Crack Detection

Crack detection is accomplished by various methods, such as mechanical proof testing, metallography, and filtered particle or magnetic particle inspection. Promising nondestructive testing methods for P/M applications also include electrical resistivity testing, eddy current, and magnetic bridge testing, magnetic particle inspection, ultrasonic testing, x-ray radiography, gas permeability testing, and 7-ray density determination. The capabilities and limitations of these techniques are summarized in Table 6.

Table 6 Comparison of the applicability of various nondestructive evaluation methods to flaw detection in P/M parts

Method

Measured/detected

Applicability to P/M parts(a)

Advantages

Disadvantages

Green

Sintered

X-ray radiography

Density variations, cracks, inclusions

C

C

Can be automated

Relatively high initial cost; radiation hazard

Computed tomography

Density variations, cracks, inclusions

C

C

Can be automated; pinpoint defect location

Extremely high initial cost; highly trained operator required; radiation hazard

Gamma-ray density determination

Density variations

A

A

High resolution and accuracy; relatively fast

High initial cost; radiation hazard

Ultrasonic imaging: C-scan

Density variations, cracks

D

B

Sensitive to cracks; fast

Coupling agent required

Ultrasonic imaging: SLAM

Density variations, cracks

D

C

Fast; high resolution

High initial cost; coupling agent required

Resonance testing

Overall density, cracks

D

B

Low cost; fast

Does not give information on defect location

Acoustic emission

Cracking during pressing and ejection

C

D

Low cost

Exploratory

Thermal wave imaging

Subsurface cracks, density variations

D

C

No coupling agent required

Flat or convex surfaces only

Electrical resistivity

Subsurface cracks, density variations, degree of sinter

A

A

Low cost, portable, high potential for use on green compacts

Sensitive to edge effects

Eddy current/magnetic bridge

Cracks, overall density, hardness, chemistry

C

A

Low cost, fast, can be automated; used on P/M valve seat inserts

Under development

Magnetic particle inspection

Surface and near-surface cracks

C

A

Simple to operate, low cost

Slow: operator sensitive

Liquid dye penetrant inspection

Surface cracks

C

D

Low cost

Very slow; cracks must intersect surface

Pore pressure rupture/gas permeability

Laminations, ejections, cracks, sintered density variations

A

A

Low cost, simple, fast

Gas-tight fixture required; cracks in green parts must intersect surface

(a) A, has been used in the production of commercial P/M parts; B, under development for use in P/M; C, could be developed for use in P/M, but no published trials yet; D, low probability of successful application in P/M

The problem of forming defects in green parts during compaction and ejection has become more prevalent as parts producers have begun to use higher compaction pressures in an effort to achieve high-density, high-performance powder metallurgy (P/M) steels. Proper press setup for molding P/M parts also is critical to prevent cracking. In a flanged part that experiences a change in diameter, density in the hub and flange should be nearly equal. Unequal density leads to powder displacement from one part level to the next and to the formation of shear cracks. Such cracks often occur at 45° to the pressing direction and at surfaces at the junction (radius) between the hub and flange. At press setup, equal density should be obtained in the hub and flange. A high green strength powder and a press that maintains a small counter pressure on the top of the part during ejection from the tools (top punch hold-down) should be used.

Mechanical Proof Testing. A sampling of sintered parts can be broken to confirm the presence of a suspected cracking problem--for example, pushing flanges off hubs in such a manner that in addition to shear effect there is some stretch effect at the crack. The presence of a few unexplained low readings indicates that an initiating crack is present.

Metallography. Low-powered binocular microscopes can be used to detect cracks at changes in diameter. Metallography is a more time-consuming method. A sampling of parts are sectioned parallel to the pressing direction. When mounted and carefully polished to expose open pores and cracks, the presence of minute cracks is readily apparent (Fig. 8).

Fig. 8 Cracks and unbounded particles at the junction (radius) of a horizontal flange and vertical hub resulting from shear during compaction. Unetched

Liquid-Penetrant Crack Detection. During liquid-penetrant testing, the surface of the part is covered with a colored or fluorescent penetrating fluid that fills all cracks and some pores. Surface liquid is then washed off, leaving the penetrant in the cracks. A layer of developer powder, which acts as a blotter, is applied to the surface. The penetrant leaves the cracks and is absorbed by the developer layer near the crack. Crack indications are visible by color contrast or by fluorescence under ultraviolet light. Most sintered parts have porous surfaces that absorb and then release sufficient penetrant in all regions so that it is impossible to distinguish the crack from the porosity background.

