Surly
Contributor
Has anyone tried to do a metal injection molded tank?
Stainless Steel Tanks?
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For scuba or scba service, I don't think so. Consistent production would be very hard that way.
Surly
Contributor
The reason I ask is that the technology appears to be improving. Kershaw is now selling a knife w/ a 440C MIM blade (the Offset). The reviews have been quite impressive.
A huge difference between a 4" folding knife blade and a 28" cylinder of course.
A huge difference between a 4" folding knife blade and a 28" cylinder of course.
Most modern steel tanks are stamped 3AA or with a special permit. The 3AA tanks are built under CFR-49-I-C Section 178.37 (Code of Federal Regulations 49).
These steel tanks per the code are seamless steel cylinder made of chrome-molly steel and the most common alloy is 4130, but several other similar alloys are also specified in CFR49. Per the code the maximum allowed design stress using the design testing pressure (as the applied load) is 80% of Yield Strength, but not to exceed 70,000 PSI.
Aluminum tanks are stamped 3AL and are built under CFR-49-I-C Section 178.46. The maximum design stress using the design testing pressure is again 80% of yield Strength. The code also specifies the yield strength for 6061-T6 is 35,000 PSI. This makes the allowable 28,000 PSI.
Since the strength ratio from steel to aluminum is 2.5 to 1, the aluminum tank wall thinness has to be approximately 2.5 times thicker than a steel cylinder. This works out to be great because aluminum is approximattedly1/3 the density of steel (Aluminum = 0.1 LB/ cu. inch, Steel = 0.29 LB/ cu. inch).
To be specific the wall thickness of aluminum 80 is about 0.491 inches. For a traditional steel 72 the wall thickness is 0.185 inches. This is a ration of 2.6 to 1.
IMO the biggest problem with most stainless steels is that the strength is nowhere near the high strength chrome-molly steel (including the new special permit steels). If I remember correctly (I am at home, my engineering data books are at work) the yield strength for 304SS is only 30,000 PSI. There are other higher strength stainless like 17-4PH, but IMO it would be impractical and cost prohibitive to make a tank out of 17-4PH.
The density of Stainless Steel is essentially the same as any steel.
A tank made out of common stainless like 316 etc. would have to weight about 70 to 80lbs (due to the required wall thickness) rather than the 30 to 40 Lbs for most SCUBA tanks (steel or aluminum). This would make the SS tank about 40 Lbs negative in the water (or more).
Cousteau had tanks made out of Titanium, but I dont know the reason why they werent successful. Titanium is a relatively new metal for western engineers. Most Titanium comes from Russia so it was considered exotic during the cold war era. We are quickly developing new Titanium alloys, fabrication, and machining practices.
On the other hand I have steel tanks that are over 50 years old and they should last another 50 easily. Galvanized steel tanks will last for a very long time if no water or humidity is allowed inside.
I am sorry for the long post. My figures should be correct, but as I mentioned my engineering references are at work. Professionally, I have performed plenty of pressure vessels calculations, but mostly ASME code or to meet customer specifications. Due to personal interest in SCUBA tanks, I have also performed the design calculations per CFR49 on a number of DOT cylinders. If you read CFR49, the design calculations are actually very simple.
These steel tanks per the code are seamless steel cylinder made of chrome-molly steel and the most common alloy is 4130, but several other similar alloys are also specified in CFR49. Per the code the maximum allowed design stress using the design testing pressure (as the applied load) is 80% of Yield Strength, but not to exceed 70,000 PSI.
Aluminum tanks are stamped 3AL and are built under CFR-49-I-C Section 178.46. The maximum design stress using the design testing pressure is again 80% of yield Strength. The code also specifies the yield strength for 6061-T6 is 35,000 PSI. This makes the allowable 28,000 PSI.
Since the strength ratio from steel to aluminum is 2.5 to 1, the aluminum tank wall thinness has to be approximately 2.5 times thicker than a steel cylinder. This works out to be great because aluminum is approximattedly1/3 the density of steel (Aluminum = 0.1 LB/ cu. inch, Steel = 0.29 LB/ cu. inch).
To be specific the wall thickness of aluminum 80 is about 0.491 inches. For a traditional steel 72 the wall thickness is 0.185 inches. This is a ration of 2.6 to 1.
IMO the biggest problem with most stainless steels is that the strength is nowhere near the high strength chrome-molly steel (including the new special permit steels). If I remember correctly (I am at home, my engineering data books are at work) the yield strength for 304SS is only 30,000 PSI. There are other higher strength stainless like 17-4PH, but IMO it would be impractical and cost prohibitive to make a tank out of 17-4PH.
The density of Stainless Steel is essentially the same as any steel.
A tank made out of common stainless like 316 etc. would have to weight about 70 to 80lbs (due to the required wall thickness) rather than the 30 to 40 Lbs for most SCUBA tanks (steel or aluminum). This would make the SS tank about 40 Lbs negative in the water (or more).
Cousteau had tanks made out of Titanium, but I dont know the reason why they werent successful. Titanium is a relatively new metal for western engineers. Most Titanium comes from Russia so it was considered exotic during the cold war era. We are quickly developing new Titanium alloys, fabrication, and machining practices.
On the other hand I have steel tanks that are over 50 years old and they should last another 50 easily. Galvanized steel tanks will last for a very long time if no water or humidity is allowed inside.
I am sorry for the long post. My figures should be correct, but as I mentioned my engineering references are at work. Professionally, I have performed plenty of pressure vessels calculations, but mostly ASME code or to meet customer specifications. Due to personal interest in SCUBA tanks, I have also performed the design calculations per CFR49 on a number of DOT cylinders. If you read CFR49, the design calculations are actually very simple.
Aircraft fire suppressant pressure vessels can be stainless or titanium. These are welded spheres. That is the ideal shape for a compressed gas vessel, although not practical for diving. You may see this shape used on rockets and spacecraft.
I believe there is some carbon in the chrome moly alloy. Carbon does things to help the process. You can anneal carbon steel to make it soft and easy to draw and spin, then heat treat it for hardness and strength after you have the shape you want. Other steels can do this, but not as well.
If you haven't noticed, I am a metallurgist, and was in the business.
Happy diving!
I believe there is some carbon in the chrome moly alloy. Carbon does things to help the process. You can anneal carbon steel to make it soft and easy to draw and spin, then heat treat it for hardness and strength after you have the shape you want. Other steels can do this, but not as well.
If you haven't noticed, I am a metallurgist, and was in the business.
Happy diving!
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