This is a discussion on Heat Treatment and Warping within the The M14 forums, part of the M14 M1A Forum category; Question for the researchers/experts on receiver production....or perhaps more accurately...metallurgy
I know things can "move around" some during heat treatment
was/is there any identified metallurgical ...
Upon heating, steel parts change volume as they change crystal structure. When these heated parts are quenched, their internal crystal structure changes again, and that volume change is not necessarily sufficient to offset the change upon heating. This change of volume can cause dimensional distortion. The rule of thumb used for medium carbon alloy steels is to expect a change in linear dimensions of about 0.125% maximum. That is, one eighth of a percent of the linear dimensions could be the change encountered from heat treatment and quench. In generally it is less, but 0.125% gives a rule of thumb to evaluate capability to hold dimensions after heat treat. Part growth as a result of heat treat warpage or shape distortion as a result of heat treat is different because it is usually a result of process and design issues rather than the expected phase changes of the material. 8 reasons steel parts can warp upon quench and tempering:
1. Rapid heating.
3. Non-uniform heating.
4. Non-uniform cooling.
5. Non-uniform agitation.
6. Water contamination in oil.
7. Large changes of mass and section.
8. Asymmetric features.
Rapid heating can cause stresses to develop in parts due to excessive temperature gradients. Overheating similarly lowers mechanical properties, potentially leading to parts sagging or creeping depending on orientation in the furnace. Non-uniform heating also creates differences in properties within the parts as well as leading to incomplete transformation products or hybrid structures upon quenching. Non-uniform cooling allows unbalanced stresses to develop during the quench, as does non-uniform agitation of quench medium. Often non-uniform heating or cooling result from the way parts are stacked or piled in the basket or on the belt such that gradients of temperature are created.
Overheating caused by short preheat time or excess temperature delta (if you want 900 degree part temperature the oven needs to be set for a higher temperature, set delta to high and the surface temperature with a gradient into the part exceeds desired temperature). Most of the problems are related to process, location in the oven, orientation in the chamber, improper removal from chamber, location of transfer device (the contact between transfer device which is colder than the part), time between removal after chamber is open. The ideal is to subject the part to exactly the same process time/temperature cycle which in a production cycle is difficult at best to achieve. Any variation(and the number of these is limitless) in the time/temperature cycle changes the characteristics of the metal in a non uniform pattern leading to internal stress resulting in distortion.
Hope this helps
Plus one Jim...I would go further and say that the number one reason would be from non-uniform heating as well. The other variables amplify this of course. I am just in my infancy in the study of this and it's ramifications upon the finished product, but have learned a good deal about it. Your post is a great help in my continued education in mettalurgy.
Jim that was an outstanding answer and the kind of info I was hoping to read
One of the follow-ons I have in the realm of heat-treating/quenching etc is the relative thickness of various aspects of a given piece....
for instance we know that the clip guide dovetail on the M14 is quite thin whereas the rest of the "lump" has good mass....
would this area of the receiver be an Achilles Heel of sorts? I know we have all read or heard of someone snapping the dove tail off one way or another and I am curious aside from its dimensions if the heat-treatment process does not lend itself well when it comes to the dove tail
My first post was a general reference concerning heat treat effects and physical characteristics of medium carbon steel, the reference asking for comparison of forge vs cast, the metal is the same. The difference is in process, cast steel starts out as a molten material, it flows into a cavity to form a near finished product (in this example assume a receiver). The heating process, electric elements, magnetic induction, forced gas/air, vacuum chamber, or artificial atmosphere furnace can alter the metal in either positive or negative ways (not + or - for the steel but its application). When a hot cast receiver is in its mold as compared to a hot rough forged receiver, the cast receiver distorts more than forged because it has thinner cross sections, a wider variation of cross sections, random alignment of its grain structure and random small inclusions. The extremely high temperature (molten metal) combined with thin variable cross sections with random alignment of the grain structure impacts severely the cool down quench process. The rough forged (red hot, cooler than molten metal) part has more material, comparative equal cross sections, greater mass, grain structure alignment, inclusion distribution throughout the metal and a partial stress relief from the forge. The larger mass/equal cross section forged receiver will cool/quench with less distortion because of these factors. The ingot or bar stock used in the forged receiver was molten poured and rolled into shape(either hot or cold rolled) the rolling process is a form of forging which starts the process described above, the second forging of the receiver blank develops the positive characteristics further, both of these process steps are missing from the cast receiver.
As a general rule, certified steel from a major supplier has a higher probability of uniformity than small batch melts at a final processor, these variables are not from the metal but from variations during melt and pour.
Forged parts are more consistent by process than cast parts. Can a forged part be processed wrong? Yes! Can a cast part be processed right, absolutely!
A properly cast part does not have the same characteristics (neither good nor bad it just is) as a properly forged part. If working forces applied to the finished cast part don’t exceed the limits of the part and the casting process produces a part within dimensional designs, a cast part will perform superbly. One limit on cast parts shows up in the attempts to design and build a cast bolt for the M14/M1a. Even forged bolts manufactured by the Military complex had difficulties with proper process.
It is not about which is better or worse, it is about which metal process (the metal is the same) provides a consistent, repeatable, mass producible, properly functioning part.