Failures related to metalworking Introduction WROUGHT FORMS are produced by a wide variety of metal working operations that can be roughly divided into bulk-working operations and sheet-forming operations(Ref 1). The general distinction here is that bulk working imposes material flow in all directions, while sheet-forming operations are typically limited to two- dimensional deformation. Metalworking operations are also classified as either primary metalworking(where mill forms such as bar, plate, tube, sheet, and wire are worked from ingot or other cast forms)or secondary metalworking(where mill products are further formed into finished products by hot forging, cold forging, drawing, extrusion, straightening, sizing, etc. ) These metal working operations have a two-fold purpose. First, they are obviously designed to produce parts with the desired configuration. Secondly, metal working can develop a final shape with internal soundness and improved mechanical properties by Improved internal quality due to compressive deformation Grain refinement Uniform grain structure Elimination of casting porosity and breakup of macrosegregation patterns Beneficial grain-flow pattern for improved part performance Improved toughness and/or fatigue resistance due to grain flow and fibering Burnished surface and controlled surface quality However, the beneficial factors of deformation processing can become a potential problem if the process is not carefully understood. Potential problems of the deformation process also include fracture-related problems: for example, internal bursts or chevron cracks, cracks on free surface cracks on die-contacted surfaces Metal-flow-related problems: for example, end grain and poor surface performance; inhomogeneous grain size; shear bands and locally weakened structures; cold shuts, folds, and laps; flow-through defects Control, materials selection, and use problems: for example, underfill, part distortion, and poor dimensional control; tool overload and breakage excessive tool wear; high initial investment due to equipment cost; poor material use and high scrap loss The movement of metal during these processes, whether performed at room temperature or at elevated temperatures, makes them common sources of surface discontinuities, such as laps, seams, and cold shuts Oxides, slivers or chips of the base material, or foreign material also can be embedded into the surface during working. These surface imperfections produce a notch of unknown severity that acts as a stress raiser, which may adversely affect strength under load Subsurface and core discontinuities may also occur. Subsurface flaws often(but not always)originate from the as-cast ingot due to shrinkage, voids, and porosity that form during solidification. For example, this is shown schematically in Fig. 1 for rolled bar that contains ingot porosity and pipe imperfections(discussed later in the fabulon,"Imperfections from the Ingot" ). These imperfections can also serve as sites for crack initiation during frication or in service Thefileisdownloadedfromwww.bzfxw.comFailures Related to Metalworking Introduction WROUGHT FORMS are produced by a wide variety of metalworking operations that can be roughly divided into bulk-working operations and sheet-forming operations (Ref 1). The general distinction here is that bulk working imposes material flow in all directions, while sheet-forming operations are typically limited to two￾dimensional deformation. Metalworking operations are also classified as either primary metalworking (where mill forms such as bar, plate, tube, sheet, and wire are worked from ingot or other cast forms) or secondary metalworking (where mill products are further formed into finished products by hot forging, cold forging, drawing, extrusion, straightening, sizing, etc.). These metalworking operations have a two-fold purpose. First, they are obviously designed to produce parts with the desired configuration. Secondly, metalworking can develop a final shape with internal soundness and improved mechanical properties by: · Improved internal quality due to compressive deformation · Grain refinement · Uniform grain structure · Elimination of casting porosity and breakup of macrosegregation patterns · Beneficial grain-flow pattern for improved part performance · Improved toughness and/or fatigue resistance due to grain flow and fibering · Burnished surface and controlled surface quality However, the beneficial factors of deformation processing can become a potential problem if the process is not carefully understood. Potential problems of the deformation process also include: · Fracture-related problems: for example, internal bursts or chevron cracks, cracks on free surfaces, cracks on die-contacted surfaces · Metal-flow-related problems: for example, end grain and poor surface performance; inhomogeneous grain size; shear bands and locally weakened structures; cold shuts, folds, and laps; flow-through defects · Control, materials selection, and use problems: for example, underfill, part distortion, and poor dimensional control; tool overload and breakage; excessive tool wear; high initial investment due to equipment cost; poor material use and high scrap loss The movement of metal during these processes, whether performed at room temperature or at elevated temperatures, makes them common sources of surface discontinuities, such as laps, seams, and cold shuts. Oxides, slivers or chips of the base material, or foreign material also can be embedded into the surface during working. These surface imperfections produce a notch of unknown severity that acts as a stress raiser, which may adversely affect strength under load. Subsurface and core discontinuities may also occur. Subsurface flaws often (but not always) originate from the as-cast ingot due to shrinkage, voids, and porosity that form during solidification. For example, this is shown schematically in Fig. 1 for rolled bar that contains ingot porosity and pipe imperfections (discussed later in the section, “Imperfections from the Ingot”). These imperfections can also serve as sites for crack initiation during fabrication or in service. The file is downloaded from www.bzfxw.com