Materials Selection and Design (2010) Part 17 docx

In the second case, P is increased from 0 to 534 kN (120 kips) and then brought back to zero. The entire history of stresses in the three bars is shown in Fig. 1(b). When P exceeds 400 kN (90 kips), the two outer bars deform plastically and, because of the reduced modulus, begin to share less load. The stress in the two outer bars follows the path ABCD, whereas that in the middle bar follows the path ABEF. It can be seen that when P is again zero (unloading), the stresses in the three bars do not go back to zero. Instead, the middle bar has a residual tensile stress of 78.8 MPa (11.4 ksi), and each of the two outer bars has a residual compressive stress of 39.4 MPa (5.7 ksi). Because there is no external load on the assembly, the residual stresses in the three bars are in self-equilibrium. A comparison of the two loading histories indicates that the presence of inhomogeneous plastic deformation in the three bars is responsible for the generation of residual stresses. Similarly, mechanical residual stresses occur in any component when the distribution of plastic deformation in the material is inhomogeneous, such as the surface deformation in shot-peening operation. Thermal Loads. A similar three-bar model explaining the generation of residual stresses due to inhomogeneous plastic deformation caused by thermal loads is discussed by Masubuchi (Ref 6, presumably adopted from Ref 7). In this model, three carbon-steel bars of equal length and cross-sectional area are connected to two rigid blocks at their ends. The middle bar is heated to 593 °C (1100 °F) and then cooled to room temperature, while the two outer bars are kept at room temperature. Some of the details are not clearly explained in Ref 6, but the problem is very similar to the previous example. When the temperature in the middle bar is raised, the requirements of compatibility and equilibrium imply that a compressive stress be generated in the middle bar and tensile stresses in the two outer bars; the stress in each of the two outer bars being half of that in the middle bar. If the temperature in the middle bar is so high that its stress exceeds yield but in the two outer bars the stresses are still below yield, residual stresses will occur in the three bars when the temperature of the middle bar is brought back to room temperature (i.e., on unloading). Similarly, if the stresses in all three bars exceed yield but by different amounts, residual stresses will still occur when the temperature of the middle bar is brought back to room temperature. Indeed, this case is very similar to that of a cylinder immersed vertically in a quenchant where, during the initial stages of quenching, the temperature in the outer layer is much lower than that in the inner core. The three-bar model can be further utilized to explain the generation of residual stresses due to the mismatch in coefficients of thermal expansion. For example, suppose the two outer bars represent the layers of matrix in a composite lamina and the inner bar represents a layer of fibers. The coefficient of thermal expansion of the two outer bars is equal but, in general, different from that of the middle bar. It is assumed that the initial temperature of all the three bars is equal, which corresponds to a certain processing temperature much higher than room temperature. When the assembly is brought to room temperature, the requirements of compatibility and equilibrium will be satisfied if a system of forces (residual stresses) is established such that the sum of the forces in the two outer bars is equal and opposite to that in the middle bar. In this case, the presence of unequal plastic deformation is not a prerequisite for the generation of residual stresses. This explains why, while selecting the constituent materials for a composite or for a coating, the designers try to minimize the mismatch between their coefficients of thermal expansion. Solid-State Transformation. In quenching, welding, and casting processes, many metals such as steels undergo one or more solid-state transformations. These transformations are accompanied by a release of latent heat, a change in volume, and a pseudoplasticity effect (transformation plasticity). All of these affect the state of residual stresses in the part. The release of latent heat during solid-state transformation is similar to that during the liquid-to-solid transformation, albeit of a smaller amount. The change (increase) in volume occurs due to the difference in mass densities of the parent phase (e.g., austenite) and the decomposed phases (pearlite, ferrite, bainite, and martensite). In steels, the volumetric change due to phase transformation is in contrast to the normal contraction or shrinkage during cooling (Ref 8). A simple example of transformation plasticity is shown in Fig. 2, which is based on the results of a constrained dilatometry experiment (Ref 9). The figure shows that during cooling in the phase transformation regime, the presence of even a very low stress may result in residual plastic strains. Two widely accepted mechanisms for transformation