1. Prediction of Temperature profile in the HAZ of Maraging steel GTAW weldments Arpan Kumar Sahoo 1, Niladri Maitra 2, Venkateswaran T 2,Chakravarthy P 3, Gurpreet Singh 3, Sivakumar D 2 1 ISRO Satellite centre (ISAC), Bangalore | 2 Vikram Sarabhai Space Centre (VSSC), Trivandrum | 3 Indian Institute of Space Science and Technology (IIST), Trivandrum Prediction of Temperature profile in the HAZ of Maraging steel GTAW weldments Arpan Kumar Sahoo 1, Niladri Maitra 2, Venkateswaran T 2,Chakravarthy P 3, Gurpreet Singh 3, Sivakumar D 2 1 ISRO Satellite centre (ISAC), Bangalore | 2 Vikram Sarabhai Space Centre (VSSC), Trivandrum | 3 Indian Institute of Space Science and Technology (IIST), Trivandrum ABSTRACT This paper discusses the modeling of Gas- Tungsten Arc Welding (GTAW) process being adopted for maraging steel plates used in aerospace applications. Primary objective of this work is to predict the temperature profile of heat affected zone in a maraging steel weld. Finite Element modeling was done utilizing a Gaussian heat flux distribution on a weldment. Model also incorporated a moving heat source along with provision for heat sinks in the form of copper backup plates. For validating the model, welding experiments were conducted on 2 mm and 8 mm thick 250 grade Maraging steel plates. Thermal profiles were acquired at different locations of heat affected zone using thermocouples and compared with the predictions from the model. Heat Source Modelling  Primary objective - to predict the temperature profile of heat affected zone in a maraging steel weld.  Finite Element modeling was done utilizing a Gaussian heat flux distribution on a weldment.  Model incorporated a moving heat source along with provision for heat sinks in the form of copper backup plates.  For validating the model, welding experiments were conducted on 2 mm and 8 mm thick 250 grade Maraging steel plates.  Thermal profiles were acquired at different locations of HAZ using thermocouples and compared with the predictions from the model. Fig.1 Rosenthal Heat equation schematic with Gaussian heat source distribution Simulation  Simulation carried out by assuming a spot weld movement of the heat source with incremental time step. to simulate a moving heat source.  Gaussian heat source, thus acts at a point for a delta time and in the next time increment step moves to the next point defined by the velocity of the heat source. This approximation was found to be very reasonable.  By controlling the delta time step and simulation parameters, movement of heat source was fine-tuned.  Necessary boundary conditions and cooling effect due to backup plate was also accounted in the simulation. Fig 5(a) &(b) Longitudinal temperature slice view of plate at 6mm from weld center line for 2 mm and 8 mm thick plate respectively at t=30s captured from simulation. ResultS For various locations of thermocouples in experimental setup, probe points of simulation were captured and data was compared correspondingly using thermocouples. Fig.6 (a) & (b) shows the plot of temperature vs. time for various thermocouples (K1 to K4) for both the thicknesses. Fig. 6. (a) Temperature vs. time plot for both experimental and simulation for 2 mm thick plate Fig. 6 (b) Temperature vs. time plot for both experimental and simulation for 8 mm thick plate Experimental Setup  To correlate the results of simulation, GTA welding of maraging steel plates were carried out. The plates were solution annealed at 820 o C for 1hr and machined to final sizes.  2 mm and 8 mm thick plates were prepared for welding trails.  Temperatures of the HAZ were measured using standard K-type thermocouples fixed using high temperature cement. The work piece was mounted on the backup plate in the weld setup as shown in Fig. 2.  Distance of various thermocouples from weld central line is provided in Table 1.  The temperature data was acquired using high speed data acquisition system to capture temperature with intervals of 10 ms.. DISCUSSIONS  From Fig.6(a) for 2mm plate, the agreement between simulation and acquired data is very close for K1 and K4 thermocouples. The deviation in the peak temperature for K2 and K3 thermocouple is minimum and within the experimental scatter.  It is evident from the Figure 5(a) that in the case of 2 mm thick specimen, the cross section available for heat conduction is lower and subsequent cooling by backup copper plate through conduction is significantly effective. This is the plausible reason behind the close match of experimental and simulated results in 2 mm thick plate.  Figure 6 (b) shows significant deviation of the peak temperature between the simulated and acquired data for 8 mm thick plate. Same trend has been observed for all the thermocouples (K1 to K4).  However, it is important to note that the trend of the graph (heating and cooling rates) for simulation and experiments data was in good agreement. Overall for both 2 mm and 8 mm thick plates, the predicted peak temperature is lower than the experimentally measured values.  In the case of 8 mm thick plate the shift is larger in comparison to 2 mm thick plate. Possible reasons for this deviation are;  The contact between the plate and the heat sink (backup plate) is not perfect due to limitation of fixture  Minor bending due to thermal stress  Non availability of thermal conductivity & specific heat data at high temperatures for maraging steel and  Non availability of coefficient of heat transfer data for convection and radiation for maraging steel. The model can be refined with required thermal data to obtain good prediction matching to experimental data. Conclusion  Simulation model and experimental results are found to be in sound agreement when applied to thin plate specimens (2mm).  However for the specimen of thickness 8 mm, the shift in the peak temperature is significant, but the trend of the temperature history is well matched.  The possible reason for the deviation has been analyzed.  Modified experimental setup can be used to have better result correlation with simulation data.  For further work to proceed, more work needs to be done on accurate characterization of maraging steel properties at high temperatures.  These properties can be incorporated into the simulation model for better prediction of HAZ temperature profiles. M-250 2mm plate Copper Backup Plate M-250 2mm plate Copper Backup Plate References ] R F Decker and S. Floreen, Maraging steels-the first 30 years, Proceeding of symposium: Maraging steels-recent developments and applications, Minerals, Metals and Materials Society (1988) 1-38. ] ASM Handbook, 1, 10th edition, ASM International (1991) 793. ] P.H. Salmon Cox, A.J. Birkle, B.G. Reisdorf and G.E. Pellisier, An investigation of the mechanical properties and microstructure of 18Ni250 Maraging steel weldments, Trans. ASM 60 (1967) 125. ] E. Friedman, Thermomechanical analysis of the welding process using the finite element method, Journal of Pressure Vessel Technology, 97 (1975) 206-213. ] T.W. Eager and N.S. Tsai:Welding J., 1983, vol. 62 (12), pp. 346s-355s. ] V. Pavelic, R. Tanbakuchi, O.A. Uyehara, and P.S. Myers: Welding Res. Suppl., 1969, vol. 43, pp. 295-s-304-s. Fig. 4. Isothermal Contours at various times for 2mm plate Fig.2. Experimental Setup showing weld specimen with thermocouples bonded clamped to copper backup plate Table1. Thermocouple Location from weld line Fig. 3. FEM model showing meshed maraging plate (grey) seating on copper backup plate( blue)