you need to use heat to prevent cracking. The towel and torch method mentioned before is in my opinion the best option. You dont even have to go to red hot, anything above 400 F greatly reduces the chances of cracking.
If you haven't done this already I second the heat option. I heat with a torch and straighten in a bench vice. As JMJones said it doesn't need to be red hot, you can do it easily at tempering temps, 400f ish.