ATOM-PROBE INVESTIGATION OF PRECIPITATION IN 12-PERCENT CR STEEL WELDMETALS

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 n itrogen content, was studied using TEM (transmission electron microsco py) and APFIM (atom-probe field-ion microscopy) in post-weld heat-trea ted condition. The microstructure of the 12% Cr weld metals consisted of tempered martensite, retained delta-ferrite, an irregular low-dislo cation alpha-ferrite and precipitates. Precipitates in the weld metals were dominantly M23C6 on different boundaries. Plate-like and fine cu bic MN and M2N were found inside the alpha-ferrite. APFIM analysis sho wed that M23C6 was almost a pure carbide and MN was almost a pure nitr ide. Carbon and nitrogen in the weld metals mainly existed in the prec ipitates. High nitrogen content did not change the composition of the precipitates, but increased the quantity of nitrides. Therefore, in th e high nitrogen weld metal, the content of strong nitride-forming elem ents in the matrix decreased. These results are important in order to understand the strengthening mechanism of the high Cr steel weld metal s, as well as of other high Cr heat-resistant steels.