More "Tool Steel Simplified" Questions: Aging

[deker]

So I've been doing more reading, and I've come across something that's realy of not a lot of interest to me as a bladesmith, but it does make me wonder what is happening inside the steel....

First I read the following:

"As ordinarily made, a hardened tool may size or shape to the extent of a few ten thousandths over a period of years."

Uhhh...ok....can anybody explain why? Is it continued martensite transformation?

Then I read this about "aging" the steel more rapidly:

"After the tool has been properly hardened and thoroughly drawn, it is ground to the point of lapping. It is then heated for an hour or two to the temperature of boiling water and cooled back to room temperature. This is followed by cooling in contact with "dry ice" for a similar time and allowing it to warm up to room temperature. It is then put through the same cycle of heating and cooling four or five times more. This brings the steel to a stable condition after which the final lapping operation is completed."

So, aside from dimensional stability, what else is happening here?

Enquiring minds want to know

-d

 

[Sam Salvati]

Cast iron lathe beds and other frames for precision machines are put out in the raw elements to "season" them.

 

[mete]

Yes Sam they still season castings that way. A friend had a machine that they forgot to age and he developed problems.I asked ,'did they age it? ' He said they are supposed to as he had seen the factory. But checking ,they had forgotten !
In making precision dies you don't want the tool steel to change dimension with time . This is why we want to minimize retained austenite and stabilize any RA remaining. The first change in steel is the long dimension of the martensite crystal .The higher the tempering temperature the shorter that dimension becomes. The second change is that of transformation of austenite to martensite. Not only are there dimension changes but there are problems when RA changes to martensite and is not tempered .That means a brittle condition.
I don't know if I'd follow their proceedure for rapid aging. These days we would use liquid nitrogen [-300 F] rather than dry ice [-100 F] .That would be followed by a triple temper .That should stabilize things as tranformation means dimensional changes.

 

[sunshadow]

as opposed to ageing non-ferrous alloys to harden them (sterling silver likes 700 degrees f for a couple hours, some of the alloys I work with at work like an ageing in the neighborhood of 1000F) which makes them tougher AND harder, kinda like letting the crystal structure act like it's own hammer work hardening them

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