Steel is a vital raw material in several manufacturing industries. Steel quality is crucial as it directly impacts the strength and durability of the final product. The chemical composition of steel provides valuable information about its properties, but testing is necessary to validate its strength, hardness, and quality.
However, before looking into the types of tests, it is essential to understand some key terms:-
1.Coupon: A coupon is a sample material taken from a master plate or produced through the same forging process, ensuring that it represents the master plate and is used for testing purposes.
There are two types of coupons: -Longitudinal coupons (LCVN) and transverse coupons (TCVN). An LCVN coupon is taken in the direction of the rolling direction of the plate, while a TCVN coupon is taken transverse to the rolling direction. LCVN coupons generally have higher yield and tensile strength than TCVN coupons, as they follow the steel grain. Structural engineers often consider LCVN and TCVN coupon test results while choosing materials.
2. Lamination: A lamination is a flaw or defect in a steel or metal alloy material that can weaken its structural performance. Various factors, including folds, layers, foreign materials, and trapped gases during the rolling process, can cause it. Lamination occurs when the defect or foreign material in the ingot spreads as the material is rolled into the desired size and thickness. Some steel materials have an acceptable lamination threshold, where some laminations are permitted until the risk of material failure becomes too high.
3.Mill Test Reports: A mill test report (MTR) is a document that shows the origin and composition of the material, as well as the results of its testing. It serves as a quality assurance document that provides engineers with the exact composition and performance of the material. MTRs are often included with every steel material purchase and can always be requested if not provided.
With the advancement of technology, the methods of testing steel quality have also evolved to provide more accurate and reliable results. Here are some of the commonly used modern methods of testing steel quality:
1.Spectrometer Analysis - A spectrometer is an instrument that uses light to analyse the chemical composition of a material. Steel samples are subjected to a beam of light, and the absorbed or emitted light is analyzed to determine the chemical composition. This method is quick, precise, and non-destructive, making it a popular choice for testing steel quality.
2. Ultrasonic Testing – Laminations can occur during the production of steel. To detect these imperfections, ultrasonic testing (UT) is utilised. UT utilises high-frequency soundwaves to identify laminations within the steel material. It is a non-destructive test performed directly on the steel material, not on a coupon.
During UT, a probe emits a soundwave into the material. When the soundwave encounters an obstacle, such as air, it bounces back to the probe. The probe's ultrasonic transducer converts the soundwaves into electrical energy, which is then displayed on a testing machine screen.
3. The Charpy V-Notch test, also known as the Charpy Impact Test, evaluates a material's ability to withstand impact energy within a specified temperature range. The test is named after Georges Charpy, who established the standard for impact testing. Brittle materials can withstand less impact before breaking, and low temperatures increase brittleness. For projects involving steel materials exposed to cold temperatures, the Charpy test helps engineers determine the appropriate steel grade. The Charpy test involves a small sample of the material being struck by a heavy pendulum hammer. The sample has a V-shaped notch in the center, giving the test its name. The pendulum strikes the sample, which can bend or break, and the amount of energy absorbed is measured.
4. Magnetic Particle Testing - This method uses magnetic fields to detect surface and slightly subsurface defects in steel. A magnetic field is applied to the steel, and iron oxide-coated magnetic particles are introduced to the surface. The magnetic particles are attracted to any areas of discontinuity, such as cracks, and form an easily visible indication on the surface.
5. Hardness Testing - Hardness measures a material's resistance to permanent deformation. There are several methods of hardness testing, including Rockwell, Brinell, and Vickers, each of which uses a different type of indenter and load. Hardness testing is used to determine the strength and durability of steel and is often used in conjunction with other testing methods to provide a complete picture of the material's quality.
6. Tensile Testing - Tensile testing is a method of measuring the force required to pull a material apart. This test is used to determine the strength and ductility of steel. A steel sample is subjected to an increasing load until it breaks, and the force required to break the sample is recorded. The results of this test are used to determine the material's ultimate tensile strength, yield strength, and elongation.
Methods of testing steel quality have advanced significantly, providing more accurate and reliable results. These methods ensure that steel products meet the required standards and provide the desired strength and durability. By utilising these methods, industries can ensure that the steel they use is of the highest quality, which in turn helps to reduce the risk of product failure and improve overall safety.
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