plasticity were developed by Greenwood and Johnson (Ref 10) and Magee (Ref 11). According to the former, the difference in volume between two coexisting phases in the presence of an external load generates microscopic plasticity in the weaker phase. This leads to macroscopic plastic flow, even if the external load is insufficient to cause plasticity on its own. According to the Magee mechanism, if martensite transformation occurs under an external load, martensitic plates are formed with a preferred orientation affecting the overall shape of the body. Fig. 2 Transformation plasticity. Source: Ref 9 Material Removal. A fact that is often overlooked in discussing residual stresses caused by various manufacturing processes is the effect of material removal on the state of stresses in the product. Consider, for example, that a casting mold must be finally broken and removed, or a forging die must be retracted. Likewise, in making a machined part some of the material has to be removed. All of these operations change the state of stress in the part. In order to fully understand this concept, three examples discussed in Ref 5 should be considered. The first example entails an assembly of two concentric springs of slightly different lengths, L i and L o , as shown in Fig. 3(a); the subscripts i and o refer to inner and outer springs, respectively. The bottom ends of the two springs are fixed. Then, the upper ends are tied to a rigid block that is free to move only in the vertical direction. The two springs adopt a compromise length, L, which is in between L i and L o , as shown in Fig. 3(b). As a result, the two springs develop equal and opposite forces: compressive in the longer inner spring and tensile in the outer shorter spring. The assembly of the two springs may be viewed as analogous to the assembly of a cast part and its mold or to the assembly of the forged part and the die, or to a machined part before some portion of it is removed. Then, the removal of the outer spring becomes analogous to removal of material during machining (Ref 5, 12), of the casting mold (Ref 13, 14, 15, and 16), or of the forging die (Ref 17). Two cases are considered. In the first case, the stresses in both springs are assumed to be within their elastic limits. When the outer spring is removed, the force acting on it is transferred to the inner spring in order to satisfy equilibrium and the inner spring returns to its original length. In the second case, it is assumed that the inner spring has undergone a certain amount of plastic deformation. When the outer spring is removed, the inner spring does not return to its original length, L i . In either case, because the two springs and, therefore, the forces, are concentric, the residual stress in the inner spring becomes zero when the outer spring is removed. Fig. 3 Residual stresses in an assembly of two springs with unequal initial lengths. Source: Ref 5 For the second example, reconsider the three-bar model from the section "Mechanical Loads" in this article. After creating residual stresses in the three bars by loading and unloading the assembly, bar 3 is removed, by (for example) machining. As shown in Fig. 4(a) and 4(b), a redistribution of stresses in the remaining two bars takes place. The resultant stresses at the centroids of the two bars become -14.8 MPa (-2.14 ksi) in bar 1, and 14.8 MPa (2.14 ksi) in bar 2. Also, the assembly rotates (distorts) by an angle of 4.3 × 10 -3 radians. Fig. 4 Effect of asymmetric material removal in the three-bar model of Fig. 1. Source: Ref 5 The third example in Ref 5 is of a thick-walled cylinder with an internal diameter of 101.6 mm (4 in.) and an outer diameter of 203.2 mm (8 in.) as shown in Fig. 5(a). Both ends of the cylinder are restrained axially, and the cylinder is subjected to an internal pressure. A 25.4 mm (1 in.) thick (along the axis) slice of the cylinder is analyzed by subdividing it into 10 equal finite elements (5.08 mm, or 0.2 in., thick each) in the radial direction (Fig. 5b). The residual stresses are created by increasing the pressure from zero to 345 MPa (50 ksi), and then back to zero. The elements 1 and 2 are removed successively. The variation of the three stress components along the radius is shown in Fig. 6, before material removal (i.e., the residual stresses) and after removing the two layers. It may be noted that in an overall sense, the level of residual stresses goes down as the material is removed. However, this is not necessarily true in a local sense. Consider, for example, the circumferential stress at the centroids of elements 3 and 4 in Fig. 6(b); it increases as the material is removed. Fig. 5 Cylinder with internal pressure and its finite element mesh. Source: Ref 5 Fig. 6 Effect of removing layers of elements of material from the inside of the cylinder. Source: Ref 5 Important conclusions from the three examples discussed above can be summarized as follows: • When the material removal is symmetric with respect to the stress distribution (Fig. 3 ), the residual stresses in the remainder of the assembly or part are very small or even zero. • When the material