Chris Carolan

Metals Consultant at Metal Analysis Group

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Carbon Steel PMI Testing: The Truth

steel-pipes

Chris Carolan

Metals Consultant at Metal Analysis Group

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Who rules the world? Steel.

Carbon steel is literally in everything: bolts, bridges, airplanes, cars, appliances… pretty much any machine. So, it’s really important to accurately and precisely measure carbon values in steel.

pressing-steel-GIF

That means finding the best way to perform a carbon steel PMI test. (PMI is Positive Material Identification. Here’s an article about it.)

What’s the best way to conduct a carbon steel PMI test? To conduct a successful carbon steel PMI test, you’ll need to know:

  1. How and where the material is being applied
  2. The properties associated with specific alloys
  3. Whether or not you need a very portable PMI analyzer
  4. Whether or not you need to test for other elements as well
  5. Whether or not you’re testing for very low levels of carbon

Before jumping into how we positively identify carbon values in materials, let’s talk about the why.

Psst: This post is based on a podcast with co-host Chris Carolan. To hear this episode (and more like it), subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.

Why carbon steel PMI testing is crucial

Carbon steel became widely used throughout the late industrial revolution. Steelmakers reveled in the fact that adding carbon to raw iron made a stronger, more malleable material — aka, carbon steel.

steam-engine-b&w-GIF

Where carbon steel is used

Carbon steel shows up in droves of applications.

Depending on the level of carbon, steel can be used for:

  • Vehicle bodies
  • Wire
  • Bolts
  • Home appliances
  • Fences
  • Buildings
  • Bridges
  • Pipelines
  • Cutting tools
  • Springs

Clearly, carbon steel is a versatile material. Nonetheless, it’s absolutely essential to identify the carbon value of steel before using it for any of its specific properties.

Carbon steel PMI testing is important

Reaching the accurate carbon content is a grave concern across industries, including foundries, laboratories, and factories. Failing to use material with the specified level of carbon can negatively affect its… 

  • Melting point
  • Compatibility
  • Brittleness
  • Malleability
  • Weldability

Imagine if an integral part of a pipeline in a processing plant had too low of a melting point. Or, if a bridge’s guard rail was too brittle and easy to break.

Yikes!

Take this animation of the 2015 explosion at the ExxonMobil refinery in Torrance, CA for example.

If you’re ready to learn how to prevent disasters like this one, keep reading.

Let’s move on to the first step in successfully testing for carbon in steel: Know how and where the material is being applied.

Understanding how and where carbon steel is being applied

Getting a comprehensive understanding of where and how a material is being applied is essential for all PMI testing. Otherwise, how do you know what levels of certain elements need to be present?

Take into consideration the following questions:

Will the material be… 

  • Under a great amount of pressure?
  • Used in extreme temperatures?
  • Transporting corrosive chemicals?

These questions can be used to determine the material’s chemical compatibility. In other words, understanding the material’s application will let you know what kinds of characteristics it needs.

Carbon steel properties

Carbon steel is classified into three main categories according to the level of carbon present: low-carbon steel, medium-carbon steel, and high-carbon steel.

Low-carbon steel

As the most widely used type of carbon steel, low-carbon steel — sometimes known as mild-carbon steel — has a carbon content of less than 0.25 wt.% and has numerous applications.

Low-carbon steel properties:

  • Relatively soft
  • High ductility
  • Formable
  • High toughness
  • Great weldability
  • High machinability
  • Low cost

Low-carbon steel applications:

  • Pipes
  • Nails
  • Wire
  • Vehicle panels
  • Structural steel
  • Low-temperature pressure vessels

Medium-carbon steel

Medium-carbon steel has a carbon content ranging from 0.25 and 0.60 wt.% as well as a manganese content of 0.60-1.65 wt.%

By means of heat treatment, medium-carbon steel can be strengthened. However, this comes at the expense of the material’s ductility and toughness.

Medium-carbon steel properties:

  • Low hardenability
  • Medium strength
  • Some ductility
  • Medium toughness

Medium-carbon steel applications:

  • Gears
  • Bolts
  • Axles
  • Machinery parts
  • Shifts
  • Crankshafts
  • Couplings

High-carbon steel

With a carbon content of 0.60-1.25 wt.% and manganese content of 0.30-0.90 wt.%, high-carbon steel has the highest hardness and toughness and the lowest ductility.

Due to the fact that they are almost always tempered and hardened, high-carbon steels are extremely wear-resistant.

High-carbon steel properties:

  • High hardness
  • High toughness
  • Low ductility
  • High wear-resistance
  • High cost

High-carbon steel applications:

  • Pocket knives
  • Blades
  • Drillbits
  • Milling cutters
  • Springs
PropertiesApplications
Low-carbon steelSoft; formable; ductile; tough; high weldability; low costPipes, nails, wire, vehicle panels, structural steel
Medium-carbon steelLow hardenability; some ductility; medium strength and toughnessGears, bolts, axles, crankshafts, couplings
High-carbon steelHigh hardness, toughness, wear-resistance, & cost; low ductilityPocket knives, blades, drillbits, springs, milling cutters

Understanding the properties of each type of carbon steel will help you in the next step: choosing the best PMI analyzer for your application.

Choosing the best PMI technique for carbon steel

To successfully measure the carbon value of steel, you need to employ the right kind of PMI analyzer for the specific job.

There are three popular ways of determining carbon value in steel:

  1. Combustion (normally reserved for labs)
  2. Mobile OES (optical emission spectroscopy)
  3. LIBS (laser-induced breakdown spectroscopy)

For our purposes today, we’re only going to be comparing mobile OES and LIBS.

Portability

Testing materials out in the field often means you need a portable PMI solution. In those cases, it’s best to go with a handheld LIBS device.

handheld-LIBS-instrument-for-carbon-steel

If the steel is difficult to access or to transport, handheld LIBS is the clear winner when compared to mobile OES.

Testing for other elements

Need to test for low levels of other elements in your carbon steel alloy? Mobile OES will probably be your best bet.

mobile-OES-device-for-carbon-steel

Sometimes manufacturers add alloying elements to carbon steel in order to improve its strength, melting point, or any other desired properties. The PMI technique you’ll want to use for those elements is mobile OES.

Mobile OES can provide a more extensive chemistry than LIBS, including low levels of elements like boron, phosphorus, sulfur, and nitrogen.

Testing for low levels of carbon

Because it’s a more sensitive form of positively identifying materials, mobile OES is recommended for measuring levels of carbon at 800 ppm or less. Example: L-grade Stainless Steel

When it comes to extreme amounts of carbon, such as >1-2%, it’s difficult for mobile OES as well as LIBS. Gray cast iron and ductile iron often contain too much carbon for either PMI technique to measure precisely.

“You definitely have to know which carbon levels you’re looking for to determine the right analyzer.”

Chris Carolan

Using XRF for testing carbon steel

Because handheld XRF cannot provide a value for carbon, it should not be used for positive material identification when true carbon values are needed.

However, since XRF can effectively measure other elements, the technique can be used for sorting carbon steels. As long as the XRF instrument understands it’s testing carbon steel, it can use the other present elements to determine the category of carbon steel.

Carbon steel takeaways

There are a few key ideas I hope you walk away from this article with.

  1. Always do your research on PMI techniques and which ones are best to use in certain situations.
  2. Gain a full understanding of your application.
  3. Speak with a trusted consultant if you’re questioning anything in regards to PMI or testing carbon steel.

All in all, take the proper precautions when you’re positively identifying any material. Understand the process, the device, and the application.

Until next time!

For more information on PMI testing for carbon steel, subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.

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