Volume 16 - Machining Part 2 docx

Table 7 Summary of possible surface alterations resulting from various material removal processes R, roughness of surface; PD, plastic deformation and plastically deformed debris; L & T, laps and tears and crevicelike defects; MCK, microcracks; SE, selective etch; IGA, intergranular att ack; UTM, untempered martensite; OTM, overtempered martensite; OA, overaging; RS, resolution or austenite reversion; RC, recast, respattered metal, or vapor-deposited metal; HAZ, heat-affected zone Process Conventional Nontraditional Material Milling, drilling, or turning Grinding Electrical discharge machining Electrochemical machining Chemical machining R R R R R PD PD MCK SE SE Nonhardenable 1018 steel L & T RC IGA IGA R R R R R PD PD MCK SE SE L & T MCK RC IGA IGA MCK UTM UTM UTM OTM OTM OTM Hardenable 4340 and D6ac steels OTM R R R R R PD PD MCK SE SE L & T MCK RC IGA IGA MCK UTM UTM UTM OTM OTM D2 tool steel OTM R R R R R PD PD MCK SE SE L & T MCK RC IGA IGA MCK UTM UTM UTM OTM OTM Type 410 stainless steel (martensitic) OTM R R R R R PD PD MCK SE SE Type 302 stainless steel (austenitic) L & T RC IGA IGA R R R R R PD PD MCK SE SE L & T OA RC IGA IGA 17-4 PH steel OA OA R R R R R PD PD RC SE SE L & T RS RS IGA IGA RS OA OA 250 grade maraging (18% Ni) steel OA Nickel and cobalt-base alloys HAZ HAZ Inconel alloy 718 R R R R R René 41 PD PD MCK SE SE HS 31 L & T MCK RC IGA IGA IN-100 MCK HAZ HAZ R R R R R PD PD MCK SE SE Ti-6Al-4V L & T MCK RC IGA R R R R R L & T MCK MCK SE SE Refractory alloy TZM MCK IGA R R R R R L & T MCK MCK SE SE Tungsten (pressed and sintered) MCK MCK MCK IGA IGA Source: Ref 7 Table 8 Comparison of depth of surface integrity effects observed in material removal processes Maximum observed depth of effect (a) Turning or milling Drilling Grinding Chemical machining Electrochemical machining Electrochemical grinding Electrical discharge machining Laser beam machining Property and type of effect Condition mm in. mm in. mm in. mm in. mm in. mm in. mm in. mm in. Mechanically altered material zones Finishing (b) 0.043 0.0017 0.020 0.0008 0.008 0.0003 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Plastic deformation Roughing (d) 0.076 0.0030 0.119 0.0047 0.089 0.0035 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Finishing (c) (c) (c) (c) 0.013 0.0005 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Plastically deformed debris Roughing (c) (c) (c) (c) 0.033 0.0013 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Finishing 0.013 0.0005 0.025 0.0010 0.038 0.0015 0.025 0.0010 0.036 0.0014 0.018 0.0007 0.025 0.0010 (c) (c) Hardness alteration (e) Roughing 0.127 0.0050 0.508 0.0200 0.254 0.0100 0.079 0.0031 0.051 0.0020 0.038 0.0015 0.203 0.0080 (c) (c) Finishing 0.013 0.0005 0.013 0.0005 0.013 0.0005 (c) (c) 0.008 0.0003 0.000 0.0000 0.013 0.0005 0.015 0.0006 Microcracks or macrocracks Roughing 0.038 0.0015 0.038 0.0015 0.229 0.0090 (c) (c) 0.038 0.0015 0.025 0.0010 0.178 0.0070 0.102 0.0040 Finishing 0.152 0.0060 (c) (c) 0.013 0.0005 0.025 0.0010 0.000 0.0000 0.000 0.0000 0.051 0.0020 0.005 0.0002 Residual stress (f) Roughing 0.356 0.0140 (c) (c) 0.318 0.0125 0.025 0.0010 0.000 0.0000 0.000 0.0000 0.076 0.0030 (c) (c) Metallurgically altered material zones Finishing (c) (c) (c) (c) 0.013 0.0005 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Recrystallization Roughing (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Finishing (c) (c) (c) (c) (c) (c) 0.008 0.0003 0.008 0.0003 0.000 0.000 (c) (c) (c) (c) Intergranular attack Roughing (c) (c) (c) (c) (c) (c) 0.152 0.0060 0.038 0.0015 (c) (c) (c) (c) (c) (c) Finishing 0.010 0.0004 (c) (c) 0.005 0.0002 0.015 0.0006 0.010 0.0004 0.003 0.0001 0.013 0.0005 (c) (c) Selective etch, pits, protuberances Roughing 0.025 0.0010 0.076 0.3000 0.010 0.0004 0.038 0.0015 0.064 0.0025 0.013 0.0005 0.041 0.0016 (c) (c) Finishing 0.010 0.0004 0.038 0.0015 0.013 0.0005 (c) (c) 0.000 0.0000 0.003 0.0001 0.015 0.0006 0.015 0.0006 Metallurgical transformations Roughing 0.076 0.0030 0.508 0.0200 0.152 0.0060 (c) (c) 0.005 0.0002 0.008 0.0003 0.127 0.0050 0.038 0.0015 Finishing 0.003 0.0001 (c) (c) 0.018 0.0007 (c) (c) (c) (c) (c) (c) 