420 is a martensitic stainless. Roughly 14% Cr with no Ni or Mo. It is hardenable by heat treatment, magnetic, and has limited corrosion resistance. The mechanical properties are a function of the heat treatment.
The room temp modulus is 30 million psi,
coef of thermal exp at room temp is 5.7 ppm/F,
the density is 0.276#/cuin,
and a thermal cond of 13.5 BTU/hr-ft-F.
304 is an austenitic stainless, roughly 18% Cr and 8% Ni. When annealed it is non-magnetic, very ductile and not very strong. This alloy can only be strengthened by cold working.
E rt = 28 million psi
CTE rt = 9.6 ppm/F
density = 0.287#/cuin
therm cond rt = 9.4 BTU/ft-hr-F
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Corrosion never sleeps, but it can be managed.