The possibility of welding stainless steel is a used in various fields, thanks to the technical and mechanical characteristics of the metal itself. But is it really always possible, making steel the best usable material, or are there any contraindications?
Let’s find out together what we need to know about weldability and welding of stainless steel.
Weldability of steel
First of all, it is necessary to define what the weldability of steel is and on what factors it depends: it is the ability of the steel itself to bind, through the use of welding techniques, to elements of different materials (heterogeneous welding) or of the same material (autogenous welding).
Being a property of steel, it is not a measurable physical quantity and is closely linked to the desired final characteristics. Depending on the application of the metal, a certain mechanical strength, high toughness or magnetic permeability may be preferred.
Generally, a steel with high weldability, has the following fundamental characteristics:
- mechanical resistance suitable for use
- high tenacity and workload bearing capacity
- lack of surface and internal defects such as oxides, porosity, gluing, etc.
In order to guarantee the respect of these characteristics, the conditions and the procedure with which it is intended to weld (operational weldability), the chemical-physical transformations that take place in the structure of the steel itself (metallurgical weldability) and the properties of the welded structure (constructive weldability) must be considered.
Metallurgical weldability heavily affects the other two factors, making it necessary to identify three critical areas in the weld joint:
- Altered thermal zone, ZTA: is the point directly affected by heating, at a temperature between 700° and 750°.
- Under-beam zone, ZSC: is the point closest to the Melted Zone, which is heated to more than 750°.
- Melted zone, ZF: is the point where the mixing takes place between the filler metal and the base metal, where the most important chemical variations occur.
The ability of a steel to maintain, within certain limits, the hardness values in the ZTA is considered an index of metallurgical weldability: it depends on the chemical composition of the steel itself, on the hardenability and on the cooling rate.
The chemical composition of steel
The chemical composition of steel determines the structural variations that occur in the Altered Thermal Zone and is expressed through an index, called carbon equivalent (CE).
The weldability of steel decreases considerably as the CE coefficient and the thickness of the weld seam increase, while it increases with the specific heat input.
Up to a carbon content of 0.30% weldability is considered good, between 0.30% and 0.50% is still acceptable although it requires some precautions, while over 0.50% even all possible precautions can be useless as a guarantee of the finished work.
Welding stainless steel, therefore, is always possible when it has very specific characteristics: welding is a complex and highly precise process, knowledge of materials is essential to ensure a finished process of high quality.