Carbide Evolution in Heat Affected Zone of T91 Steel Weld and Its Relationship to Creep Behavior

Abstract

T91 steel tubes were welded without addition of any filler material by gas-tungsten arc welding. The welds were progressed as bead-on-plate joints with double side technique at the middle of work pieces. The welding parameters were set as welding current of 90A, welding voltage of 10V, arc distance of 3 mm, welding speed of 12 mm/min, and post-weld heat treatment at 760 °C for 30 minutes. The creep-testing furnace was designed and installed at the Department of Production Engineering, King Mongkut’s University Technology Thonburi. The temperature inside the furnace was controlled and the pulled static load was adjusted corresponding to the cross-section area of the specimens subjected to creep to achieve the tension stress of 165 MPa, which is about 90% of yield strength at maximum service temperature of Grade 91 steels. The selected temperature for creep testing, i.e. 552 °C, was calculated from the pre-determined Larson-Miller parameters with the expected rupture time of 1,000 hours. A specimen was subjected to creep until rupture and the actual rupture time was 1,138 hours, which was hence determined as 100%creep. Consequently, other specimens were subjected to creep with the exposure time for obtaining required percentages of creep at 10%, 20%, 30%, 50% and 80%. The information about precipitate evolution in T91 steel weldment during creep rupture is controversial. Therefore, the present work focused on carbide and precipitate evolution in fine-grain, heat-affected zone (FGHAZ) during creep of T91 steel weldment.Microstructure and precipitate evolution in FGHAZ have been investigated by light microscopy, scanning electron microscopy and transmission electron microscopy by carbon-film extraction replica technique. Grain size in FGHAZ was interpreted by a circular intercept method adapted from the standard procedure of ASTM E112. Increasing trend of grain size in FGHAZ during creep was found from about 1.70 𝜇𝜇m at 0%creep to about 2.30 𝜇𝜇m at 100%creep. Empirical correlation of grain size in FGHAZ during creep of T91 steel weldment can be expressed as: y=0.1417ln(x)+2.31, where y = FGHAZ grain size (𝜇𝜇m) and x = %creep. Regarding precipitate evolution in FGHAZ, the total area fraction of precipitates during creep was increasing at 10%creep up to the maximum value at 30%creep. Transmission electron microscopy revealed that precipitates in FGHAZ during creep of T91 weldment were M23C6, M7C3, M6C, MC, M2C and Laves phase. M23C6 was found in all creep conditions, mainly at grain and subgrain boundaries. M7C3, M6C and Laves phase were observed only during intermediate percentage of creep (20% to 80%), mainly nearby grain boundaries. MC and M2C were found from 20%creep until 100%creep or rupture, mainly at grain interior. Two types of M23C6 were found as typical and high-V M23C6. For typical M23C6, consecutive major elements in M were Cr, Fe, Mo and V, whereas the average wt% ratios of Fe/Cr, Mo/Cr and V/Cr were 0.50, 0.14 and 0.02, respectively. The stoichiometric formula of typical M23C6 can be written as (Cr14.43Fe6.77Mo1.06Si0.40V0.29Nb0.04)C6. For high-V M23C6, consecutive major elements in M were Cr, Fe, V and Mo. As compared to typical M23C6, higher wt% ratio of V/Cr of 0.11 was observed with comparable Fe/Cr and Mo/Cr as 0.49 and 0.13, respectively. The stoichiometric formula of high-V M23C6 can be written as (Cr13.53Fe6.23V1.53Mo0.94Nb0.38Si0.38)C6. M7C3 was found with consecutive major elements in M as Fe, Cr, Mo and V. The values of Fe/Cr and Mo/Cr wt% ratios higher than 1.0 are characteristics of M7C3. The average wt% ratios of Fe/Cr, Mo/Cr and V/Cr were 2.03, 2.20 and 0.23, respectively. Faulting-like contrast and streaking in electron diffraction patterns are also characteristics of M7C3. The stoichiometric formula of M7C3 can be written as (Fe2.89Mo1.82Cr1.53V0.36Si0.36Nb0.04)C3. High V/Cr wt% ratio as compared to M6C should be noted. M6C was observed with consecutive major elements in M as Fe, Mo comparable to Cr, and Si. The average wt% ratios of Fe/Cr, Mo/Cr and Si/Cr were 2.55, 2.77 and 0.45, respectively. The Fe/Cr and Mo/Cr wt% ratios were higher than 1.0 as found in M7C3, but relatively higher Mo/Cr and Si/Cr wt% ratios are characteristics of M6C. The stoichiometric formula of M6C can be written as (Fe2.45Mo1.55Cr1.03Si0.86V0.10Nb0.02)C. MC was found with consecutive major elements in M as V, Cr, Nb and Si. High V/Cr and Nb/Cr wt% ratios of 3.74 and 2.72 were characteristics of MC. The stoichiometric formula of MC can be written as (V0.53Nb0.25Cr0.14Si0.05Fe0.02Mo0.01)C. Higher Nb/Cr wt% ratio as compared to that in M2C should be noted. M2C was observed with consecutive major elements in M as V, Cr and Nb comparable to Si. High V/Cr wt% ratio of 2.88 and lower Nb/Cr wt% ratio as compared to MC are characteristics of M2C. The stoichiometric formula of M2C can be written as (V1.23Cr0.42Nb0.15Si0.13Fe0.05Mo0.02)C. Finally, A2B Laves phase was also found with relatively high Si/Cr wt% ratio as compared to carbides. The average wt% ratios of Fe/Cr, Mo/Cr, V/Cr and Si/Cr are comparable as 0.98, 0.94, 1.07 and 0.90, respectively. The stoichiometric formula can be written as (V0.72Cr0.66Fe0.62)( Si0.68Nb0.09Mo0.22).