This study will explore the carbide precipitation behavior in the heat affected zone (HAZ) of Grade 22 Cr-Mo steel subjected to temper bead welding (TBW). During repair welding of large low alloy steel components, underlying HAZs become austenitized and experience rapid cooling rates to form martensite. Martensite formation is undesirable due to its low impact toughness properties which greatly increases susceptibility to cracking and premature failure during service application. Therefore, postweld heat treatment (PWHT) is performed to temper the martensite and homogenize mechanical properties. TBW is the preferred method for repair in the nuclear industry since it can be achieved without interrupting plant operations for in-service components as would be required for PWHT. This technique utilizes the heat generated by subsequent weld passes/layers to effectively temper the underlying HAZ. Martensite tempering involves carbide precipitation at high energy sites such as grain boundaries, laths, and dislocations which reduces lattice distortions and provides effective barriers to crack propagation to increase impact toughness. The goal of this work is to develop thermo-kinetic models using CALPHAD-based methods to predict carbide formation behavior in the HAZ of low alloy steels subjected to TBW. These predictive models were validated through TEM analysis for identification and quantification of the carbide precipitation behavior.
Thermocouples were pre-installed into Grade 22 low alloy steel plates at depths corresponding to the HAZ. Next, weld overlays were developed with GTAW according to the proper bead spacings and inter-pass temperatures to qualify as an effective TBW application. Afterwards, thermal histories were extracted from the thermocouples to obtain accurate peak temperatures, heating, and cooling rates experienced in TBW.
ThermoCalc-PrismaTM was utilized to develop a kinetic model for carbide precipitation in the Grade 22 steel HAZ. This model was calibrated to replicate experimental data according to the carbide volume fraction. Calibration was performed using a trial-and-error process through adjusting model parameters such as interfacial energies, number of nucleation sites, and dislocation densities until the simulation results corresponded to the experimental data found in literature. Afterwards, the thermal histories obtained from the experimental weld overlays were programmed into the TC-Prisma models to simulate TBW and predict the carbide precipitation behavior in the HAZ.
Since TBW involves short excursions to elevated temperature, there was limited particle coarsening and nano-scale carbides were formed. As a result, TEM was used for imaging and diffraction analysis. Sample preparation involved using the extraction replica technique which involved deep etching the sample surface to dissolve the martensitic matrix and expose carbides at the top surface. Acetate tape was placed on the surface to adhere to the carbides, then extracted and coated with carbon. Next, the tape layer was dissolved until only the carbon films containing the carbide particles remained. These carbon replicas were then imaged in the TEM for identification and quantification of the carbide formation behavior.
Results and Discussion:
Analysis of the extracted thermal histories revealed peak temperatures between 842℃ – 1350℃ during the hardening HAZ cycle which corresponded to temperature just above Ac1 until slightly below the solidus temperature. Tempering temperatures ranged from 500℃ - 820℃ corresponding to a range between the minimum tempering temperature and slightly below the Ac1 temperature. Heating rates ranged from 50 - 100 ℃/s depending on the heat input level with rates decreasing with heat input. Cooling rates ranged from 20 - 50 ℃/s during the hardening thermal cycle and 15 - 30 ℃/s during the tempering cycles.
The TC-Prisma model for Grade 22 steel after an as-welded CGHAZ hardening cycle predicted a minuscule volume fraction of M3C cementite on cooling from the austenite phase field. Next, a typical PWHT procedure for Grade 22 was simulated for the CGHAZ with heating/cooling rates of 200 ℃/hour and a 10-hour isothermal hold at 650℃. Simulation results showed that M3C cementite first formed followed by M7C3, M2C, and M23C6. However, M23C6 became the equilibrium carbide towards the end of the PWHT as the remaining carbides dissolved during the isothermal hold. Simulating the CGHAZ after applying a single TBW-type reheat at 710℃ predicted greatly increased cementite precipitation compared to the as-welded condition. Multiple TBW reheats 785℃ predicted significant volume fractions of M3C cementite which increased with each reheat application.
The TEM carbide characterization is ongoing, and the results would be used for validation of the PrismaTM simulations results. Initial results show good correlation between predicted and TEM identified carbide types. The TEM results will be used for calibration of the PrisnaTM model, which will be implemented in a computational design of experiment (CDoE) framework for process-microstructure-property optimization in temperbead welding.
ThermoCalc-PrismaTM was used to predict the carbide formation behavior in the HAZ of Grade 22 low alloy steel during temperbead welding and conventional PWHT. Cementite formation was predicted in the as-welded CGHAZ and both single and multiple reheat TBW conditions. The CGHAZ after the PWHT predicted an equilibrium distribution of M23C6 carbides. Ongoing TEM characterization work demonstrated a good correlation with the carbide precipitation predictions. The developed carbide precipitation model can be utilized in computational design of experiment process-microstructure-property optimization for welding and additive manufacturing of low alloy steels.