0.015 0.0006 0.015 0.0006 Heat-affected zone or recast layers Roughing 0.025 0.0010 0.076 0.0030 0.318 0.0125 (c) (c) (c) (c) (c) (c) 0.127 0.0050 0.038 0.0015 Source: Ref 8 (a) Normal to the surface. (b) Finishing, gentle, or low-stress conditions. (c) No occurrences or not expected. (d) Roughing, off-standard, or abusive conditions. (e) Depth to point at which hardness becomes less than ±2 points HRC (or equivalent) of bulk material hardness (hardness converted from Knoop microhardness measurements). (f) Depth to point at which residual stress becomes and remains less than 140 MPa (20 ksi) or 10%, of tensile strength, whichever is greater. Surface Alterations Produced in Traditional Machining Operations. Traditional machining methods, which are the principal means of metal removal, include chip cutting (such as milling, drilling, turning, broaching, reaming, and tapping) and abrasive machining methods (such as grinding, sanding, and polishing). These machining operations can produce surface layer alterations when abusive machining conditions are used (Fig. 10 and 11). In general, abusive machining promotes higher temperatures and excessive plastic deformation. Gentle machining operations occur when a sharp tool is employed and when machining conditions result in reduced machining forces. Fig. 10 Surface alterations produced from drilling with dull tools. (a) Section perpendicular to the drill- hole axis in a 4340 steel (48 HRC). Abusive drilling produced a cracked untempered martensite s urface alteration. Also note the softer overtempered zone below the untempered martensite layer. (b) Cross section through a hole drilled in type 410 stainless with a dull drill (1.5 mm, or 0.060 in., wearland). The drill broke down at the corner during the test and friction welded a portion of the high- speed steel drill bit to the workpiece. The base metal exhibits a rehardened and subsequently overtempered zone as a result of the high localized heating. 20×. Source: Ref 9 Fig. 11 Surfaces produced by the face milling of Ti-6Al- 4V (aged, 35 HRC). (a) With gentle machining conditions, a slight white layer is visible, but c hanges in microhardness are undetected. 1000×. (b) With abusive conditions, an overheated white layer about 0.01 mm (0.0004 in.) deep and a plastically deformed layer totalling 0.04 mm (0.0015 in.) deep are visible. 1000×. (c) Microhardness measurements sh ow a total affected zone 0.08 mm (0.003 in.) deep from abusive conditions. Source: Ref 9 Surface Effects in Grinding. In grinding, there are five important parameters that determine gentle or abusive conditions: wheel grade, wheel speed, downfeed or infeed, wheel dressing, and grinding fluid. As grinding parameters become more aggressive (that is, harder wheels, higher wheel speeds, increased infeed, and so on), the grinding process becomes more abusive and therefore more likely to produce surface damage. Gentle, or low-stress, grinding conditions for a variety of alloys are summarized in Table 9. Table 9 Low-stress grinding procedures Grinding parameters Steels and nickel-base high-temperature alloys (a) Titanium Surface grinding Wheel A46HV C60HV Wheel speed, m/s (sfm) (b) 13-15 (2500-3000) 10-15 (2000-3000) Downfeed per pass, mm (in.) 0.005-0.013 (0.0002-0.0005) 0.005-0.013 (0.0002-0.0005) Table speed, m/min (sfm) (c) 12-30 (40-100) 12-30 (40-100) Crossfeed per pass, mm (in.) 1-1.25 (0.040-0.050) 1-1.25 (0.040-0.050) Grinding fluid Highly sulfurized oil (d) Traverse cylindrical grinding Wheel A60IV C60HV Wheel speed, m/s (sfm) (b) 13-15 (2500-3000) 10-15 (2000-3000) Infeed per pass, mm (in.) 0.005-0.013 (0.0002-0.0005) 0.005-0.013 (0.0002-0.0005) Work speed, m/min (sfm) (c) 20-30 (70-100) 20-30 (70-100) Grinding fluid Highly sulfurized oil (d) Source: Ref 10 (a) For a wide variety of metals (including high-strength steels, high- temperature alloys, titanium, and refractory alloys), low- stress grinding practices develop very low residual tensile stresses. In some materials, the residual stress produced near the su rface is actually in compression instead of tension. (b) Low- stress grinding requires wheel speeds lower than the