Microstructural And Welding Impacts Of Minor Boron Additions In Austenitic Stainless Steel
Restricted (Penn State Only)
- Author:
- Andrewlavage, Eric
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 09, 2024
- Committee Members:
- Todd Palmer, Thesis Advisor/Co-Advisor
Jay Keist, Committee Member
John Mauro, Program Head/Chair
Jingjing Li, Committee Member - Keywords:
- Stainless Steel
Boron
Weld Cracking
Liquation Cracking
Chromium Boride
Kinetics
Microstructural Analysis
Phase Development
Laser Welding - Abstract:
- Austenitic stainless steels are complex, multi-component material systems in which small changes in minor alloying elements can produce significant changes in microstructures and properties with only minuscule changes in the alloying element compositions. Many of these alloying elements are not typically monitored and are only now appearing in commercial alloys as the steel scrap stream is becoming more contaminated with undesirable trace elements. For example, small amounts of boron, even at levels of only approximately 0.0020 wt.%, can lead to the formation of tetragonal Cr2B precipitates in cast and wrought austenitic stainless-steel components that impact the resulting microstructure, mechanical properties, and weldability. By characterizing the microstructures in 304L austenitic stainless steels with boron contents ranging from 0.0019 and to 0.0340 wt.%, the formation and evolution of both the Cr2B precipitates and delta-ferrite phases were tracked across a range of heat treatment temperatures between 900ºC and 1100ºC and times up to 30 minutes. Even at these low boron levels, tetragonal Cr2B precipitates appeared on austenite grain boundaries as well as regions where delta-ferrite was present. Increasing boron levels led to increases in the amount of Cr2B phase present, while increasing heat treatment temperatures led to shifts in the locations of the Cr2B precipitates to interfaces with delta-ferrite phases. High resolution imaging of the regions where Cr2B precipitates appeared highlighted unexpected differences in the precipitate morphologies and the appearance of small delta-ferrite grains. Changes in the composition, even over these small distances, can lead to a degradation in the mechanical properties of the 304L austenitic stainless steel. Laser weldability is also impacted by the presence of Cr2B precipitates, particularly when present along austenite grain boundaries, with these alloys seeing an increased susceptibility to liquation cracking along the fusion zone boundary. Mimicking heat affected zones with high temperature heat treatments up to 1250ºC saw an increase in ferrite content in the cases of 0.0019 and 0.0340 wt.% boron. Ferrite growths were predominantly along high angle grain boundaries and previous delta-ferrite stringers. Low boron containing samples saw a shift of the Cr2B away from the austenite grain boundaries back to the ferrite. The high boron containing samples illustrated a buildup of Cr2B and new, fine ferrite growth along the high angle austenite boundary possibly giving insight to root causes of liquation cracking issues