ATOM-PROBE INVESTIGATION OF PRECIPITATION IN 12-PERCENT CR STEEL WELD METALS

The microstructure of two types of 12% Cr steel weld metals, one with the composition of a common 12% Cr steel and the other with a higher nitrogen content, was studied using TEM (transmission electron microscopy) and APFIM (atom-probe field-ion microscopy) in post-weld heat-treated condition. The microstructure of the 12% Cr weld metals consisted of tempered martensite, retained delta-ferrite, an irregular low-dislocation alpha-ferrite and precipitates. Precipitates in the weld metals were dominantly M23C6 on different boundaries. Plate-like and fine cubic MN and M2N were found inside the alpha-ferrite. APFIM analysis showed that M23C6 was almost a pure carbide and MN was almost a pure nitride. Carbon and nitrogen in the weld metals mainly existed in the precipitates. High nitrogen content did not change the composition of the precipitates, but increased the quantity of nitrides. Therefore, in the high nitrogen weld metal, the content of strong nitride-forming elements in the matrix decreased. These results are important in order to understand the strengthening mechanism of the high Cr steel weld metals, as well as of other high Cr heat-resistant steels.