Comparing Common Steel Material Tests

Steel is an essential material and is used in various different applications. It is necessary to evaluate the chemical composition and mechanical properties of steel before choosing the right grade.

This article will compare and explore common steel material tests and provide help with the interpretation of the results.

There are over 3500 grades of steel available and so engineers rely upon accurate material testing in order to determine which material is needed for each specific project. Chemical composition of steel materials tells a lot about the tendencies of the material, but to prove the hardness, strength and quality of the final material, testing is required.

Key Phases of Steel Testing

It is important to understand a few phrases and keywords before exploring common material tests.

Coupon

A coupon is a test sample which is either cut from a master plate or prepared from the same forging process. It is cut or produced in a way which ensures that it is representative of the master plate and can be used for testing.

There are two common types of coupon: transverse coupon and longitudinal coupon. A transverse coupon (TCVN) is taken transverse to the rolling direction of the plate, whilst a longitudinal coupon (LCVN) is taken longitudinally.

LCVN coupons will have a higher yield and higher tensile points than TCVN coupons because LCVN coupons go with the grain of the steel. Structural engineers often take both LCVN and TCVN coupon test results into account when choosing their material.

Lamination

A lamination is a defect or imperfection in a steel (or any other metal alloy) material that can impair the structural performance. Laminations can be caused by various things including foreign materials and trapped gases during the rolling of steel or the presence of folds or layers.

Steel is formed into an ingot (a large block) and rolled to the desired size and thickness during production. If there is a defect or any foreign material present in the ingot it will spread as the material is rolled.

Some steel materials have an acceptable threshold for laminations which means that some laminations are acceptable up until a point at which the threat of material failure becomes too great.

Mill Test Reports

A mill test report (MTR) is a document that shows the material composition and origin, as well as material testing results. It is a quality assurance document that tells engineers the exact makeup and performance of the material.

Often, MRTs are provided with every steel material purchase but if they are not provided initially, they can be requested.

MRTs are required to show strength test results and material chemistry, but they can also be used to show results for toughness, quality and hardness.

Material Strength Test: Yield, Tensile and Elongation

Material yield point, tensile strength and elongation percentage are calculated in one of the most common and important tests. The test indicates the maximum load the material can bear before failure.

Yield point is the point where the shape of the material is permanently changed due to stress of a force. For example, the yield point of a bookcase would be when the weight of the books causes the shelf to bow permanently, even when the books are removed.

Tensile strength is the force at which the material will break or fail. In the bookshelf example, the tensile strength of the shelf would be the force of the books (book mass multiplied by acceleration of gravity) that would cause the shelf to snap in two.

Yield and tensile are most commonly expressed in pounds per square inch (psi) or thousand pounds per square inch (ksi).

Elongation is how much the material can bend or stretch in relation to its original length. This is calculated as a percentage difference between the yield point (when the material does not return to its original shape) and the tensile strength (the point where the material breaks).

Elongation is also often used to indicate ductility. A material will have more ductility if it has a greater elongation.

Below are yield, tensile and elongation measurements for two common grades of steel: A572-50 high strength low alloy structural steel and A36 mild steel. The data show that A572-50 can bear a greater load than A36 but that A36 is more ductile.

Ultrasonic Testing for Defects

As previously mentioned, laminations can occur during steel manufacturing. Ultrasonic testing (UT) detects laminations in the body of steel material using high frequency soundwaves. It is a non-destructive test that is performed on the steel material itself rather than on a coupon.

During UT, a probe sends a sound wave into the material. When the sound hits a barrier (e.g. air), it will bounce back towards the probe. The probe’s ultrasonic transponder converts those sound waves into electrical energy and this can be read on a test machine screen.

UT is highly accurate with accuracy values between +/-0.025 mm and +/-0.001 mm. Ultrasonic testing supplies the engineer with important information regarding their material which ensures safe and proper future use.

Charpy V-Notch Test for Toughness

A material’s ability to absorb energy, or impact, within a given temperature range is measured using the Charpy V-Notch test (or Charpy Impact Test). The test is named after Georges Charpy, who was the first to standardize impact testing.

Brittle materials can absorb less impact before fracturing and colder temperatures increase brittleness. The Charpy test helps engineers to choose the correct grade of steel for projects that need the steel material to be exposed to cold temperatures.

The Charpy test pits a small piece of test material against a heavy pendulum hammer. The name comes from a V-shaped notch carved in the middle. The pendulum swings into the test piece (often bending, if not snapping, the piece) and the absorbed energy is measured.

The test can be certified and conducted at different temperatures and this will be reflected in the mill test report.

Brinell Hardness Test

Hardness is the ability of a material to withstand friction and abrasion. This is different from strength (the amount of force a material can withstand before deforming or failing) and from toughness (the ability to resist fracturing under force).

For example, a diamond is very difficult to scratch (high hardness) but it is comparatively easy to shatter (low toughness).

The Brinell hardness test was standardized by and named after Johan August Brinell and uses a small steel indenter and applied force to dent a material. The brittle hardness number (BHN) is calculated using the size of the dent and is a standard unit of hardness.

An abrasion resistant grade steel has a BHN greater than 300 whilst mild A36 steel only has a BHN of 133.

For applications that require abrasion resistance instead of strength or toughness, hardness is important. Examples where hardness is important include: grates, body armor, buckets, and conveyors.

Not every material need will require every test discussed in this article. Understanding testing options and the differences in what they measure will help engineers to make informed decisions about materials for their projects.

Lecco Steel can supply a wide variety of steel plate material, depending on engineering specifications, and MRTs are included with every order invoice.

This information has been sourced, reviewed and adapted from materials provided by Leeco Steel, LLC.

For more information on this source, please visit Leeco Steel, LLC.