Introduction: Mechanical properties in the HAZ of low alloy steels during tempering are heavily dependent on the selected heat treatment method and ensuing microstructural changes. Tempering procedures are often performed isothermally for extended time periods to promote extensive softening and residual stress relief. Past work has shown that short term tempering can produce advantageous impact toughness properties compared to longer procedures via the fine dispersion of carbides that form, as well as a shift in the tempering kinetics. This work outlines the methodology to correlate the carbide formation behavior, structural changes, and influence on impact toughness properties when low alloy steels forming martensite are subjected to multiple short term tempering cycles.
Experimental Procedures: The tempering cycles were imposed experimentally through Gleeble thermal simulations and observation of microstructure was completed using optical and scanning electron microscopy (SEM). The fine carbides that form after short-term tempering were identified using transmission electron microscopy (TEM) and these identified carbides were validated through precipitation simulations using the TC-PRISMA software package. Charpy impact testing was conducted after tempering to measure absorbed energies and mils lateral expansions to evaluate the impact toughness properties.
Results and Discussion: After the application of multiple short term, sub-Ac1 tempering cycles to F22 2.25Cr-1Mo steel, fine intergranular carbides formed along the prior austenite grain boundaries (PAGBs). These nano-scale carbides were imaged with TEM techniques and identified as cementite after analysis of the selected area diffraction pattern (SADP). Furthermore, the TC-PRISMA model predicted cementite to form along the PAGBs in this steel after simulating the short-term tempering procedure and provided a reliable validation for the carbide identification.
The effect of short-term tempering and influence on impact toughness was compared to long-term PWHT through Gleeble simulation and Charpy-V Notch testing. Test results showed that short-term tempering performed ~2X better than long-term PWHT in terms of the absorbed energy and demonstrates the strong potential for using short-term tempering procedures to optimize the tempered martensite microstructure and carbide precipitation behavior to improve impact toughness properties.
Conclusion: The results of this study are relevant to applications that require high impact toughness or in the production of components where minimal softening is desired after the tempering procedure. In such cases, short term tempering is beneficial and can be imposed through a variety of processing methods such as induction heating, temper bead welding, laser heat treatment, and additive manufacturing.