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At ARP, the threads are rolled after heat treating, to produce a beneficial compressive stress in the root of the thread. This counteracts the fluctuating tensile stresses that can lead to fatigue cracking. If heat treatment were to happen after rolling, the compressive stresses would be eliminated and it would reduce the fatigue resistance of the bolt. The bolts and studs are heat-treated for strength in this oven after they have been machined.