removal is not symmetric with respect to the stress distribution (Fig. 4, 6 ), the residual stresses in the remainder of the assembly or part are not necessarily small. • Material removal may result in an increase in stresses at some locations of the assembly or the part ( Fig. 6). References cited in this section 5. U. Chandra, Validation of Finite Element Codes for Prediction of Machining Distortions in Forgings, Commun. Numer. Meth. Eng., Vol 9, 1993, p 463-473 6. K. Masubuchi, Analysis of Welded Structures, Pergamon Press, 1980, p 94-96 7. W.M. Wilson and C.C. Hao, Residual Stresses in Welded Structures, Weld. J., Vol 26 (No. 5), Research Supplement, 1974, p 295s-320s 8. W.K.C. Jones and P.J. Alberry, "The Role of Phase Transformation in the Developmen t of Residual Welding Stresses," Central Electricity Generating Board, London, 1977 9. J B. Leblond, G. Mottet, J. Devaux, and J C. Devaux, "Mathematical Models of Anisothermal Phase Transformations in Steels, and Predicted Plastic Behavior," Mater. Sci. Technol., Vol 1, 1985, p 815-822 10. G.W. Greenwood and R.H. Johnson, The Deformation of Metals under Small Stresses During Phase Transformation, Proc. Royal Soc., Vol 283, 1965, p 403-422 11. C.L. Magee, "Transformation Kinetics, Microplasticity and A ging of Martensite in FE31 Ni," Ph.D. Thesis, Carnegie Institute of Technology, 1966 12. U. Chandra, S. Rachakonda, and S. Chandrasekharan, Total Quality Management of Forged Products through Finite Element Simulation, Proc. Third International SAMPE Metals and Metals Processing Conf., Vol 3, F.H. Froes, W. Wallace, R.A. Cull, and E. Struckholt, Ed., SAMPE International, 1992, p M379- M393 13. U. Chandra, Computer Prediction of Hot Tears, Hot Cracks, Residual Stresses and Distortions in Precision Castings: Basic Concepts and Approach, Proc. Light Metals, J. Evans, Ed., TMS, 1995, p 1107-1117 14. U. Chandra, Computer Simulation of Manufacturing Processes Casting and Welding, Comput. Model. Simul. Eng., Vol 1, 1996, p 127-174 15. U. Chandra, R. Thomas, and S. Cheng, Shrinkage, Residual Stresses, and Distortions in Castings, Comput. Model. Simul. Eng., Vol 1, 1996, p 369-383 16. A. Ahmed and U. Chandra, Prediction of Hot Tears, Residual Stresses and Distortions in Castings Including the Effect of Creep, Comput. Model. Simul. Eng., to be published 17. U. Chandra, S. Chandrasekharan, and R. Thomas, "Finite Element Analysis of the Thread Rolling Process," Concurrent Technologies Corporation, submitted to Knolls Atomic Power Lab, Schenectady, NY, 1995 Control of Residual Stresses U. Chandra, Concurrent Technologies Corporation Computer Prediction of Residual Stresses In recent years, the finite element method has become the preeminent method for computer prediction of residual stresses caused by various manufacturing processes. A transient, nonlinear, thermomechanical analysis software is generally employed for that purpose. Some of the mathematics that form the basis of such software is common for all manufacturing processes. Such common mathematics is summarized by this section. However, because every process is unique, some mathematical requirements are, in turn, dependent on the process. Also, for the simulation of certain processes a sequential thermomechanical analysis is adequate, whereas for others a coupled analysis may be preferred or even essential. Such subtleties are pointed out later when individual processes are discussed. Ignoring convection, the following conduction heat-transfer equation is solved with appropriate initial and boundary conditions: (Eq 1) where T is the temperature at an arbitrary location in the workpiece at time t, k is the thermal conductivity of the material, c is the rate of heat generated per unit volume, is the density, C p is the specific heat, and is the differential operator; all material properties are assumed to vary with temperature. The term c accounts for the release of latent heat during liquid-to-solid transformation in casting and welding processes or during solid-state phase transformation in quenching, welding, or casting processes. It also accounts for the heat of plastic deformation in forging and other bulk deformation processes. The initial and boundary conditions are process dependent. Details of converting Eq 1 into its finite element form and of numerical solution are available in a number of technical papers and textbooks and are not repeated here. For a general treatment of the subject, the reader is referred to Ref 18, 19, 20, and 21. The transient temperatures computed above are used as loading for the subsequent transient stress/displacement analysis. Using the incremental theory, the total strain increment { } at time t can be divided into various components (Ref 22, 23, 24, 25, and 26): { } = { e } + { t } + { p } + { cr } + { v } + { tr } (Eq 2) where superscripts e, t, p, cr, v, and tr refer to elastic, thermal, plastic, creep, volumetric change, and transformation plasticity components, respectively. The first three strain terms are needed in the simulation of every manufacturing process discussed here, whereas the use of the other three terms is dependent on