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.
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.
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.