The full transcript is below. In this video we try to answer the following questions: What are the PH grades of Stainless Steel? What is PH stainless steel? What does PH stand for? How do the many conditions of heat treating differ?
Good morning everyone! We’re back again to talk steel. Today we’re going to talk about PH stainless steels.
What the heck are “PH” stainless steels? Great question.
Now this video will just give the basics of this group of stainless steels, but before we get started, if you have been watching this series and enjoying it, please consider hitting that little subscribe button which is the MMI button in the bottom right hand corner. If not, I get it but how about liking the video then, hit that little thumbs up 🙂 Now back to PH stainless steel!
High strength, excellent corrosion performance, and a simplified heat treatment process are advantages of the PH types as compared to the conventional martensitic types.
PH is shorthand for “Precipitation Hardening” which is a heat treatment that is a bit different from that of the conventional heat treatment of the Martensitic types discussed in the prior two videos. (Martensitic Part One and Martensitic Part Two)
An initial “solution treatment” at high temperature …..typically 1900 degrees F….assures that all the alloy elements needed for the hardening reaction are uniformly distributed within the metal structure. At these temperatures the structure is austenitic. From this temperature, the alloy is cooled at a rate that retains the distribution of the hardening elements in solution.
Depending on the chemical composition of the specific alloy, the resulting structure after the “solution treatment” is either martensitic, semi-austenitic, or austenitic. These structures contain more of the hardening elements than would be completely stable, so it’s just waiting for an additional heat treatment to cause things to happen within the structure. But things are stable enough that we can choose to fabricate components prior to the final hardening heat treatment This additional relatively low temperature heat treatment is called “aging”. The increased temperature and time allows the elements mobility to combine and form the “precipitates” (think particles) that then strengthen the structure.
Before we go further let’s split the PH alloy types by the solution treated structure…….
First up is the Martensitic types. They form a low carbon martensite ( relatively soft but also brittle) when solution treated. Alloys should not be used in the solution treated condition. When reheated to the aging temperatures the particles that form further strengthen the structure and improve toughness and corrosion performance.
The resulting heat treated condition is designated by the letter H followed by the aging temperature. For example: H900 indicates that it has been solution treated and then aged at 900 degrees F. Hardness increases and yield strength minimums of 170,000 psi are achieved with this second simple heat treatment.
Conditions range from H900 thru to H1150, and even a Double H1150 (with two aging sessions at 1150 degrees F). The higher the aging temperature the lower the strength but toughness is increased.
H1150M is an overaged condition producing the lowest hardness.
Solution treated, solution annealed, annealed, and Cond A are synonymous in describing condition.
Often, these types are solution treated by the producing steel mill and then the aging treatment is performed after additional fabrication to parts.
If already in the required aged condition, then no further heat treatment is required. Its all a matter of what provides the best manufacturing plan.
Common grades in this group include 17-4 (aka 630), 15-5, 13-8, 450, and 455
15-5 and 13-8 are examples of premium vacuum melted grades…the additional melting under vacuum minimizes any harmful impurities for critical applications like highly stressed aerospace components. Grades can also have multiple vacuum melting that we will touch on in the next video.
Corrosion performance and suitability for any application is best left to the professionals to specify the alloy and condition required.
Remember to always be on the lookout for any specifications that apply. The devil is always in the details …… and sometimes hiding.
Stay tuned for part two……for the austenitic and semi austenitic types.