TWI Industrial Member Report Summary 152/1981
By N Bailey
The present report summarises four earlier reports (1-4) on the strength and toughness of mild steel submerged-arc weld metal, generally in 30mm thick plates. Two-pass and multipass welds were made with unalloyed wire and fluxes and, from the results, regression equations were developed to identify the compositinnal and microstructural factors controlling strength and toughness. The investigation has been extended to include published work including, where appropriate, alloyed weld metals. Previously developed correlations between yield or tensile strength and carbon equivalent have been modified to include alloyed weld metals with yield strengths up to 780N/mm2 and tensile strengths up to 900N/mm2.
Toughness behaviour is influenced by flux type. For multipass welds at 3kJ/mm, the less basic flux types give coarse lamellar microstructures whose toughness is controlled by the strength. As the degree of alloying with Mn and the proof stress increases, toughness falls. More basic fluxes give weld metals whose toughness is dependent on microstructure. To achieve a fine tough fully acicular ferrite microstructure, alloying beyond the levels readily achievable with Mn alone is necessary.
With higher heat input two-pass welds, the influence of flux type is less direct. Toughness behaviour is strongly influenced by elements such as phosphorus, silicon and nitrogen, which derive in part from wire and plate. With such welds, it is more difficult without further alloying to eliminate grain boundary ferrite and to produce acicular ferrite which is fine enough to give improved toughness as the yield strength is increased.