Best thing to do is call your airlines and find out what they say . . .
Call the airlines you are traveling on and ask them for their recommendation for traveling with your Kettlebell. You will ultimately have to describe in detail what is is and what it weighs.
I highly recommend that you travel with a catalogue or book and make sure to pack the Kettlebell with whichever item you bring along, so they can inspect the book or catalogue, while they are trying to figure out what exactly you are traveling with.
I'd also recommend that in whatever type of case you pack the KB in, that you also pack the KB in a box and make sure that you mark the box with a description of the item:
35 pound Weight Training Tool
Cardiovascular Training Equipment
Anything that you can do to describe the item that you're traveling with will help your cause.
Whatever you decide to do, make sure that you pack it in a heavy duty luggage or backpack or a hard-sided case. If it isn't reinforced or a very strong piece of luggage, you may pay the price. I just recently heard from a woman who lost her 12kg, because it ripped through the seams of her baggage.