Cold forming is perhaps the best description of the thread rolling process.
Here's how it works:
Static tensile tests on parts formed through thread rolling show overall increases in strength of about 30 percent. Fatigue strength is reported to be improved by 50-75 percent. Tests on bolts first heat-treated to a hardness of 35-40 Rockwell C and subsequently rolled show even greater fatigue strength.
In thread rolling, thread grain structure is not severed; instead, it's reformed in continuous, unbroken lines following the thread contours. Rolled threads have increased resistance to stripping because such failures are compelled to take place across, rather than with, the grain flow.
Threads are produced with burnished roots and flanks, free from surface imperfections that might prove to be starting points for fatigue failure. Surface layers of the thread, particularly those in the roots, are stressed in compression. These compressive stresses must be overcome before the tensile stresses that cause fatigue failure can be built up.