It's important to note that the first stage (which is about 70% of the rocket) is the part that is being reused. The top of the rocket (the second stage, the fairings, and the payload) are not reused (At least not yet. They are working on recovering fairings and there are rumors of second stage reuse).

When the stages separate, the first stage is going about 1.5 - 2.5 km/s. Then the second stage takes the payload the rest of the way to orbital speeds of 7+ km/s, and the first stage returns to earth.

So the first stage is going fast (several thousand miles per hour) but not as fast as things that re-enter from orbit. It still does need heat shielding, but not as much as the space shuttle or a capsule.

They've worked hard to get rid of pieces that have significant re-engineering. A good example is the grid-fins which have been moved to titanium, because they old ones were being worn away.

There is a heat-shield in the button of the rocket. PICA-X is a technology that SpaceX is working on and have been for a long time.

There is heat coating on the new rest of the rocket.

presumably a combination. Most parts on the rocket don't actually see that much stress. Find the parts that take the most stress. Then either make them stronger; or make them quick/easy to replace quickly and cheap to manufacture.

The retropropulsion actually helps shield the rocket from heat.

https://space.stackexchange.com/questions/13935/how-does-the...