This article presents six case studies of failures with steel forgings. The case studies covered are crankshaft underfill; tube bending; spade bit; trim tear; upset forging; and avoidance of flow through, lap, and crack. The case studies illustrate difficulties encountered in either cold forging or hot forging in terms of preforge factors and/or discontinuities generated by the forging process. Supporting topics that are discussed in the case studies include validity checks for buster and blocker design, lubrication and wear, mechanical surface phenomenon, forging process design, and forging tolerances. Wear, plastic deformation processes, and laws of friction are introduced as a group of subjects that have been considered in the case studies.

This chapter discusses the role of plastic deformation in forging and the effect of strain and strain rate on metal flow. It demonstrates the use of stress tensors and shows how metal flow can be represented qualitatively by the displacement of volume elements and quantitatively by the distribution of velocity components and strain rates. It describes the conditions associated with homogeneous deformation in a frictionless upset forging and explains how they can also be obtained using engineering and true stress-strain curves.

This chapter covers the fundamentals of metal flow and the tools and techniques used to predict and control it. It begins by illustrating the local state of stress in a metal cylinder during upset forging and showing how stress components can be expressed in matrix form. It then explains how to determine the onset of yielding, which corresponds to the start of plastic deformation and the flow of metal within the workpiece. The chapter then goes on to present two important yield criteria, one based on shear stress (Tresca criterion), the other on distortion energy (von Mises criterion). It compares and contrasts the two methods and demonstrates their use as flow rules. It also explains how to calculate effective strain and strain rate and includes a brief discussion on the mechanical energy consumed during deformation.

This chapter explains that the key to forging is understanding and controlling metal flow and influential factors such as tool geometry, the mechanics of interface friction, material characteristics, and thermal conditions in the deformation zone. It also reviews common forging processes, including closed-die forging, extrusion, electrical upsetting, radial forging, hobbing, isothermal forging, open-die forging, orbital forging, and coining.

This chapter discusses the factors that influence temperature in forging operations and presents equations that can be used to predict and control it. The discussion covers heat generation and transfer, the effect of metal flow, temperature measurement, testing methods, and the influence of equipment-related parameters such as press speed, contact time, and tooling geometries.

This chapter addresses the issue of die failures in hot and cold forging operations. It describes failure classifications, fatigue fracture and wear mechanisms, analytical wear models, and the various factors that limit die life. It also includes several case studies in which finite-element modeling is used to predict die failure and extend die life.

This chapter discusses bulk deformation processes and how they are used to reshape metals and refine solidification structures. It begins by describing the differences between hot and cold working along with their respective advantages. It then discusses various forging methods, including open-die and closed-die forging, hot upset and roll forging, high-energy-rate forging, ring rolling, rotary swaging, radial and orbital forging, isothermal and hot-die forging, precision forging, and cold forging. The chapter also includes information on cold and hot extrusion and drawing operations.

Prior to forging, it is often necessary to preform billet stock to achieve adequate material distribution. This chapter discusses the equipment used for such operations, including transverse rolling machines, electric upsetters, ring-rolling mills, horizontal presses, and rotary (orbital) and radial forging machines. It describes their basic operating principles as well as advantages and disadvantages.

This chapter discusses the process of cold forging and its effect on various materials. It describes billet preparation and lubrication procedures, cold upsetting techniques, and the use of slab analysis for estimating cold forging loads. It likewise describes extrusion processes, explaining how to estimate friction and flow stress and predict extrusion loads and energy requirements. The chapter also discusses the tooling used in cold forging, the parameters affecting tool life, and the relative advantages of warm forging.