Dear all,

I think this will spark a healthy debate:

Assume you have a piece of steel and let's say its a square 1x1x.5"
Assume you drill a small hole 1/8" at the center of the 1x1 square all the way through.

Now you carbonitride it which involves putting it in an atmosperic furnace and quenching it. You end up with a thin layer of martensite on the surface.

Now concentrate on the whole. Taking into account that the martensite transformation has a 5% expansion what do you think will happen?

Will the hole get bigger or smaller?



Herc

Bigger.

The hole will get non-round.

LOL. The hole will change in size. The real question is in which direction?  

Well let me throw something else out since I ve been thinking about it for couple of days.

1) assume you have a carbon steel doughnut and somehow we carbonitride it through its whole thickness. Upon quenching all transforms to martensite and all of it expands 5%.
Now if we think of the atoms around the perimeter of the ID hole they should expand too and the only way is if the hole is bigger.
Ok that's easy as ALL Atoms expand.

2) HOWEVER on a real part you are actually making a 100micron martensitic layer only. The rest of the metal remains ferrite and pearlite as it was.

Here is the deal:
 - Will the martensitic ring layer grow inwards and therefore get compressive stresses and make the hole smaller ...OR
 - Will the martensitic ring grow circumferentially and push the base carbon steel through its elastic limit?

And that my friends is the question! Happy Friday!
  

When you use terms "bigger" and "smaller", do you mean bigger or smaller than the hole was prior to quenching, when it was in the furnace, at carbo-nitriding temperature?  Or, bigger or smaller than the hole was prior to being put into the furnace?

In general, holes in low-alloy steels will grow in size during heat treatment, as compared to their size prior to heat treatment.  But, there are many contributing factors and, depending on how the contributions of these factors combine, the observed result can be opposite the theoretical result.  


rp
 

I'm not really sure what you are trying to determine.
Are you trying to resolve a problem or are you just presenting us with a test of our knowledge?

Carbonitrided, Nitrocarburized, Gas Nitrided, Plasma/Ion Nitride, Carburized or just plain old through hardening; determining what geometrical changes are likely to occur as a result of any of these forms of heat treatment is virtually impossible to predict............there are far too many variables that are extremely difficult to control.
I've known bores to shrink, expand, become tapered & even end up elongated.
I've even seen parts where all of the above have occurred in the same bore!
While it is certainly possible to limit the distortion, my experience has been that it's impossible to eliminate entirely.
 

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181