Straightening: straightness after quenching and tempering

During hardening and tempering, stresses and changes in the microstructure develop in the steel that can distort a long, slender part: the bolt comes back from heat treatment slightly bent. For short bolts, this does not matter, but for long studs, tie rods, reduced-shank bolts and threaded rods several hundred millimetres long, it matters a great deal. A bent bolt is difficult to pass through the bore, the nut bears on one side, and thread rolling produces an uneven thread. Straightening brings the part back within the straightness tolerance.

What straightness is required

ISO 4759-1 specifies straightness tolerances for bolts depending on length and diameter; drawings for turbine or press bolts often specify tighter values in the form of runout. Measurement is carried out between centres or on V-blocks with a dial gauge. At RS Randack, the straightness check is part of the final inspection, but for long parts it is already done before thread rolling, because the thread needs a straight shank.

How straightening is done

On the straightening press, the bolt is pressed between two supports in a controlled manner against the curvature until it springs back elastically and remains straight. This requires experience: high-strength materials from 10.9 tolerate only small straightening deflections; with 12.9 and hardened parts, straightening is avoided wherever possible, and distortion is instead prevented by machining allowance and finish machining after quenching and tempering. After straightening, the part is measured again and crack tested if necessary. Where drawings exclude straightening, quenching and tempering remains the responsibility of the heat treatment shop, for example by hardening long parts in a suspended position.

What you should specify

Straightness or runout tolerance, whether straightening is permitted, whether a dimensional report is required.