February 2022 Volume 4

MATERIALS

As indicated in the previous issue of this column, we will move on to discussing a very important family of stainless steels known as precipitation hardening stainless steels , with an added focus on the heat treatment of these alloys. Due to their tailored chemical compositions, precipitation hardening (PH) stainless steels possess unique strengthening mechanisms that result in significant improvements in both strength and corrosion resistance as compared to other families of stainless steels. PH stainless steels are commonly classified with a “AA-B” numbering system, where in the “AA” is the approximate chromium content and “B” is the approximate nickel content. For example, a common PH stainless grade is 17-4, which contains approximately 17% chromium and 4% nickel. These high levels of alloying elements allow PH stainless grades to attain very high strength levels without significant sacrifice in corrosion resistance. Additionally, the very low carbon contents of these allows (< .10%) gives them great toughness properties, even at high strength levels. Not surprisingly, these are considered premium grades of stainless steel and are typically reserved for critical applications in industries such as aerospace, medical, or power generation. PH stainless steels are divided into three sub-categories depending on alloy design. First, there are the martensitic grades. These form a martensitic microstructure at room temperature following solution anneal treatment. Common examples of martensitic grades are the aforementioned 17-4, as well as 13-8 and 15-5. Next, there are the Semi-austenitic grades which form a metastable austenitic microstructure at room temperature and can either be cold-worked or refrigerated to complete the martensitic transformation prior to age hardening. Finally, austenitic PH steels are chemically tailored such that the austenitic microstructure is completely stabilized. While age-hardening still occurs, these grades are not capable of attaining the same strength levels as the martensitic or semi-austenitic grades but have advantages of high temperature properties. The common thread amongst all types of PH stainless steels is that they utilize precipitation hardening as their primary strengthening mechanism. Precipitation hardening occurs when these alloys are re-heated to relatively low temperatures (typically 900°F-1200°F) which causes precipitation of intermetallic compounds into the martensitic or austenitic matrices. These compounds are formed by adding elements such as Al, Nb, N, or Ti to the alloys. Upon formation, these intermetallic compounds (such as Ni3Al) hinder atomic slip and thus strengthen the alloy. While this is an extremely simplistic overview of the primary strengthening mechanism of PH stainless steels, it is of more benefit to use the overall heat treating process as a discussion guide for further explanation. Heat Treating Corner By Chuck Hartwig

It must be understood that the necessary starting point for the precipitation hardening process to occur is a fully solution treated (or solution annealed ) microstructure. As with the austenitic stainless steels, these terms should not be confused with simply stating “annealed” as this word describes heating with slow cooling which would NOT be a desirable outcome for any stainless grades. In solution annealing of PHgrades, the material must be heated to a very specific temperature range as defined by the chemical composition of the particular grade – typically in the range of 1700°F-1900°F - followed by rapid cooling . The cooling protocol for PH grades needs to be carefully assessed by the heat treater, as there is no single black and white answer. In selecting a cooling method for solution treatment, the thickness and geometric complexity of the component must be considered, but water or oil quenching is most common for forged products. Air or fan cooling can be successfully employed for thin cross-sections or distortion-prone components so long as final properties can be attained post-age harden. During solution treating, it is absolutely critical that each part is uniformly heated to the desired temperature range and soaked at this temperature for a sufficient time. In aerospace practice, load contact thermocouples are typically employed to verify that this has occurred. This must also be carried out in a furnace of known temperature uniformity, as even slight deviations outside of the required solution treating temperature range could have disastrous effects on the material’s ability to respond to subsequent age- hardening treatments. It is also critical that the entire workload be thoroughly cooled to ambient temperature after quenching, prior to undertaking any subsequent thermal operations. This is to ensure stabilization of the martensitic microstructure. The proper execution of the solution treating cycle is a critical starting point for the strengthening process of PH stainless steels as it puts all chemical elements into the solution and subsequently provides the driving force for the precipitation hardening response to occur when the material is re-heated to a lower temperature. This concept is very similar in practice to quench hardening an alloy steel component; however, the precise mechanisms are very different as the PH stainless steels are in a relatively soft condition post solution treatment and alloy steels are at their highest hardness after quenching and must be softened by tempering. The final thermal process step for PH stainless steels is the age- hardening treatment. Most PH grades are denoted by a final temper callout, such as 17-4 H900, which specifies 17-4PH age-hardened at 900F for one hour. Each grade designation is also defined by specific tensile properties and typical hardness ranges. It is critical that reference standards/industry specifications be consulted to ensure that the exact time and temperature protocols are followed for any given temper callout for PH steels.

FIA MAGAZINE | FEBRUARY 2022 36