What safety factor do I have to put on the stress in a butt weld in mild steel?

TWI Frequently asked questions

Almost all steel weld metals are stronger than mild steel and you do not need to apply any factor for static strength in most simple steel structures. In higher strength steels, the choice of welding consumables and welding procedure must ensure a matching strength if possible and this must be verified in the welding procedure tests. Some pressure vessel standards impose factors depending on the amount of NDT and you must consult the relevant standard or code.