And by: Dr. Gabriel McBain, Dr. Hendrik Schafstall

Die life is a major concern for the forging industry, and cracking followed by die wear are the most prevalent failure mechanisms for forging dies (see Figure 1.)  Improving die life by avoiding premature die failure due to cracking is a one of the dominating challenges that die and forging process engineers are facing.

During recent seminars conducted by Simufact Engineering GmbH, one third of the participants identified die failure as a significant challenge for their company. This is not a representative result, but it motivated us to demonstrate how the numerical process simulation tool Simufact.forming can be used to increase die life significantly, optimizing the forging process by optimizing the pre-shape produced in the blocker stage. This is achieved by removing excess material. This has three major positive effects that will reduce manufacturing costs significantly: Increased die life, material savings, and greater productivity due to less frequent die change.

overview of forging die failure mechanisms

The optimization can be done for components in production, and for new products by introducing die-life studies during the forging die design phase.

When complete fill of the die cavity is reached too early, before the final stroke is reached, very high (hydrostatic) stresses develop in the forged workpiece because the material can flow only into the flash area. This results in high (tensile) stresses in the forging dies. These stresses are so great that after a limited number of forgings the dies will fail at the point where the high stresses are felt, resulting in a visible crack.

The life span of steels under high cyclic loading is described by S-N curves. Figure 2 is a graph of the magnitude of a cyclic stress (S) against the logarithmic scale of cycles to failure (n.) It shows the expected number of loadings (number of parts that a blocker or finisher die can produce) depending on the stress (max. tensile stress) loads on the die for a particular die material.

These curves indicate approximately how many load cycles a certain material can withstand. Because these curves are determined in laboratory conditions (uni-axial stresses, uniform peak stresses, sinusoidal loading, constant temperature) the load cycle numbers given in the S-N diagram cannot predict the exact number of components a die can produce. Nonetheless, the principle behavior is correctly described and reveals the key to improve the die-life.

At low stress values the material can withstand an infinite number of loads, which is called the fatigue limit and equals about one million load cycles (10°.) Forging dies are subject to much higher loads and their life span is much shorter — a few hundred to approximately 40,000 load cycles (forgings) and their behavior is determined by the low-cycle fatigue characteristics where slight changes of the stress can have a large impact on the life span. Hence, even a slight reduction of the stress (e.g., 20%) has a large impact on the die-life.

This means that if die life is to be increased, the primary goal should be to reduce the tensile peak stresses in the dies.For this task the details of the S-N curve are not even required.

The stresses in the dies reach their maximum when complete die filling is reached. Thereafter, excess material can leave the cavity only through the flash, which requires very high pressure (hydrostatic stresses) within the workpiece. This causes high contact stresses on the tools and finally high tensile stresses in the edges of the tool cavity.

This mechanism applies not only to complete die filling of the entire cavity but also to locally premature die filling, which can significantly increase the die stresses in a local region. An adjusted material distribution by optimized pre-form is the key to reducing the excessive die stresses for significant die life improvement. Let’s study this on a real life example.