Welding stainless steel in the fabrication of engineering components depends greatly on the weldability of the steel. Welding is not a precise quantifiable property, but rather implies the ability of the material to be joined by the standard welding processes.
Since the group “Stainless Steel” represents a huge class of different iron base materials only having some chromium content in common, welding stainless steel requires a basic knowledge of the stainless steel types. Stainless steel types include austenitic, ferritic, martensitic, precipitation hardenable or duplex, according to the microstructure or arrangement of their crystal structure.
The Challenges of Welding Stainless Steel
The microstructure of the stainless steel has an influence on the metal’s properties and on their welding characteristics. The weldability of the various classifications of stainless steel varies due to the different chemical compositions, crystal structure as well as the resultant reactions of these to the thermal cycles of welding.
A major problem of welding stainless steel is the prevention of sensitization which is when the corrosion resistant characteristics of the steel may be adversely affected by heat in the heat affected zone (HAZ).
The addition of chromium is what gives iron-based materials a certain resistance to rusting and corrosion as it helps produce a tough and impervious layer on the surface of the material. On a sensitised joint, on both sides of the weld bead, a certain portion of the metal becomes depleted of chromium and loses its anticorrosive properties which means the welded metal is now prone to corrosion.
While the physical properties may differ between the various classifications of stainless steel, the general properties of major influence are:
- The coefficient of thermal expansion
- The thermal conductivity
- The electrical resistivity
- The melting temperatures.
Coefficient of Thermal Expansion
Austenitic stainless steel will expand at a greater rate than mild steel and allowances must be made for this expansion and contraction to control distortion and thermal stress, particularly during cooling which can cause weld zone cracking.
Austenitic steels in particular conduct heat more slowly. This promotes sharp temperature gradients which along with high thermal expansion causes distortion to be confined to a small area. When welding stainless steel, it is important that the weld zone remains at a higher temperature for a longer time to control interpass temperatures.
The higher electrical resistance of stainless steel will result in the generation of more heat for the same current. This, coupled with low heat conductivity, is an advantage when using resistance welding processes. However, if not controlled in other welding processes, this will result in too great a heat input.
In manual metal arc (MMA) welding, the use of too high a welding current will cause overheating of the core wire of the electrode and also damage to the flux coating.
Stainless steel has a lower melting temperature than mild steel, which is an advantage because less heat is required to produce fusion. This results in faster welding for the same heat. One must be careful, however when changing from welding mild steel to welding stainless steel.
Appropriate actions should be taken to avoid or minimise possible detrimental effects on the steel such as distortion and excessive heat input. These can include using smaller and more frequent tack welds, step welding and the use of copper back-up bars and chill plates.
Steelmor is the leading supplier and manufacturer of stainless steel in South Africa and is dedicated to supplying cost effective quality products on time. Give us a call to find out how we can help you with your stainless steel requirements.