There are so many factors involved with the selection and subsequent construction of the RAN’s SSN’s, one of them is what steel is to be used in the pressure hull. Obviously, strength is the primary consideration but there are a number of other factors such as corrosion resistance, reduced magnetic field disturbance, etc and DST has been involved in the research to find the optimum steel alloy as part of the planned Attack class build process. According to this
Australian Defence article, Bissalloy delivered 250 tonnes of a moly steel that had been found to have the best balance of properties for prototyping work before the Attack class was cancelled. Bissalloy had been involved (with Bluescope) in the development and supply of the steel used in construction of the Collins class which has proven to meet all the requirements, including longevity with the imminent Life of Type Extension (LOTE).
Now that the focus has switched to the future SSN’s, the selection of steel takes on another dimension. The Astute class is manufactured from British Q1N grade steel which has similar strength to the HY80 used in the Los Angeles class and the Virginia class has HY100 steel which is stronger but is difficult to weld and requires special welding techniques. The BIS812EMA steel used in the Collins is a little bit stronger (including bulge testing) than HY100 and has a lot of improved properties, including being lighter and easier to weld. An advanced version of this would make a good choice for the SSN’s but then creates more design decisions as it’s slightly lighter and thus affects the ballast requirements of the vessels. It could also cause more complex issues if sections of the vessel have to be constructed overseas, eg, if the Virginia class is chosen and it was decided that the hull section containing the reactor compartment is to be manufactured in the US, would a section made from HY100 be able to be welded to hull sections made from a different steel, or would the Bissalloy steel have to be shipped to the US for use in their construction of the reactor hull section? This would require them to adjust to different handling and welding techniques from those currently in use.
When people scoff at the idea of local steel being used instead of the OE steel, the US discovered that substandard steel was used for building Navy submarines between 1985 and 2017, a problem that investigators discovered in 2017 and that was first reported in 2020 so, we would be able to better manage quality control of the steel locally.
The following table gives an indication of steels used in submarine construction.
Table courtesy of Submarine Matters