Comparison of mechanical properties between HSS Gleeble simulated specimen and welded specimen

Abstract

High Strength Steel is receiving increasing attention from various industries due to its superior strength-to-weight ratio. There is a growing demand for researches dedicated to how HSS, welded with different heat inputs, will behave. One of the methods to achieve this purpose is to utilize Gleeble simulating system which imitate the heat cycles of welding process easily with desired input parameters. However it is very little known how similarly the Gleeble could simulate the welding. This thesis investigates difference in mechanical properties between welded and thermally simulated (Gleeble-simulated) steels which are affected by same heating curve and T8/5 cooling time.

Initially, 8mm thick plate of S700MC Plus High strength steel from SSAB was used for the research. The plate was welded with three weld runs, but in the Gleeble simulation, Gleeble specimen was heat-treated with a single heat cycle. Consequently, their tensile strengths became vastly different. In order to eliminate potential errors and simplify the experiment, the new experiment was conducted with a 4mm single-welded plate. Two plates (T5 and T6) were welded with two different heat inputs (0.38 KJ/mm and 0.68 KJ/mm). On contrary to expectation, Tensile strength of welded specimens from both T5 (t8/5=13.45) and T6 (t8/5=33.03) turned out to be almost same. However, as for the Gleeble-simulated specimen, there was a greater reduction in tensile strength from T6 than T5. Significant hardness drop was observed in HAZ of all the samples. Welded specimens from T5 with T8/5 cooling time of 13.45s had higher hardness values in HAZ compared to the T6 with T8/5 of 33.03s. Hardness value in HAZ of Welded specimens is almost identical to that of corresponding Gleeble specimens.

It is unclear why the tensile strengths of welded specimens from both T5 and T6 are so similar, while those of Gleeble specimens show different values in general pattern of strength reduction proportional to longer t8/5 cooling times. One possible explanation could be that strength reduction in welded specimen may not occur significantly any more once t8/5 reaches 13 seconds, but samples that are Gleeble-simulated may continuously show linear strength reduction even after t8/5 exceeds 13 seconds. Another explanation is that this could be simply just an error in tensile testing process for welded specimens. In future research, more samples from larger range of heat inputs and t8/5 cooling times are required, so that resulting data can be validated enough.

Because of seemingly unreasonable result of tensile test, this comparison between welded and Gleeble specimen could not be validated. Consequently, meaningful correlation could not be made as well. However, this thesis still successfully set the standard methodology and procedures for comparing welded and Gleeble-simulated steels. Furthermore, it has clearly laid down the next steps of experiment that could help identify exact problem and validate the result.