A shaft should have a hardness between 28 and 32 Rockwell C... but Why?

One of the first things I was taught as an engineer involved in specifying materials for steam turbine or compressor parts was that the material hardness for shafts was supposed to be between 28 and 32 Rockwell C Scale hardness.

What is hardness?

What does range of numbers mean?

Why is this the proper range?

Where is this written?

What is hardness?

A simple definition: Hardness is a measure of a material’s resistance to local plastic deformation, usually from pressing.

Let’s break this down:

• Resistance to local plastic deformation

• Usually from pressing

Engineers use the term plastic deformation to represent a condition where material deforms under a force and stays permanently deformed once the force is removed.

It does not necessarily mean you tear or break the material; you simply deform it.

Imagine playdoh. You make a nice flat brick of playdoh, and then you stick in your finger, pressing down.

You end up making a finger shaped indentation!

The human hardness tester Romero Scale.

When you remove the force, you pull your finger out, the material stays deformed.

This is the opposite to elastic deformation. Imagine pulling on a rubberband.

You apply a force to pull, the rubberband stretches, becomes longer.

When you let go of the pulling force, the rubberband returns to the original size.

So, in a material science, hardness is a measure of a material’s resistance to permanent deformation due to an indentation.

So, what does knowing the hardness of a material tell us?

When working with metals like Steel, hardness is not just a number or a value. It is like looking through a window into the past that reveals how a material was manufactured and processed.

It tells a whole story that helps us understand how the material was treated and lets us know what to expect when it is put into service.

And in terms of the steel alloys that are used in manufacturing turbines and compressors, it so happens that hardness and tensile strength are closely correlated.

Conversion table from API 687

This means that if you know one, you can get a pretty good idea of the other!

And this is extremely valuable.

It is like going to the store and searching for the perfect avocado. We are not allowed to cut avocados in half at the store to know how ripe they are!

Avocado lovers all over the world know this, and we have developed a method of gently holding avocados and squeezing them to gauge how ripe and perfect it is inside.

When we get home and we are ready to eat said delicious fruit, we slice it and we feast!

I know people that put this delicious fruit on everything!

This need to know a material without cutting it also exists in a turbomachinery repair shop.

We are not allowed to cut every steam turbine or compressor part to perform a destructive tensile test. So, we rely on measuring hardness. Measuring hardness is fast; it’s non-destructive.

Knowing the tensile strength of a material is extremely important. Since turbomachinery are high stressed machines with rotors that spin fast, they must be able to withstand a lot of forces and stresses. Knowing the strength of a material is fundamental to making decisions in terms of repairing or replacing parts.

Another awesome thing about hardness is that it not only tells us how strong a material is, it reveals how it got there. It tells us how the material was heat treated during the manufacturing process.

We’ve all seen that scene in the movie where the medieval armies are preparing to go to battle, and you see warriors testing swords and picking helmets, and the camera cuts to a blacksmith pounding a hammer on a sword that glows bright yellow and red from heat.

And when he is done pounding, he dunks the sword in a bucket of water, and it makes a swishing sound, and a bunch of steam comes out.

Yes! We have all seen that movie.

All those steps the movie Blacksmith took are real steps one would take when manufacturing a compressor shaft, for instance.

We will forge a shaft; we will heat the material up to make it malleable and to upset and deform it, improving its strength. Then we may rapidly cool it or quench it to lock in desirable mechanical properties.

The use of heat when manufacturing and processing a metal is called heat treating. And heat-treating steel is done to affect the way it is structured.

Steel alloys have crystalline structures. Imagine them as if they were structures made from Legos. If you make a sword out of small 1x2 Legos (meaning very short Legos) you may end up with a fragile sword that breaks as soon as you lift it.

But if you make a sword with longer Legos 1x4 or 1x6 Legos, with a lot of overlapping connections, you end up with a pretty strong sword.

A metal’s crystalline structure is just like that: it is the configuration of the “grain size” or Legos and how you stack them.

So when we pick up a turbine or compressor part with no drawings or design paperwork and measure its hardness, I am not just getting a number.

I am reading the grain structure, I am reading the heat treatment, I am reading the decisions the original design engineers made when they designed that part. Hardness is a very powerful thing.

Why is this important?

Well, in a turbomachinery repair shop, it is likely we will have to repair parts or make replacements or even duplicate an existing part.

When you are going to copy or duplicate a part you need to know:

• What is the shape?

• What is the material composition?

• What are the material properties?

We answer all these questions by a process called reverse engineering, which is a term we clever engineers describe how we copy things.

We determine the shape of an object by physically measuring it. We use lots of tools, and another day we will get into that on a separate post.

We know material composition by performing Positive Material Identification. This basically means, we know what elements are found on an alloy and at which proportions.

We use sophisticated tools for this as well, that tell us in terms of percent how much Iron, Nickel, Chrome, and other elements are on a sample.

