Nondestructive testing: 5 ways to ensure defect-free deliveries.

As end-users continue to demand higher quality castings, this article provides foundries with five options to their inspection needs.

Nondestructive testing (NDT) of castings provides your operation with quality assurance. It is a step within the production process to affirm your foundry's ability to deliver defect-free castings. Whether or not NDT is required by your end customer, an industry regulation or your own standards, it should be an integral step in your operation's quality control.

The process of NDT is a variety of physical inspection methods that can be 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 following testing.

There are five methods of NDT available to metalcasting - magnetic particle testing (MT), liquid penetrant testing (PT), ultrasonic testing (UT), radio- graphic testing (RT) and eddy current testing (ET). Each method has definite advantages and limitations, but none can provide 100% coverage or inspection of a casting. Because of this, a combination of NDT methods is usually used to achieve the desired inspection parameters.

MAGNETIC PARTICLE TESTING

MT is a NDT method that 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 is present (crack or other type of linear indication), it will disrupt the magnetic flux field that is present from the current flow and will result in a flux leakage. The inspection medium (iron particles) that is applied simultaneously with the current will be attracted to the areas of flux leakage and form a visible indication (the particles will pile up over the area of the discontinuity as shown in [ILLUSTRATION FOR FIGURE 1 OMITTED]).

Another form of MT inspection, electromagnetic yokes, uses equipment that induces the magnetic field without current. This method of magnetization is performed after inspection when there is concern over the surface condition of the casting (a finish machined surface), as the before mentioned method can cause arcing or burning of the surface of the casting because it is part of the actual electrical current flow circuit.

MT Testing Steps

1. Magnetize the casting to be inspected.

2. Apply an inspection medium of fine iron particles while the casting is magnetized.

3. Inspect the casting surface for any flux leakage fields.

4. Clean the casting of any inspection medium residue and demagnetize.

The iron particles that are used can be either dry or suspended in a liquid carrier. The dry method uses a visible or color contrast particle and requires only ambient or white light for inspection. The wet method uses a particle that has been dyed with a fluorescent material that gives off a green/yellow color when exposed to ultraviolet light. The wet method of inspection is more sensitive than the dry method and can detect smaller or finer discontinuities due to the smaller size of the iron particles.

Advantages: This test method is quick and simple in principle and application. It is highly sensitive to the detection of shallow (0-0.003 in.) surface cracks and other linear indications. In addition, the MT indications appear on the actual casting. This method may often work through contaminant layers and thin coating thickness.

Limitations: This test method is applicable to ferrous materials only. It provides a limited potential for the detection of subsurface indications (0.003-0.007 in.). Care is required to avoid burning and arcing of the casting surface at the points of electrical contact. The magnetic field direction must intercept the major dimension of the discontinuity for maximum detection capabilities. This method will require the demagnetization of the casting following the inspection.

LIQUID PENETRANT TESTING

PT 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 penetrant process is the pre-cleaning of 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.

With 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 0.001-0.002 in. of material.

Two types of penetrants - visible and fluorescent - are used. Visible dye penetrants are usually red and only require ambient or white light for use. Fluorescent dye penetrants are green/yellow and require the use of an ultraviolet light. The fluorescent penetrant method is capable of detecting finer discontinuities.

Penetrant materials are further sub- divided by the method used to remove excess surface penetrant from the casting. Water-washable penetrants require a water spray to remove the excess surface penetrant. Solvent-removable penetrants require a solvent/cleaner to remove the excess surface penetrant.

PT Testing Steps

1. Pre-clean and dry the surface of the casting.

2. Apply the penetrant material to the surface area to be inspected and allow it to remain for a predetermined time.

3. Remove the excess surface penetrant. (This is a critical step to the process, as it is possible to overclean the casting and render any discontinuities undetectable.)

4. Allow the casting surface to dry from the penetrant removal process.

5. Apply a developer to the surface to be inspected. (The developer is a white talc-type product that provides a contrasting background for the penetrant material. It also performs a secondary function of blotting or helping to draw the penetrant material back out of the discontinuity.)

6. Inspect the casting and clean to remove all traces of the process materials. These steps are illustrated in Fig. 2.

Advantages: This method is highly sensitive to fine, tight surface discontinuities such as cracks and cold shuts. It also is effective in detecting founded indications, such as porosity or gas. The discontinuity indications are viewed on the casting surface.

Limitations: The discontinuities must be open to the inspection surface. Subsurface discontinuities cannot be found with this method. There are temperature considerations with this method due to the penetrant material that is used. The typical temperature of the casting at the time of inspection should be between 60-125F (16-52C).

ULTRASONIC TESTING

UT is a method that uses high-frequency sound waves to detect surface and subsurface discontinuities in both ferrous and nonferrous castings. UT also can be used to gauge the thickness of a casting. Because UT allows investigation of the cross-sectional area of a casting, it is considered a volumetric inspection method. The frequency of the sound is in the Megahertz (MHz) range and is therefore not audible to the human ear.

