To improve the machineability of steels, elements are added that promote chip formation, rather than the formation of long lengths of swarf. Those elements comprise sulphur (S), lead (Pb) and selenium (Se).
- Sulphur typically 0.1 to 0.5% promotes the formation of low melting point constituents at the grain boundaries during welding. This results in cracking at the grain boundaries of both the weld metal and the HAZ as the joint solidifies and cools.
In addition, high levels of sulphur are known to promote weld metal porosity.
- Lead or selenium of up to 0.12% are also used to give improved machineability, since they are almost insoluble in steel and exist as distinct globules which act as crack-initiators to promote chip formation during machining.
On welding, three effects occur:
- The lead or selenium remain liquid to very low temperatures, giving rise to hot cracking.
- There is an increased risk of porosity and embrittlement in the steel.
- Lead volatilises and is then present in the welding fume. Since lead is toxic, additional precautions are then necessary to ensure that adequate ventilation is provided to remove the fumes. See also health & safety FAQ on fume
When welding is part of a fabrication procedure, the use of free-machining steels should be eliminated at the material selection stage.
If a free-machining steel has to be welded, then it is essential that low-hydrogen electrodes and low welding currents are used to limit dilution and reduce the risk of porosity and cracking.
Slow travel speeds should be used to reduce the risk of weld metal solidification cracking. In addition, low arc energies must be used to reduce the risk of liquation cracking in the HAZ.