Deformation Effects during Variable High Frequency Welding of Modern Steels

Thursday, June 5, 2008 - 2:20 PM

Deformation Effects during Variable High Frequency Welding of Modern Steels

R. W. Warke, Y. Adonyi, J. Swenson, LeTourneau University, Longview, TX

Fusion-welded high performance plate steels such as the 100 ksi minimum yield strength weathering HPS 100W have been used successfully in bridge fabrication. As other welding technologies are being investigated for large tubular structures, there was a need to predict their weldability during Electric Resistance Welded pipe manufacturing. The present modeling and simulation work was performed in support of a project aimed at characterizing Variable High Frequency Welding, a new solid-state controller-based modification of HFIW in which frequency can be modified during welding throughout the 200-800 kHz range. This modification allows for steeper edge temperature gradients prior to welding, as well as overall improvements in weld quality.

Weld simulations performed in the Gleeble 1500 showed evidence of dynamic recrystallization during the forging state of welding even in low-carbon steels. A series of separate high temperature compression tests were performed at strain rates varying from 0.5 sec^–1 to 40 sec^–1 and at different constant peak temperatures. Flow stresses were measured and the Holloman-Zener “Z” parameters were calculated.

It was found that the typically monotonic stress curve for dynamic recrystallization (DRX) was experienced only at high temperatures and low strain rates (at a low Z-parameter). At the same time, the peak stress before dynamic recrystallization caused flow softening, effect that increased at higher strain rate and lower deformation temperatures. The critical strain was estimated from the q-σ plots that were derived from the true stress–true strain plots. This critical strain ε_cr for the onset of DRX increased as the temperature decreased, thus making the kinetics for DRX heavily strain dependent. As heating rates can be expected to strongly affect DRX, work continues at heating rates of up to 1000ºC/second.

This research can be expected to benefit the ERW tubular product users such as the oil and gas and automotive industries.

Summary: High Performance Steels were evaluated for Variable High Frequency ERW tubular welding. High temperature compression tests were performed in the Gleeble 1500 at different strain rates following numerical modeling. Monotonic stress curve for dynamic recrystallization was experienced at high temperatures/low strain rates. This research will ultimately benefit ERW product users.