Casting Inspection Methods 101
True success in producing a quality casting depends on control of the entire casting operation. Quality is the responsibility of the metalcasting facility, starting with the raw materials that are used, but the customer and supplier must agree on what the critical part and process variables are. This leads to the inspection and testing needed.
The inspection and testing of castings can be grouped into five categories:
- Casting finish.
- Dimensional accuracy.
- Mechanical testing.
- Chemical composition.
- Casting soundness.
This article focuses on two categories: Mechanical testing and chemical composition.
Hardness testing is the most commonly used procedure for mechanical property evaluation. It quickly provides a numerical value and is usually nondestructive.
Hardness numbers are closely related to several key properties of metal alloys, such as machinability and wear resistance. For a known grade of alloy, hardness is a useful indicator of tensile properties.
A common hardness test for castings is the Brinell hardness test, using a 10-mm diameter carbide ball indenter and a 3,000-kg load. The Brinell impression is large enough in area to provide a dependable average hardness. Rockwell hardness tests, which make smaller indenter impressions, may also be satisfactory if the median of several values is used.
Tensile and impact testing is conducted on test specimens of standardized dimensions. These specimens may be cut from special test coupons produced as part of a casting or from cut specimens taken from selected castings set aside for destructive mechanical testing.
Tensile and Charpy impact are the two most common types of mechanical testing. Tensile testing provides ultimate tensile strength, yield strength, elongation and reduction of area data. Charpy impact testing determines the amount of energy absorbed during fracture and involves both ductility and strength.
Service load testing can be conducted on an entire casting to evaluate its properties. For castings that must carry a structural load, a test load can be applied in a fixture while measuring the deflection. Pressure-containing parts can be hydraulically tested to a proof load or to destruction. Rotating parts, such as clutch plates, can be spin tested.
Service-type tests on components check the soundness of the casting, as well as its properties and its appropriateness for the design.
The performance properties of an alloy are determined to a significant degree by the chemical composition of the alloy, with minor alloying elements often having a significant effect. This has lead to the practice of specifying casting alloys by using ASTM, Society of Automotive Engineers (SAE) and AMS alloy specifications.
These alloy specifications provide a commonly accepted chemical composition for a wide range of different alloys. Depending on the sensitivity of a given alloy’s properties to variations in chemical composition, chemical analysis of the alloy may be called for in a given specification. The chemical analysis is commonly done on a sample of molten metal poured into a special mold and then evaluated by spectrographic atomic absorption or x-ray florescence analysis.
The performance of metal components can be markedly affected by the presence of both internal and surface flaws, which may not be readily apparent in a simple visual inspection. Nondestructive testing (NDT) consists of a variety of physical inspection methods that are used to determine the integrity of a casting without causing physical damage to it. This differs from the destructive testing methods that render the casting useless.
No matter whether the customer, an industrial regulation or the metalcasting facility’s internal standards require NDT, the process provides the metalcasting facility and the buyer with a measure of quality assurance.
Five NDT methods commonly are used in the metalcasting industry: magnetic particle testing, liquid penetrant testing, ultrasonic testing, radiographic testing and eddy current testing.
Magnetic particle testing detects linear surface and near surface discontinuities in ferromagnetic materials using the principles of magnetization. Typically, a high-amperage, low-voltage current is passed through the casting, which in turn establishes a magnetic field. If a discontinuity (crack or other type of linear indication) is present, it will disrupt the magnetic field and result in a flux.
Electromagnetic yokes, another form of magnetic particle inspection equipment, induce the magnetic field without a current. This method of magnetization is performed after inspection when there is concern about the surface condition of the casting (a finish machined surface). Traditional magnetic testing can cause arcing or burning of the casting surface because it is part of the actual electrical current flow circuit.
Magnetic particle testing dictates the following four steps:
- Magnetize the casting to be inspected.
- Apply an inspection medium of fine iron particles while the casting is magnetized.
- Inspect the casting surface for any flux leakage fields.
- Clean the casting of any inspection residue and demagnetize.
Magnetic particle testing has the advantage of being quick and simple in principle and application. It is highly sensitive to the detection of shallow (0.003 in.) surface cracks and other linear indications. In addition, the indications appear on the actual casting. This method may sometimes work through contaminant layers and thin coating thickness.
Magnetic particle testing has the limitation of being applicable only to ferrous materials, and it provides limited potential for the detection of subsurface indications (0.0034–007 in.).
