Quenching small round 'bubble' marks?

[JohnPW]

I made some blades from ground flat stock. After quenching in oil and tempering, some had a sort of round bubble marks on them.

This one is 13mm wide and the marks are raised:

This one is 11mm wide but the marks are much more faint and not raised:

I've read that the marks are caused by the steel getting too hot. I've sharpen the narrower blade and it's really sharp (cuts paper). but will it be too brittle to use as a plane iron?

 

[Richard T]

How did you heat them? I got the marks of the coke in my forge impressed onto the last plane iron I made. When the steel got up to the optimum temperature, it relaxed into the fire and the coke made impressions on the surface.

And how hot did you get them? The temperature to aim for is the point that the steel becomes non magnetic. Instead of ruining magnates and fingers by trying to test it, it's quite easy to see the right colour. It should pass through a dull red to bright red and no further.

Have you tempered them?

Just a bit of info - if you do get it too hot, let it cool slowly back to black before going up to bright red. Steel must be quenched on the way up and not down.

 

[JohnPW]

Heating was done with a blow torch, then quenched in motor oil. Tempering was done in a kiln at 200 degrees.

I don't know the temperature but did it by colour. Actually this was done at college and the colour was supposed to be "cherry red", as I've never done this before I just went by when teacher said it's ready for dunking in the oil. The colour was a sort of bright red as far as I could tell, "cherry red" seems to me a darker sort of red.

 

[CHJ]

 

[MMUK]

What grade steel did you use? The only grade of tool steel that should be oil quenched is O1. W types should be water quenched and stainless types should all be air quenched (ie; left to cool naturally).

 

[Spindle]

Hi

You don't appear to have followed the normal process for DIY tempering which would be to clean the steel back to bright metal in order to allow sight of the tempering colours, are the marks just the remains of carbonised oil from the quenching process? can they be removed with fine wet and dry?
Tempering at 200 degrees will result in a very hard and brittle edge for the knife, dark straw colour is a more normal temper for knife blades.

Regards Mick

 

[JohnPW]

Going by that colour chart, I think I quenched when it was 'light cherry'.

Tempering was done in an electric kiln with it set at 200 degrees. Basically it was put blades in kiln, turn it on, set to 200 degrees, leave for an hour, turn it off, then take out when cool enough to touch.

 

[Richard T]

Sounds right to me John.

On a second look I think that it is just scale that has appeared in pretty patterns. Now it's time for the emery cloth and the elbow grease.

Tempering is right too, (straw) so they should be fine for any edge use and not brittle. Personally I put my bits in the oven at gas mark 6.

 

[Random Orbital Bob]

fascinating thread