January 12, 2011
This is part of a multi year Navy prototyping contract. This project is one of the largest and definitely the heaviest piece of steel we have worked with. Both the dome and plate are HY-100, an alloy developed in the sixties at the request of the US Navy. HY-100 is most notably used in ship hull and submarine applications. The plate is a massive 2.5″ thick and weighed in just under 8,000lbs before cutting. We had schedule to have a forming company here in San Diego handle the cutting of the ring halves for the base plate, but unfortunately they could not handle the size or weight of the plate. With help from RJ Kates Co, a whole lot of oxygen, propane, and a little robot we designed a jig to guide the robot and made all the cuts ourself. As you can see in the photos, the cut quality was amazing for a torch. The ring halves received a 60* chamfer on both sides then welded to form the base ring.
Welding HY-100 not only requires special wire, but a specialized fabrication spec in order to ensure the material’s stability. Preheating is required for the thicker plates to slow down the cooling rate after welding. This helps in avoiding HAZ cracking and weld cracking. Preheating is required to overcome susceptibility to Hydrogen Induced Cracking in the weld heat affected zone (HAZ). This form of cracking occurs especially in high strength steels.
During the cutting process, it is necessary to ensure plate steel temperatures were at a minimum of 60 degrees fahrenheit.
During the welding process it is necessary to preheat the weld zone to a minimum of 250 degrees fahrenheit and maintain that temperature throughout the welding process. It is also necessary that the interpass temperature does not exceed 400 degrees fahrenheit.
Preheating, interpass temperature control and post weld heating operations essentially reduce the dissolved hydrogen content by allowing the hydrogen to escape by diffusion from the steel while also allowing transformation of the weld metal and more importantly the adjacent HAZ to a less susceptible microstructure that might reduce the peak residual tensile stress as well.