Pressure Hull Steel Choices Depend on Missions/Countries

 The debate on pressure hull steel with some relevance to the future Netherlands submarine (Walrus Replacement) issue continues. Below this article on February 5, 2021 an Anonymous commented:

“Concerning the use of austenitic stainless steel (non magnetic) versus the use of dedicated low carbon steel (extra high yield stress) in submarine construction - I did understand that the choice had to do more with the compromise selection relevant to the expected missions; I might be wrong however [Pete comment: Expected missions of the specific submarine class and consumer navy nationality sounds right.]

For a coastal sub (operating in the Baltic [an essential Swedish, German or Polish  characteristic is minimal] magnetic signature. This is very relevant (magnetic anomaly signature decreases at the cube with depth and the sensor has to be very close, may be a fraction of a mile) while the need to operate at great depth (250 /400 meters) and endure the associated cycling fatigue /cracks is not the prerequisite [for Baltic subs].

AIP for short distance , low speed makes also sense . (Israeli Dolphins are really in the "parking lot" off Haifa or the Syrian coast for a second Nuclear strike [especially at Tehran]..)

For an oceanic sub which needs to hide below the accoustic diopter at 150m+ typically, magnetism becomes secondary versus the need to have a lighter structure allowing more batteries for a conventional sub, for instance (we are talking a hundred tons probably for equivalent depth).

Magnetic anomaly detection is probably not really pertinent for a nuclear attack sub à la US Seawolf/UK Astute/French Barracuda.

Within a few min of a datum [a SSN] can cruise at 30 knots at 350 m ..How long does it take for a helicopter or plane to be on the spot and launch an attack in the real tactical world ?

The US, UK or French subs use HY100 or equivalent [pressure hull] steel. I do not know about the Japanese choice.”
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On the same thread a different Anonymous responded (at February 6, 2021 at 12:32 AM) about the Japanese choice of pressure hull steel [for Japan's varying oceanic travel and deep seafloor sitting missions]:

“Pressure hull for Japanese submarine is made of [Japanese naval standard] NS80 (proof strength 784MPa [around HY120 see Table 2 below. ]) and NS110 (1078MPa)[around HY156 also see Table 2 below.]. Welding strength of NS80-NS110 and NS110-NS110 were experimentally proven.”
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The following was originally from the article Submarine Pressure Hull Steel (Tables) Especially Swedish of March 7, 2019:

Pete Comment

All submarines have pressure hulls that are made of special steel alloys. In the past the Soviets/Russians bankrupted their economy by also using Titanium alloys. Essentially pressure hulls protect the crew, sensors and engines from excessive water pressure and provide the airtight conditions the crew rely on to breathe.

Pressure hulls need special high yield steel alloys that are flexible enough to:

-  contract as water pressure increases (during diving), 

-  is also strong enough to stop contracting (at maximum diving depth) and

-   can then expand as water pressure reduces on the way to the submarine surfacing.  

High proof or yield steel for submarines can be measure in pounds per square inch (psi), (eg. HY80 = 80,000psi) or Mega/million pascals (MPa) with this conversion
 https://www.unitconverters.net/pressure/megapascal-to-psi.htm.

A submarine can only go through a limited number of contraction-expansion cycles (especially if it has to deep dive) before it loses flexibility, putting it in danger of metal fatigue.

For a diesel-electric submarine with AIP (eg. a Swedish Gotland class ) on a typical short 12 day mission in the Baltic Sea it may dive and the operate on its Stirling AIP for 10-12 days. It then surface only once = 1 cycle. Perhaps its pressure hull is good for 200 cycles (?) - although its service life is more rapidly limited by hull rust and corrosion - all contributing to a 34 year service life. 

ANONYMOUS' TABLE 1.

Many thanks to Anonymous advising that the pressure hull details in Table 1. below apply to the:

-  Swedish (Gotlands, future A26s), 
-  Swedish designed Australian Collins, and
-  future SAAB-Damen entry for the Walrus Replacement competition 

In the [cylinder like] design of a submarine, the diameter of a pressure hull significantly effects on many other feature of a submarine [including space, propulsion choices and acoustics]. In terms of diameter (which may sometimes equal Beam) Gotland's are 6.2m, A26s 6.75m, Collins 7.8m and 8m for a SAAB-Damen Walrus replacement. Part outer hulls and sonars on hulls will effect the overall beam of a submarine.

Table 1 Similarity of pressure hull of Swedish and Australian submarines

Submarine	Pressure hull steel	Proof (or Yield) strength of steel [MPa]	Beam [m]	Diameter of pressure hull	Estimation
Gotland	Weldox700EM [1]	700	6.2	Same	In A26, sound absorption rubbers (thickness 100mm) are presumably attached on the surface of prssue hull. In A19, no sound absorption rubbers are attached, or very thin rubbers are used.
A26	Strenx700E	700	6.75
Collins	BIS 812 EMA	700	7.8	Same	Difference in beam is due to difference in position or shape of flanl arry sonar.
SAAB-Damen Walrus Replace-ment	Strenx700E	700	8

[1] Weldox is old brand name of Strenx.

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Below is a broader comparison of Chinese, Russian & Western Submarine Pressure Hull Steel Strengths originally published on July 20, 2015. Note the Swedish measure Weldox700 and Australian BIS 812 EMA in both Tables with both at 700MPa equivalent to HY100..

TABLE 2.

YS = Yield Strength or proof stress. HY = High Yield

YS	US	UK	France	Germany	China	Russia and India	Japan	Australia	Sweden
550MPa	HY80	Q1N		1.3964		India DMR- 292 #	* 1) NS56
590MPa			60HLES		921A TA5-A	AK-25 48-OT3
630MPa				1.3974
690MPa	HY100	Q2N				AB-2	* 2) NS70
700MPa			80HLES					BIS812 EMA	Weldox 700
785MPa					980	AK-29	NS80
890MPa	HY130		100HLES		*4) Ti80	AB-3?	NS90		Weldox 900
980MPa						AK-33
1075MPa						AK- ??
1178MPa	HY156						* 3) NS110

* 1) steel of 550MPa and NS56 is equivalent to HY80 grade steel.

* 2) steel of 690MPa and NS70 is equivalent to HY100 grade steel.

* 3) steel 1078MPa and NS110 is equivalent to the HY156 grade steel.

* 4) Ti80 Titanium alloy under development 880MPa equivalent to NS90 or HY128 steel - source.

The Table is from towards the end of Japanese Document "High-tensile steel Summary of national submarine" or Summary of High Tensile Steel for Submarines by Country 2013 http://seesaawiki.jp/w/doramarine/ .
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COMMENTS FOR TABLE 2.

The higher the yield (stronger) the steel alloy or Titanium alloy in a submarines pressure hull the deeper it can dive in operations. As well as alloy strength other factors also influence diving depth including:

- supports for the the pressure hull
- compartments or multiple pressure hulls, and
- strength of the welds used to join sections of the pressure hull.

It is not possible to verify the accuracy of most of the figures. Commenters who know more are invited to supply corrections.

FURTHER READING
 Soryu Submarine Steel Details - Japan Offer to Australia, January 20, 2015

Anonymous and Pete