Metallurgical Factors

A wide variety of metallurgical effects can occur during a finishing process: formation of untempered or overtempered martensite, retained austenite, volume changes due to phase transformation, oxidation, and other effects can become important, depending on the mechanical and thermal aspects of the finishing operation. Reference 9 provides a good introduction to these phenomena in the case of grinding, while Ref 1 reviews various metallurgical alterations in the cases of milling, reaming, drilling, electrodischarge machining, and laser beam machining.

Most of these metallurgical characteristics can be qualitatively assessed through a wide variety of observation techniques, such as metallography, scanning electron microscopy, and chemical etching. New techniques have also been developed for better quantification. For example, the percentage of retained austenite present at the surface of material can be measured by x-ray diffraction on the same diffractometer used for residual stress measurements. Reference 47 describes measurements of both residual stress and percentage of retained austenite on various heat-treated steels ground with cubic boron nitride. Acoustic microscopy has also been used to evaluate the thickness of the metallurgically damaged layer on a ground surface (Ref 48). Table 3 lists various testing techniques and references that provide additional information. Some of these metallurgical factors that may restrict a finishing operation can also be turned to advantage. For example, significant toughening of ceramic materials can be obtained by forcing phase transformations to occur at the surface through a grinding operation. Reference 21 gives an example of a controlled stress-induced phase transformation from the tetragonal to monoclinic phase of ZrO2. The 3 to 5% volume expansion associated with this phase transformation resulted in highly compressive residual stresses with values exceeding 1 GPa at the surface, which strengthened the ceramic.

Table 3 Testing techniques for surface integrity evaluation

Surface integrity factor considered

Evaluation technique

References

Cracks

Acoustic emission

57

Acoustic microscope

48

Eddy current

Etching

9, 46

Magnetic particle

9

Metallography

9

Optical microscope

58

Penetrant

9

Scanning electron microscopy

Transmission electron microscopy

Ultrasonic signal

4

Visual observation

Metallurgy

Magnetic particle

Metallography

1, 9, 58

Scanning electron microscopy with microprobe analysis

4

X-ray diffraction

47, 56

Hardness

Microhardness

58

Superficial hardness

4

Ultrasonic signal

4

Residual stresses

Barkhausen noise analysis

4, 24

Eddy current

Hole drilling

59, 60, 61

Interferometry

62

Layer removal

27, 30, 42

Neutron diffraction

26, 63

Ultrasonic signal

4

X-ray diffraction (XRD)

27, 28, 30, 42

XRD with electropolishing

4

This table, by no means exhaustive, provides references that describe various techniques one can use to qualify or quantify key surface integrity factors and that present practical examples of applications for various finishing methods. Volume 9 Metallography and Microstructures, and Volume 17, Nondestructive Evaluation and Quality Control, of the ASM Handbook also provide specifics on most of these techniques.

Other Surface Integrity Considerations

This table, by no means exhaustive, provides references that describe various techniques one can use to qualify or quantify key surface integrity factors and that present practical examples of applications for various finishing methods. Volume 9 Metallography and Microstructures, and Volume 17, Nondestructive Evaluation and Quality Control, of the ASM Handbook also provide specifics on most of these techniques.

Other Surface Integrity Considerations

In some applications, additional specific properties of the surface layers have to be considered. The chemical, magnetic, optical, or electrical properties of a finished part can sometimes be dramatically affected by a finishing process. For instance:

• Chemical glass strengthening by ion exchange can result in high compressive residual stress when large sodium ions replace smaller lithium ions in a salt bath rich in sodium ions (Ref 49).

• For manganese-zinc and nickel-zinc surfaces, the deformation and residual stress induced during a lapping operation can generate magnetically dead layers detrimental to the performance of a magnetic recording head system (Ref 50).

• Optical properties of infrared materials may be influenced by various processing techniques (Ref 51).

• The surface condition, particularly the presence of cracks due to brittle fracture during the finishing process, can affect the electrical behaviors of insulators (Ref 52).

In other cases (Ref 53), the finish surface might require special consideration due to the risks of hydrogen embrittlement (cracking without corrosion) or liquid metal embrittlement (as in the case of aluminum alloys with mercury). The breakdown of machining fluid, especially sulfurized ones, can result in high levels of hydrogen in ground steel samples (Ref 54). Other examples include a sanding technique that influences the wettability and shear strength of wood (Ref 55) and abrasive jet machining finishing techniques that are used where there is risk of explosion.

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