High-strength steel welding, monitoring and other functions are now highly automated including computer dependent, particularly in well established assembly lines. (Photo courtesy Design News and Bosch Rexroth)
Japan is particularly concerned that defence exports to customers/allies might increase the chances that Japan's submarine secrets (including its hull steel characteristics, cutting and welding techniques) might pass to potential enemies. A particular threat is passage of secrets to China. Particular concerns are China's ability to reverse engineer or develop counter-actions against allied weapons like the F-35.
Regarding the specific and sensitive issue of the operational depth of submarines. This depth is not soley dependent on the steel strength estimates discussed below. There are so many other determining factors including weld strength and hull structure.
Noone knows operational or crush depth except designers and naval officers who operate the submarines. Detailed computer simulation and materials testing is required to calculate pressure distribution on submarine hulls.
Chinese submarines appear to be rapidly improving. Chinese submarines used to be clearly inferior to US or Japanese submarines, but now, the situation is changing. Ignorance of such rapid changes makes people too optimistic. The latest Chinese subs may be superior to Collins subs at least in hull strength. Australian and Japanese hull strengths therefore need to be improved.
Documentary Comparisons of Chinese and Japanese Submarine Steel Strengths
Chinese submarine hulls appear to made of "980 steel" with a yield strength of 800 MPa. China's "980" and Japans NS 80 are nearly the same. Their chemical components and mechanical strengths are very similar - see http://www.mod.go.jp/trdi/data/pdf/G/G3111C.pdf , page 3, table 3, chemical composition (NS80E) 4.2.1 left column.
See the following document: Shanghai Jiaotong University, School of Naval Architecture and Ocean Engineering's, "High Crack Propagation of Steel 980 Welded Joiont under Spectrum Load” (Chinese), http://image02w.seesaawiki.jp/d/e/doramarine/16dc6f08dbd1d96c.pdf
Page 5, “ 2.2 Fatigue Testing indicates:
“Test material for high-strength structural steel submarine 980 steel welded joints. 980 steel (VHD402), yield strength σs> = 800MPa, China developed high strength, high toughness, can be welded with a submarine pressure hull.” See Page 5, Table 1 Chemical component (Steel and Weld for upper and lower rows respectively), Table 2 Mechanical Property (ibid).
Using conversion site http://www.convertunits.com/from/psi/to/MPa 800 MPa = 116,030 psi = "HY-116".
Returning to G3111C.pdf
Soryu Submarine Steel Details - Japan Offer to Australia of January 20, 2015 very briefly describes http://www.mod.go.jp/trdi/data/pdf/G/G3111C.pdf , Within that document see page 4, table 4.2.1 left column, which seems to indicate the Soryu has Japanese naval steel measure NS80 = 80kgf/mm2 which converts to 113,760 lbf/in2 or HY-114.
So assuming the translations and conversions are correct China's use of HY-116 steel is very similar to the Soryu's HY-114.
Soryu Submarine Steel Details - Japan Offer to Australia also discusses NS110 which may also be used in the Soryu. NS110 means 110kgf/mm2 proof stress converting to 156,414 lbf/in2 or HY-156 . HY-156 using simple linear calculations allow the Soryu an operating depth of just over 600 metres.
It has to be pointed out, however, that operational depth is not soley dependent on steel strength. There are so many other determining factors including weld strength and hull structure.
Noone knows a submarine's operational or crush depth except designers and naval officers who operate the submarines. Detailed computer simulation and materials testing is also required to calculate pressure distribution on submarine hulls.
Anonymous and Pete