Full Body Annealing (Heat Treatment)
Effects on Mild Steel Microstructure after Welding and Quenching
The most distinctive property of steel over iron is the ductility with strength. Iron in general contains higher carbon and other impurities which make it a hard material but lack ductility. With production process to control chemical composition and microstructure, steel has therefore replaced iron in many applications where ductility is required in addition to strength. Microstructure of mild steels is in the form of ferrite and pearlite (see photo 1). White area represents ferrite while black for pearlite. Area with higher carbon content will become more pearlite. This microstructure results in the mechanical properties which allow material to be ductile enough while maintaining the required strength. The applications of such mechanical properties have since become an integral part of construction industry and engineering solutions.
Mild steel metallography showing microstructure of ferrite (white area) and pearlite (black area)
Zoom‐in metallography of pearlite structure
<Photo 1: showing microstructure of ferrite and pearlite in most steels>
When steel goes through its melting phase and is quenched as exampled in the ERW steel pipe making industry, such ferrite/pearlite microstructure is lost and martensite microstructure (see photo 2) is formed at the melting zone. Martensite is a needle like structure of carbon which turns ductile steel into a hard but brittle material. Steel with martensite structure is therefore not desired in many applications and mechanical failure often results from the martensite structure. In a stringent application, untempered martensite is prohibited in order to prevent any damages caused by failure of steel.
<Photo 2: showing steel being melted and quenched to form a martensite microstructure>
In order to fix such problem, annealing or normalizing is used in curing this martensite structure. This, however, incurs higher cost and time consumption. This prevents many manufacturers from investing in such facilities. Though most steel can be used when enough safety factor has been allowed for, some performance‐based design applications falls in the high risk category if steel is used without understanding the consequences of the steel microstructure. Fire sprinkler industry is a great example for our reference. Generally, steel pipe of ASTM A53 or ASTM A795 schedule 40 is used to convey fluid to sprinkler heads. This schedule 40 pipe has enough
safety factor to allow for the possible failure resulting from the martensite microstrucutre. However, when a Schedule 10 pipe which is thinner than Schedule 40 pipe (see comparison table below) is used, care must be taken to prevent possible failure occurred from the transformation of ferrite/pearlite to martensite phase at the steel pipe weld seam after welding and quenching. In such case, weld seam annealing or normalizing is mandatory. Rigorous testing is needed and third party certification, such as FM or UL, is highly recommended to prevent the building from possible damage and to ensure safety of the buildings. Such institutes lay out frame work on production quality process and control, testing, and auditing, by writing their own specification required for the industry. These specifications are relatively much more stringent than general industry standards
such as ASTM A53 or ASTM A795. By following the FM or UL standards, sprinkler pipes are relatively fail‐safe and the building is protected with proper engineering knowledge.
<Table comparing thicknesses of Schedule 10 and 40 steel pipe>
In summary, as industrialists and engineers, we urge for prudence in choosing proper materials for any construction applications. Designers and contractors need to understand basic engineering materials in order to select the right material and to prevent a catastrophe from happening. When specifying a steel material, care needs to be taken when design is pushing the limit of such material, and the steel microstructure comes to play an important role in deciding the life and usage of the material which in turn determines the protection level of the building.