conventional 30 m/s (6000 sfm). To apply low- stress grinding, it would be preferable to have a variable-speed grinder. Because most grinding machines do not have wheel-speed control, it is necessary to add a variable- speed drive or to make pulley modifications. (c) Increased work speeds even above those indicated are considered to be advantageous for improving surface integrity. (d) Cutting f luids should be nitrate free for health reasons. Some manufacturers also prohibit cutting fluids with chlorine when machining titanium. Figure 12(a) illustrates the surface characteristics produced by the low-stress grinding of AISI 4340 steel quenched and tempered to 50 HRC. The low-stress condition produced no visible surface alterations, while the abusive grinding condition (Fig. 12b) produced an untempered martensite layer 0.03 to 0.13 mm (0.001 to 0.005 in.) deep with a hardness of 65 HRC. Below this white layer there was an overtempered martensitic zone with a hardness of 46 HRC. The hardness returned to its normal value at a depth of 0.30 mm (0.012 in.) below the surface (Fig. 12c). Fig. 12 Surface characteristics produced by the low-stress and abusive grinding of AISI 4340 steel. (a) Low- stress grinding produced no visible surface alterations. (b) The white layer shown from abusive conditions has a hardness of 65 HRC and is approximately 0.025 to 0.050 mm (0.001 to 0. 002 in.) deep. (c) Plot of microhardness alterations showing a total heat- affected zone of 0.33 mm (0.013 in.) from abusive conditions. (d) Plot of residual stress. (e) Effect on fatigue strength. Source: Ref 11 Abusive grinding also tends to produce a residual stress within the altered surface layer. A residual stress profile can be readily obtained by using x-ray diffraction techniques both at the surface and then by stepping into the surface with repeated x-ray diffraction readings after successive surface etching. The abusive grinding produced high tensile stresses in the altered zone, while low-stress grinding produced a surface with small compressive stresses (Fig. 12d). Fatigue tests conducted on flat specimens indicate that abusive grinding may seriously reduce the fatigue strength, as shown in Fig. 12(e). In this example, the abusive grinding depressed the endurance limit from 760 MPa (110 ksi) for low- stress grinding to 520 MPa (75 ksi). Surface Alterations Produced in Nontraditional Machining Operations. Nontraditional machining includes a variety of methods for removing and finishing materials. Examples of nontraditional operations are electrical discharge machining, laser beam machining, electrochemical machining, electropolishing, and chemical machining. Electrical discharge machining (EDM) tends to produce a surface with a layer of recast spattered metal that is usually hard and cracked and porous to some degree (Fig. 13). Below the spattered and recast metal, it is possible to have some of the same types of surface alterations that occur in abusive or traditional machining practices. Fig. 13 Surface characteristics of cast Inconel 718 (aged, 40 HRC) produced by EDM. (a) Finishing conditions produced a variable recast layer 0.005 mm (0.0002 in.) thick. 860×. (b) Roughing conditions produced an extensively cracked variable recast la yer up to 0.05 mm (0.002 in.). The random acicular structure is not related to the surface phenomena. 660×. (c) The recast structure has a hardness of about 53 HRC, and slight overaging due to localized surface overheating was also noted. Source: Ref 9 The effect is more pronounced when EDM is used under abusive or roughing conditions. Figure 14 shows a surface produced by EDM under finishing and roughing conditions. Under roughing conditions, globs of recast metal are spattered onto a white layer of rehardened martensite. An overtempered zone up to 46 HRC is also found beneath the surface. The surface produced under finishing conditions contains discontinuous patches of recast metal plus a thin layer of rehardened martensite 0.003 mm (0.0001 in.) deep. Fig. 14 Surface characteristics of AISI (quenched and tempered, 50 HRC) produced by EDM under finishing and ro ughing conditions. (a) Finishing conditions produce discontinuous patches of recast metal plus a thin layer (0.0025 mm, or 0.0001 in.) of rehardened martensite. 