Electrical energy from the ultrasonic equipment is transformed to mechanical energy in the form of sound pressure waves through an ultrasonic transducer. In UT, the generated sound pulse initiates at the transducer, travels through the casting and is reflected back by the rear wall of the casting or the discontinuity. This inspection method is similar to the ultrasound that is performed on a developing human fetus or to the application used by fishermen to determine the depth of a body of water or to identify fish.

All of this information is presented to the individual performing the inspection in one of four presentation styles - A-scan, B-scan, C-scan and digital numeric. In an A-scan presentation, the initial sound pulse and the resulting rear wall or discontinuity reflections are displayed on a cathode ray tube (CRT). The inspector must interpret what these signals represent in order to perform the inspection. In a B-scan presentation, the UT equipment displays the material being inspected as a cross-sectional view. This presentation type is beneficial when the inspector wishes to see how the cross section of a casting is configured. In a C-scan presentation, the UT equipment displays the casting in a topographical perspective. This presentation is useful when plotting thickness of material over a given area. With digital numeric presentation, the UT equipment converts the "flight time" of the ultrasonic pulse from the transducer until it is received back at the transducer and displays this "flight time" in a whole or decimal fraction representing material thickness.

UT Testing Steps

1. Calibrate the ultrasonic equipment against a known defect or standard.

2. Connect the ultrasonic equipment to the casting via the transducer. The transducer is coupled to the casting either by direct contact or by a water column.

3. Transmit the sound energy into the casting from the UT equipment via the transducer.

4. Receive the reflected sound energy back to the UT equipment where it is changed into one of the four presentation formats.

5. Interpret the displayed information.

Advantages: Ultrasonic inspection is accurate for material thickness measurements. UT can be performed through coatings on the casting. UT inspection only requires access to one surface of the casting. Ultrasound can be "bounced" or reflected internally inside the casting to achieve 100% inspection coverage for discontinuities.

Limitations: The UT inspection method requires a vast amount of knowledge and experience to properly establish inspection techniques and interpret the results. The surface roughness of a casting and dimensional variations may scatter the sound pulse and make discontinuity detection or thickness measurement difficult. The velocity of the sound pulse is dependent on the temperature of the material so adjustments must be made for high-temperature applications.

Castings do present a problem with ultrasonic inspection if the grain size of the casting is larger than the size of the sound pulse generated by the transducer. This condition causes many reflections of the ultrasound and reduces the penetration of the sound pulse into the casting. This may render the UT inspection inconclusive. To help overcome this problem, ultrasonic transducer manufacturers can design special transducer configurations or increase the penetrating power of the ultrasonic sound wave.

RADIOGRAPHIC TESTING 

RT is a method that uses X-ray or gamma energy to pass ionizing radiation through the casting to reveal internal discontinuities on a film medium. X-rays are mechanically produced ionizing radiation. Gamma rays are the product of a nuclear disintegration from a radioactive isotope, which produces ionizing radiation. The RT inspection method can be used on both ferrous and nonferrous castings.

This inspection method makes use of the ionizing radiation to penetrate the cross-sectional area of a casting and expose a piece of radiographic film. The concept is similar to how a camera produces a photograph by exposing a piece of photographic film to visible light or how an X-ray of a broken bone is performed at the hospital. Because this inspection method examines the cross-sectional area of the 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, less radiation is absorbed by the casting and more radiation reaches the film. This increased film exposure to the radiation ultimately produces an image of the discontinuity on the film. Figure 3 is an X-ray of a gray iron casting with a hot tear.

RT Testing Steps

1. Place the film on one side of the casting and position the radiation source on the opposite side.

2. Place the appropriate quality indicators (penetrameters or image quality indicators) and identification markers on the source side of the casting.

3. Expose the film and chemically process it using a manual or automatic process.

4. Evaluate the processed film with regard to both the quality of the film and the quality of the casting.

Advantages: Internal discontinuities are revealed on the film. The radiograph can be a permanent visual record of the casting quality that can be reviewed by multiple personnel.

Limitations: The orientation of the radiation source, the object and the film may cause distortion in the projected discontinuity images on the film. The inspection process requires access to both sides or surfaces of the casting. The discontinuities in the casting must be parallel to the radiation beam for the best detection probability. The casting thickness and density will limit the range of inspection possible (dependent on the energy level of the radiation).

EDDY CURRENT TESTING

ET is a NDT method that utilizes an induced lower-energy electrical current in a conducing and observes the interaction between the casting and the induced electrical current. The observation is performed with electronic equipment designed to measure the inspection method variables.

In principle, as shown in Fig. 4, an AC current is applied through coil windings that are located in a probe or a 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 that 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.

ET Testing Steps

1. Calibrate the eddy current instrument against a known defect or standard.

2. Exhibit the probe or coil to the casting and monitor the instrument displays for various changes.

3. Interpret and document the display changes.

Advantages: This inspection method 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 give an immediate response on the monitoring equipment. This inspection method can be readily adapted to high-speed automatic scanning equipment.

