Weld able steels are commonly supplied in heat treated
conditions. The manufacturers provide a material testing certificate which
contains manufacturing conditions for that particular material. An inspector
conforms materials certificate to that required in codes and standards.
One such manufacturing condition is heat treatment of prepared steel plates
(for welding). Below is a brief overview on types of thermal treatments which
are normally employed to steels before supplying for welding.
1. Hot Rolling
In order to give strength and toughness to low strength
carbon steels, hot rolling is performed during manufacturing. What is Hot
Rolling? It is working (rolling) of material to required size, at a specific
temperature and after that material is allowed to cool in air. Variation in
rolling temperature implies variation of toughness and strength in material
plates.
2. Normalizing
When the material is rolled to certain plate size, it is
subjected to normalizing. What is Normalizing? It is a type of thermal
treatment in which the material is given heat about 900 - 980 degree Celsius
and after that it is allowed to cool in air at a particular cooling rate. This
imparts strength and toughness in low alloy steels.
It’s quite different from hot rolling because later involves
rolling at high temperature and variation in properties occurs while in
normalizing the material is heated after rolling and uniform mechanical
properties are achieved for a specific material grade.
3. Thermo-control Rolling
It is a type controlled hot rolling in which the material is
subjected to rolling within a controlled temperature range. The working
(rolling) of material to specific size is also mechanically controlled along
with thermal treatment. That is why the technique is also know by the term
Thermo-Mechanical Rolling. It is employed to cryogenic steels to give sound
toughness at low temperature applications. Moreover High strength low alloy
steels are also processed by this technique to optimize their properties.
4. Quenching and Tempering
It’s a type of heat treatment in which the rolled material
is heated to about or above nine hundred (900) degree Celsius normally above
upper critical temperature line and then subjected to rapid cooling by mean of
some cooling medium; oil or water. This rapid cooling is normally referred to
as quenching which may optimize strength but the material loses ductility. In
order to gain required ductility the quenched materials is processed by softening
technique, called Tempering. The technique is normally employed at wide range
of different temperature to improve the required mechanical properties of weld
able low alloy steels.
5 Solution Treatment
In this technique, the cold or hot worked material is
subjected to heat (at about eleven hundred degrees Celsius 1100 oC).
After heating to required temperature it is allowed to cool rapidly by the
process called quenching. Quenching is the fast cooling method by simply
immersing the hot material in the water or oil. Fast cooling wouldn’t allow the
formation of other phases in microstructure. This type of heat treatment is
normally referred to as Solution Annealing. For welding austenitic stainless
steel family are normally supplied in solution annealed condition.
Last Comments
The above described thermal treatments are applied on
weld-able steels before supply for welding and joining. The type heat treatment
which is employed to weldment for stress relieving is referred to as Post Weld
Heat Treatment.