Little was known about the correlation between preheating of High Performance Steel (HPS), hydrogen induced cracking, and certain specialty electrodes. The purpose of this project was to independently test, observe, and record Lincoln Electric’s UltraCore 712c and UltraCore8Ni1c-H flux cored welding electrodes susceptibility to hydrogen induced cracking (HIC) in HPS, and, in doing so, to establish a minimum preheat temperature.
The G-BOP test was first introduced in 1974 by Graville and McParlan, although several refinements have been made from the original test. The G-BOP test is a self-restrained cracking test in which a transverse weld is deposited across a gap introducing a large stress concentration. This stress concentration causes cracking to originate from the base of the weld pool and grow upward into the weld bead. Depending on the hydrogen concentration and the microstructure, the resulting crack will range from 0-100% of the total bead cross sectional area, and the crack can be quantified as the area fraction of the total nugget area which failed by hydrogen induced cracking.
The electrodes were tested using three different heat inputs on three different types of base metals at four different preheat temperatures. The electrodes were tested at heat inputs of 20(kJ/in), 30(kJ/in), and 40(kJ/in) with the voltage, wire feed speed, and travel speed being adjusted for each electrode. The base metals used were HPS 50W, HPS 70W, and HPS 100W; all tested at preheats of 225⁰F, 175⁰F, 125⁰F, and room temperature.
The welder used was an ESAB Aristo Mig 500 power source with an ESAB Aristo Feed 30 wire feeder. The welder used carbon dioxide as the shielding gas, and the travel speed was set by mounting the torch to a Gullco KAT travel carriage. After setting up the welder the proper torch angle and welding parameters were set for both electrodes.
The base metal blocks were then heated on all sides to about 20 degrees above the desired preheat temperature and allowed to sit until the temperature equalized to about 10 degrees above the desired preheat temperature to allow for time to place the blocks in the testing fixtures. The time was recorded at the point the weld bead was started and finished.
The blocks sat for 48 hours in order to allow time hydrogen induced cracking develop, and were removed from the testing apparatus and tinted using a rosebud torch as the time of tinting was recorded. Immediately after tinting, the welds were broken and photographed with a ruler in order to provide a scale reference so they could be measured on the computer. The test was repeated several times using the different heat input, preheats, and base metals until a sufficient amount of data was recorded for each electrode. Trends in weld data variations were observed and compared, and a minimum preheat temperature was established.
Overall, the experience was not only relevant and timely for learning, as well as industrial application, but it also gave LETU students the opportunity to better understand fundamentals of hydrogen-induced cracking in modern steels.
AISI (American Iron and Steel Institute): $650,000 / 10 years
See more of this project here.