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Hardening Steel

Critical Temperatures. 

The "critical points" of carbon tool steel are the temperatures at which certain changes in the chemical composition of the steel take place, during both heating and cooling.

Steel at normal temperatures has its carbon (which is the chief hardening element) in a certain form called pearlite carbon, and if the steel is heated to a certain temperature, a change occurs and the pearlite becomes martensite or hardening carbon.
If the steel is allowed to cool slowly, the hardening carbon changes back to pearlite. The points at which these changes occur are the decalescence and recalescence or critical points, and the effect of these molecular changes is as follows:

1) When a piece of steel is heated to a certain point, it continues to absorb heat without appreciably rising in temperature, although its immediate surroundings may be hotter than the steel. This is the decalescence point.
2) Steel cooling slowly from a high heat will, at a certain temperature, actually increase in temperature, although its surroundings may be colder. This takes place at the recalescence point. The recalescence point is lower than the decalescence point by anywhere from 85 to 215 degrees F., and the lower of these points does not manifest itself unless the higher one has first been fully passed. 

These critical points have a direct relation to the hardening of steel. Unless a temperature sufficient to reach the decalescence point is obtained, so that the pearlite carbon is changed into a hardening carbon, no hardening action can take place; and unless the steel is cooled suddenly before it reaches the recalescence point, thus preventing the changing back again from hardening to pearlite carbon, no hardening can take place.

The critical points vary for different kinds of steel and for AISI 1080 or 1084 (0.83% carbon) that temperature is 1333F.

 Determining Hardening Temperatures.


The temperatures at which decalescence occurs vary with the amount of carbon in the steel, and are also higher for high-speed steel than for ordinary crucible steel. The decalescence point of any steel marks the correct hardening temperature, and the steel should be removed from the source of heat as soon as is has been heated uniformly to this temperature.

Heating the piece slightly above this point may be desirable, either to insure the structural change being complete throughout, or to allow for any slight loss of heat which may occur in transferring the work from the furnace to the quenching bath. When steel is heated above the temperature of decalescence, it is non-magnetic.


Iron/Carbon Phase Diagram

The important boundaries (the lines) separating phases have some universally used abbreviations:
  • A1: The upper limit of the ferrite / cementite phase field (horizontal line going through the eutectoid point).
  • A2: The temperature where iron looses its magnetism (so-called Curie temperature). Note that for pure iron this is still in the a-phase.
  • A3: The boundary between the g austenite and the austenite/ ferrite field.
  • A4: The point in this case where austenite changes to delta iron at high temperatures.- 2552F and not used in knife making.
  • ACM: The boundary between the g austenite and the austenite / cementite field.



After all the above reading and head scratching you will find that after reviewing a couple of heat treating schedules that the most important point to reach for hardening steel is the A3 or ACM line plus a little to allow for cooling between heat source and quenching medium.

While heating to the non-magnetic point and quenching is often suggested you do fall short of the temperature needed to reach the change to Austenite. It is best to say non-magnetic plus a couple hundred just to be safe. Austenite temperature has to be reached in order for Martensite to be formed during quenching.

Martensite

Martensite is formed in steels when the cooling rate from austenite is sufficiently fast.
It is a very hard constituent, due to the carbon which is trapped in solid solution. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process.

By sudden they mean Martensite plates can grow at speeds which approach that of sound in the metal so when something goes ping it is understandable. In order to prevent the dreaded "Ping" or at least reduce the chances of it happening it helps to have all of your edges smooth without little nicks or notches that can act as mirrors to the shockwave or stress risers that focus the energy.

Now putting a mirror polish on the edges would be way past overkill and simply sanding with a 200 grit paper will suffice. On knife shapes I have had crack in the past and at re-entrant corners I will also break the edges of the steel just to be sure I have covered all bases.

The martensite transformation normally occurs in a temperature range that can be defined precisely for a given steel. The transformation begins at a martensite start temperature (Ms), and continues during further cooling until the martensite finish temperature (Mf) is reached. Ms can occur over a wide range, from 500°C to below room temperature, depending on the hardenability of the steel.

Below is a CCT diagram and on the lower left is the Martensite start Ms and M50 (50%) and the M90 or (90%) points.

By looking over the above CCT diagram for AISI 1095 you will see the dotted line forming what is commonly referred to as the nose. In order to have 100% martensite or as close as possible, there will always be some retained Austenite you must cool the steel down below the nose (1000 degrees F) in under 1 second from the hardening temperature of 1475 degrees F.

This cooling rate is why a fast oil in the six second category is needed and not a slower oil which will have you move into the Upper Bainite range which is a composition Austenite+ Ferrite+ Carbide. This crystal structure is not as hard as Martensite and as such less desirable.

There is the option of trying to achieve a Bainite structure in some cases and this would be covered under subject Austempering.

For crack sensitive steels it helps to perform an interrupted quench and this typically stops the quenching above the Ms start point then has the steel stabilize and slowly drop below the Ms start point. To learn more about interrupted quenching read up on Marquenching / Martempering.

If you are the type who learns best by watching there are plenty of videos on YouTube that show decalescence and recalescence very well.

All the above are bits and pieces from notes taken from my Metallurgy class and a very old copy of the Machinist handbook (very good reading) and a whole bunch of translucent memories :)


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