HEAT STRAIGHTENING> Heat straightening is a repair procedure in which controlledheat is applied in specific patterns to the plastically deformedregions of damaged steel in repetitive heating and coolingcycles to gradually straighten the material.> Heat straightening is distinguished from other methods inthat force is not used as the primary instrument ofstraightening.
THE PROCESS IS CHARACTERIZED BY THE FOLLOWING CONDITIONS WHICH MUST BE MAINTAINED:1. The temperature of the steel does not exceed either (a) thelower critical temperature (the lowest temperature at whichmolecular changes occur), or (b) the temper limit for quenchedand tempered steels.2. The stresses produced by applied external forces do notexceed the yield stress of the steel in its heated condition.3. Only the regions in the vicinity of the plastically deformedzones are heated.
A clear distinction should be made for two other methodsoften confused with heat straightening:Hot Mechanical straightening and Hot WorkingHot mechanical straightening- differs from heat straightening in that external force is appliedafter heating to straighten the damage. These applied forcesproduce stresses well above yield, resulting in largemovements during a single heat cycle.
The results of this type of straightening are unpredictable andlittle research has been conducted on this procedure.Specific concerns about hot mechanical straightening include:1. Fracture may occur during straightening.2. Material properties may be adversely affected.3. Buckles, wrinkles or crimps may result.Its use should be considered only for non–load carryingelements when replacement or other methods are not viable.
Hot working- is distinguished from heat straightening in that both large external forces and high heat are used. This method is similar to hot mechanical straightening in that external forces are used.- In addition, the steel is heated well above the lower critical temperature.
The results of this process are highly unpredictable and mayresult in:1. Fracture during straightening.2. Severe changes in molecular structure which may not bereversible.3. Severe changes in mechanical properties including a highdegree of brittleness.4. Buckles, wrinkles, crimps, and other distortions.Hot working should not be used to repair damaged structuralsteel.
Why Heat Straightening WorksThe basic concept of heat straightening is relatively simple andrelies on two distinct properties of steel:- If steel is stretched or compressed past a certain limit(usually referred to as yield), it does not assume its originalshape when released. Rather, it remains partially elongated orshortened, depending on the direction of the originally appliedforce.