So far, we know what the part looks like and what it is made from. But we don’t know how strong it is yet.

How do we find out how strong the part is, especially if we are not allowed to destroy the sample part?

For the reasons I presented already, knowing the hardness will reveal the strength properties and will describe to the engineers and metallurgists what heat treatments were applied.

Let’s imagine I am hired to copy a compressor disk on a gas turbine.

I will likely measure it and make a 3D model, that then I can use to make drawings, machining instructions, or even run simulations if I need to verify and qualify the design of the part.

I will request a chemical composition test. This test will check the elements on the sample disk and report that the composition best matches a material called AISI 4340.

AISI stands for American Iron and Steel Institute. It is an organization that developed a naming or numbering system to identify steels. AISI only gives alloys their name.

AISI names like 4340, 4140, 1045, 410, 422, and so on, are only designations of chemical composition. AISI only lists the ingredients that go in an alloy. They don’t write the recipe on how to cook up the alloy.

The recipe that determines how strong or what microstructure or what heat treat an alloy is is called a material specification or a procurement specification.

And the AISI does not write material specifications, other groups do, like the American Society for Testing and Materials (ASTM), or the Society of Automotive Engineers (SAE) or the Aerospace Material Specifications (AMS).

The AMS specification we use the most when purchasing and treating AISI 4340 is the AMS 6414 or AMS 6415. It basically describes how the metal must be melted, what type of furnaces are used, how it is heated, cooled, and so forth.

AMS 6415 is the standard specification. AMS 6414 is the premium version, produced using a process called Vacuum Arc Remelting or VAR. The VAR process produces a cleaner, more homogeneous steel with fewer internal defects. For a high-stress rotating component like a compressor disk, that difference matters.

So, what about 28-32 HRC? What do those numbers mean?

Before I can answer that, I need to introduce you to the people that invented the hardness scales in the first place.

Throughout history, engineers have invented different tools and methods for measuring things. Think Celsius and Fahrenheit, millimeters and inches.

In practical terms, in turbomachinery, the most relevant or useful methods for measuring metal hardness were invented around the same time by some really clever people.

Those who have read my previous posts will know I am an avid Pokémon fan. And when the opportunity presents itself, I love imagining engineering historical figures as if they had their own Pokémon card.

So here we go.

Finally, we are ready for the numbers.

We have covered a lot of ground to get here. Hardness. Plastic deformation. Heat treatment. Grain structure. AISI, ASTM, AMS. Brinell, Rockwell, Vickers. You have earned this answer.

Let's start with what those numbers actually mean in the real world.

Looking at the API 687 hardness conversion table, 28 HRc corresponds to approximately 269 Brinell, which translates to a tensile strength of roughly 133,000 psi. And 32 HRc sits at approximately 302 Brinell, which translates to roughly 150,000 psi.

So, 28 to 32 HRC is really saying: this shaft must have a tensile strength between approximately 130,000 and 150,000 psi.

That is not an arbitrary range. That is an engineering decision.

Why not softer?

Below 28 HRC, you are looking at tensile strength under 130,000 psi.

The shaft has plenty of toughness, but it starts losing the strength needed to resist the stresses of rotating equipment service. Journal surfaces wear faster. Seal and fit areas gall. Press-fit interfaces can yield under interference stress and loosen over time. The shaft is not strong enough to do its job reliably for the long term.

Why not harder?

Above 32 HRC, the shaft is stronger but more brittle.

Every keyway, every step in diameter, every surface finish scratch becomes a potential fatigue crack initiation point. Turbomachinery shafts experience millions of stress cycles over their service life. A slightly harder shaft that is less forgiving of stress concentrations is a shaft that may fail in fatigue before a softer one would ever wear out.

The 28 to 32 range is the sweet spot.

Strong enough to handle mechanical stress. Tough enough to tolerate the dynamic loading and the inevitable imperfections of real-world machining and service. Sort of like the love of a sweet old Ecuadorian grandma.

 

And finally, where is this all written?

The truth is, before I wrote it here, it was not written anywhere in one place. No API document has it, no turbomachinery design book in academia has it.

It is not in one single document as a universal law. It lives in the accumulated engineering practice of the industry, like a secret message codified in OEM design specifications, in material standards like AMS 6414, ASTM A470, and in the hardness conversion tables of API 687 which connect the number directly to the tensile strength your part needs to have.

When my mentor told me, “That shaft should have a hardness of 28 to 32 HRC” on my second day at work back in September in 2002, he was not reciting a paragraph from a standard. He was bestowing on me decades of engineering judgment, field experience, and failure analysis compressed into four numbers.

Thank you, Mr. East.

And now you know exactly what those four numbers mean, where they come from, and why they matter.

What is hardness? A window into the past.

What does the range mean? A tensile strength target.

Why is it the right range? Because rotating equipment demands both strength and toughness.

 Where is it written? In the hands of every engineer who ever had a good mentor.