Lunar Stainless Steel Production and Applications

Stainless Steel is an attractive material for lunar industrial projects, because of its resistance to heat and corrosion. Reaction vessels and plumbing in conditions of moderately high reactivity and temperature, such as in resource extraction, are likely to be the most common applications. The alloy is of predominantly iron, with a large component of chromium, and small amounts of carbon and sometimes nickel. Type 420 stainless steel contains 86.65% iron, 13% chromium, and 0.35% carbon. Type 302 stainless steel contains 73.9% iron, 18% chromium, 8% nickel, and 0.1% carbon.

Due to common terrestrial use, Type 420 stainless steel is well understood. Its combination of exceptional strength, high melting temperature and corrosion resistance make it ideal for reaction vessels of lunar industrial processes up to moderate temperature of around 1400^o C, although pressure vessels may need to operate at lower temperatures to prevent softening of the metal. Alloys commonly have a number of hard "refactory" phases in performance across a temperature range, which may determine the exact composition of the steel for a given application temperature range.

Properties

Stainless steel is so named for its corrosion-resistant properties which, due to the formation of a passive layer, stop further corrosive action when exposed to corrosive material. Stainless steel has low thermal conductivity, several times less than other iron alloys and bronzes, and up to ten times less than aluminum alloys or brasses. Type 302 stainless steel resists corrosion without the heat-treatment type 420 requires, although it is not hardenable by the heat-treatment process as type 420 is. Annealed stainless steel of both types is several times weaker than the heat-treated type 420 stainless steel:

Alloy                      Yield     Tensile    Elongation   Melting   Specific  
                          Strength   Strength                 Range    Gravity
Type 302, Annealed           30        90          55       1400-1420    8.02     
Type 420, Annealed           60        98          28       1415-1510    7.70
Type 420, Heat-treated      240       250           8       1415-1510    7.70
Aluminum 75S, Heat-treated   80        88          10        480-640     2.80

Yield Strength is 0.2% offset, measured in 1000 lb/square inch. Tensile Strength is also in 1000 lb/square inch. Elongation is in 2 inch, by percent. Melting range is in degrees Celsius.

Strength properties of type 420 steel are very attractive compared to aluminum, as the higher weight-to-strength ratio is not an issue for ground-based applications. While other aluminum alloys, such as Alcoa 214 and Alcoa 52S, have excellent resistance to corrosion, their melting ranges remain between 600^o and 650^oC, and their strength is much less than Alcoa 75S (above). Inconel, a nickel-base alloy with 13% chromium and 6.5% iron, has temperature and corrosion resistance characteristics similar to type 420 heat-treated stainless steel, however, nickel is available in much lower concentrations than iron, usually less than 0.1% of bulk regolith, and inconel is half as strong as the steel.

Availability of Raw Materials

Type 420 steel is composed of elements available on the lunar surface in reasonable concentrations. Iron, which composes 15% of the lunar maria and 6% of highland terrain, can be harvested with a simple magnet, although much of the iron is incorporated into nickel alloys or glassy agglutinates and microbreccia fragments. A magnet can also be used to locate local concentrations of iron for extraction. Finally, iron is a byproduct of processes such as illemite oxygen extraction.

Chromium is available in much smaller quantities than iron, between 0.2% and 0.7% in mare basalts, and generally from 0.1% to 0.5% in breccias. However, there is a strong, nearly linear correlation between iron and chromium concentrations, so not only can the two be mined from the same locations and possibly at the same time, but local concentrations of chromium may be located magnetically. (Other elements are similarly correlated with iron and chromium, such as titanium, manganese, vandium, scandium, and, to an extent, sodium. It should be noted that titanium and manganese are both important alloying elements.)

Significant concentrations of carbon are present in the lunar regolith, although unless apparatus to extract volatile gasses is in place, the small carbon component would require importataion from Earth. In the long-term, asteroidal sources may become the most cost-effective.

References

Collier's Encyclopedia
The Lunar Sourcebook: A User's Guide to the Moon