Are class 150 & 300 malleable iron pipe fittings steam rated?

Yes, both classes of malleable iron pipe fittings are rated for saturated steam pressure.

CLASS 150 PIPE FITTINGS - Black and Galvanized Malleable Iron pipe fittings are rated by pressure class. Class 150 is rated at 150 psi for saturated steam pressure which has a maximum rating of 300 psi WOG (Water Oil Gas) @ 150°F.

USES : Low-pressure plumbing applications including air, water, natural gas, and steam.

CLASS 300 PIPE FITTINGS - Black and Galvanized pipe Malleable Iron fittings are rated by pressure class. Class 300 is rated at 300 psi for saturated steam pressure which has a maximum rating of 300 psi WOG (Water Oil Gas) @ 72°F.

USES: Higher pressure plumbing applications including air, water, natural gas, and steam

The chart below can be found on page 5 of the ASME B16.3 Standard.