7 minute read

Steel

Quality Control

To specify the various physical and mechanical properties of the finished product, various tests, both destructive and nondestructive, are performed. Metallurgical, hardness, hardenability, tension, ductility, compression, fatigue, impact, wear, corrosion, creep, machinability, radiography, magnetic particle, ultrasonic, and eddy current are some of the major tests that are performed by quality control personnel.

Metallurgical testing is used to determine the quality of steel by analyzing the microstructure of a sample under a microscope. A cross section of a sample is first highly polished and then examined at a magnification from 100-500 diameters. The microstructure of steel consists of grains of different compositions and sizes. Generally, a sample of steel with fine grains is tougher than one with large grains. Different characteristics are produced through alloying the steel with other substances. It is possible to determine grain size and the size, shape, and distribution of various phases and inclusions (nonmetallic material) which have a great effect on the mechanical properties of the metal. Some grains are made of ferrite, or pure metallic iron; graphite, a crystal form of carbon; pearlite, an alloy of iron of carbon; cementite, also called iron carbide, a hard compound of iron and carbon and other carbide-forming elements; austenite, a solid solution of carbon in gamma iron, a nonmagnetic form of iron; and martensite, an extremely hard constituent of steel produced by heat-treating. The sample can also be etched to make visible many structural characteristics of the metal or alloy by a preferential attack on the different constituents. The microstructure will reveal the mechanical and thermal treatment of the metal, and it may be possible to predict its expected behavior under a given set of conditions.

Hardness is not a fundamental property of a material, but is related to its elastic and plastic properties. The hardness value obtained in a particular test serves only as a comparison between materials or treatments. The test procedure and sample preparation are fairly simple and the results may be used in estimating other mechanical properties. Rockwell and Brinell are two popular hardness tests that are widely used for inspection and control. These tests are usually performed by impressing into the test specimen, which is resting on a rigid platform, an indenter of fixed and known geometry, under a known static load.

Hardenability is a property that determines the depth and distribution of hardness induced by quenching. The standardized test used is called the end-quench hardenability test, also known as the Jominy test. A 1-in (2.54 cm) round 4-in (10 cm) long sample is heated uniformly to the austenitizing temperature (this temperature depends on the material composition, ranging from 1,500–1,900°F [816–1,038°C]). The sample is removed from the furnace and placed on a fixture where a jet of water contacts the bottom face of the sample. After ten minutes on the fixture, the sample is removed and two flat parallel surfaces are ground on the sample. Rockwell hardness readings are taken along the ground surfaces at certain intervals from the quenched end. The results are expressed as a curve of hardness values versus distance from the quenched end. Plain carbon steels tend to be hard on the surface, near the quenched end, but remain relatively soft at the core, or further away from the quenched end. Alloyed steels, in general, have an increased depth of hardenability which is one of the main advantages of using alloyed steels.

Next to the hardness test, the tensile test is the most frequently performed test to determine certain mechanical properties. A specifically prepared tensile sample is placed in the heads of a testing machine and an axial load is placed on the sample through a hydraulic loading system. The tensile test is used to determine several important material properties such as yield strength, where the material starts to exhibit plastic or permanent deformation, and the ultimate tensile or breaking strength.

Ductility of a material is indicated by the amount of deformation that is possible until fracture and can be determined by measuring elongation and reduction in area of a tensile sample that has been tested to failure.

Compression tests are performed on small cylinders, blocks, or strips to determine the ability of a material to undergo large plastic deformations (a mechanical property also known as malleability) and its limits. Stress-strain relations determined from this testing are used to predict the pressures and forces arising in industrial forming operations such as rolling, forging, or extrusion. Samples are placed between anvils or pressure plates and are compressed (friction is also a factor to consider as the material slides sidewise over the anvils).