the process and are pointed out as appropriate. Also, mathematical details for the first four strain terms are discussed in most standard references (Ref 22, 23), whereas the details for the last two terms are discussed often in the context of the simulation of quenching and welding processes (Ref 24, 25, 26). In forging and other large deformation processes, the term c in Eq 1 represents the heat of plastic deformation and leads to a coupling between Eq 1 and 2. At present, no single computer code is capable of predicting residual stresses caused by all manufacturing processes. However, several general-purpose finite element codes are capable of predicting these stresses to a reasonable degree of accuracy for at least some of the manufacturing processes (Ref 27, 28, 29). In addition, some of these codes permit customized enhancements leading to more reliable results for a specific process. Before attempting to predict residual stresses due to a manufacturing process, it is advisable to compare the capabilities of two or three leading codes and use the one most suited for the simulation of the process in consideration. Examples of such comparisons are given in Ref 12 and 13 for forging, quenching, and casting processes. It must be noted that, due to continuous enhancement in these codes, it is always advisable to compare the capabilities of their latest versions. References cited in this section 12. U. Chandra, S. Rachakonda, and S. Chandrasekharan, Total Quality Management of Forged Products through Finite Element Simulation, Proc. Third International SAMPE Metals and Metals Processing Conf., Vol 3, F.H. Froes, W. Wallace, R.A. Cull, and E. Struckholt, Ed., SAMPE International, 1992, p M379- M393 13. U. Chandra, Computer Prediction of Hot Tears, Hot Cracks, Residual Stresses and Distortions in Precision Castings: Basic Concepts and Approach, Proc. Light Metals, J. Evans, Ed., TMS, 1995, p 1107-1117 18. G. Comini, S. Del Guidice, R.W. Lewis, and O.C. Zienkiewicz, Finite Element Solution of Non- linear Heat Conduction Problems with Special Reference to Phase Change, Int. J. Numer. Methods Eng., Vol 8, 1974, p 613-624 19. R.W. Lewis, K. Morgan, and R.H. Gallagher, Finite Element Analysis of Solidification and Welding Processes, ASME Numerical Modeling of Manufacturing Processes, PVP-PB- 025, R.F. Jones, Jr., H. Armen, and J.T. Fong, Ed., American Society of Mechanical Engineers, 1977, p 67-80 20. A.B. Shapiro, "TOPAZ A Finite Element Heat Conduction Code for Analyzing 2- D Solids," Lawrence Livermore National Laboratory, Livermore, CA, 1984 21. B.G. Thomas, I. Samarasekera, and J.K. Brimacombe, Comparison of Numerical Modeling Techniques for Complex, Two-Dimensional, Transient Heat-Conduction Problems, Metall. Trans., Vol 15B, 1984, p 307- 318 22. A. Levy and A.B. Pifko, On Computational Strategies for Problems Involving Plasticity and Creep, Int. J. Numer. Methods Eng., Vol 17, 1981, p 747-771 23. M.D. Snyder and K J. Bathe, A Solution Procedure for Thermo-Elastic-Plastic and Creep Problems, Nuc. Eng. Des., Vol 64, 1981, p 49-80 24. S. Sjöström, Interactions and Constitutive Models for Calculating Quench Stresses in Steel, Mater. Sci. Technol, Vol 1, 1985, p 823-829 25. S. Das, G. Upadhya, and U. Chandra, Prediction of Macro- and Micro- Residual Stress in Quenching Using Phase Transformation Kinetics, Proc. First International Conf. Quenching and Control of Distortion, G.E. Totten, Ed., ASM International, 1992, p 229-234 26. S. Das, G. Upadhya, U. Chandra, M.J. Kleinosky, and M.L. Tims, Finite Element Modeling of a Single- Pass GMA Weldment, Proc. Engineering Foundation Conference on Modeling of Casting, Welding and Advanced Solidification Processes VI, T.S. Piwonka, V. Voller, and L. Katgerman, Ed., TMS, 1993, p 593- 600 27. "ABAQUS," Version 5.5, Hibbitt, Karlsson and Sorenson, Pawtucket, RI, 1995 28. "ANSYS," Release 5.3, ANSYS, Inc., Houston, PA, 1996 29. "MARC," Version K 6.2, MARC Analysis Research Corporation, Palo Alto, CA, 1996 Control of Residual Stresses U. Chandra, Concurrent Technologies Corporation Measurement of Residual Stresses It is generally not possible to measure residual stresses in a product during its manufacture; instead, they are measured after the manufacturing process is complete. Smith et al. (Ref 30) have divided the residual stress measurement methods into two broad categories: mechanical and physical. The mechanical category includes the stress-relaxation methods of layer removal, cutting, hole drilling, and trepanning, whereas the physical category includes x-ray diffraction (XRD), neutron diffraction, acoustic, and magnetic. The layer-removal technique as originally proposed by Mesnager and Sachs (Ref 4) is only applicable to simple geometries such as a cylinder with no stress variation along its axis or circumference, or to a plate with no variation along its length or width. Thus, whereas it could be used to measure quench-induced residual stresses in a cylinder or a plate, it is not suitable for measuring complex stress patterns such as those caused by welding. The layer removal and cutting techniques, however, have been applied to