go to the track view curve editor under Graph editors>track view curve editor… of course you gotta have your sphere selected the curve editor shows u the rotation curve
drag select both keys (1-80) pick the “set tangents to linear” and thats it
If you animate the sphere further you can explore whit other tangents properties, if you want linear interpolation in the whole animation you may right click in the transform axis at the left and go to assign controller and pick linear float but if u do so the animation curve would be linear all the time for the transform you assigned, remember that rotation, position and scale have its own curves for each xyz axis
if you use max 6 the default key interpolation is Bezier float, i think is not bad.
You can also use the “open mini curve editor” wich is before the time line down to the left
Hope it helps, once you use the curve editor you’ll find that it is not complicated