Abstract
Thermal embrittlement of 2 1/4Cr-1Mo electroslag weld metals in high temperature operation was studied in this report, with particular emphasis on grain boundary segregation of solute elements proved by Auger spectrum analysis.
The main conclusions obtained in the study were as follows:
(1) The embrittlement were mainly caused by grain boundary segregation of phosphorus.
(2) Silicon and manganese acted as promote the segregation of phosphorus, and also manganese itself did embrittled grain boundaries by segregation like copper and some carbides.
(3) Small precipitates on grain boundaries, which were supposed chromium and/or molybdenum carbides, gave favorable site for segregation of phosphorus.
(4) Sulphur in weld metals occupied grain boundaries after welding and postweld heat treatment, being in contrast with no occupation in base metals. This might be main reason why weld metals were much less tough, compared with base metals when fracture occurred in grain boundaries such as thermal embrittlement.
(5) Segregation of phosphorus and other solute elements increased with holding time in isothermal treatment and reached to a saturated value. Degree of segregation was larger for phosphorus than for carbon, chromium and molybdenum.
(6) Progress of segregation of solute elements was more rapid than that of embrittling.
(7) Correlation of embrittlement between step cooling and isothermal treatment was not definite and changed depending on chemistry and thermal history of weld metals.
(8) C-curves of isothermal embrittlement of 2 1/4Cr-1Mo welds had an intermediate mode between that of Ni-Cr steel by Jaffe and that of Cr steel by Vidal.
