I have been searching the Web (including this Forum) for some time now looking for specific information/data regarding the "softening" processes of steel (i.e. Annealing, Normalizing, Spheroidizing and Stress relieving...), but I could not find much definitive information specifically relating to treating of knife steels - not even from the steel manufacturers' websites!
The best description/explanation that I could find was from efunda.com:
- Full annealing is the process of slowly raising the temperature about 50 ºC (90 ºF) above the Austenitic temperature line A or line A in the case of Hypoeutectoid steels (steels with < 3CM 0.77% Carbon) and 50 ºC (90 ºF) into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon). It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate of about 20 ºC/hr (36 ºF/hr) in a furnace to about 50 ºC (90 ºF) into the Ferrite-Cementite range. At this point, it can be cooled in room temperature air with natural convection.
- Normalizing is the process of raising the temperature to over 60 º C (108 ºF), above line A or 3 line A fully into the Austenite range. It is held at this temperature to fully convert the structure CM into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite. The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling.
The main difference between full annealing and normalizing is that fully annealed parts are uniform in softness (and machinablilty) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part. This may not be desirable if further machining is desired, since it makes the machining job somewhat unpredictable. In such a case it is better to do full annealing.
- Stress Relief Anneal is used to reduce residual stresses in large castings, welded parts and cold-formed parts. Such parts tend to have stresses due to thermal cycling or work hardening. Parts are heated to temperatures of up to 600 - 650 ºC (1112 - 1202 ºF), and held for an extended time (about 1 hour or more) and then slowly cooled in still air.
- Spheroidization is an annealing process used for high carbon steels (Carbon > 0.6%) that will be machined or cold formed subsequently. This is done by one of the following ways:
- Heat the part to a temperature just below the Ferrite-Austenite line, line A or below the 1 Austenite-Cementite line, essentially below the 727 ºC (1340 ºF) line. Hold the temperature for a prolonged time and follow by fairly slow cooling.
- Cycle multiple times between temperatures slightly above and slightly below the 727 ºC (1340 ºF) line and slow cool.
- For tool and alloy steels heat to 750 to 800 ºC (1382-1472 ºF) and hold for several hours followed by slow cooling.
This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.
Here on SA Blade, I've found the following:
Regarding stress relieving:
the rule of thumb states that should you work (drill, machine, grid, forge, form) an material to the extent where 10% or more of the components structure (shape) is affected or removed then a stress releiving cycle is required. these opperations introduce residual stress into the steel which makes it more prone to distortion and cracking during the quenching stages of heat treatment. Stress releiving is a heat treatment process that will not hurt your product if carried out, it will only have a positive influence on it. This cycle is carried out just before you hardening cycle. place the item in the furnace, heat to the specified temperature and cool inside the furnace. in this instance the item must be allowed to cool down as slowly as possbile as to avoid picking up stresses during the cooling cycle.
steels above the eutectoid ,especially the 1.00 % carbon grades present a problem that you must understand . The slow cooling from the austenitizing range gives lots of time for the carbon to diffuse to the grain boundaries .You can get continuous grain boundary carbide and that is extremely brittle !!! So no cooling in the furnace or sand . Air cool to give a pearlitic structure or quench and temper at 1200-1300 F to give a fine spheroidized structure .
On hypefreeblades.com I found the following:
So the standard three step process (for spheroidizing) is:
1) Austenitize at about 200° above Ac1 ( 1550F), and quench. This clearsall previous structures in the steel, and forms a martensite structure.
2) Austenitize at just above Ac1 ( 1350F), and quench to form martensite with a very fine grain.
3) Spheroidize at 1250F for 10-15 minutes and cool to below 900F slowly.
This should give you a steel that will drill, mill, and grind the easiest.
It appears to me that some of these processes are more applicable to large industrial steel manufacturing processes, and might not be necessary for knife making. Some of these processes might also be more applicable to knife forgers, and not necessarily to knife grinders?
All I want when making a knife is for the steel to be workable (and new steel is usually sold in an annealed state...), and to not distort or crack during hardening. Obviously, I would also want the steel to be as "good" as possible in it's final heat treated state (i.e. tough, hard, retain edge, etc.), without having to take extraordinary steps for only very small improvements.
Now, with regards to applying these various processes to knife making, specifically utilising the STOCK REMOVAL (GRINDING) method of knife making, can someone please clarify the following:
- What process/es do I use before starting to work on a knife (if any...)?
- What process/es do I use after completion of shaping & drilling the knife (I grind after hardening...) before heat treatment(hardening), and why?
- Are these recommended processes applicable to all types of steel (i.e. tool steels such as O1, D2, 5260, etc. AND stainless steels such as N690, 12C27, 44C, etc. What about powder steels?
- When annealing/spheroidizing, etc. should the knife also be wrapped in foil to prevent oxidation/decarburation?
- When "quenching" as described for spheroidizing (above quote from hypefree blades), how is this done?
Regarding the specifics of the above-mentioned processes, can someone also please post the actual recommended treatment "recipes" (i.e. stress relieve temp, annealing temp, spheroidising temps, normalizing temp, hardening temp, and soak times for each process), and either the TTT diagrams or the A3/Acm temperatures (I assume the A1 temperature for all steels is the same, being 727°C, but if this is not so, then also the A1 temperature...) for the following steels (and any others that you have information of...):