A Study On The Relationship Between CTOD Toughness And Microstructure Of Welded Joints For High Strength Steel EQ70

Because of the welding structure developing to the large scale, the high strengthening, the deep and freezing water, the problem of brittle fracture of welded joints becomes particularly urgent and important. In order to adapt to the needs of the production, fracture mechanics continue to put forward a series of fracture toughness. CTOD as an indicator of fracture toughness, other parameters cannot be compared with whose authenticity and accuracy. However, many studies are carried out for the relationship of the Charpy impact test results and the microstructure. There is no report on relationship of CTOD toughness and microstructure. With the widespread use of the CTOD assessment criteria, the study on the relationship between the CTOD toughness and microstructure of high strength steel is a very meaningful work.This paper relies on the national industry and information technology program "CTOD toughness mechanism and allowed values of EQ56/EQ70welding joints". Item number:Industry and informatization ministry <2009>,382. The EQ70high strength steel CTOD toughness and welding process were studied by correlation method. That is to determine the relationship between the toughness of the material and microstructure or fracture morphology, and explore the suitable welding process and try to avoid harmful microstructure, and ultimately achieve the goal of improving the toughness of steel. This work includes the following content:1. This paper describes common microstructure combined with characteristics of high strength steel EQ70. Then electron microscopes were compared and Scanning electron microscope was proved more suited to analyze clearly the microstructure and fracture morphology at welded joints.2. The toughness of welded joints for high strength steel EQ70with60mm thickness by two welding processes has been determined by CTOD test. The results showed that the CTOD average value at the weld by the welding process B is greater than the value at the weld by welding processes A. Corresponds to CTOD average value on the fusion line, there is not much difference. But the CTOD discrete nature at the weld is less than that on the fusion line by two welding processes. 3. Metallographic test was made on the basis of the CTOD test. Under the high-powered scanning electron microscope, the effect of microstructure on CTOD of welded joints for offshore high-strength steel has been analyzed. The results show that there are two reasons for the embrittlement of the EQ70CGHAZ. One is that the granular bainite in CGHAZ was distributed on the lath ferrite in the form of the M-A. It promotes the cracks crack in its boundary. The coarse carbides in the upper bainite weaken the strengthened effect, which make the cracks crack easily and reduce the toughness. The other is that coarse grains decrease the toughness. In this paper, the microstructure of different samples by two different welding processes has been compared and a method to improve the welding process has been put forward. Reduce the welding heat input appropriately can improve the toughness of welded joints.4. The CTOD allowed value was discussed in this paper, which provides a suggestion for determining the allowed value.