||Erin Barrick, Jeffrey Rodelas, James (Tony) Ohlhausen, Johnathon Brehm, Jack Herrmann, Peter Duran, Khalid Hattar, Kathryn Small, Ryan DeMott, Charlie Robino
Introduction: In 304L stainless steel containing between 5 - 20 wt ppm B, Cr-rich M2B borides have been identified on delta-ferrite/austenite interphase boundaries in the bar formed condition. While stainless steel is often micro-alloyed with boron for varying desired benefits, it can also be deleterious for weldability. Boron-rich phases have been associated with liquation cracking in the heat-affected zone (HAZ) during high energy density welding processes1. Previous investigations at Sandia on boron micro-alloyed 304L established that the as-received condition of borides co-located with delta-ferrite is not susceptible to cracking during laser welding for boron concentrations less than 40 wt. ppm. However, when material is exposed to elevated temperatures during complex, application-specific heat treatments prior to welding, the borides dissolve and re-precipitate on austenite/austenite grain boundaries, and this microstructure has exhibited HAZ liquation cracks. The morphology and microstructural evolution during heat treatment of Cr-borides located along delta-ferrite/austenite boundaries, and their ultimate effect on weldability, have not previously been reported. Therefore, the objective of this work was to develop an overall understanding of the boride and associated delta-ferrite phase transformation kinetics to enable quantitative predictions of liquation crack susceptible microstructures resulting from part-specific heat treatments that precede welding.
1.Chen, W., et al. Met Mat Trans: A, Volume 32A, April 2001, 931-939.
Procedure: A series of isothermal heat treatment and weldability experiments were performed to assess the kinetics of boride transformation and their effect on HAZ cracking. The experiments utilized 304L stainless steel with 5 – 20 wt. ppm B. A Gleeble 3500 thermal-mechanical simulator was used to isothermally heat treat samples. The influence of isothermal soak temperature, soak time, and cooling rate on phase transformations was investigated. A rapid heating rate of 500°C/s was utilized to minimize phase transformations occurring during the non-isothermal heating portion of the test, thereby restricting phase transformations to the isothermal soak temperature. Soak temperatures ranged from 1000°C to 1300°C, soak times ranged from 1 min to 64 min, and cooling rates ranged from 100°C/s to 0.1°C/s. To achieve 100°C/s, He gas quenching was utilized. Samples for weldability studies were first isothermally heat treated in the Gleeble at various time-temperature combinations described above, ground to remove surface oxide, and then laser welded using a Mundt DB-2412 workstation with an IPG Yb-fiber laser. Laser weld arrays were made along the bar direction using both continuous wave and pulsed seam laser welding schedules. The bars were cross-sectioned and the HAZ was inspected for liquation cracks with light optical microscopy. At least 60 weld cross-sections were examined for each heat treatment condition. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) mapping was used to characterize the distribution of boron in the microstructure as a function of heat treatment. Complimentary scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was performed to understand the state of boron on austenite grain boundaries (i.e., elemental B vs. Cr-boride).
Results: To understand which heat treatments are crack susceptible during welding, it was first important to understand the temperature above which the borides on the delta-ferrite/austenite boundaries dissolve, which enables boron diffusion to the austenite/austenite grain boundaries creating the deleterious liquation crack-susceptible microstructure. Experiments were performed at a series of different isothermal hold temperatures followed by rapid cooling (100°C/s). The boride dissolution temperature was identified to be between 1025°C and 1050°C. These dissolution experiments also demonstrated the rapid nature of boron diffusion to the austenite/austenite grain boundaries; ToF-SIMS revealed boron on austenite/austenite grain boundaries after only one minute of exposure to temperatures above the solvus temperature. Though substantial changes in boron distribution occur, the delta-ferrite exhibits minimal differences in morphology.
Initial weldability experiments were performed on samples that were heat treated above the solvus temperature and either rapidly cooled at 100°C/s or slowly cooled at 0.5°C/s. The results demonstrated that the generation of the crack susceptible microstructure is dependent on cooling rate, as the rapidly cooled samples were crack resistant, yet the slowly cooled samples exhibited cracking. This is significant in that cracking can be prevented with rapid cooling rates, but as most heat treatments in actual applications involve slower cooling rates, experiments were conducted a range of cooling rates between 100°C/s and 0.5°C/s and will be presented to demonstrate the critical cooling rate below which liquation cracking is realized. The kinetic observations will be rationalized in terms of known boron segregation behavior (i.e., equilibrium vs. non-equilibrium segregation), boride precipitation kinetics, and thermodynamic/kinetic predictions.
Conclusion: These results have established the kinetics of Cr-boride transformation that are responsible for generating a liquation crack-susceptible microstructure in 304L stainless steel. This work is significant as the morphological distribution of borides co-located with delta-ferrite is unique and newly described, and therefore the kinetics were unknown. These kinetics, which were determined during carefully controlled isothermal heat treatments can be applied to complex, non-isothermal application-specific heat treatments. Ultimately, this work will be used to help refine procedures for thermal processing applied to boron-containing 304L parts that circumvent liquation cracking when subsequently welded.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.