2. HEAT STRAIGHTENING
> Heat straightening is a repair procedure in which controlled
heat is applied in specific patterns to the plastically deformed
regions of damaged steel in repetitive heating and cooling
cycles to gradually straighten the material.
> Heat straightening is distinguished from other methods in
that force is not used as the primary instrument of
straightening.
3. THE PROCESS IS CHARACTERIZED BY THE FOLLOWING
CONDITIONS WHICH MUST BE MAINTAINED:
1. The temperature of the steel does not exceed either (a) the
lower critical temperature (the lowest temperature at which
molecular changes occur), or (b) the temper limit for quenched
and tempered steels.
2. The stresses produced by applied external forces do not
exceed the yield stress of the steel in its heated condition.
3. Only the regions in the vicinity of the plastically deformed
zones are heated.
4. A clear distinction should be made for two other methods
often confused with heat straightening:
Hot Mechanical straightening and Hot Working
Hot mechanical straightening
- differs from heat straightening in that external force is applied
after heating to straighten the damage. These applied forces
produce stresses well above yield, resulting in large
movements during a single heat cycle.
5. The results of this type of straightening are unpredictable and
little 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 carrying
elements when replacement or other methods are not viable.
6. 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.
7. The results of this process are highly unpredictable and may
result in:
1. Fracture during straightening.
2. Severe changes in molecular structure which may not be
reversible.
3. Severe changes in mechanical properties including a high
degree of brittleness.
4. Buckles, wrinkles, crimps, and other distortions.
Hot working should not be used to repair damaged structural
steel.
8. Why Heat Straightening Works
The basic concept of heat straightening is relatively simple and
relies 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 original
shape when released. Rather, it remains partially elongated or
shortened, depending on the direction of the originally applied
force.
