Tooling Fatigue Life Enhanced with Laser Peening

Laser peening was demonstrated to produce deeper compressive residual stresses, which led to longer service life and more constrained failure regions in these dies

Posted: February 14, 2017
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Pilger rolling illustration — **Figure 1:** Illustration of pilgering process courtesy of thefabricator.com

Cold pilgering is an important metalworking process for producing seamless pipes and tubes for critical high stress applications. The process involves feeding prefabricated tubes through rotating steel dies to reduce their diameter and wall thickness (Figure 1). Cold pilgering is applied to many different metals – steel, copper, titanium, etc. – and has applications across industries ranging from zirconium tubes in nuclear power plants, hydraulic tubes in aircraft, umbilical tubes for offshore oil platforms, and high-performance golf club shafts.

The Challenge

Fatigue Failure

Pilger dies are subjected to cyclic loading at significant pressures (up to 1500 MPa for some alloys) and are thus prone to fatigue failures within the die grooves. When a pilger die begins to fail, small cracks are introduced into the forming groove surface requiring post-process sanding to remove the defect. These cracks will eventually grow to the point where tube defects can no longer be eliminated, forcing the die to be removed from service.

The Solution

Laser Peening for Fatigue Enhancement

LSP Technologies partnered with Pacific Northwest National Laboratory and Sandvik engineering group to study the impact of laser shock peening on the life and failure mode of cold pilger dies. The aim of the study was to assess the metal fatigue improvements provided by laser peening for pilger dies made of high strength tool steel. A set of standard dies (Figure 2) was laser peened at LSPT’s facility, then inserted into normal pilger production equipment at Sandvik Special Metals and operated to failure. The failed dies were analyzed for differences in service life, fracture appearance, and residual stress, and compared to standard dies that had undergone shot peening treatment but were not laser peened. The enhancement benefits of laser peening were significant.

Residual stress distribution - standard vs. laser peened pilger dies — **Figure 3:** Residual stress comparison between standard and laser peened (LSP) pilger dies.

The Results

Residual Stresses Improve Material Strength

Residual stresses were measured using X-ray diffraction analysis, and the laser peened dies were shown to have significantly deeper residual stress profiles (Figure 3). At the surface, each set of dies showed similar compressive residual stresses (≈ 1.05 GPa), consistent with the shot peening treatment applied to all standard dies. However, at surface depths as shallow as 0.01 mm, the residual stress of the standard dies dropped to nearly zero, while the laser peened dies maintained residual stresses of about 900 MPa. The beneficial compressive stresses of the laser peened dies did not drop to zero until about 1.5 mm in depth, extending nearly fifteen times deeper than those produced by shot peening alone.

Service Life Extensions Reduce Costs

The laser shock peened dies were operated to failure under standard production conditions, and produced approximately 300% more tubing than the average standard production die set. This dramatic life improvement represented the highest die life ever measured at Sandvik for these production conditions.

Conclusions

This study was an important confirmation of the benefits of laser peening in the metal forming industry. Laser peening was demonstrated to produce deeper compressive residual stresses, which led to longer service life and more constrained failure regions in these dies. Post-analysis by scanning electron microscope revealed shorter cracks and flaking in the laser peened dies, compared to deeper, longer, continuous cracks in the standard dies (Figure 4). Thanks to LSP Technologies’ groundbreaking work in the surface enhancement of pilger dies, the company was named co-recipient of the Federal Laboratory Consortium’s Award for Excellence in Technology Transfer. LSPT continues to regularly laser peen pilger dies for Sandvik Special Metals.

SEM crack images — **Figure 4:** SEM images of failure regions for **(a)** laser peened die, and **(b)** standard die

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