Can I use a 9sp chainring set on my MTB which has a 8 speed Shimano CS-HG70 Cassette Just installed the CS-HG70 cassette and while doing some tuning, noticed that my chainring is no longer true, especially around the crank so am thinking about replacing the chainring set but can not find a decent 8 speed set but see a lots of 9 speed chainring set, can I use that instead?

Thanks
Ideally, you should stay with gearing designed for the number of gears on your bike. If you must use a different design, stay as close as possible to the original, and make sure you go to a larger number of gears, like the 9 speed you suggest.

If you do use a 9 speed chainring set, be prepared for your shifting to suffer to some degree. (How much depends on the exact brands and specifications of your original, and the new set you pick. No, I don't have any way to tell you what to pick.)

It's also possible that you may occasionally drop your chain unpredictably. If it happens even once, then you need to find a good 8 speed set again. 

This is the 9 speed compatibility chart from Shimano.



This is the 6/7/8 speed compatibility chart from Shimano.



As you can see, there are no cranks listed as duplicated on both charts. /There is also a note indicating sub par shifting if even some closely interchangeable parts are used.