Precipitation hardenable (PH) stainless steels are one of the most popular types of stainless steel alloys in use today, but they are often not as well known as the austenitic and ferritic categories. In fact, while some people refer to the five types of stainless steel, others might only mention four: the two already named above, plus martensitic and duplex alloys, leaving precipitation hardening off the list.
Austenitic and ferritic alloys account for 95% of the commercial stainless steel applications on the market today, so it’s easy to explain why they are the most recognized alloys. Why is precipitation hardening often overlooked? In today’s post, we’ll be looking at the material’s beneficial attributes and the process of precipitation hardening. At the same time, we’ll highlight some of the most popular applications, and which industries you’ll be most likely to find them.
What is precipitation hardening?
The categories of stainless steel are mainly determined by a material’s chemistry; however, with precipitation hardening, it’s more about the process than the alloying agents, which is why other metals can also undergo a similar process. The precipitation hardening process can be used with aluminum, nickel or titanium.
Precipitation hardening is a form of heat treatment that can increase the strength of malleable materials such as steel. The technical process involves heating the metal in such a way as to produce fine particles or impurities that impede the movement of defects in the metal’s crystal lattice structure. This decreased plasticity strengthens the material.
When heating the metal, it can undergo several different responses depending on the techniques used. You can imagine it as when water vapor freezes in the air. Depending on the atmospheric conditions, the new particles can take the form of clouds, hail or snow. Similarly, when precipitation forms in solids, it can result in vastly different properties.
Precipitation hardening differs from ordinary tempering in that the alloys must be kept at a constant elevated temperature for a specified duration to allow for the precipitation to occur. This is referred to as aging the metal.
It should also be noted that once they have been hardened in this manner, the alloys may still technically be referred to as semi-austenitic or semi-martensitic. This is because they still retain some of the properties of the original alloy.
What is the typical process of precipitation hardening?
The hardening process generally involves three main steps: First, the metal must undergo a solution treatment. During this stage, the metal will be heated to a high temperature to dissolve any precipitates and alloying agents into a supersaturated solution. Typical temperatures range from 1800° to 1950°F and can occur in conjunction with the hot-rolling process.
Next, the metal will undergo a quenching step to cool it down to room temperature. This can occur in air, oil or water, rapidly enough to induce a supersaturated solid solution. A slow cool down will be more likely to produce a coarser grain size than a faster cool down. Generally, the finer the grain size, the better performing the finished alloy will be.
The third step is known as precipitation (or age) hardening. The supersaturated solid solution will decompose as small precipitate clusters form, significantly strengthening the metal. With stainless steels, this process involves holding the metal at a constant elevated temperature for a specific time and air cooling to room temperature.
What are the most popular precipitation hardening alloys?
Martensitic precipitation hardening stainless steels are the most popular PH grades in widespread use. During the heat treatment process, these alloys typically have an austenitic structure, but when they are cooled to room temperature, they undergo a transformation that results in them more closely matching martensitic alloys. Some of the more common grades include 17-4 (17% chromium 4% nickel), 13-8 (13% Cr 8% Ni) and 15-5 (15% Cr 5% Ni). These grades can be heat treated for high strength, while offering excellent corrosion resistance and machinability. They are all magnetic.
The age hardening of martensitic PH grades is performed to achieve specific conditions, such as Condition H900, H1025, H1100 and H1150. These conditions indicate the temperature of the age hardening process. Each condition will produce different mechanical properties in the metal, but the machinability and corrosion resistance will also differ between them. The martensitic PH grades also exhibit very good dimensional stability after age hardening.
The inherent value of PH stainless steels lies in their flexibility. While the aforementioned grades have corrosion resistance approaching or meeting that of 304 austenitic stainless, the age hardened conditions allow for mechanical properties to be tailored to a wide variety of applications. This is the reason the production of PH grades has radically increased over the last few decades as designers have learned to take advantage of this material’s adaptability.
There are also austenitic precipitation hardening alloys, which maintain their basic austenitic structure even after the heat treatment. These alloys are typically not as strong as the other two categories but have the advantage of being fully austenitic. An example of this category is A286, suitable for applications requiring high strength and corrosion resistance up to 1300°F.
Next are the semi-austenitic alloys, which are austenitic in the annealed condition and martensitic in the age hardened condition. These alloys are still soft enough to be cold worked. A typical example is 17-7 PH, which is alloyed with chromium, nickel, and aluminum. It is generally regarded as the most formable of precipitation hardening alloys, while still showing excellent strength and hardness. For these reasons it is mostly available in sheet form. It also shows minimal distortion after heat treatment.
What applications commonly rely on precipitation hardening stainless steel?
Because of their superior strength, precipitation hardening stainless steels are most often found in high-tech industries such as aerospace and energy. For instance, 17-4 is commonly used in the aerospace industry, as well as for petroleum and chemical applications. Other uses for precipitation hardening alloys include gears, valves, and other engine parts, turbine blades, high-strength shafts, molding dies, and nuclear waste casks.
To learn more about precipitation hardening stainless steels, get in touch with an experienced and knowledgeable member of the Clinton Aluminum sales team. We are dedicated to working with our clients to find just the right material for every application.