pipe welds in combination with conventional strain gages and XRD measurements. The layer-removal technique is also used to measure residual stresses in coatings. Hole-drilling and trepanning techniques can be used in situations where the stress variation is nonuniform, but they are generally restricted to stress levels of less than one-third of the material yield strength. Also, these two techniques can be unreliable in areas of steep stress gradients and require extreme care while drilling a hole or ring in terms of its alignment as well as the heat and stress generation during drilling (Ref 31). For such reasons, and others, these two techniques have found little application in the measurement of weld-induced residual stresses. Of the methods in the physical category, XRD is probably the most widely used method, the neutron diffraction method being relatively new. These two methods measure changes in the dimensions of the lattice of the crystals, and from these measurements the components of strains and stress are computed. The XRD technique has undergone many improvements in recent years. With the development of small portable x-ray diffractometers, the technique can be used for on-site measurement of residual stresses. It should be noted, however, that this technique is capable of measuring strains in only a shallow layer (approximately 0.0127 mm, or 0.0005 in., thick) at the specimen surface. To measure subsurface residual stresses in a workpiece, thin layers of materials are successively removed and XRD measurements are made at each exposed layer. For reasons discussed in the section "Material Removal" in this article, the measurements at an inner layer should be corrected to account for the material removed in all the previous layers. Reference 32 gives analytical expressions for such corrections in cases of simple geometries and stress distributions. For more complex cases, it still remains difficult to determine subsurface residual stresses accurately. In contrast to the x-rays, neutrons can penetrate deeper into the metals. For example, in iron the relative depth of penetration at the 50% absorption thickness is about 2000 times greater for neutrons than for x-rays. Only a few materials, such as cadmium and boron, absorb neutrons strongly. However, to gain the advantage of greater penetration of neutrons requires the component to be transported to a high flux neutron source (Ref 30), which limits the use of the technique. References cited in this section 4. R.G. Treuting, J.J. Lynch, H.B. Wishart, and D.G. Richards, Residual Stress Measurements, American Society of Metals, 1952 30. D.J. Smith, G.A. Webster, and P.J. Webster, Measurement of Residual Stress and the Effects of Prior Deformation Using the Neutron Diffraction Technique, The Welding Institute, Cambridge, UK, 1987 31. C.O. Ruud, A Review of Nondestructive Methods for Residual Stress Measurement, J. Met., Vol 33 (No. 7), 1981, p 35-40 32. M.E. Hilley, J.A. Larson, C.F. Jatczak, and R.E. Ricklefs, Ed., "Residual Stress Measurement by X- Ray Diffraction," SAE J784a, Society of Automotive Engineers, 1971 Control of Residual Stresses U. Chandra, Concurrent Technologies Corporation Residual Stresses Caused by Various Manufacturing Processes Casting. In the past, little attention has been paid to the control of residual stresses in casting; much of the interest was focused on the prediction and control of porosity, misruns, and segregation. A review of the transactions of the American Foundrymen's Society or of earlier textbooks on casting (e.g., Ref 33) reveals practically no information on the subject; even the ASM Handbook on casting (Ref 34) provides little insight. In a recent book, Campbell (Ref 35) has included a brief discussion of residual stresses summarizing the work done by Dodd (Ref 36) with simple sand-mold castings. Dodd studied the effect of two process parameters: mold strength, by changing water content of sand or by ramming to different levels, and casting temperature. The conclusions of these costly experiments could have been more economically and easily arrived at by using the basic concepts discussed in the section "Fundamental Sources of Residual Stresses" and further amplified in the following paragraphs. When a casting is still in its mold, the stresses are caused by a combination of the mechanical constraints imposed by the mold, thermal gradients, and solid-state phase transformation. Also, creep at elevated temperature affects these stresses. Finally, when the casting is taken out of its mold, it experiences springback that modifies the residual stresses. As discussed in Ref 13, 14, 15, and 16, the computer prediction of residual stresses in castings requires a software that is capable of performing coupled transient nonlinear thermomechanical analysis (see the section "Computer Prediction of Residual Stresses" in this article). In addition, it should be able to account for the following: • Release of latent heat during liquid-to-solid transformation, that is, in the mushy region • Mechanical behavior of the cast metal in the mushy region • Transfer of heat and forces at the