The fatigue test is used to determine the behavior of materials when subjected to repeated or fluctuating loads. It is used to simulate stress conditions developed in materials under service conditions. The fatigue potential, or endurance limit, is determined by counting the number of cycles of stress, applied first in one direction and then another, to which the metal can be subjected before it breaks. Fatigue tests can be used to study the material behavior under various types and ranges of fluctuating loads and also the effect of corrosion, surface conditions, temperature, size, and stress concentrations.

Impact tests are used to determine the behavior of materials when subjected to high rates of loading, usually in bending, tension, or torsion. The quantity measured is the energy absorbed in breaking the specimen in one blow, two such tests are called the Charpy and the Izod, which use notched bar specimens. A swinging pendulum of fixed weight raised to a standard height is used to strike the specimen. Some of the energy of the pendulum is used to rupture the specimen so that the pendulum rises to a lower height than the standard height. The weight of the pendulum times the difference in heights indicates the energy absorbed by the specimen, usually measured in foot-pounds.

Wear resistance is represented by few standardized tests because of its complex nature. One test is the "pin on disk" method, where a pin is moved against a disk of the test material. Usually, wear testing is application specific and the equipment is designed to simulate actual service conditions.

Corrosion involves the destruction of a material by chemical, electrochemical, or metallurgical interaction between the environment and the material. Various types of environmental exposure testing is done to simulate actual use conditions, such as salt bath immersion testing. Zinc coating, or galvanizing, is commonly applied to sheet and structural steel used for outdoor applications to protect against corrosion.

Creep tests are used to determine the continuing change in the deformation of a material at elevated temperatures when stressed below the yield strength. This is important in the design of parts exposed to elevated temperatures. Creep may be defined as a continuing slow plastic flow under constant load conditions. A creep test is a tension test run at a constant load and temperature. The percent elongation of the sample is measured over time.

Machinability is the ease with which a metal may be machined. Many factors are considered in arriving at machinability ratings. Some of the more important factors are the rate of metal removal, quality of the finished surface, and tool life. Machinability ratings are expressed as a percentage, in comparison with AISI 1112 steel, which is rated at 100%. Metals which are more difficult to machine have a rating of less than 100% while metals which machine easily have a rating more than 100%.

Radiography of metals involves the use of x rays or gamma rays. The short-wavelength electromagnetic rays are capable of going through large thickness of metal and are typically used to nondestructively test castings and welded joints for shrinkage voids and porosity.

Magnetic particle inspection (also called "Magnaflux") is a method of detecting cracks, tears, seams, inclusions, and similar discontinuities in iron and steel. This method will detect surface defects too fine to be seen by the naked eye and will also detect discontinuities just below the surface. The sample is magnetized and then covered with a fine iron powder. The presence of an imperfection is indicated by a pattern that assumes the approximate shape of the defect.

Ultrasonic testing utilizes sound waves above the audible range with a frequency of 1-5 million Hz (cycles per second). Ultrasonics allow for fast, reliable, nondestructive testing which employs electronically produced high-frequency sound waves to penetrate metals and other materials at speeds of several thousand feet per second. If there is a flaw in the path of the ultrasonic wave, part of the energy will be reflected and the signal received by a receiving transducer will be reduced. Ultrasonic inspection is used to detect and locate such defects as shrinkage voids, internal cracks, porosity, and large nonmetallic inclusions.

Eddy current inspection is used to inspect electrically conducting materials for defects and variations in composition. Eddy current testing involves placing a varying magnetic field (which is produced by connecting alternating current to a coil) near an electrically conducting sample. Eddy currents are induced in the sample which then produces a magnetic field of its own. A detection unit measures this new magnetic field and converts the signal into a voltage which can be read on a meter for comparison. Properties such as hardness, alloy composition, chemical purity, and heat treat condition influence the magnetic field and may be measured through the use of eddy current testing.


Additional topics

Science EncyclopediaScience & Philosophy: Spectroscopy to Stoma (pl. stomata)Steel - Raw Materials, Manufacturing Processes, Quality Control, Byproducts/waste, The Future