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What condition is your PH or age-hardenable steel in?

There is a lot of confusion about the condition that age hardenable materials, such as 17-4 or 718 are in.  Are they annealed, solution annealed, or aged and how can you tell?  These materials are heat treated to a very specific sequence:  

  1. solution treating (also called solution annealing or just annealing)
  2. quenching (air cool or liquid quench)
  3. aging

The solution treatment is done at a high enough temperature to facilitate diffusion of all the constituents into solution.  Quenching is to freeze everything in solution that would normally not stay in solution.  Aging can be a one step or two step process (a step is a temperature or a cycle) that allows a second phase to precipitate.  These precipitates increase the strength of the material.  

Solution treating can produce a very soft material or a hard material depending on the phase equilibrium.  Solution aging 718 produces a soft material.  In 17-4, a hardened material is obtained.  The aging process hardens 718.  It also hardens 17-4 at the lower aging temperatures but as the aging temperature increases, the effect of tempering overtakes the increases imparted by the precipitation process.

So how can one tell whether a material is supplied in the solution treated condition?

  1. The specification itself.  AMS 5662 and 5604 require materials to be solution treated and precipitation HARDENABLE.  Hardenable is the key word, implying that they are not hardened.  Furthermore, within the specification, it states that each product form is furnished in the solution treated condition.
  2. Mill test certification will show the applicable specification.  It MAY also show that material is CAPABLE of meeting a different specification requiring the actual aging heat treatment.
  3. Also on the mill certification, under product description, it will show the alloy and that it is either annealed, solution annealed, or solution heat treated.
  4. Some mill certifications will list the actual product heat treatment and show a separate capability heat treatment.  When you see the word capability, that means the material itself is only solution treated.
  5. Some other mill certifications will separately list the mill heat treatment as solution anneal, and show an additional laboratory heat treatment of a sample only.

Most precipitation hardenable materials are supplied in the solution heat treated condition, and will be aged much later.  In order to meet specification, samples are precipitation hardened to demonstrate that the lot is capable of being heat treated (at a later time) and meeting all specification requirements.

Conversely, the information that informs one that material has been fully heat treated including aged:

  1. The specification itself.  AMS 5663 and AMS 5643 require material to be furnished in the solution heat treated and precipitation hardened condition. 
  2. The mill certification will show the applicable specification requires material to be solution heat treated and precipitation hardened.
  3. The mill certification, under product description, will show material is supplied in the solution heat treated and precipitation hardened condition.
  4. Mill certifications will either show that the product received both heat treat steps or there will be no statement of laboratory heat treatment; only product heat treatment.

The Rolled Alloys Metallurgical Services team can help you interpret your mill certification if necessary.