In the first two articles we published in this journal, we described defects that can occur in forgings due to the geometry of the die and the material of the workpiece. In this article and one to follow, we examine failures that can occur not in the workpiece but in the tooling. This article presents the fundamental aspects of stress within a die that has been loaded during the forging process. It is imperative in analyzing die failures to have a basic understanding of the stresses in the die. The second part will describe some examples of die failures, the analysis performed based on the stresses in the die and remedies to prevent future failures.
There are a number of sound economic and engineering reasons why prevention of tool failures is important. Tooling costs are significant in most forging companies. Tool and die cost is typically well over 5% of the cost of sales. Total cost is frequently underestimated, as tooling is subject to considerable hidden cost. Replacement of broken or worn dies is clearly expensive.
Operating a lean manufacturing facility both with regard to schedule and to inventory can be very problematic if tool life is short or erratic. Delivery problems are routine when tooling failures occur. Lean manufacturing is harder to implement with inconsistent die life.
Safety risks also increase in the presence of overstressed tooling, especially if the overloads lead to catastrophic die failures. Dies are subject to high stresses in the best of cases. When the ultimate strength of a die material is significantly exceeded, a catastrophic fracture can occur, resulting in injury or even death.
Quality problems have been documented from premature die failures at most companies. The unpredictability of die life results in higher inventory levels as companies try to “keep the shop running.”
In order to prevent die failures, dies can be analyzed by a structural analysis, which provides a measure of the potential for failure. If failures do occur, they should be analyzed to determine a root cause so that appropriate corrective actions can be taken.
MODES OF DIE FAILURES
There are four principal reasons to take a die out of service:- Catastrophic failure due to an overload causing fracture
- Plastic deformation due to large-scale or local yielding of the die
- Low-cycle fatigue (LCF), thermal or mechanical, due to the repeated temperature or stress cycles that the die receives
- Wear due to the flow of the workpiece across the die surface
Figure 1. This typical stress-strain curve compares a high-strength brittle material to a low-strength ductile material. In the linear (left side) region, the material remains elastic. The “X” represents the yield point, where additional strain results in plastic deformation. The y-axis is stress (s), and the x-axis is strain (e). This type of curve is used to characterize the mechanical behavior of forging-die steels.
DIE MATERIAL BEHAVIOR
Before we can understand and evaluate die failures, we need to understand how the die responds to loads imposed on it during forging. Figure 1 shows the normal method of characterizing the die material using a stress-strain curve. The die strength is measured by the maximum stress value that a die material can withstand. The area under a stress-strain curve is the fracture toughness of the material. A brittle material, although it can be very strong, has a small area under the stress-strain curve and can withstand only a small amount of impact energy before fracture. Very strong (and brittle) dies have low toughness and are not appropriate for use on hammers. In contrast, a ductile material has a large area under the stress-strain curve and can withstand a high amount of impact energy before fracture even though it may have lower strength. Dies are generally subjected to very high forging pressure, resulting in little remaining safety factor.In order for the die not to deform plastically, the stresses must remain below the die-material yield strength where the material behaves elastically. An elastic material will have a linear stress-strain curve at a constant temperature. The slope of the stress-strain curve (Young’s modulus) in this linear region is the stiffness of the material. The stiffness of steels does not vary significantly with yield strength. The material simply remains elastic until a higher stress level. As temperature increases, the elastic modulus (stiffness) and yield strength of die steels decrease.
Figure 2. The stress state in a forging die: a) As the workpiece is pressed, the forging die is loaded. These loads produce stresses in the die steel. b) A point in the die can be examined in detail. c) The position is “extracted” from the die for closer examination. d) The stress components for the one point on the die are shown on a free-body diagram.
DIE STRESS STATE
The die shown in Figure 2 is experiencing stress due to the applied loading. To evaluate the stress state, an arbitrary block of material is “removed” from the die, and the stress acting on that material is shown. Figure 3 shows this block for the die location shown in Figure 2. This extracted block is called a free-body diagram. A free-body diagram can be used to understand the stress state at any part of a die or other structural component. Even though the forging load may seem to be a case of axial compression, the die will experience induced-stress components in different directions based on the structure, material, forging, support, thermal gradients and process conditions. Even the simplest forging results in a complex die stress state. Evaluating the individual components of the three-dimensional stress state can be used to study problems and potential solutions. Stress components are generally used to troubleshoot problem jobs, especially when a low-cycle fatigue failure is observed.A typical stress state, such as the one shown in the free-body diagram of Figure 3, includes both normal-stress components and shear-stress components. Rotating the free-body diagram will result in a stress state being represented by only normal stresses (i.e. no shear). Figure 4 shows the rotation for the stress state at the die location shown in Figure 2. The normal-stress components at this orientation, when there are no shear components, are called the principal stresses. The most tensile of the principal stresses is called the maximum principal stress. The maximum principal stress is the primary stress to use in the design and analysis of fatigue failures in forging dies. The minimum principal stress is the most compressive of the principal stresses. In Figure 4, only two of the three orthogonal x, y, z stress components are included for clarity.
