What exactly is ‘PH’ stainless steel?”
Excellent question.
Compared to traditional martensitic grades, PH grades offer advantages such as high strength, excellent corrosion resistance and simplified heat treatment processes.
PH stands for “precipitation hardening,” a type of heat treatment that differs slightly from the traditional heat treatment of martensitic grades.
An initial “solution treatment” is conducted at high temperatures, typically 1,900 degrees Fahrenheit, to ensure that all alloying elements required for the hardening reaction are evenly distributed throughout the metal structure.
At these temperatures, the structure is austenitic. From this temperature, the alloy cools at a rate that maintains the distribution of hardening elements in the solution.
Based on the chemical composition of specific alloys, the structures produced after “solution treatment” are martensite, semi-austenite, or austenite.
These structures contain more hardening elements than those that are fully stable, so they are just waiting for additional heat treatment to induce changes inside.
However, they are stable enough that we may choose to manufacture components before final hardening heat treatment.
This additional heat treatment, at a relatively low temperature, is referred to as “aging”. Increasing temperature and time allows the mobile elements to combine and form “precipitates” (think particles), which subsequently strengthen the structure.
PH alloy type
We will categorize the types of PH alloys based on the structure obtained through treatment in solution…
1. Martensitic Type
They form low-carbon martensite, which is relatively softer but also brittle, during solution treatment. The alloy should not be used under solution treatment conditions.
When reheated to the aging temperature, the particles formed further strengthen the structure and improve resistance and corrosion performance.
The resulting heat treatment condition is represented by the letter H followed by the aging temperature.
For example: H900 indicates that it has undergone solution treatment, followed by aging at 900 degrees Fahrenheit. Through a second simple heat treatment, hardness increases and minimum yield strength reaches 170,000 psi.
Condition ranges from H900 to H1150, or even double H1150 (aged twice at 1150 degrees Fahrenheit). The higher the aging temperature, the lower the strength, but the toughness increases.
H1150M is the aging condition that produces the lowest hardness.
Solution treated, solution annealed, annealed, and Cond A are synonymous when describing conditions.
Typically, these types undergo solution treatment in the steel mill, followed by aging treatment after additional parts are manufactured.
If it is already at the desired aging state, no additional heat treatment is necessary. This entirely depends on the content of the ideal production plan provided.
Common stainless steels in this group include 17-4 (also known as 630), 15-5, 13-8, 450, and 455.
2. Semi-austenitic type
The chemical composition of these alloys results in a range of distinct structures during the heat treatment steps.
Like all PH stainless steels, the first step is “solution treatment”. This achieves a uniform distribution of the elements involved in the hardening reaction within the austenitic structure.
After cooling from solution temperature, the structure of these alloys remains in the austenitic state at room temperature… but only temporarily.
This relatively soft and ductile austenitic structure gives us the opportunity to perform a wider range of fabrication than martensitic types allow before hardening.
Well… looks like we found a way to have our cake and eat it too. We can get a softer, more moldable metal at this stage, and then we can harden it to the high strength of fully martensitic PH stainless steel.
To achieve final hardening of the material, we first need to allow the austenitic structure to transform into the martensitic structure. There are three methods to form the martensitic structure.
Any of the following:
Cooling to about -100 degrees Fahrenheit and holding for up to 8 hours.
Or
Heat to about 1,400 degrees Fahrenheit and hold for up to 3 hours.
Or
Cold working (such as cold rolling plate)
Now that we have the martensitic structure, the familiar aging treatment can perform the final hardening on these types.
The two-step hardening process for these grades is indicated by a prefix followed by H and the aging temperature. The prefix indicates the method of martensite formation.
For example:
- R H950 denotes cryogenic treatment (hence R), followed by aging at 950 degrees Fahrenheit.
- T H1050 means a thermal process and then ages at 1050 degrees Fahrenheit.
- C H900 represents cold working, subsequently aged at 900 degrees Fahrenheit.
Common examples of semi-austenitic steel grades include 17-7 (AISI 631), 15-7 (632), AM-350 (633), and AM-355 (634).
Applications typically require superior cleanliness of the remelted steel, with the precise details of the heat treatment required varying depending on the steel type and specification.
3. Austenitic Type
The last category of PH stainless steels are those that retain an austenitic structure from solution treatment through the aging process.
Although their strength is much lower than that of the other two types of PH, they are non-magnetic and have higher strength than 300 series stainless steels.
Solution treatment typically occurs at higher temperatures compared to other types. Aging also occurs at higher temperatures, generally above 1300.
In most cases, only one aging treatment is applicable to the alloy. As aging temperatures are higher, these alloys can be used at temperatures where other types of PH would lose resistance.
An example of this type is A286 stainless steel, which has superior vacuum reflow cleanliness, ideal for aerospace engine or turbine applications.
