Visual Factors

Discoloration/Burn. In many manufacturing operations, surface damage can be detected with simple visual observations. See, for example, the 1018 steel cutoff sample shown in Fig. 4. Grinding burns are simply a discoloration due to the oxidation at high temperature. However, the absence of discoloration does not imply that the surface was not heated excessively. Work can be badly overheated in grinding without any trace of discoloration remaining on the finished surface if the conditions are such that the wheel cleans off the discoloration almost as soon as it appears. In cross-feed grinding, if the leading edge of the wheel takes a heavy cut so that the work is heated severely to a depth of several thousandths of an inch, the trailing portion subsequently may remove a layer a fraction of a thousandth inch deep, thus eliminating all evidence of the prior discoloration and leaving practically all of the heat-affected layer of metal (Ref 7).

Fig. 4 Measurement and analysis of surface damage using x-ray diffraction. (a) 38 mm (1.5 in.) 1018 steel cutoff sample showing burn-related discoloration. Two residual stress measurements by x-ray diffraction were made on the sample at the locations marked by the four concentric markers. (b) Typical d versus sin 2 curve (where d is the interatomic distance, which varies according to stress level, and is the angle of the incoming x-ray beam relative to the surface) obtained from an x-ray diffractometer. Seven different orientations (from -45° to +45°) were used. The d-spacing can be looked at as a natural strain gage in a material: a compressive stress tends to diminish the d-spacing value, whereas a tensile stress increases it by bringing two interatomic spaces farther apart. The x-ray diffractometer allows one to measure this value by providing a measurement for d from Bragg's law (2d sin ©= nk, where © is the angular distance between the incident and diffracted beams, n is the order of reflection, and k is the wavelength of the x-rays). The slope of the curve, obtained by

Fig. 4 Measurement and analysis of surface damage using x-ray diffraction. (a) 38 mm (1.5 in.) 1018 steel cutoff sample showing burn-related discoloration. Two residual stress measurements by x-ray diffraction were made on the sample at the locations marked by the four concentric markers. (b) Typical d versus sin 2 curve (where d is the interatomic distance, which varies according to stress level, and is the angle of the incoming x-ray beam relative to the surface) obtained from an x-ray diffractometer. Seven different orientations (from -45° to +45°) were used. The d-spacing can be looked at as a natural strain gage in a material: a compressive stress tends to diminish the d-spacing value, whereas a tensile stress increases it by bringing two interatomic spaces farther apart. The x-ray diffractometer allows one to measure this value by providing a measurement for d from Bragg's law (2d sin ©= nk, where © is the angular distance between the incident and diffracted beams, n is the order of reflection, and k is the wavelength of the x-rays). The slope of the curve, obtained by linear regression, is directly proportional to the residual stress. (c) Two analyses of the curve shown in (b). The first is a simple linear regression that relates the lattice d-spacing to the sin2^ at various angles, with the result an = 110 MPa (16 ksi); it is assumed that ct13 = 0. The second model is a multiple linear regression with the result a11 = 110 MPa (16 ksi) and a13 = 10 MPa (1.5 ksi). Additional information about these measurement techniques is available in Ref 28.

As demonstrated in Fig. 4, measurement of residual stresses by x-ray diffraction now offers a practical way to quantify what used to be only a visual observation. A stress measurement in the burned area (darkest zone in Fig. 4a) indicated a tensile residual stress of 110 MPa (16 ksi), whereas the unburned area (clearest zone) indicated a compressive stress of -30 MPa (-4 ksi). The Eddy-current technique or careful microhardness measurements can also lead to meaningful information (Ref 8).

Micro- and Macrocracks. Cracks or even spalling may occur in finishing operations that have significant thermal and mechanical effects, such as grinding. For flat-surface grinding, cracks appear primarily perpendicular to the grinding direction, with typical depths from 0.010 to 0.020 in. (Ref 9). Interesting crack formations have been described (Ref 1) resulting from additional operations, including milling, reaming, drilling, electrodischarge machining, and laser beam machining.

Susceptibility to Corrosion/Stress-Corrosion Cracking. Finishing operations have a significant influence on the stress-corrosion resistance of a part, and it is well known that the introduction of some surface roughness may improve resistance in corrosive atmospheres. Crack propagation in a stressed material in a corrosive environment, termed stress-corrosion cracking (SCC), can occur in a finished part due only to residual stress, without any applied load.

Extensive research has been performed on the relationship between residual stress and SCC susceptibility for 304 and 316 austenitic stainless steel pipe weldments used in nuclear power plant boiling water reactors. In one study (Ref 10), intergranular SCC in a weld heat-affected zone (HAZ) was detected by ultrasonic testing long before any rupture occurred. Measurements of residual stress, by x-ray diffraction and stress relief techniques, were used in multipass gas-tungsten arc welding with reported accuracy of ±35 to 50 MPa (±5 to 7 ksi) and 135 MPa (15 ksi), respectively. It was found that the welding operations, along with the grinding operations used before and after welding, generated tensile stress. This stress contributed to the intergranular SCC in the weld microstructure, which was sensitized by the reactor water coolant containing small amounts of dissolved oxygen (200 to 300 ppb). A significant reduction in SCC was obtained by deliberately introducing compressive residual stress on the inside pipe surface. This was accomplished by creating a thermal gradient during the welding operation--the outside diameter of the pipe was maintained at a temperature between 500 and 550 °C (930 and 1020 °F), while the inner diameter was kept at 100 °C (212 °F) with circulating water.

Rough-turning operations studies have shown that tensile residual stresses at the surface of a part are quite detrimental to its corrosion resistance. Fluids used in finishing operations should also be considered when dealing with corrosion issues. For instance, based on hot-salt stress-corrosion tests (Ref 11), it has been shown that chemical milling can destroy the compressive stress layers that normally exist after machining, which gives a finished part reduced resistance to stress-corrosion cracking. Aluminum parts can show corrosion if they are machined for an extended period of time, due to galvanic action: current flow can occur between the machine tool base and the part if no special fluid is used (Ref 12). Even blowing off parts after machining can lead to rust if the compressed air used contains any moisture.

Aesthetic considerations are also important. When performance requirements are nearly equal, the finished component of choice is the one that has the better appearance. Cast iron sinks, brass plumbing equipment, hand levers, golf clubs, faucets, hammers, and silverware are examples of parts deburred, polished, or buffed by the thousands to improve their appearance. Even very large parts can be buffed to a bright finish. For example, Ref 13 describes the buffing of an entire aircraft fuselage (32.3 m, or 106 ft, long) using machines with 457 mm (18 in.) buffing wheels mounted on four computer numerically controlled floating heads on a movable gantry.

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