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Methods for Testing the Casting Quality
Inspection and testing of castings encompasses five main categories: casting finishing, dimensional accuracy, mechanical properties, chemical composition and casting soundness.
Casting Finish
The surface finish of a metal casting can be influenced by the type of pattern or molding sand, mold coating, and method of cleaning. So far, instrumentation for measuring surface roughness has not provided a useful evaluation, so it is performed largely through simple visual comparison using a series of test panels with increasing surface roughness.
Dimensional Accuracy
Variation in the dimensions of a casting can be the result of mold cavity expansion caused by the heat and head pressure of molten metal, the contraction of the metal as it cools and heat treatment. These expansions and contractions are predicted by the patternmaker who will compensate for the variations in the pattern’s design. For large volumes of castings, casting facilities may measure the critical dimensions of the castings more often to check for possible drift, particularly drift due to pattern wear. If a casting requires tight tolerances that are critical to the part’s application, those tolerances should be specified by the customer.
Casting customers should check how their casting supplier will verify the dimensions of the parts they produce. The accuracy of the measuring tools is just as important as the dimensional accuracy of the castings. In many instances, the gauges or fixtures needed to routinely check the dimension are supplied to the casting facility by the customer.
Expecting exact dimensions over the course of a production run will result in frustration. The dimensions of each casting will vary slightly, so castings are specified by setting a range of values that the dimensions can fall within. The range between the lower tolerance limit and upper tolerance limit can be set by the supplier, but the narrower the range, the more difficult to produce and test and therefore more costly the casting will be.
Mechanical Properties
Mechanical testing gives an evaluation of the metal and the casting to determine whether the properties are in compliance with the specified mechanical requirements. Following are common mechanical tests used in metalcasting facilities.
Hardness testing—the most commonly used procedure for mechanical property testing, it provides a numerical value and is nondestructive. Hardness values generally relate to an alloy’s machinability and wear resistance. The Brinell hardness test uses a 10-mm diameter carbide ball to indent a 3,000-kg load. The impressions are large enough to provide a dependable average hardness. Rockwell hardness tests make smaller indented impressions, which also can be satisfactory if the median of several values is used.
Tensile and impact testing—conducted on test specimens of standardized dimensions, the two most commont types are tensile and Charpy impact. 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 is used to gauge ductility and strength.
Service load testing—usually conducted on the entire casting to evaluate its properties, it can be conducted in a number of ways. Castings that must carry a structural load can have a load applied in a fixture while the deflection and the load is measured. Pressure-containing parts can be hydraulically tested to a proof load or destruction. Rotating parts can be spin tested. These types of tests check the soundness of the casting, as well as its properties.
Chemical Composition
The chemical composition of an alloy has a significant bearing on its performance properties. Chemical composition can be further affected by minor alloying elements added to the material. Casting alloys are typically specified accorded to ASTM, SAE and AMS alloy specifications. Depending on how susceptible an alloy is to variation of its chemical composition, chemical analysis may be required to verify the proper composition is present to achieve a certain set of properties.
Chemical analysis often involves a sample of molten metal poured in to a special mold and evaluated by spectrographic atomic absorption or x-ray fluorescence analysis. Many metalcasting facilities check the chemical composition of the alloys they are pouring throughout the course of a day, so melt shop personnel can make required adjustments to the alloy composition as needed.
Casting Soundness
The performance of metal components can be notably affected by internal and surface defects that can not be detected through the regular course of visual inspection. Several nondestructive methods can be employed to inspect castings for these “invisible” flaws. Nondestructive tests determine the integrity of a casting without causing physical damage, so once it passes the tests, it can be used for its intended application. Below is a detailed list of nondestructive tests.
NON-DESTRUCTIVE TESTING METHODS
Non-destructive testing gives the metalcasting facility the capability of assuring the quality of a casting without destroying it. A metalcasting facility may have internal standards regarding nondestructive testing, but it is up to the customer to specify specific tests or frequency of testing. While various methods of nondestructive testing exist to measure mechanical properties, chemical composition, casting soundness or maximum service loads, a single test that encompasses all these factors does not exist. A combination of nondestructive methods may be required to document the soundness and quality of a casting. The most common methods available are described below.
Visual Inspection
Visual inspection is based on the use of the human eye to identify surface defects, improper filling and molding errors. Casting defects that can be detected via visual inspection include sand holes, excessively rough surface, surface shrinkage, blowholes, misruns, cold shuts, and surface dross or slag.
Dimensional Inspection
To ensure a part meets dimensional requirements, such as tolerances, a metalcasting facility can check the dimensional accuracy of a part manually or with a coordinate measuring machine (CMM). Checking the dimensional accuracy of a part helps guarantee the customer won’t have to perform further costly machining on a part to meet the specified dimensions.
