Large heat-to-heat variation of creep rupture strength in weldments of mod.9Cr-1Mo steels was observed in the creep rupture tests conducted for two different heats at 600°C and 650°C. One heat showed consistently lower time-to-rupture than the other for 130-60MPa at 600°C. Detailed microstructural investigations revealed that the number density of precipitates in the weaker heat was remarkably lower than that associated with the stronger heat through most of the creep region. Accordingly, heat-to-heat variation of creep rupture strength was attributed to the difference in the precipitate strengthening effects throughout creep. Equilibrium calculation predicted that the smaller phase fraction of M 23 C 6 and VN precipitates due to the lower content of chromium and lower ratio of nitrogen/aluminum in the weaker heat. However, given that long-term creep rupture strength at 650°C converged for the two heats, the microstructure including precipitates may settle into a similar level for subsequent longer hours even at 600°C.

Grade 91 steel has been found to be susceptible to Type IV cracking in the base metal heat affected zone (HAZ). In order to better understand this type of failure, a study on the metallurgical reactions occuring within the HAZ was conducted, particularly within the fine grained (FG) and intercritical (IC) regions where Type IV cracking is most commonly found to occur. The course grained (CG), FG and IC regions of the HAZ in Grade 91 steel were simulated using a Gleeble 3800 Thermo-Mechanical Simulator. A dilatometer was used to determine the phase transformations occuring during simulation of weld thermal histories. For the first time, it was shown that ferrite can form in the IC HAZ of Grade 91 steel welds. The magnitude of the ferrite transformation was observed to decrease with faster cooling rates. The presence of ferrite in the simulated IC HAZ microstructure was shown to decrease the high temperature tensile strength and increase the high temperature elongation compared to HAZ regions that did not undergo ferrite transformation. Welding parameters such as heat input, preheat and interpass temperature can be selected to ensure faster cooling rates and reduce or potentially avoid formation of ferrite in the IC HAZ.

Type IV damage is observed in creep-strength-enhanced ferritic (CSEF) steel used in USC plants and the research on the evaluation of such damage has been carried out in the world. Type I failure is recently reported in welded joint of Gr.91 so that the importance of the evaluation of the creep strength of the weld metal is increasing. In this study, the change in hardness with aging and creep strength before and after aging were evaluated to determine the creep strength of the weld metal of Gr.91. The hardness of the weld metal subjected to aging significantly decreased compared with that of the base metal and the heat-affected zone (HAZ). The creep strength of the weld metal was also decreased by aging. From these results, it is suggested that the failure morphology of Gr.91 steel welded joint used for a long term may change from type IV to type I.

There is a constant need for improved knowledge of the influence of non-standard processing on the expected performance of creep strength enhanced ferritic (CSEF) materials as the total installed tonnage of these materials is rapidly increasing across the power generation industry. Cr-Mo-V steel grades micro-alloyed with niobium and titanium designed for pressurized equipment operating in the supercritical steam range proved to be very sensitive to relative minor variations in the principal heat treatment parameters time and temperature, when compared to the traditional Cr-Mo-V grades. A key component for successful welds is optimised post weld heat treatment (PWHT). Under certain conditions premature failures of welds can occur when incorrect weld and heat treatment performance result in a reduction of specified mechanical properties and high temperature creep performance, it is therefore of significant importance to have a good understanding of actual material properties for effective operation and plant life studies. This study investigated the effect and impact variations of post weld heat treatment time and temperature on mechanical properties of tungsten inert gas (TIG) and manual metal arc (MMA) welds on Grade 91 pipes from a set of reference samples. This is in preparation of establishing a benchmark set of tests to determine the integrity and expected long-term performance of butt-welds from limited site sample volumes, providing a non-intrusive methodology to identify welds suspected to have received non-standard PWHT cycles on Grade 91 pipework systems.

Fossil fuels continue to be the primary source of energy in the U.S and worldwide. In order to improve the efficiency of fossil power plants, advanced structural materials need to be developed and deployed to meet the need of high temperature creep resistance and corrosion resistance. Examples include creep strength enhanced ferritic (CSEF) steels, austenitic stainless steels, nickel-based superalloys, and oxide dispersion strengthened alloys. Welding is extensively used in construction of fossil power plants. The performance of the weld region can be critical to the safe and economical operation of fossil power plants. Degradations in performance such as reduced creep strength and premature failure in the weld region (e.g. Type IV failure in ferritic steels) are examples of longstanding welding and weldability problems for boiler and other components. In the past, extensive studies have been carried out to characterize the different microstructures in different regions of a weld, and to a certain extent, to establish the correlations between the microstructure and the creep strength. However, the metallurgical or microstructural induced local stress/strain variations have been seldom quantified. In addition, it has been long recognized that, due to the sharp microstructure and property gradients in the weld and HAZ, the standard creep testing procedure for the base metal can produce erroneous results when used for weld testing.

Unpredictable failures near the phase boundary in Grade 91 dissimilar metal welds (DMW) with nickel based filler metals represent a significant problem for the power generation industry. In order to determine the root cause for these failures, it is necessary to understand the formation of the microstructure in the weld regions around the site of failure. Thermal histories were therefore measured inside the Grade 91 steel heat affected zone (HAZ) of an autogenous weld and of a DMW in the form of bead on plate with Alloy 625 to study the effect of the weld thermal cycle on microstructural formation. It was found that the HAZ in the DMW experienced longer dwell time at high temperatures because of the latent heat of fusion released during Alloy 625 solidification (1350 - 1125 °C). This allowed longer time for carbide dissolution and phase transformations in the DMW than in the autogenous weld. Additionally, the 625 filler metal created a large chemical potential gradient for carbon, which when combined with longer dwell times, yielded carbon depletion in the heat affected zone. Retention of δ ferrite in the coarse grained HAZ (CGHAZ) of DMWs was found to be an indicator for these mechanisms.

Carbon migration in narrow-gap welding joints of dissimilar steels has been studied using bead-on-plate specimens to determine the factors that influence the formation of a soft ferrite structure in the carbon-depleted zone. Carbon migration was found to occur during tempering, with a ferrite structure formed at the intersection of multiple layers due to severe carbon migration. This was attributed to a steep gradient in Cr content caused by the low fusion penetration at the intersection. Experimental results and the relationship between fusion penetration and weld bead alignment confirmed that low fusion penetration is the main cause of ferrite-structured carbon depleted zones.