|This phenomenon applies to carbon and
low alloy steel. It involves ferrite forming a compound with nitrogen;
iron-nitride (Fe4N). Temperatures around 250°C, will
cause a fine precipitation of this compound to occur. It will tend
to pin any dislocations in the structure that have been created by cold
work or plastic deformation.
Strain ageing increases tensile strength but significantly reduces ductility and toughness.
Modern steels tend to have low nitrogen content, but this is not necessarily true for welds. Sufficient Nitrogen, approximately 1 to 2 ppm, can be easily picked up from the atmosphere during welding.
Weld root runs are particularly at risk because of high contraction stresses causing plastic deformation. This is why impact test specimens taken from the root or first pass of a weld can give poor results.
Additions of Aluminium can tie up the Nitrogen as Aluminium Nitride, but weld-cooling rates are too fast for this compound to form successfully. Stress relief at around 650 degrees C will resolve the problem.
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Page last updated 08 May 2002