Phase Transformation of P91 Steels upon Cooling after Short Term Overheating above Ac₁ and Ac₃ Temperature

Abstract:

Advanced ferritic steels containing 9 wt% Cr are widely used in the construction of supercritical and ultra supercritical boiler components. Grade 91 is one of the most common alloys used in this application. The microstructure of the as supplied 91 materials consists of a tempered martensite matrix, a fine dispersion of intergranular chromium rich M23C6 precipitates and intragranular carbonitrides MX particles rich in V and Nb. This steel requires post weld heat treatment (PWHT) to produce a tempered microstructure after welding to develop excellent creep strength for high temperature service. Based on past experience, situations may arise whereby the components are subjected to an accidental ‘overshoot’ in temperature during PWHT. The consequence is the formation of deleterious phases which will result in undesirable changes in material property. In this research, P91 base metal specimens were heated to various peak temperatures in a laboratory furnace. Heat treatment parameters, as practiced at site, were applied. Peak temperatures applied were below Ac1, between Ac1 and Ac3, and above Ac3. Hardness measurement demonstrated a significant reduction once the Ac1 temperature was exceeded, due to the presence of soft α-ferrite matrix. As the temperature was increased towards Ac3, newly transformed fresh martensite which is hard and brittle in nature would form the dominant matrix. The phase transformation and precipitate morphology changes were studied using optical microscopy and scanning electron microscopy techniques. Three factors were identified to determine the phase transformation: (1) the homogeneity level and amount of precipitates dissolved in austenitic matrix upon heating; (2) slow cooling rate that may shift the cooling curve to enter ferrite nose and (3) deviation in chemical composition.

• Periodical: Advanced Materials Research (Volumes 634-638)
• Pages: 1756-1765
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.634-638.1756
• Online since: January 2013
• Authors: Ng Guat Peng, Badrol Ahmad, Mohd Razali Muhamad, Mohd Ahadlin
• Keywords: Ac₁, Ac₃, Ferritic, Martensitic, Phase Transformation, PWHT

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