Study On Fracture Behavior For Weld Of High Strgenth Steel Of 980MPa

In this paper.Microstructure and Properties of weld HAZ of a 980MPa low carbon bainite high strength steel is studied under the conditions of at room temperature and -50℃.the various regional of welded joint HAZ are simulated by welding thermal simulation technique.Microstructure observation,phase analysis,and corresponding tensile and impact and COD test for various regional of welded joint HAZ are done.The new results obtained are as follows:(1)Ultra-low carbon bainitic high strength steel show good mechanical properties,high strength,high toughness and low sensitivity of the crack.Simulation of welding HAZ mechanical properties of all regions of higher and show good low temperature performance.FGHAZ(peak temperature 900℃) for the highest intensity, Plastic good.At room temperature for CGHAZ(peak temperature 1320℃) strength less than the ICCGHAZ,but the plastic Not less than FGHAZ.under the conditions of at room temperature and -50℃,900℃+900℃show best tensile properties.1320℃+1320℃show poor tensile properties,and lower with temperature tensile properties has increased.Other rule change is not obvious.Hardness tests showed that the hardness of FGHAZ maximum,the hardness of ICGHAZ and CGHAZ slightly higher than the base material.SCCGHAZ show the lowest hardness, which is consistent with the mechanical test results.[2]In a single thermal cycle:CGHAZ the impact toughness of the best,FGHAZ the impact toughness of the worst for the weak links of welded joint.[3]In repeated thermal cycling:1320℃+730℃the strength of the best,1320℃+900℃the strength of the worst,900℃+900℃and 1320℃+1320℃strength is not very different,which is located between the 1320℃+730℃and 132℃+900℃.From the toughness of the material considerations,1320℃+1320℃show best toughness,900℃+900℃show worst toughness become weak part welded joint,1320℃+730℃and1320℃+900℃toughness at between CGHAZ and FGHAZ.Repeatedcycle CGHAZ can improve the material toughness,FGHAZ can reduce the material toughness.[4]Observed through SEM analysis:Although CGHAZ very large grain size,uniform interweaved coarse and fine lath bainite.formed large dimples at fracture regional.fracture is made by the connection and integration of the large dimples.FGHAZ fracture regional for the middle and a large number of small dimples.Fracture is caused by a large number of small dimples connect the middle of dimples.1320℃+1320℃fracture regional is the expansion of dimples integration,less brittle,no carbon,no impurity phase, the small deformation around of large holes,it inhibited the small dimples.900℃+900℃dimples form of fracture regional in the majority of small and shallow,only a small number of large dimples,which is due to island and plastic poor,that is in development to a certain large around the plastic deformation has been induced by small dimples.[5]Observed through TEM analysis:Microstructure of CGHAZ is uniform interweaved coarse and fine lath bainitics,and precipitation acicular Carbide are distributed between the lath bainitics.Toughness of coarse grain zone was increased owing to the austenite thin film adherent lath bainitics interface.Microstructure of FGHAZ are twin martensite and a small number of lath bainitics,only minor acicular Carbide are distributed in the twin martensite.Toughness of Fine-grained zone was decreased owing to twin martensite.