Effect Of Heat Input On The Microstructure And Mechanical Properties In Heat Affect Zone Of S690QL Steel

S690QL steel in quenched and tempered (Q&T) condition with fine grain and highstrength belongs to ASTM A517GR Q(Modified) grade, and is increasingly used inoffshore platforms. However, welding of such a steel is difficult due to its high carbonequivalent. The obvious problems occurred in welding are cold cracking, high tendency ofhardenability and hydrogen-induced cracking in the heat affect zone(HAZ). Therefore inthis subject, thermal simulation was used to evaluate the effect of heat input onmicrostructure and toughness of the simulated HAZ. In the meantime, microstructure andproperties of150mm thickness S690QL welded joints were studied, which were welded inconditions of GMAW, GMAW, FCAW and SAW.The results of thermal simulation test show that grain enlargement is the vital factorthat affects toughness of S690QL simulated HAZ. In condition of t85≤40s, the toughness ofHAZ is increased with the prolongation of cooling time. Toughness of HAZ can beimproved when the HAZ gains the ML and a small amount of BLwith control of heat inputat20kJ/cm more or less. When the peak temperature is1150℃，toughness of HAZ isimproved significantly by the second thermal cycle. And the alternate permutation of MLand BLis the reason of toughness improvement.Mechanical properties of the welded joint indicates that, thickness center is the weakestlink of the S690QL welded joint. And the structural embrittlement in weld metal, fusionzone and5mm from fusion line are the main factors that restrict the impact performance.When heat input is at20kJ/cm around, impact performance of HAZ is optimal. Neitherreducing nor increasing heat input is good for toughness. When welding in the actual,GMAW is the preferred method. The tensile strength of welded joint with asymmetric anglegroove is superior to the one with symmetric angle. However, effect of groove type onwelded is not yet conspicuous.The analysis of microstructure and Fracture morphology indicates that the mainmicrostructure of weld metal are intragranular ferrite, acicular ferrite, and a little granularbainite. As the heat input increases, grain coarsening, width of bainite and ferrite lathincreasing occurs in the HAZ. While, content of high angle grain boundary is reduced. Atthe same time, extent of cleavage fracture is enlarged. The main structure of CGHAZ arelath martensite and bainite. Both the size of these lathes and morphology character of bainite are main crucial factors of impact performance in CGHAZ. Fine grain area is temperedmartensite. Affected by the multiple thermal cycles, there is coarse grain morphology inHAZ of5mm from fusion line. In the interior of grain, martensite lath with the characteristicof tempering is directional distribution. Meanwhile, the size of adjacent grain isheterogeneous with a little M-A constituent existing. So that, the low temperature impacttoughness comes to be poor,and the fracture shows the characteristics of cleavage.