cooled ,say to black,then reheated to just past magnetic the grain size would shrink accordingly?

[MattBower]

So if the piece is heated to a yellow heat for ease of working the grain size would be large. But if it was cooled ,say to black,then reheated to just past magnetic the grain size would shrink accordingly?

Yes. This is one reason many folks recommend reducing the working temperature as the forging nears completion.

[thingmaker3]

So if the piece is heated to a yellow heat for ease of working the grain size would be large. But if it was cooled ,say to black,then reheated to just past magnetic the grain size would shrink accordingly?

You can demonstrate this to yourself with a simple excercise. Get a quarter inch or so square or round piece of steel and file some notches in it every half inch or so along the length. Heat to yellow, let it sit at temp for a bit, and quench. Put the steel in a vice with the last 1/2" sticking out and break it off. Heat the remaining steel again but only to orange. Quench & break off the next 1/2". Keep going this way, and keep track of which pieces were which. Look at the broken surfaces - a magnifier or one of those cheap pocket microscopes come in handy here.

The above excercise will work with plain carbon steels and low alloy steels.