Bad technique or bad steel?

[woodbloke]

That's also how I do it. The ideal temp. for tempering is 200 - 210 degrees C, but I find that my oven does the job best when set to 230 degrees - basically you're looking for a hint of light brown colouring. I just leave them in the oven for around 15 - 20 minutes and then turn the oven off and let them cool naturally. The length of time an iron is tempered for is not critical so long as the metal is heated all the way through.



Cheers

Aled

Aled - however you do it, tempering requires a second quench after hardening. There's an account here of three different highly technical ways of tempering, all of which require a quench of some sort. Again here in this description of the process:

"Color tempering is based on the oxide colors that appear on the surface of steel, as it is heated. When you slowly heat a piece of polished hardened steel, you can see the surface turn various colors as the temperature changes. These colors indicate structural changes are taking place within the metal. Once the proper color appears, the part is rapidly quenched to prevent further structural change. In color tempering, the surface of the steel must be smooth and free of oil. The part may be heated by a torch, in a furnace, over a hot plate, or by radiation. "

Whichever way you do it, if it's not quenched, the metal ain't being tempered - Rob

 

[CHJ]

Some rule of thumb charts that may be worth printing out.

 

[Harbo]

The blades I have made from gauge plate come with these specs to achieve Rockwell C62:
Hardening 790degrees C (Nearly Cherry Red) - I quench in oil
Tempering 250degrees - (which I do in the oven)
I follow Tubal Cain's "Hardening, Tempering & Heat Treatment" No.1 in the Workshop Practice Series - an excellent little book on the subject.

The metal for both Hardening and Tempering should be kept at the correct temperature for an hour per inch of thickness.

Annealing the metal should be brought up to temperature slowly (Cherry Red for the above steel) then cooked for the times as above and then let to cool down as slowly as possible.

Rob - according to TC, quenching after tempering is only required if flame heating has been used.

Rod

 

[Aled Dafis]

Whichever way you do it, if it's not quenched, the metal ain't being tempered - Rob

I hate to disagree Rob, but the only reason for quenching is to stop the material from being heated past the desired tempering point. It's all to do with converting Austenite to Martensite or the other way round. I'm no metallurgist, but that's the basic jist of what I remember from my metallurgy lectures. How I wish now that I'd have paid more attention back then. Matthew is much more of a metallurgist than myself, and will chime in soon I hope.

If a tool is tempered in a flame, then I would definately advise quenching, to avoid the temerature raising unnecessarily and over softening the tool. When tempering say a scriber, I always heat the tool away from the point and watch the colour bands travel to the point, when the desired colour is achieved I quench in oil.

However, if a tool is tempered in a temperature controlled environment i.e. my kitchen oven, then there's no risk of overheating the tool, and therefore no risk of over tempering. No quenching is therefore required

Cheers

Aled

 

[Philly]

Rob
I'd say take some of the stuff you read on the 'Net with a pinch of salt. Aled has it in a nutshell.
Cheers
Philly

 

[Racers]

Hi.


If they deleted all the stuff on the net that was wrong, there wouldn't be much left :wink:

Pete

 

[CHJ]

If a tool is tempered in a flame, then I would definately advise quenching, to avoid the temerature raising unnecessarily and over softening the tool. When tempering say a scriber, I always heat the tool away from the point and watch the colour bands travel to the point, when the desired colour is achieved I quench in oil.

Precisely, the 'rear' of the tool will now be softer than the business end but in most instances such as a centre punch or scriber this is of no consequence as in most cases it is never sharpened beyond its working temper zone, in some cases such as a centre punch this is an advantage because the increased resilience reduces shattering risk.

However, if a tool is tempered in a temperature controlled environment i.e. my kitchen oven, then there's no risk of overheating the tool, and therefore no risk of over tempering. No quenching is therefore required

And therefore has an even cutting temper (hardness) for the whole of its length.

 

[woodbloke]

Whichever way you do it, if it's not quenched, the metal ain't being tempered - Rob

I hate to disagree Rob, but the only reason for quenching is to stop the material from being heated past the desired tempering point. It's all to do with converting Austenite to Martensite or the other way round. I'm no metallurgist, but that's the basic jist of what I remember from my metallurgy lectures. How I wish now that I'd have paid more attention back then. Matthew is much more of a metallurgist than myself, and will chime in soon I hope.

If a tool is tempered in a flame, then I would definately advise quenching, to avoid the temerature raising unnecessarily and over softening the tool. When tempering say a scriber, I always heat the tool away from the point and watch the colour bands travel to the point, when the desired colour is achieved I quench in oil.

However, if a tool is tempered in a temperature controlled environment i.e. my kitchen oven, then there's no risk of overheating the tool, and therefore no risk of over tempering. No quenching is therefore required

Cheers

Aled

Aled - Ahhh...understando now! It's just that I've always flame hardened and quenched and never done any tempering using an oven. Apologies for the misunderstanding - Rob

 

[bugbear]

I can also add that the colour you see depends on temperature AND time.

In other words, the colour does not solely indicate temperature, and (thus) the rate of heating must be taken into account when tempering according to colour.

Fortunately, if your heating process is consistent, the colour/temper relationship will also be consistent.

BugBear