Characterizing Creep Behaviour of Modified 9Cr1Mo Steel by using Small Punch Impression Technique for Thermal Powerplants

Abstract

Modified 9Cr1Mo (P91) having ferritic/martensitic structure is a candidate material for thermal powerplant industry due to its enhanced creep resistance. Fusion welding of P91 steel nurtures heat affected zone (HAZ) which gives rise to infamous type IV failure/cracking. The origination of type IV cracking still remains ambiguous mainly due to sampling constraints from P91 - HAZ subzones owing to thin width of HAZ. In present work, this issue is investigated with the help of small punch impression creep technique. Thermo – mechanical simulator Gleeble®-3800 is used to physically simulate HAZ subzones of P91 steel representing HAZ – CG, HAZ – FG and HAZ – IC. Impression creep tests done on parent metal P91 and its simulated HAZ subzones at a stress level of 270 MPa and 600℃ reveal HAZ – IC as the weakest subzone followed by HAZ – FG. Coarsening of chromium rich carbides on prior austenite grain boundaries (PAGBs) associated with blended phase transformations in HAZ – IC render it as the most degraded HAZ subzone in P91 steel thus becoming an initiating location for type IV cracking. Furthermore, presence of relatively fine carbides combined with less inter-lath spacing in P91 steel and its HAZ – CG contribute towards retarding creep rate leading to an increase in heterogeneous creep behaviour among HAZ subzones and parent metal.