Study Of Weldability Of Q390 High Strength Low AlloySteel

In this paper, weldability of high strength low alloy steel Q390 had been researched systematically. The main contents included:structure and property of Q390 steel base metal; structure and properties of welded joints for gas shielded arc welding and submerged arc welding; With different conditions in the welding process, glycerin method was used to determine the diffusible hydrogen in deposited metal, these conditions included:wire type, humidity, shielding gas; thermal simulation test was used to welding heat affected zone; thermal simulation test was combined with metallographic method, Lever method to mapped heat affected zone continuous cooling transformation diagram of Q390 steel. At 0℃factory state, the tendency to cold cracking of welded joints of Q390 steel was analyzed thoroughly.The results showed that, microstructure of Q390 steel of hot rolling state was banded structure which was composed of the ferrite banded structure and pearlite banded structure roughly parallel and alternate arrangement in the rolling direction. Carbon equivalent for Q390 steel was 0.44, so there was a certain hardening tendency, preheat should be done before welding, preheating temperature should be controlled at 150-230℃. The mechanical properties of base metal met the requirements, These properties included:tensile strength, impact value and reduction in cross-section of "Z" direction.Q390 steel which was weld by submerged arc welding:The weld was proeutectoid ferrite-based columnar grain structure; coarse grain area was mainly bainite. Q390 steel which was weld by gas shielded arc welding:low temping was carried out immediately after welding, the weld was polygonal ferrite-based equiaxed grain structure; coarse grain area was tempered martensite.The maximum of diffusible hydrogen in weld was 4.19ml/100g which was determined by using glycerin method, so this diffusible hydrogen in weld met the requirements of high-rise steel structure. When humidity was high, and we used flux cored wire, high welding current, as shielding gas in the welding process, diffusible hydrogen in weld would increase remarkably. It was found that with the decrease of cooling time from 800℃to 500℃, different constituent in heat affected zone can be seen from SH-CCT diagram, they were proeutectoid ferrite, pearlite, upper bainite, lower bainite, lath martensite. Cold crack of Q390 steel in welding process could be prevented only when the welding cooling time was greater than 15s. The success of mapping Q390 SH-CCT diagram had provided a theoretical basis for determining the structure and property of heat affected zone of Q390 steel and selecting welding parameters. In the On-Scene cold crack test, cold crack had appeared in butt joint which was located between the upper annular plate and tubular pillar. According to three factors of causing cold cracks:the hardening tendency of Q390 steel was not serious; the content of diffusible hydrogen was also within reasonable bounds; so the reason of the cracks was binding degrees of steel structure,improper welding techniques and so on.