620×. (b) Roughing conditions produce globs of recast metal and a white layer of rehardened mar tensite 0.075 mm (0.003 in.) deep. 620×. (c) Microhardness measurements show a total heat- affected zone approaching 0.25 mm (0.010 in.). Globs of recast and the white layer are at 62 HRC. An overtempered zone as soft as 46 HRC is found beneath the surface. Source: Ref 9 Laser beam machining (LBM) tends to produce the same types of surface alterations as EDM. Figure 15 illustrates the heat-affected zone produced by LBM on Inconel 718. The intense heat generated by the laser beam resulted in a recast surface layer at the entrance and exit of the hole produced by the laser beam. Fig. 15 Surfaces from the laser bea m drilling of Inconel 718 shown at magnifications of 185× (a) and 1160× (b). Note the grain structure in the heat-affected zones of the entrance (A) and the exit (B). Source: Ref 9 Electrochemical machining (ECM) is capable of producing a surface that is essentially free of metallurgical surface layer alterations. However, when ECM is not properly controlled, selective etching or intergranular attack may occur (Fig. 16). Abusive ECM conditions also tend to degrade surface roughness (Fig. 17). Fig. 16 Surface characteristics of Waspaloy (aged, 40 HRC) produced by ECM. (a) Gentle conditions produce a slight roughening of the surface and some intergranular attack. (b) Abusive conditions produce severe intergranular attack. (c) Microhardness is unaffected by the abusive conditions. Source: Ref 9 Fig. 17 Surface characteristics of 4340 steel (quenched and tempered, 30 HRC) produced by ECM. (a) Gentle conditions produce slight surface pitting but no other visible changes . (b) Abusive conditions produce surface roughening but no other visible effect on microstructure. (c) Gentle and abusive conditions both produce a slight hardness loss at the surface. Source: Ref 12 Electropolishing (ELP) and Chemical Machining (CM). Surface softening is produced on most materials by electrochemical machining as well as by electropolishing and chemical machining, also referred to as chemical milling. Figure 17 illustrates the surface softening produced by both gentle and abusive ECM on 4340 steel. The surface is about 5 HRC points lower in hardness than the interior to approximately 0.05 mm (0.002 in.) in depth. With CM, the same steel had its surface softened by about 5 HRC points to a depth of about 0.05 mm (0.002 in.) (Fig. 18). This softening may be severe enough and deep enough to affect the fatigue strength and other mechanical properties of metals and may necessitate postprocessing. Fig. 18 Surface characteristics of 4340 steel (annealed, 31 to 36 HRC) produced by CM. (a) Gentle conditions produce no visible surface effects and a surface finish of 0.9 m (35 in.). (b) Abusive conditions produce a slight roughening and a surface finish of 3 m (120 in.). (c) Gentle and abusive conditions both produce a softening at the surface. Source: Ref 9 Residual Stress. Machining processes impart a residual stress in the surface layer. In grinding, the residual stress tends to be tensile when more abusive conditions are used (Fig. 19). By using gentle grinding conditions, the stress can be reduced in magnitude and can even become compressive. In milling, the residual stress tends to be compressive (Fig. 20). In facing milling 4340 steel (Fig. 20), the stresses are tensile at the surface but go into compression below the surface. [...]