| Abstract Scope |
The use of higher strength reinforcing steel, specifically ASTM A706 Grade 80 and Grade 100, is growing in popularity across the structural and construction industries. The current edition of AWS D1.4:2018 does not include provisions for these higher grades, creating the need to validate or develop new minimum preheat temperature recommendations to avoid hydrogen-induced cracking during welding. The manufacturing processes used to achieve these higher strengths, whether through micro-alloying (MA) or quench- and- self-tempering (QST), increase hardenability and produce harder microstructures in the heat-affected zone, making these steels inherently more susceptible to cracking than the lower grades currently covered by the code.
To address this, a series of self-restraint cold cracking tests were performed on ASTM A706 Grade 80 and Grade 100 bars in sizes #6, #11, and #14. Both MA and QST bar types were evaluated using shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) with filler metals appropriate for each strength grade. Preheat temperatures were varied in 50°F increments and crack evaluation was conducted on polished weld cross-sections using light optical microscopy a minimum of 48 hours after welding.
Results show that the current requirements in Table 7.2 are not sufficient to prevent cracking in the higher strength reinforcing steel, particularly for Grade 100 bars and larger bar diameters. The majority of cracks initiated at the weld root and propagated transgranularly through the coarse grain heat-affected zone (CGHAZ) of the bars. New minimum preheat temperatures required to preclude cracking were developed. Notably, while QST bars required higher preheat temperatures than MA bars for Grade 80, this relationship reversed for Grade 100, suggesting that the influence of bar type on hydrogen cracking susceptibility warrants further investigation. Based on these findings, new suggested minimum preheat values have been developed to inform the next revision of AWS D1.4. |