Robert Hamlin - Research
The research is funded by the Defense Logistics Agency and is concerned with optimizing welding procedures for cast precipitation hardened stainless steels 17-4 and 13-8+Mo. The mechanical properties of these alloys has made them potential replacements for materials currently used in high strength military applications. The applications involving these materials will require welding, therefore it is important to develop welding procedures that will allow strength retention in the heat affected zone (HAZ) and fusion zone during fusion welding processes.
Preliminary work involved using a Gleeble 3500 thermomechanical simulator to recreate the heat affected zone and test the mechanical properties of 17-4 and 13-8+Mo. Thermal cycles that were characteristic of four major regions of the HAZ were determined using the Sandia’s SmartWeld program and applied to samples of each material in the aged condition and samples in the solution treated condition. Samples that were subjected to thermal cycles in the solution treated condition were then given a post simulation age. Gas metal arc (GMA) welds were also created with both materials in the same conditions. Tensile testing was performed on the Gleeble samples and cross weld tensile testing was performed on the GMA welds. Microstructures were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), dilatometry, and hardness measurements. Light optical microstructures of the base metal and each region of the HAZ for 17-4 and 13-8+Mo can be seen in Figure 1 and Figure 2, respectively. The microstructural and mechanical property trends observed in the Gleeble samples matched closely with the GMA welds. As shown in Figure 3, it was determined that welding in the solution treated condition and giving a post weld age provided higher and more uniform strength through the HAZ when compared the samples that were welded in the aged condition. It was also found that over 90% of the base metal strength was retained in the cross-weld tensile samples that were welded in the solution treated condition and given a post weld age. The observed trends in mechanical properties could not be understood based solely on the change in matrix microstructure (e.g., relative amounts of ferrite and martensite), suggesting that precipitation was the primary factor controlling the mechanical properties. Therefore, precipitate modeling was performed using MatCalc thermodynamic and kinetics modeling software. The modeling was used predict the precipitate evolution in the HAZ of both materials. It was determined that the low strength values correlated to low volume fraction and small mean radius of precipitates while the high strength values correlated to an equilibrium volume fraction of precipitates with a larger mean radius. It was concluded that welding in the solution treated condition and giving a post weld age will provide uniform strength at base metal levels and that precipitation was the primary factor controlling the strength of these materials.
The current work for this project is concerned with multi-pass welding of aged 17-4 and 13-8+Mo. Welding in the solution treated condition and giving a post weld heat treatment can provide uniform strength at base metal levels, however post weld heat treating is not always possible when welding large components or if repairs must be made in the field. This has necessitated evaluation of welding procedures to determine if controlled weld metal deposition can be used to promote precipitation in the HAZ and fusion zone during welding and eliminate the need for a post weld heat treatment. The Gleeble 3500 has been used to subject samples of each material to multiple thermal cycles in order to simulate multi-pass welding. Hardness measurements were then taken from each sample to determine the strengthening effect. It was observed that a decrease in strength occurs during primary thermal cycles, but that secondary thermal cycles can increase the strength to near base metal values or higher as can be seen in Figure 4. Work is currently underway to predict the precipitate growth and dissolution kinetics for both materials over a range of thermal cycles using isothermal aging data, Avrami analysis, and hardness measurements.
Figure 1: LOM photomicrographs of 17-4 simulated HAZ samples in the aged condition showing the (A) BM, (B) SCHAZ, (B) ICHAZ, (C) AQMHAZ, and (D) MFHAZ. TM = Tempered Martensite, AQM = As-Quenched Martensite, δ = δ-Ferrite
Figure 2: LOM photomicrographs of 13-8+Mo simulated HAZ samples in the S-A-W condition showing the (A) BM, (B) SCHAZ, (C) ICHAZ, (D) AQMHAZ, and (E) MFHAZ. TM = Tempered Martensite, AQM = As-Quenched Martensite, δ = δ-Ferrite
Figure 3: Yield strength, tensile strength, percent reduction in area for simulated HAZ samples (A) 17-4 welded in the aged condition (B) 13-8+Mo welded in the aged condition (C) 17-4 welded in the solution treated condition and post weld aged (D) 13-8+Mo welded in the solution treated condition and post weld aged
Figure 4: MatCalc predicted values of (A) phase Fraction and (B) mean radius of Cu Precipitates in 17-4 samples for the base metal and each region of the HAZ in both conditions. MatCalc predicted values of (C) phase Fraction and (D) mean radius of Cu Precipitates in 13-8+Mo samples for the base metal and each region of the HAZ in both conditions
Figure 5: Hardness for the base metal and four different HAZ peak temperatures accompanied by hardness values for material subjected to secondary and ternary thermal cycles for aged 17-4 samples