Rocketman
Geesh; hard Questions… Hope you bear with me while I try and answer this off the top of my head w/o aid of my metrology and heat treating books.
These old frames were made out of low carbon steel, what we would call SAE 1018 – 1020 today, 0.1 -0.2 % Carbon. Steels with less than 0.25% carbon are generally classified as mild or non hardening steel.
Medium Carbon Steels have 0.25% - 0.5% carbon (SAE 4140)
High Carbon Steels 0.5% and higher carbon content, (SAE 1095)
Generally the last two digits of steel’s SAE number will tell the carbon content.
Case hardening is used on mild steels.
Thru hardening process is used on medium and high carbon steels.
Hardening occurs during heat treating when the steel (containing sufficient carbon) is cooled rapidly (quenched) from above its critical temperature. Mild steel heated and quenched from above its critical temperature will show no increase in hardness. In order to harden mild steel, its carbon content must be increased. To do this, the steel is placed in a carbon rich environment (Bone & Wood Charcoal) at a high temperature (above critical point). As bone & wood charcoal (almost pure carbon) is heated; it starts to produce Carbon Monoxide and a small amount of Carbon Dioxide gas, which is readily absorbed by the steel at higher temps. The longer the steel is held in this environment the deeper the penetration of carbon or “Case”.
When I anneal a frame, I generally heat it above its critical temp in a low oxygen environment (prevent scaling) and let it cool gradually, generally 12-14 hours. Annealing will remove the hard outer (case) hardening, and normalize the steel, relieving internal stresses, the frame in total is normalized.
Your Question : “it would seem that the "skin" would anneal as soon as it reaches critical” is interesting, and yes I wondered that myself, “wouldn’t the steel self anneal when heated above critical temps”. The simple answer is no. Annealing takes time and slow cooling, think of an ice cube melting at 36 degrees, it will melt but it takes time. Also since the bone/wood mixture starts producing Carbon Monoxide far below the steel's critical temp, and the steel will start to absorb this carbon below its critical temp, you are actually adding to the carbon already in the outer layer.
Does warpage come from unequal "skin" stress early in the heat or from unequal "skin" stress coming out of heat, like in quench? Or, does it come from within the base metal? Yes.. The majority of warpage comes from uneven cooling of the metal during quench, the more mass (thicker the material), obviously the more retained heat and the longer it takes to cool, this can be from milliseconds to seconds. I’m always concerned with thin areas attached to a larger mass. The temperature of quench and the ability of the quench to absorb/transfer heat away from the part have a significant impact. Quenching parts in boiling water will greatly reduce the shock of quenching, but will also reduce the amount and depth of hardening. Another example: SAE 1095 is an oil quench steel. Oil’s reduced ability to transfer and absorb heat buffers the shock of quenching this high carbon steel. If 1095 were quenched in water (greater heat transfer) you would have significant issue with warpage, cracking and uneven hardening.
So did I pass?
