I have well water that is very hard, a lot of iron, and has sulphur. Will the salt used in a water softener hurt the concrete septic tank and or the leach field?

Not if you'll tell me where the backwash terminates. Back to you, Tom

Salt is an antiseptic, it does two things to damage your septic and drain field.
One, it kills the microbes that you need to convert your toilet into carbon dioxide and methane gases, that means the tank will fill up quickly and cost to have it emptied.
Two, it hardens the soil round your drain field and makes it water proof, then you need a new drain field that costs thousands.
Used correctly, that is to say, only used for things you have already eaten, a septic tank and drain field will need to be emptied every 20 to 30 years. That means, used by the correct number of people.