Microstructure and Mechanical Properties of High Chromium Steel Weld Metals

Doctoral thesis, 1994

The microstructure of three types of Cr steel weld metals, containing 5%, 9% and 12% chromium, was studied in different heat treated conditions using optical microscopy, analytical transmission electron microscopy and atom probe field-ion microscopy. The mechanical properties of the weld metals were investigated. Scanning electron microscopy (SEM) and optical microscopy were used to carry out the fractography investigation of the impacted specimens. In the 5% Cr steel weld metal, it was found that M7C3 and M2C started to precipitate during tempering at 500 °C. M23C6 precipitated out from 600 °C. The composition of the precipitates also changed according to the tempering temperature. The development of microstructure during tempering was discussed. The microstructure of three 9% Cr steel weld metals consisted of lath martensite, some retained .delta.-ferrite and precipitates. The predominant precipitates were M23C6. In weld metals containing V and Nb, MC and M2C were present. The failure mode of the impacted weld metals was quasi-cleavage. Non- metallic inclusions were found to be the nucleation sites for cleavage cracks during impact. The cleavage stress of the weld metals was determined by the size of the inclusions and the size of martensite packets or recrystallized ferrite grains (in the case that the quantity of recrystallized ferrite was high). The impact toughness of the weld metals depended on the yield stress and fracture stress of the materials. The microstructure of three varieties of 12% Cr steel weld metals consisted also of martensite, retained .delta.-ferrite and precipitates. The precipitates included M23C6, MX and M2X. The morphology, quantity and distribution of the precipitates were determined by composition and heat treatment of the weld metals. The microstructure development in the weld metals in different heat treatment conditions was investigated. The failure mode of impacted specimens was mainly cleavage. Intercooling the weld metals to 125 °C before post-weld heat treatment did not affect the tensile strength of the weld metals very much, but improved their impact toughness dramatically. The mechanism by which the microstructure of the weld metals affects their mechanical properties was discussed.