This work evaluates the strain ageing behaviour of prior cold-strained thermomechanically processed steel S460ML in multipass welding and artificial ageing. The results show that S460ML tolerated cold forming much better than conventional CMn steels. This is due to the smaller shift in the initially low transition temperature of S460ML caused by cold deformation, which can be attributed to its low content of cementite, i.e. low C content. The high resistance to the ageing process itself stems from a low free nitrogen content due to titanium.
In light-weight structures the use of cold-formed members combined with welding enables structural work to be carried out economically. It allows the use of pre-fabricated products with high dimensional accuracy, which in many cases reduces the need for pre- and post-weld treatments. When cold-formed steel is heated, as in welding, the lowest temperature at which the steel structure can be used safely increases due to strain ageing in the low-temperature heat-affected zone (HAZ) raising the ductile/brittle transition temperature (Sorsa and Vierros,1987, Herman et al.,1987). Earlier work on older types of structural steels has" hence led to the introduction of codes (Anon,1981, Anon,1993) restricting welding near cold-formed areas. The increase in transition temperature of cold-formed ferritic steel comprises two parts: that due to cold strain alone and that caused by subsequent ageing. For conventional grades the greatest increase occurs after heating to temperatures in the range 200 to 400°C (Sorsa and Vierros,1987, Herman et al., 1987, Bathke, 1988). This has been ascribed to the locking of dislocations caused by prior cold deformation by C and N atoms that diffuse to. form atmospheres around them (Herman et al., 1987, Bathke,1988). At temperatures below 200°C ageing is reduced by the decreasing solubility of C, and N-induced ageing becomes dominant provided that free N is available (Herman et al., 1987, Hautala,1992).
