Generally the plastic like Polypropylene can be used in microwaves. The plastic spur gear with steel core can also be used inside the microwave chamber.

Steel might get hot, particularly if it is magnetic.

Hi,

It's a good idea to have metal parts of non-magnetic material - stainless steel.

Since the wavelength in the microwave system is about λ = 12.23 cm (at 2.45 GHz), the length of the metal elements should not be a multiple of λ / 4.