Common Methods of Testing Steel Material
Posted on Jan 03, 2019
Steel is an essential material used in a variety of applications. Choosing the right grade of steel requires evaluating chemical composition and mechanical properties. This article will explore and compare common steel materials tests and provide insight into understanding the results.
With over 3,500 grades of steel available, engineers depend upon accurate material testing to determine the materials needed for each specific project. Chemical composition of steel materials tells a lot about the material's tendencies, but testing ultimately proves the strength, hardness and quality of the final material.
Steel Testing Key Phrases
Before exploring common material tests, it is important to understand a few keywords and phrases:
A coupon is a test sample cut from a master plate (or prepared from the same forging process). The coupon is cut or produced in a way that ensures it is representative of the master plate and is used for testing.
There are two common types of coupons: longitudinal coupon and transverse coupon. A longitudinal coupon -- often abbreviated as LCVN -- is a coupon that is taken longitudinal to the rolling direction of the plate.
A transverse coupon (or TCVN), is taken transverse to the rolling direction of the plate.
LCVN coupons will have higher yield and tensile points than TCVN coupons, since LCVN coupons go with the steel grain. Structural engineers often look at both LCVN and TCVN coupon test results when choosing their material.
A lamination is an imperfection or defect in a steel (or other metal alloy) material that can impair structural performance. Laminations are caused by a variety of things, including the presence of folds, layers, foreign materials and trapped gases during the rolling of steel.
During steel production, steel is formed into an ingot -- or a large block -- and rolled to the desired size and thickness. Whatever defect or foreign material is present in the ingot will spread as the material is rolled.
Some steel materials have an acceptance threshold for laminations. This means that some laminations are acceptable, up until a point when the threat of material failure becomes too great.
Mill Test Reports
A mill test report (MTR) is a document that shows material origin and composition, as well as material testing results. An MTR is a quality assurance document that tells engineers the exact makeup and performance of the material.
MTRs are often provided with every steel material purchase. If not initially provided, an MTR can always be requested.
MTRs are required to show material chemistry and strength test results, but will also show results for quality, toughness and hardness if those tests were run.
Material Strength Test: Yield, Tensile and Elongation
One of the most common and important tests conducted on steel material is a mechanical properties test that evaluates the material's yield point, tensile strength and elongation percentage. The test ultimately indicates the maximum load the material can bear before failure.
Yield point is the point where the stress of a force permanently changes the shape of the material. Take a bookshelf as an example. The yield point occurs when the weight of the books causes the shelf to bow permanently, even after the books are removed.
Tensile strength is the force required to cause the material to break or fail. Thinking of our bookshelf example, the tensile strength of the shelf would equal the force of books (book mass multiplied by acceleration of gravity) it would take to snap the shelf in two.
Yield and tensile are typically expressed in pounds per square inch (psi) or thousand pounds per square inch (ksi).
Elongation is simply how much a material can bend or stretch in relation to its original length. Elongation is calculated as the percentage difference between the yield point (remember, this is the point at which the material DOES NOT return to its original shape) and the tensile strength (or the point where the material breaks).
Elongation is often used to indicate ductility, as well. The greater the elongation, the more ductility a material has.
Below are yield, tensile and elongation measurements for two common grades of steel: A36 mild steel and A572-50 high strength low alloy structural steel. The data show that A572-50 can bear a greater load than A36, but A36 is more ductile.
|Strength Point||ASTM A36||ASTM A572-50|
|Yield Point (psi)||36,000||50,000|
|Tensile Strength (psi)||58,000-80,000||65,000|
|Elongation % (200 mm)||18%||20%|
Ultrasonic Testing for Defects
As mentioned above, laminations can occur during the manufacturing of steel. Ultrasonic testing (UT) uses high-frequency soundwaves to detect laminations in the body of steel material. UT is a nondestructive test that is performed on the steel material itself (not a coupon).
In UT, a probe sends a sound wave into the material. When the sound hits a barrier (such as air), it will bounce back towards the probe. The probe's ultrasonic transponder converts those sound waves into electrical energy, which can be read on a test machine screen.
UT is highly accurate, with accuracies between +/-0.025 mm and +/-0.001 mm. Ultrasonic testing gives engineers important information regarding their material, ensuring safe and proper future use.
Charpy V-Notch Test for Toughness
The Charpy V-Notch test -- or Charpy Impact Test -- measures a material's ability to absorb energy, or impact, within a given temperature range. The test gets its name from Georges Charpy, who first standardized impact testing.
Brittle materials can absorb less impact before fracturing, and colder temperatures increase brittleness. For projects that require steel material exposed to cold temperatures, the Charpy test helps engineers choose the correct grade of steel.
The Charpy test pits a small piece of test material against a heavy pendulum hammer. The test piece has a V-shaped notch carved in the middle, which gives the test its name. The pendulum swings into the test piece (often bending, if not snapping the piece), and the absorbed energy is measured.
This test can be conducted and certified at different temperatures, which will be reflected in the mill test report.
Brinell Hardness Test
While strength is the amount of force material can withstand before deforming or failing and toughness is the ability to resist fracturing under force, hardness is the ability to withstand friction and abrasion.
For example, a diamond is very difficult to scratch (high hardness) but is relatively easy to shatter (low toughness).
The Brinell hardness test -- coined after engineer Johan August Brinell who standardized the test -- uses a small steel indenter and applied force to dent a material. The size of the dent is used to calculate the Brinell hardness number (BHN), which is a standard unit of hardness.
Mild A36 steel has a BHN of 133, whereas an abrasion resistant grade of steel has a BHN greater than 330.
Hardness is important for applications that require abrasion resistance instead of strength or toughness. Examples include conveyors, buckets, body armor and grates.
Not every test discussed in this article may be required for every material need. Understanding testing options and the differences in what they measure will help engineers make informed material decisions for their projects.
Leeco Steel can supply a wide range of steel plate material, depending on engineering specifications, and MTRs are included with every order invoice. Browse Leeco Steel's steel plate offering and build your quote today.