If they’re stainless steel throw them in the dishwasher.
Vinegar and baking soda.
Krud Kutter is my go to for anything that needs degreasing. Spray on and let it sit and then rinse off. #affiliatelink https://www.amazon.com/KK32-Original-Concentrated-Degreaser-32-Ounce/dp/B000S8EQFO/ref=as_li_ss_tl?crid=3E3KZDEYJ6YJL&keywords=krud+kutter&qid=1555260203&s=gateway&sprefix=krud+kutter,aps,605&sr=8-3&linkCode=ll1&tag=craftylittleg-20&linkId=070896e0e96c568ee989408a792918b2
I find the easiest way to clean greasy grates or even the aluminum exhaust fan filter is to use Cascade automatic dishwasher detergent, hot water and and soak in a pan or bucket overnight, so you can empty outside. You do not want the grease to go down your drain.
If they are bad try using Greased Lightening. It is very strong so make sure to clean them outside and wear a mask. https://amzn.to/2GlSb2M