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Comment by ortusdux | Hacker News Reader

There are plenty of things to control for when designing and producing steel. An important consideration for some applications is the brittle to ductile transition temperature. Some steels have a DBTT around 0°C, which means that they loose most of their impact resistance below that temp. By adjusting the chemistry, processing methods, and post-processing methods, you can adjust the DBTT of steel down below -70°C. My guess is that they were being paid to supply steel with a DBTT below -70°C, and that the contract included proof testing. The standard testing method would be a Charpy impact test at various temperatures. It sounds like they were making complex castings, so they probably had some non-standard testing requirements.

More info:

https://www.punchlistzero.com/dbtt-ductile-brittle-transitio...

https://knifesteelnerds.com/2018/12/21/why-cold-steel-is-bri...

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Its interesting, but poor quality steel with sulphur contamination is one of the theories behind the failures of the titanic.

>Metallurgical and mechanical analyses were performed on steel and rivet samples recovered from the wreck of the RMS Titanic. It was found that the steel possessed a ductile-to-brittle transition temperature that was very high with respect to the service temperature, making the material brittle at ice-water temperatures. This has been attributed to both chemical and microstructural factors. It has also been found that the wrought iron rivets used in the construction of Titanic contained an elevated amount of incorporated slag, and that the orientation of the slag within the rivets may hold an explanation for how the ship accumulated damage during its encounter with the iceberg.

Source: https://www.govinfo.gov/content/pkg/GOVPUB-C13-17a17f71ae2f9...

Yeah, even a small % of sulfur can shave off 50% or more of your strength. A small amount of phosphorus (<1%) can raise the DBTT above 0°C. The average ocean temp at 200m is 4°C, so you could understand why the steel chemistry is important in subs. Luckily, these can be tested for non-destructively.

So sounds like designing for surface collisions (other vessels, icebergs) in a very cold area maybe?

The Navy is understandably cagey about how they surface through ice-sheets, but this Smarter Every Day video is the best summary I have seen. https://youtu.be/XFJnWp1tAdU

Basically, some part of the sub needs to be strong enough to reliably break ice at sub-zero temps. The rest needs to maintain its strength at those temps.

Yes, U.S. Navy nuclear submarines go under the polar ice caps somewhat frequently -- as did Soviet (and do Russian) nuclear submarines as well. It's a good place to hide from prying eyes, as not only can ships not (easily) follow you there, but the sound produced by all the ice moving and grinding makes passive sonar quite ineffective.

The YouTube channel Smarter Every Day recently had a video series on a nuclear submarine under the arctic ice.

Presumably it makes you invisible to any magnetic anomaly detection via the air or satellites as well.

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