CMM has improved the speed and accuracy of measuring casting dimensions, and computerization has made it repetitive and able to be used as a statistical tool.
Dye Penetrant and Fluorescent Powder Testing
For tiny cracks, pores or other surface glitches that are hard to detect by the human eye, dye penetrant testing is used for both ferrous and nonferrous materials. In this method, a colored dye solution is applied to the surface of the casting. The dye, which is suspended in penetrating oil, will find its way into the surface defects. When a special developer is applied, the defects are clearly indicated.
A similar method involves fluorescent powder suspended in penetrating oil. Again, the solution penetrates the defects, so when the casting is dusted or sprayed drying powder, the solution is drawn from the defect and glows under an ultraviolet light where defects have occurred. Fluorescent powder testing only detects surface cracks and flaws but is more effective and economical than radiographic testing.
In general, dye-penetrant techniques identify defects on the surface of the casting and do not detect internal porosity or shrinkage that is not open to the surface. But it can detect rounded indications for porosity or gas on the casting surface.
Magnetic Particle Inspection
Magnetic particle inspection is quick, inexpensive and sensitive to defects, particularly shallow (0.003 in.) surface cracks and other lineal indications.
It detects small cracks on or near the surface of ferrous alloys that can be magnetized (basically any ferrous alloy except austenitic material). A high-amperage, low-voltage current is passed through the casting, which establishes a magnetic field.
Cracks and defects have magnetic properties different than those of the surrounding material, so their presence will interrupt the magnetic field, causing distortion. Small magnetic particles show the path of the flux line that spreads out in order to detour around the distortion, thereby indicating the shape and position of the crack or void.
Ultrasonic Testing
Internal defects that are detected by radiography may also be detected by sound. Sound waves have been used by fisherman to locate hot fishing spots and depth of water and by the U.S. Navy to identify approaching objects. In casting inspection, ultrasonic testing uses high frequency acoustic energy that is transmitted into a casting. Because ultrasonic testing allows investigation of the cross-sectional area of a casting, it is considered to be a volumetric inspection method.
The high frequency acoustic energy travels through the casting until it hits the opposite surface or an interface or defect. The interface or defect reflects portions of the energy, which are collected in a receiving unit and displayed for the analyst to view. The pattern of the energy deflection can indicate the location and size of an internal defect, as well as wall thickness and the nodule count of ductile iron.
Ultrasonic testing requires a high does of knowledge and experience for an accurate interpretation of the results, which will affect the cost added to the part for the inspection.
Radiographic Inspection
Another method used to detect internal defects is radiographic inspection. When done correctly, radiographic inspection is the best nondestructive method for detecting internal defects, such as shrinkage and inclusions.
In this method, a casting is exposed to radiation from an x-ray tube. The casting absorbs part of the radiation, and the remaining portion of the radiation exposes the radiographic film. Dense material withstands the radiation penetration, so the film is exposed to a lesser degree in those areas, giving the film a lighter appearance. Less dense materials allow more penetration and correlates to darker areas on the film. Any hole, crack or inclusion that is less dense than the casting alloy is revealed as a dark area.
When done correctly, radiographic inspection is the best nondestructive method for detecting internal defects, such as shrinkage and inclusions, and the radiograph serves as a permanent record of the casting quality that can be reviewed by multiple personnel. Casting thickness and density will limit the rang of inspection possible, depending on the energy level of the radiation.
Radiographic inspection also can be performed without film. Instead, the x-ray image is viewed on a video screen. Computerized axial tomography (CAT scanning) also is being used to develop 3-D computer imagery to inspect a casting’s soundness.
Eddy Current Inspection
The eddy current inspection method is applied to the detection of cracks at or near the surface. An electrically charged coil carrying an alternate current causes an eddy current to flow in any nearby metal. The eddy current may react on the coil to produce substantial changes in its reactivity and resistance, and that reaction is used to pinpoint small cracks or defects.
Eddy current inspection is accurate for the detection of small flaws or material changes that may not be detected with other inspection methods, and the discontinuities in the casting will give an immediate response on the monitoring equipment. However, it requires a vast amount of knowledge and experience to properly interpret the results, which will affect the added cost to the part. The test only can be used with electrically conductive materials.
Pressure Leak Testing
When a casting is specified to be pressure tight or leak-proof, it is often tested by sealing openings in the casting and pressurizing it with air, inert gas or water.
When water, or hydrostatic, pressure is used, water seeping through the casting wall indicates leaks. If air or gas pressure is used, the pressurized casting is put into a tank of clear water. The appearance of bubbles indicates the air has penetrated through the casting wall and a leak is present.