Figure 3. A free-body diagram illustrates the details of the stress state at a point in the die steel. The stress state is characterized by stress components. The normal components in this case are the axial, radial and hoop stresses. The shear components are shown by the shear stresses.
The effective stress(s-) is a numerical method of converting the three-dimensional stress state into a single number that can be used in many analyses. The effective stress is a “flag” that indicates the onset of plastic deformation. Yielding will occur when the effective stress in the die becomes larger than the yield strength of the material.
After the effective stress is calculated, it is compared to the yield strength of the material to see whether the material has yielded. A material’s yield strength is a function of temperature. Also, the yield strength decreases after a tempering operation, which relieves internal stresses and decreases the brittleness of the material. Calculating the effective stress – also known as the von Mises stress – is rarely performed manually. Finite-element-modeling (FEM) programs are the common method of calculating this value.
Figure 4. Rotation of the free-body diagram can give an orientation in which the shear-stress components are zero. This orientation is the principal stress axis. The normal stress components in this orientation are called the principal stresses. The largest principal tensile stress is the maximum principal stress. The most compressive principal stress is the minimum principal stress.
FATIGUE FAILURES
Low-cycle fatigue (LCF) is a common mode of die failure. This type of failure occurs in four stages.- Fracture initiation
- Slow crack growth
- Accelerated crack growth rate
- Rapid fracture
Understanding the stress state in a die is important in understanding the root cause of any failure. A fatigue failure can occur when a metal is subjected to cyclic stresses due to either mechanical or thermal loading. The magnitude of the cyclic stresses can be below the material’s yield strength for a fatigue failure to occur. In cases where the stress is below the yield strength, it is common for the crack to initiate at a stress concentration, grinding mark, surface blemish, nonmetallic inclusion or other imperfection in the die. The size of the applied stresses will directly affect the number of cycles that the die can withstand before failure.
For steel dies, there is a stress level called the endurance limit. The endurance limit is a property of the steel. As long as the cyclic stresses remain below the endurance limit, a fatigue failure will not occur. Fatigue behavior is generally described by a stress versus number of cycles (S-N) curve. Unfortunately, these curves are rarely available for forging-die materials. Once a microscopic crack forms – usually on the surface of the die – repeated stress cycling causes it to grow on each loading. When the crack reaches a critical size, it will grow very rapidly, leading to failure. This crack-growth process requires a tensile-stress component.
Figure 5. Mohr’s circle is a graphical method of showing the stress state and the various values of the stress components that can represent the stress state. a) The values of the normal components and the shear components are plotted on a graph of shear stress (z) versus normal stress (s). Connecting the two points produces the diameter of Mohr’s circle. b) Rotation of the free-body diagram and rotation on Mohr’s circle can yield values for the principal stresses for the state of stress at one point in the forging die.
SUMMARY
In this first of two parts, we have described the reasons why forging-die failures are important and have indicated the business reasons why they should be analyzed and prevented. The various modes of forging-die failures have also been described. Each of these depends upon the stress state within the forging die. A description of the fundamental aspects of the stress state in the die has been presented. These fundamental ideas are important in developing an understanding of the root causes of die failures. Finally, the mechanism in which stresses in a die cause a low-cycle fatigue failure has been summarized. In the second part of this article, we will provide several examples of forging-die failures as well as the analysis of these failures to elucidate their root cause.ACKNOWLEDGEMENTS
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing the forging industry and the U.S. Department of Defense. These teammates are determined to ensure that the nation’s forging industry is positioned to meet the challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the Forging Fundamentals 101 course developed by Scientific Forming Technologies Corporation and the Forge-It Team.Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colo. He may be reached at (303) 273-3793 or cvantyne@mines.edu. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at (614) 451-8330 or jwalters@deform.com.
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