Clean steel technology will be incorporated into NGNN foundry standard operating procedures for hull insert castings, and is applicable to other VCS components and marine structures. Northrop Grumman Shipbuilding-Newport News photo
The objective of the project was to reduce casting costs and delivery time by improving the cleanliness of high-strength steel during melting and casting, thereby increasing casting quality, improving mechanical properties and reducing rework.
Current steel castings of VCS hull inserts at NGNN have recurring problems with inclusions (i.e., foreign particles – either metallic or non-metallic), porosity (gas and/or shrinkage) and internal hot tears. In addition to these defects detected by magnetic particle inspection, all hull insert castings are x-ray inspected, often revealing additional defects. The repairs needed to correct these defects drive up cost and time needed for delivery of these castings.
The Navy Metalworking Center worked with Northrop Grumman Shipbuilding - Newport News (NGSB-NN) and the Virginia Class Submarine (VCS) Program Office to evaluate current VCS hull insert casting problems, such as inclusions (i.e., foreign particles) and entrapped gas. Using the information gathered, test castings were prepared based on identified clean steel practices.
The identified clean steel practices are expected to lower costs by reducing rework and reheat treatments. This will result in an estimated annual savings of $700K to the Navy. Casting delivery time will also be improved by an estimated 55 days for large steel castings.
The clean steel technology developed under this project is being incorporated into NGSB-NN Foundry standard operating procedures for hull insert castings and is applicable to other VCS components and marine structures. The following technologies / activities have been successfully demonstrated and have been implemented into casting process designs: (1) filters in gating systems, (2) specialty sands in selected areas, and (3) appointment of a full-time sand manager. Improved sand processing methods have resulted in more consistent sand properties.