Destructive methods for testing casting quality. (Casting Facts).

Casting quality tests can be divided into two major categories: destructive and nondestructive. The August 2002 issue of MODERN CASTING gave an overview of the different methods of nondestructive testing (Nondestructive Methods for Testing Casting Quality, p.4243). This column focuses on the destructive testing method.

Hardness Testing

Hardness testing, one of the most common and widely used destructive testing methods, is performed by forcing a hard ball or diamond indenter into the surface of a casting. The material's ability to resist the indentation is called it's hardness. A casting's hardness is important for wear properties. If a soft material comes into abrasive contact with a harder material, the harder part will wear out the softer one.

The two main types of hardness testing used for metal castings are Brinell hardness and Rockwell hardness. The standard Brinell test uses a 10mm ball under three different load levels: 500kg, 1500kg and 3000kg. The diameter of the indentation made under the chosen load will relate to a Brinell hardness. Different metals are harder or softer than others thus the need for heavier and lighter load scales. The ball indenter provides an average hardness over an area of the surface.

Rockwell hardness testing uses a diamond point indenter, and is most commonly used for steel castings. The diamond tip is pressed into the surface of the casting, and a hardness is read off a dial indicator on the testing apparatus itself.

Although it is typically easier and faster to run a Rockwell test, it is not used for metals other than steel because they contain multiple crystal structures within the sample. The diamond tip reads the hardness of a single structure, which is not representative of the whole casting. Steel is much harder than most metals, and usually has a more uniform crystal structure, therefore the Rockwell test is sufficient.

Newer nondestructive methods of hardness testing do exist, but they are less common.

Tensile Testing

Tensile testing is another common test that determines key mechanical properties of a metal specimen. The test is fairly simple: apply an axial load to a sample, and pull it until it breaks.

An operator can measure several important parameters with a tensile. test. These parameters are used as the strength factors on which engineering design and structural usage of materials is based. A typical tensile sample would be a machined bar, strip or wire of known cross sectional area. The samples are designed such that they break at the thinnest cross sectional area. The sample is "chucked" into the machine jaws, and a constantly increasing load is applied until the sample fails, marking the end of the test.

The testing device usually is linked to a plotter that draws a "stress vs. strain diagram." From the plotted diagram, several strength values can be interpreted such as yield strength, ultimate strength, modulus of elasticity and elastic limit.

An "extensometer" device also can be connected to the test specimen. This tool relays information about the sample's "elasticity," or ability to stretch. The values obtained from this test aid the engineer in determining design criteria for the casting, and enable a casting to be created so that it will operate safely and not fail until it has exceeded it's expected service life.

When using tensile strength testing, it is important to remember that the strength of the casting is not being determined, but merely a representative sample of the material of which castings will be made.

Impact Testing

Impact force is when a load is applied to an object very abruptly, such as a hammer hitting a casting or a pile driver falling on an object to be crushed. If the part does not have sufficient impact resistance, it will shatter or fail immediately.

Many metals, especially the ferrous alloys, have a "brittle transition" point where if the temperature drops below a certain point the part becomes very brittle and looses it's impact resistance. Foundries have used impact testing to determine both brittle transition temperatures and impact resistance properties in metal castings.

The typical test apparatus is referred to as the "Charpy" impact tester. It is a vice that holds a notched specimen while a weighted hammer is allowed to swing and break the specimen. The distance the hammer travels after collision is measured and used to calculate impact resistance.

Less Common Methods

Although the following four other types of mechanical testing are less common, they deserve to be mentioned.

Torsion Testing--This is when a test specimen is subjected to twisting forces, One important criteria that can be measured with this test is the number of turns a sample can handle without failure. Testers also are interested in how much twisting force can be applied to a sample and still have it return to it's original shape after the load is released.

Fatigue Testing--This testing method is used to determine the number of force cycles an object can handle before failure occurs. A load is applied then released repeatedly until the sample breaks. This is done automatically at high speeds and it is not uncommon for a part to handle hundreds of thousands of cycles before failure. It is commonly performed with a twisting force, bending force, heating and cooling cycles, and tension and compression cycles, depending on the application of the part.

Compression Testing--This method is similar to tensile testing, and often is performed on the same apparatus. Instead of a tension load being applied to the sample, compressive forces are applied and distortion is measured. This is a common test for structural members, or parts that will be subjected to very high weight and force applications.

Shear Testing--This test applies opposite forces on the sides of a sample at or near the same point. This causes the test specimen to eventually fail due to shearing action of the test points. A shear force can then be measured and design of the part accepted of modified according to requirements.

For More Information

"Putting Tensile Specimens to the Test," P.H. Mani, F. Chiesa and G. Tackes, MODERN CASTING, September 2001, p.44-45.

Handbook of Meta1 Treatments and Testing, R. Ross, 1977, E. & F. N. Spon Ltd.