Original Date: 04/24/2007
Revision Date: / /

Eliminating residual stress or a means to identify and predict the effects of the stress will enable shipbuilders to improve their usage of automated part nesting and cutting of steel plates while minimizing material usage. A team of researchers, including researchers from the University of New Orleans, College of Engineering, Bender Shipyards, Battelle, and Caterpillar, have undertaken a research project that will assist the shipbuilders and others.

The shipbuilding industry found that nesting several different parts within the same sheet of material and then cutting the parts to near net shape had some unexpected downfalls. Typically, cutter path movement is optimized to minimize cutter (laser or plasma) movement and maximize torch-on time. This process also ensures maximum utilization of material. Residual Stress (RS) is created within the plates of steel during the steel making process. This stress is relieved during the cutting operation, resulting in movement of the plates (e.g., walking, growth, shrinkage, and warping). This movement negates the accuracy of the cut parts as programmed and creates undue waste and scrap.

The University of New Orleans, College of Engineering (UNO COE) was asked to investigate this phenomenon and develop ways of mitigating the unwanted problems. The research began with the investigation of typical steel- making processes in major steel mills. Three steel-making processes were investigated: the reverse mill process, the steckle mill process, and the hot strip mill process. Possible sources of residual stress were thoroughly investigated, researched, and analyzed in each of these processes. Mill temperature data for each step of the steel- making process was collected, and three-dimensional models were generated to analyze the different rolling, leveling, and cooling processes – all possible sources of RS.

Since there are numerous thermal and mechanical parameters involved in steel-plate making, many of them contribute to thermal stress/strain development only at high temperatures and are washed out by subsequent process steps at lower temperatures. Leveling and cooling operational non-uniformity dominate the short-range RS distribution and do not affect long-range stress factors. Consequently, it was determined that primary leveling does not contribute to the stress development in plates. Below 1500×F, cooling mechanisms dominate the long-range RS distributions; for shipyard cutting dimensional accuracy, it is the long-range RS’s that are of concern.

The research team is now conducting a systematic parametric study on long-range RS and will be developing a means to identify RS types (e.g., distribution characteristics and magnitude) for cutting optimizations. This information will assist the shipbuilders in attaining their goals of high accuracy, high- material utilization, optimized cutter-path utilization, and low material scrap. Final results and technical papers on this research project are now available.

For more information see the Point of Contact for this survey.