Many machinists are familiar with 17-4 PH, a popular stainless steel in the aerospace, nuclear, and chemical processing industries. But have you ever found yourself a little unsure about the specifics of heat treating processes for this versatile material?

You’re not alone.

In this article, we’ll help clarify 17-4 heat treating practices, focusing on the key H900 precipitation hardening process, so you can confidently approach your work.

Answers to Common Questions about H900 Precipitation Hardening & Related Treatments

There’s no shortage of discussion and debate online among machinists about 17-4 heat treating. By all means, you should hash out the specifics of your application with a materials expert. But on a fundamental level, it’s not actually that complicated, especially compared to other precipitation hardening steels.

H900 isn’t the only heat treatment you need to worry about with 17-4, but it’s easily the most common technique. We’ll get into the details of H900 and a few other heat treat processes. But first, some fundamental info.

What Is 17-4 Stainless Steel?

17-4 stainless steel is a precipitation-hardening stainless steel (it's also called 17-4 PH) valued for high strength, corrosion resistance, and toughness. Studies show its corrosion resistance about on par with 304 stainless steel and better than 400 series stainless.

Composition

All of the following are approximate ranges. Actual content will vary depending on where your material comes from. Consult your vendor for the specific composition of your 17-4 PH material.

As you can see here, the components of 17-4 PH provide a valuable combination of hardness and machinability. However, 17-4 is prone to stress cracking at its highest strength—the primary reason for using H900 precipitation hardening, which we'll get into in a moment.

Applications

17-4 stainless steel is ideal for applications that call for high strength, good corrosion resistance, and good mechanical properties.

• Aerospace & defense – Landing gear components, structural parts, engine parts, missile and aircraft parts
• Chemical processing – Valves, pumps, and other equipment
• Oil and gas – Drilling equipment, valves, and wellhead components
• Medical devices – Surgical instruments, orthopedic implants, and other medical devices

What Do You Need to Do Before Machining 17-4?

This is an important question with a pretty simple answer.

17-4 usually comes in what’s called “Condition A,” which is sometimes referred to as annealed. More accurately, Condition A means the metal has been subject to “solution annealing,” which restores corrosion resistance after welding or other fabrication processes.

Another key detail: In Condition A / solution annealed, 17-4 PH is more ductile and easier to machine. So the answer is 17-4 is ready to be machined when you get it.

This is because the H900 process involves heating the material to a high temperature of 900°F (482°C) for a certain period of time, which can cause the material to warp, distort or change its dimensions.

Machining the material to its final dimensions before performing the H900 process ensures that the material is in its final shape before the heat treatment, reducing the risk of dimensional changes and ensuring that the finished part meets the required specifications.

What Are the Steps of the H900 17-4 PH Heat Treatment?

As we’ve said, 17-4 stainless steel at its highest strength can be brittle or prone to cracking. That’s one reason you don’t want to put it through more intense, high-temperature hardening processes.

The good news is that the typical hardening treatment for 17-4, called H900, is actually quite simple. The “H” is for “hardening,” and 900 is the target temperature, 900°F.

1. To protect your workpiece from corrosion, experts recommend using a protective atmosphere. A common method is to use a vacuum or inert gas atmosphere in your furnace. Hot Shot Ovens offers an argon gas kit for this purpose.
2. Set your furnace to 900°F. Let your workpiece equalize at that temperature and hold for 1 hour.
3. Remove the metal from the furnace and allow to cool in the ambient air.

That’s it. Using 900°F will give you a hardness of Rockwell C 44. You can use temperatures up to 1150°F, but material supplier 17-4 data shows that hardness will drop off at higher temperatures and longer hold times. For example:

• 1025°- 1025°F for 4 hours: ~ Rockwell C 38
• 1150°F for 4 hours: ~ Rockwell C 33 average

What Other Ways Might You Need to Heat Treat 17-4?

Solution Annealing

Although 17-4 PH comes in the solution annealed condition, you may need to repeat the solution treatment after some fabrication processes, such as welding (see below).

1. Set your furnace to 1900 - 1950°F.
2. Hold your workpiece at that temperature for 30 minutes to 1 hour.
3. Remove from the furnace. Follow with air cooling.

How Does Welding Affect How to Heat Treat 17-4 Stainless Steel?

Welding can significantly affect the microstructure and weaken the mechanical properties of 17-4 PH steel. That’s why it’s a good idea to weld before doing the H900 process.

In addition, the more welding passes you make, the more your material might benefit from an additional solution annealing process first, before H900. This step will reduce the negative effects of thermal cycling during welding.

So do you welding first, then do solution annealing followed by the H900 process.

Other Things to Keep in Mind

Minding the specifics we’ve already discussed, here are some best practices for heat treating 17-4 stainless steel:

1. Watch your temps. When the process is called H900, you’re not supposed to be in the upper thousands. Control your temperature to achieve optimal mechanical properties.
2. Clean your workpiece pre- and post- heat treating. Remove any contaminants, residues or scale that may interfere with the heat treatment process or your desired results.
3. Stay on the lookout for cracks. Since 17-4 is prone to cracking, it’s best to inspect your workpiece regularly to make sure you’re avoiding this problem throughout heat treating and fabrication.

We hope this article helps clarify things for you. Of course, for your best results, we always recommend discussing the specifics of your application with a materials expert at your supplier. If you’re ready to take on 17-4 heat treating, be sure to check out Hot Shot Ovens for machining with rapid heating, Cool-Touch technology, and precise control for your shop.