The dye penetrant equipment found in P/M shops is generally used only for checking parts of the tooling and machinery for cracks. The dye does not preferentially reside at cracks in P/M parts, because the pore radius and the crack radius are equivalent. However, there might be an application for green parts because the surfaces of green parts are sealed against penetration by liquids through smearing of the metal powder against the die wall and through the formation of a thin coating of dry powder lubricant on the surface. Cracks intersecting the surface may form an opening in this layer that could be detected by the dye penetrant

Filtered Particle Crack Detection. One proprietary process of filtered particle crack detection (Partek) involves brief immersion of the test piece in a suspension of fluorescent particles. Particles are filtered and collect near the surface of cracks as the fluid enters. Cracks are clearly visible under black light. This one-step method is used to detect cracks in presintered porous tungsten carbide blanks (Ref 22).

To the extent that an unsintered part has open porosity, this method also can be used on green parts. Density cannot be too high, however, and excessive lubricant tends to clog the pores. Successful use of this method on presintered porous tungsten carbide blanks indicates that it may be suitable for sintered P/M parts with open pores into which fluid can enter. Small cracks fluoresce brightly, while large cracks are darker than the surrounding fluorescing surface.

Magnetic Particle Crack Detection. A magnetized part demonstrates abrupt changes in magnetic field when the field crosses a crack. When dry or wet magnetic particles are passed over the part, any magnetic leakage field at the part surface attracts and holds iron particles. These assemblies of particles over cracks are visible with the unaided eye or with black light on fluorescent particles.

This method also detects some near-surface cracks. Unsintered parts, however, are not adequately bonded to support a magnetic flux, and the method is consequently unsatisfactory. Magnetic particle detection methods have been successfully used for many years for inspecting medium-density sintered automotive parts, by both P/M parts producers and automotive manufacturers. It is possible to automate the inspection process by using digital image processing.

Direct Current Resistivity Testing. A voltage field within a conductive solid will create currents that are influenced by structural irregularities, including cracks and porosity. The arrangement shown in Fig. 9 is used to measure the voltage drop in a current field localized between two electrode probes. This method has been used to detect seeded defects in laboratory specimens. It has also been successfully applied to the production of sintered steel parts (Ref 23).

Four Point Probe Experiment

Fig. 9 Four-point probe used in the resistivity test. The outer probe pins are the current leads; the inner pins are the potential leads.

The direct current resistivity test can be used on any conductive material; it is not limited to ferromagnetic materials. Although further development is needed, resistivity measurements appear to be a promising technique for the nondestructive evaluation of both green and sintered P/M parts. In addition to detecting cracks in green parts, as well as part-to-part density variation, studies have shown that changes in resistivity due to poor carbon pickup during sintering were also detectable (Ref 23). Resistivity testing has also been used later in the processing sequence to screen heat-treated parts for incomplete transformation to martensite. Another study has yielded the relative density/conductivity relationship, suggesting that resistivity tests could be used as a rapid check for localized density variations (Ref 24). As with ultrasound, the elastic modulus and the toughness of porous steels can also be distinguished by resistivity checks (Ref 25).

Pore Pressure Rupture Testing of Green Compacts. A novel test is available for detecting ejection cracks in green compacts (Ref 26). A pressure seal is formed around a corner or area of a part where experience has shown that cracks are likely to occur. The area is then pressurized to —3.5 MPa (500 psi) using a fixture such as that shown in Fig. 10. If a crack is present, the gas pressure in the crack will be sufficient to propagate the crack the rest of the way through the part. This would be classed as a proof test rather than a nondestructive test because the part is destroyed if defects are present.

Distance

Flawed compact Unflawed compact

Fig. 10 Pore pressure rupture test for crack detection in green parts. Source: Ref 26

The test can be used in a nondestructive manner on sintered parts. The gas permeability of the pressurized area is measured at reduced pressures, and the presence of cracks or low-density areas is indicated by high permeability, as shown in Fig. 11.

Ishikawa Diagram Crack Detection Example

Flow, mL miri

Fig. 11 Detection of flawed compact using the gas permeability technique. Source: Ref 26

Flow, mL miri

Fig. 11 Detection of flawed compact using the gas permeability technique. Source: Ref 26

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