What Is Straightness?
Straightness is the shortest distance between two points. When using a compass or a laser one thinks of straightness as 180 degree line. When talking TGP a straight bar can be defined as one without a bend or curve.
Bars bend when under stress or when stresses are exerted on them. When the stresses are removed the bars, due to its elasticity, revert back to their original shape unless the stresses exceed the yield point of the material. If the stresses exceed the yield point and the stresses are removed, the deformed bar will not revert to its original shape.
In the cold finished bar industry stresses are measured in pounds per square inch or PSI. KIP can also be used where one K is equal to 1000 P. Different metallurgies have different yield points. A low carbon 1018 bars will have a lesser yield point than a medium carbon bar. The higher the carbon in a bar the higher the yield point. The addition of alloys also increases yield points.
Different metallurgies have different modulus of elasticity. The modulus of elasticity is expressed mathematically as stress divided by strain. Since the modules are "mathematical constants" stress is always proportional to strain and stain is always proportional to stress, until the yield point is exceeded.
The tensile point of a bar is greater than the yield point. When stresses reach the tensile point the bar breaks. The zone between the yield point and the tensile point is called the elastic deformation zone or plastic zone. In order to successfully straighten a bar stresses greater than the yield point but less than the tensile point must be exerted on the bar during straightening process.
Hot Rolled bars become bent and remain bent because one time stresses exceeding the yield point were exerted on the bar. Such residual stresses are the result of the rolling process or when the bars were cooled after rolling. Heat treated bars all have residual stresses due to temperature gradients in the bar and/or due to the dilation or expansion of the bar in the cooling process.
The greater the residual stresses the more difficult it is to straighten the bars.