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.
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 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.
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.
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.
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.
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 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.
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.
