[TheForge] Packing and compression

[Hochewa at aol.com]

To All,
A gauntlet has been thrown down.  Thank you Jonathan.
In ferrous materials you have to contend with both recrystallization and  
phase transformation.
Recrystallization is the materials response to prior work when thermal  
energy is applied.  The work can be cold, warm or hot.  The  recrystallization 
response occurs with time and temperature and it depends on  the amount of prior 
work and the temperature at which it was applied. Phase  transformations occur 
in response to temperature because of the thermodynamic  stability of the 
phases involved.
I do not believe in "packing".  Any one who says that it makes the  edge of 
the blade more dense probably sits on his anvil, too.  It is as  dense as it is 
going to get and a little bit of tippy-tapping does not make any  difference. 
 Working the edge a little may promote some dynamic  recrystallization which 
in turn may relieve some of the forging stresses and  promote a straighter 
edge.  Additional stress relieving is still necessary  before heat treatment. 
Transformation from a ferrite/carbide/pearlite matrix to austenite tends to  
remember where the prior austenite grain boundaries were anyway.  Austenite  
formation involves some short range diffusion and a lot of crystallographic  
changes.  Quench induced martensite initiates its lenticular shape across  the 
largest dimension of the austenite grain. So it does not really matter what  
the grain size is below 1333*F.  The hardening occurs from the grain size  
achieved during austenitization.  Grain size is mostly effected by the  temperature 
at which the work is held.  There is a minimum time needed to  achieve full 
transformation and holding it at that temperature for a longer time  does not 
radically change the grain size.  Temperature has a more  pronounced effect 
than does time.  It all has to do with the balance  between the energy of the 
volume of the grain and the energy of the grain  boundary volume.
The specific amount of reduction at the last forging pass sets the  austenite 
grain size as the material is then heat treated. Tippy-tapping tends  to make 
big grains, smacking it good reduces the grain size because of the  increase 
in potential grain nucleation sites.
 
Hochewa
 
 
In a message dated 11/11/2005 10:59:25 A.M. Eastern Standard Time,  
jonned at hvc.rr.com writes:

I  remember on a visit to a large forge shop where they made bearings, among  
other things, that they were careful to move the metal a certain amount in  
the last forging heat. This was we were told, to assure a small grain size  
in the material as it relates to the strength of the finished  bearing.

I assume that these were not going to be heat treated because,  in my limited 
knowledge of this subject, when you heat above the critical  temp, also known 
as the heat of recrystallization, you get just that - new  crystal growth, 
and this is the temp that carbon steel has to reach for  heat treatment (ok 
Bill H. you can correct me later). So all the "packing"  in the world is not 
going to get a harder edge if the grain you have  compressed in the last 
forging heat is regrown in the heat treat. Or  something to that effect.

Jon Nedbor