... model with interaction term Partial second-order model ln t = b0 + b1x1 + b2x2 ln t = b0 + b1x1 + b2x2 + b12x1x2 ln t = b0 + b1x1 + b2x2 + b12x1x2 + b11 Full second-order model + b 22 ln t = b0 + b1x1 + b2x2 + b3x3 + b12x1x2 + b13x1x3 + b23x2x3 + b11 + b 22 + b33 Konig and DePiereux (Ref 8) ln t = b0 + b1V + b3f t = tool life; x1 = ln V, where V is the cutting velocity; x2 = ln f, where f is the feed... Abusive milling Abusive grinding Gentle grinding Electrochemical machining Conventional grinding Electrical discharge machining Ti-6Al-4V, 32 HRC Inconel 718, aged, 44 HRC Endurance limit in bending, 107 cycles MPa ksi 703 1 02 620 90 430 62 480 70 430 62 350 51 22 0 32 90 13 410 60 27 0 39 165 24 150 22 Gentle grinding, % 100 88 61 113 100 82 52 21 100 65 40 37 Source: Ref 9 Effect of Grinding Endurance limits... MPa ksi 703 1 02 7 72 1 12 430 62 630 92 620 90 660 96 410 60 170 25 540 78 170 25 480 70 28 5 41 560 81 29 0 42 540 79 Gentle grinding, % 100 110 61 90 88 94 100 42 130 42 117 68 135 70 1 32 Note: Shot size: S110; shot hardness: 5 0-5 5 HRC; coverage: 300% Source: Ref 14 Retarding Stress-Corrosion Cracking It has been shown that the compressive stresses introduced by shot peening retard stress-corrosion cracking... of the CIRP, Vol 20 (No .2) , 1971 10 Machining Data Handbook, Vol 2, 3rd ed., Metcut Research Associates, 1980, p 1 8-8 7 11 M Field, J.F Kahles, and J.T Cammett, A Review of Measuring Methods for Surface Integrity, Annals of the CIRP, Vol 2 (No 1), 19 72 12 W.P Koster et al., "Surface Integrity of Machined Structural Components," AFML-TR-7 0-1 1, Metcut Research Associates, March 1970, p 2 13 A.R Werner... Material Steels Nickel-base alloys Titanium alloys Etchant 2% HNO3 and 98% denatured anhydrous alcohol 100 mL HCl, 5 g CuCl2·2H2O, and 100 mL denatured anhydrous alcohol 2% HF and 98% H2O or 2% HF, 3% HNO3, and 95% H2O Guidelines for Material Removal, Postprocessing, and Inspection The following guidelines are meant to serve only as general or starting recommendations for the machining of structural... 1 8-1 5 to 1 8-3 7 6 G Bellows and D.N Tishler, "Introduction to Surface Integrity," Technical Report TM 7 0-9 74, General Electric Company, 1970, p 3 7 L.R Gatto and T.D DiLullo, "Metallographic Techniques for Determining Surface Alterations in Machining, " Technical Paper IQ 7 1 -2 25 , Society for Manufacturing Engineers, 1971 8 Machining Data Handbook, Vol 2, 3rd ed., Metcut Research Associates, 1980, p 1 8-5 8... B46. 1-1 985, American Society of Mechanical Engineers, 1985 2 "Surface Texture Symbols," ANSI Y14.3 6-1 978, American Society of Mechanical Engineers, 1978 3 "Surface Integrity," ANSI B211. 1-1 986, Society of Manufacturing Engineers, 1986 4 J Peters, P Vanherck, and M Sastrodinoto, Assessment of Surface Typology Analysis Techniques, Annals of the CIRP, Vol 28 (No 2) , 1979 5 Machining Data Handbook, Vol 2, ... (0. 020 in.) of stock is then removed from the as-mounted metal surface on a positive-positioning automatic polishing unit, using the side of a 25 × 330 mm (1 × 13 in.) aluminum oxide 320 -grit grinding wheel as the grinding medium Water is used as a coolant Subsequent rough grinding is performed wet on silicon carbide papers or equivalent ranging from 24 0 to 600 grit For steels and nickel- and cobalt-base... condition (such as an increase in downfeed) increases the distortion of the workpiece (Fig 21 ) and creates more residual stress at the surface (Fig 19) Figures 22 and 20 illustrate a similar relation in milling As the machining condition becomes more abusive with a duller tool, the distortion (Fig 22 ) and residual stress (Fig 20 ) become greater Residual stress and distortion thus exhibit the following relation:... high-strength steels, and data illustrating the effect of some machining methods on fatigue strength are given in Table 10 The electropolishing of 4340 steel resulted in a 12% decrease in fatigue strength compared to gentle grinding Chemical machining of Ti-6Al-4V resulted in an 18% drop in fatigue strength compared to gentle grinding, while the electrochemical machining and electrical discharge machining . (sfm) (b) 1 3-1 5 (25 0 0-3 000) 1 0-1 5 (20 0 0-3 000) Downfeed per pass, mm (in.) 0.00 5-0 .013 (0.000 2- 0 .0005) 0.00 5-0 .013 (0.000 2- 0 .0005) Table speed, m/min (sfm) (c) 1 2- 3 0 (4 0-1 00) 1 2- 3 0 (4 0-1 00). (25 0 0-3 000) 1 0-1 5 (20 0 0-3 000) Infeed per pass, mm (in.) 0.00 5-0 .013 (0.000 2- 0 .0005) 0.00 5-0 .013 (0.000 2- 0 .0005) Work speed, m/min (sfm) (c) 2 0-3 0 (7 0-1 00) 2 0-3 0 (7 0-1 00) Grinding fluid. Abusive grinding 430 62 61 Gentle milling 480 70 113 Gentle grinding 430 62 100 Chemical milling 350 51 82 Abusive milling 22 0 32 52 Ti-6Al-4V, 32 HRC Abusive grinding 90 13 21 Gentle grinding