Limitations: The ET inspection method requires a vast amount of knowledge and experience to properly establish inspection techniques and interpret the results. This inspection method only can be applied to electrically conductive materials. Discontinuity detection is limited to surface and near surface discontinuities. Ferromagnetic materials usually require specialized equipment to offset the effects that magnetic permeability of the casting will have on the inspection results.

RELATED ARTICLE: Ford Develops Alternative Uses for NDT Methods to Inspect the Quality of Gray and Ductile Iron Castings

Following are descriptions of studies performed at Ford Motor Co., Detroit, to develop tests for the hardness certification of gray iron castings and the strength determination of ductile iron castings. Both studies use the NDT methods described in the adjoining article, but not for their designed intention. Instead, Ford has developed alternative uses for the tests. These alternative uses for NDT provide another benefit to foundries to embrace these inspection methods.

ET for Hardness Certification of Gray Iron Castings

In an effort to improve quality and reduce production costs, a study was performed at Ford Motor Co. in 1978 to develop a rapid and reliable NDT for hardness certification of gray iron castings. The accepted method, Brinell hardness testing, is slow, costly and sometimes not applicable in production applications.

A number of NDT methods were investigated, including ultrasonic, but the only method to yield a correlation with Brinell hardness was the eddy current test (ET) method. Since this method is rapid and repeatable and has performed well in testing steel parts in the past, it was applied to gray iron hardness determination.

The result from experimentation was an ET that yielded a good correlation with Brinell hardness of gray cast iron. The initial development of an eddy current NDT test was done on disc brake rotors and the main bearing cap cluster for an engine. The test was performed at a rate of 1200 castings per hr, with the manual handling of castings being the only limiting factor. Plant experience proved the ET to be more reliable than production Brinell hardness testing, providing results for power steering control valve housings, transmission clutch hubs and stator supports.

At the time of the test's development, the only instrumentation available was Hentschel. The best results for as-cast parts were obtained at a test frequency of 25 Hz and using a display of the resistive component.

Following is the five-step practice for establishing the hardness of a gray iron part:

1. Obtain a test coil designed to encircle the portion of the casting specified for Brinell hardness testing and having a primary winding inductance of 5-8 milihenries. Then, maximize the fill factor and build the jig to repeatably position the coil on the part on the area of interest.

2. Obtain a set of samples of the casting of interest covering as large a hardness range as possible and from several cast dates. Several castings outside of the acceptable Brinell hardness range are necessary to establish a correlation because the standard error in the Brinell hardness test is significantly large when compared with the usual hardness acceptance range for most gray iron castings.

3. Record eddy current and Brinell hardness data on the set of castings using a resistive component at a test frequency of 25 Hz. Set the sensitivity of the instrument so that the acceptable parts give readings near the center of scale and the unacceptable parts read outside this range toward the ends of the scale.

4. Select ECT accept-reject levels based on a correlation of the eddy current and Brinell hardness data. The correlation may be made mathematically with the aid of a computer or graphically.

5. If the scrap rate appears too high, the rejected specimens can be manually tested for Brinell hardness to salvage some.

Determining the Strength of Ductile Iron Castings

Quality control methods are needed to ensure the strength of ductile iron crankshafts and other parts. In the past, methods that tested only a few parts or even small samples of metal were adequate because the molten iron was treated in large batches to produce many ductile iron castings. Generally, if the last metal poured from a ladle exhibited adequate nodularity, then the entire batch of parts poured from the ladle were good.

However, with the in-mold treatment process, the treatment is provided to each mold individually. Potential failures are no longer related exclusively to large batches but also to individual parts. Therefore, each part must be inspected for nodularity - a strong correlation to strength. Automatic electronic inspection methods must be provided because metallographic examination is not practical.

Ford Motor Co. developed a prototype system in 1983 using a commercial electronic instrument and customized controls, as shown in Fig. A, to apply sonic resonance (part of the ultrasonic testing method) to the inspection of ductile iron V8 crankshafts in a foundry environment. It measures the frequency of one resonance mode to determine the quality of the iron in the part. The quality is related to the frequency through the modulus of elasticity, with the frequency being proportional to the square root of the modulus. The nodularity determines the modulus, everything else being equal. The presence of either compacted graphite or flake graphite in the part lowers the frequencies of its modes of resonance.

It was demonstrated in testing as well as in the plant that, provided the reject limits of the inspection system are set close to the frequency of a 100% ductile crankshaft, the sonic resonance inspection system can detect the presence of gray iron volumes within otherwise ductile iron crankshafts.

In the first four months of operation, the system successfully inspected over 100,000 crankshafts, well beyond the originally intended plant trial quantity of 5000, Forty rejected ductile iron crankshafts, when sectioned, showed gray iron areas similar to those in Fig. B. This metallographic examination confirmed that rejected parts exhibited rejectable metallurgical properties.

When applied to ductile crankshafts which were poured from a large batch of molten iron treated in a ladle, the system indicated low nodularity by measuring low frequency. Fifty rejected crankshafts, when sectioned, showed nodularity below the lower specification limit.

- Emmanuel P. Papadakis, Quality Systems Concepts, Inc., New Holland, Pennsylvania