mold-metal interface • Creep at elevated temperatures under condition of varying stress • Enclosure radiation at the mold surface to model the investment-casting process • Mold withdrawal to model directional solidification • Mold (material) removal The author and his coworkers have recently modified a commercial finite element code and have analyzed simple sand- mold castings (Ref 15, 16). Computer simulation of these castings indicates that: (1) for an accurate prediction of transient and residual stresses, consideration of creep is important; creep is also found to make the stress distribution more uniform; and (2) just prior to mold removal the stresses in the casting can be extremely high, but after the mold removal they become very small (owing to the springback discussed in the section "Material Removal" ) except in the areas of stress concentration. The residual stresses after the mold removal will not necessarily be small if the casting is complex and the mold removal is asymmetric with respect to the stress distribution. Also, small variations in mold rigidity are not found to have any noticeable effect on residual stresses, which confirms the observations based on trial and error using green-sand molds with various water contents (Ref 35). Although very little work is published thus far on the subject of control of residual stresses in castings, finite element simulation methodology is now sufficiently advanced to enable the study of the effect of various process and design parameters on the residual stresses in castings, for example, superheat, stiffness and design of the mold, design of the feeding system and risers, and the design of the part itself. Also, residual stresses caused by different casting practices such as sand-mold, permanent-mold, investment casting, and so forth, can be determined. As the manufacturers and end- users of cast products become more aware of the status and benefits of the computer-simulation methodology, it can be expected to play a very important role in controlling residual stresses in complex industrial castings. At present, the biggest limiting factor in the use of simulation is the lack of thermophysical and mechanical properties data for the cast metal and the mold materials. Forging. As with the casting process, little attention has been paid in the past to the control of residual stresses caused by forging; most of the interest was in predicting the filling and the direction of material flow. Now, due to recent advances in computer-simulation techniques, it is possible to predict and control the residual stresses in forged parts. Large plastic flow of the workpiece material is inherent in the forging process. The material flow is influenced by a number of factors including the die shape and material, forging temperature, die speed, and lubrication at the die/workpiece interface. Therefore, finite element simulation software used to predict and control residual stresses in the part should be capable of accounting for these factors. Because a significant amount of energy is dissipated during forging in the form of heat due to plastic deformation, a coupled thermomechanical analysis becomes necessary especially for nonisothermal forging. Other factors contributing to the complexity of the finite element simulation of this class of problems are: temperature-dependent thermal and mechanical properties of the materials (especially for a nonisothermal forging); the choice of solution algorithm and remeshing due to large plastic deformation in the workpiece; and mathematical treatment of the die/workpiece interface that includes heat transfer, lubrication, and contact. The last two terms in Eq 2 need not be considered in the simulation of the forging process. Finite element simulation of the forging process with simple geometries and of a two-dimensional idealization of the thread-rolling process (Ref 17) showed that, although the stresses in the workpiece are high during the deformation stage, the stresses after retraction of the die (residual stresses) are no longer high except in the regions of stress concentration. Again, similar to the simulation of the casting processes, it is premature to generalize this conclusion but it is clear that the technique of computer simulation of forging and many other bulk deformation processes has advanced to a stage where it can assist in controlling the residual stresses in the part by performing a detailed parametric study with much less investment of time and capital than trial and error on the shop floor. Quenching involves heating of the workpiece to the heat treatment temperature followed by rapid cooling in a quenchant (e.g., air, water, oil, or salt bath) in order to impart the desired metallurgical and mechanical properties. The choice of a quench medium is the key element; it should be such that it removes the heat fast enough to produce the desired microstructure, but not too fast to cause transient and residual stresses of excessive magnitude or of an adverse nature (e.g., tensile instead of compressive). The heat removal characteristic of a quenchant is known to be affected by a number of factors including the size, shape, orientation of the workpiece (even for simple shapes such as plates and cylinders, the heat removal is different at the bottom, top, and side surfaces); the use of trays and fixtures to hold the workpiece in the quenchant; composition of the quenchant; size of the pool and its stirring, and so forth (Ref 37, 38, 39). Additional difficulties arise when, due to economic reasons, quenching is performed in a batch process. In the past, using trial and error, shop-floor personnel have come up with some interesting strategies to control the residual stresses (and warpage), for example, air delay or an intentional delay while transporting the workpiece from the [...]