Once you know that your felony is a temporary disqualifying felony for HAZMAT background checks then you need to make sure its at least 7 years old. If you exponged your felony then you are more than welcome to apply. If you are border line, most likely no one will be able to give you a straight answer and your best option is to apply. If they deny you, you simply wait till your 7 years are up (or 5 years out of jail) and they wont take your license away either. Some people believe that they will take their CDL License if they take the HAZMAT background check but this is not true! You get to keep you normal class A License (or B Class) but you wont be able to get HAZMAT till you are approved!
If you are interested in getting your HAZMAT endorsement visit: TSA Background Checks Appointment Centers