Liquid penetrant testing can detect surface discontinuities in both ferrous and nonferrous castings. This method uses the principle of capillary action—the ability of liquids to travel to or be drawn into surface openings. The most critical step in this process is pre-cleaning the casting. Because the penetrant physically enters the discontinuity, the opening of the discontinuity must be free of any material that could inhibit the penetrant’s movement. Grease, oil, sand, welding slag or painted/anodized surfaces can inhibit the penetrant material from entering the discontinuity.
In nonferrous castings, another concern is any process that could smear the casting surface and close the discontinuity opening. Nonferrous castings that have undergone a machining process prior to penetrant inspection are usually pre-cleaned by an acid etch process that chemically removes 0.001 to 0.002 in. of material.
Two types of penetrant—visible and fluorescent—can be used. Visible dye penetrants are usually red and require only ambient light for inspection. Fluorescent dye penetrants are green/yellow and require the use of an ultraviolet light. The fluorescent penetrant method is capable of detecting finer discontinuities.
Ultrasonic testing is a method that uses high-frequency sound waves to detect surface and subsurface discontinuities in both ferrous and nonferrous castings. It also can be used to gauge the thickness of a casting. Because ultrasonic testing allows investigation of the cross-sectional area of a casting, it is considered a volumetric inspection method. The frequency of the sound in this method is not audible to the human ear.
In the ultrasonic testing method, an ultrasonic transducer transforms electrical energy into mechanical energy in the form of sound pressure waves. The generated sound pulse initiates at the transducer, travels through the casting and is reflected by both the back wall of the casting and any internal discontinuities that may be present. The transducer senses the reflected sound wave and converts it into an electrical signal. The transit time, amplitude and shape of the sound wave are monitored and measured. This inspection method is similar to the ultrasound used by fishermen to determine the depth of a body of water or to locate fish.
Radiographic testing is a method that uses x-ray or gamma energy to pass ionizing radiation through a casting to reveal internal discontinuities on a film medium. X-rays are electronically-produced ionizing radiation. Gamma rays are the product of a nuclear disintegration from a radioactive isotope that produces ionizing radiation. The radiographic testing inspection method can be used on both ferrous and nonferrous castings.
This inspection method makes use of ionizing radiation to penetrate the cross-sectional area of a casting and expose a piece of radiographic film. The concept is similar to how an x-ray of a broken bone is performed at the hospital. Because this inspection method examines the cross-sectional area of a casting, it is also known as a volumetric inspection method.
When discontinuities such as cracks, gas, shrinkage or unfused chills or chaplets are present in a casting, the casting absorbs less radiation and more radiation reaches the film. This increased film exposure to the radiation ultimately produces an image of the discontinuity on the film.
Some of the new techniques developed for medical use have been applied to the inspection of castings. An electronic image enhancement system provides a real-time image on a cathode ray tube; as a casting is passed through the X-ray beam, the image can be viewed for possible soundness. The changing angle of the view also provides the observer with a three-dimensional perspective into the casting to assist in differentiating between surface and centerline discontinuities. This system is applicable to routine inspection but can only be effectively used at present on castings of not more than one inch in metal thickness.
Heavier sectioned castings, and any castings that require an inspection record, must be radiographed on film.
Eddy current testing is an NDT method that utilizes an induced low energy electrical current in a conductive ferrous or nonferrous casting and observes the interaction between the casting and the current. The observation is performed with electronic equipment designed to measure the inspection method variables.
In principle, an AC current is applied through coil windings that are located in a probe or coil housing. The alternating current creates an expanding and collapsing magnetic field in a longitudinal direction across the coil windings. The magnetic lines of force created extend into the casting, which in turn induces the flow of eddy currents (low energy electrical currents). The induced eddy currents generate their own magnetic field, which interacts with the test coil magnetic field. When a discontinuity is present, it alters the characteristics of the eddy current magnetic field, which then alters the interaction between the two magnetic fields. This altered interaction is displayed on the eddy current instrument display.
Eddy current testing is accurate for the detection of small flaws or material changes that may not be detected with other inspection methods. The discontinuities in the casting will yield an immediate response on the monitoring equipment. This inspection method can be readily adapted to high-speed automatic scanning equipment.
However, the eddy current inspection method requires a vast amount of knowledge and experience to properly establish inspection techniques and interpret the results.
Each method has advantages and limitations, but none can provide complete assessment of mechanical properties, chemical composition, casting soundness or proof tests for maximum service loads. Therefore, a combination of NDT methods may be required to document the soundness and quality of a casting.
Click here to see this story as it appears in the March/April 2019 issue of MCDP.