... concept of design being an integral part of manufacturing and surface finishing, process equipment design restrictions and fixturing design also are very important Both can influence the quality of the resulting finish Figure 2 shows their interrelated roles schematically In this overview, emphasis is placed on issues relating to part design and process equipment Fixturing must be designed and tailored... Castings: Basic Concepts and Approach, Proc Light Metals, J Evans, Ed., TMS, 1995, p 1107-1 117 14 U Chandra, Computer Simulation of Manufacturing Processes Casting and Welding, Comput Model Simul Eng., Vol 1, 1996, p 127 -174 15 U Chandra, R Thomas, and S Cheng, Shrinkage, Residual Stresses, and Distortions in Castings, Comput Model Simul Eng., Vol 1, 1996, p 369-383 16 A Ahmed and U Chandra, Prediction of... coating, and lead to corrosion (Ref 5) Spot or tack welding is no better in this regard However, a continuous weld with a smooth bead and no weld spatter will prevent this problem and make surface finishing easier Also, the elimination of sharp edges and corners will prolong the life of grinding, polishing, and buffing belts and wheels Size and Weight The design, dimensions, and weight of a part to... are used, the blasting parameters should be tailored to the part material and design, and the part should be designed to allow easy access by the media and easy removal of the media once the desired finish (cleanliness) is obtained If a power spray washing technique is used, the part design should allow for proper drainage to conserve chemicals and minimize carryover to the next process step Providing... product-development cycle manufacturing-related design considerations are addressed, the more efficient will be the manufacturing process and the better the quality of the surface finish will be Communication with and input from the manufacturing and surfacefinishing staff are very important in establishing a satisfactory product design Design of the part or component and pretreatment selections are important, but in... Hot Tears, Hot Cracks, Residual Stresses and Distortions in Precision Castings: Basic Concepts and Approach, Proc Light Metals, J Evans, Ed., TMS, 1995, p 1107-1 117 14 U Chandra, Computer Simulation of Manufacturing Processes Casting and Welding, Comput Model Simul Eng., Vol 1, 1996, p 127 -174 15 U Chandra, R Thomas, and S Cheng, Shrinkage, Residual Stresses, and Distortions in Castings, Comput Model... cleaning, ultrasonic Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness that could deflect Avoid sharp corners and edges Avoid intricate designs and surface features Avoid features (e.g., small recesses,... prevent etching action from occurring Avoid sharp corners and edges Avoid shallow intricate designs and surface features Mask areas not to be attacked Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness Avoid sharp corners, edges, and protuberances Avoid intricate designs and surface features Surfaces must be accessible to tools... features that could trap air and prevent pickling action Avoid flat surfaces on small parts that could stick together, exclude the acid, and prevent the pickling action Surfaces must be accessible to tools and withstand the local pressure and heat buildup Avoid very thin cross sections/wall thickness Avoid sharp corners, edges, and protuberances Avoid intricate designs and surface features that could... influence on the ability to use satisfactory pretreatments and obtain quality finishes The overall part design, and the design of surface features and their size, can have an impact not only on the choice of pretreatments and finishes, but also on the efficacy of these processes and the results obtained This article provides some guidelines about general design principles for different types of surface-finishing . process and design parameters on the residual stresses in castings, for example, superheat, stiffness and design of the mold, design of the feeding system and risers, and the design of the part. the design of the part (component or assembly) can have a significant influence on the ability to use satisfactory pretreatments and obtain quality finishes. The overall part design, and the design. Ahmed and U. Chandra, Prediction of Hot Tears, Residual Stresses and Distortions in Castings Including the Effect of Creep, Comput. Model. Simul. Eng., to be published 17. U. Chandra, S. Chandrasekharan,