Improvement Of Microstructure And Toughness In High Heat Input Weld

During the development of a flux-cored wire for electrogas welding, the microstructure of the weld metal should be optimized to obtain good toughness at high heat input welding. In this paper, the effect of microalloying element Ti and B on microstructure and toughness is studied. It is concluded that acicular ferrite plays an important role in the improvement of microstructure and toughness in the high heat input welds. Moreover, there are some correlations among the amount of acicular ferrite, type of inclusions and M-A constituent with the weld toughness established. Meanwhile, the flux-cored wire with the satisfaction of AWS A5.26EG70T-G is developed based on the related study. Good weldability is obained in electrogas arc welding within the high heat input ranged between85kJ/cm and240kJ/cm, and the excellent weld toughness is achieved because of large amount of acicular ferrite formed in the high heat input weld.The effects of Ti ranged in0～0.064%and B in0～0.0088%on the weld microstructure and mechanical properties are studied. According to the results, the effective inclusions of Ti-Mn-Al-O type are formed, which serve as effective nuclei for acicular ferrite, when Ti is in0.028%～0.038%. As a result,80%of acicular ferrite is obtained and the weld toughness at low temperature is obviously promoted. With overdosed Ti up to0.064%, acicular ferrite is suppressed because of the ineffective inclusions of Ti-Al-O type is formed in the weld and bainite transformation is favored. Accordingly, the weld toughness is deteriorated with microstructure coarsening. Meanwhile, B of0.018%～0.0052%is effective for the decrease of proeutectoid ferrite in the weld and the impact toughness is obviously promoted owing to the large amount of acicular ferrite. Acicular ferrite is decreased while proeutectoid ferrite is increased due to the ineffectiveness of B to the grain boundaries when B is added to the maximum of0.0088%.The gross and mean sizes of M-A constituent are raised by Ti and B addition. However, Ti leads to the transformation of block to string with acicular ferrite formation and finally to block with bainite transformation, while B leads to blocky M-A constituent to string with acicular ferrite increase. There is an advantage in the combination of Ti and B microalloying.There is a good correspondence between the impact toughness at low temperature and fracture toughness in the weld. According to the CTOD test results, the weld fracture toughness is obviously improved by large amount of acicular ferrite in the weld. The more acicular ferrite is obtained, the better impact toughness and fracture toughness are achieved.At high heat input up to240kJ/cm, excellent impact toughness is achieved due to large amount of acicular ferrite formed, although there is a tendency of microstructure coarsening in the weld. Therefore, the designed flux cored wire is less sensitive to the welding heat input and with more wide range of application.The flux-cored wire for electrogas welding with the satisfaction of AWS A5.26EG70T-G is developed and the chemical composition of the weld is arranged as follows:(1.10%～150%)Mn,(0.30%～0.60%)Si,(0.20%～0.6%)Ni,(0.20%～0.30%)Mo, Ti<0.10%, B<0.02%, and with the restrictions of Ceq,<0.39%and Pcm<0.2%as well. Additionally, the total amount of the basic flux consisting of fluorides, SiO2, CaO and TiO2is to be restricted to8%～15%in the gross for the promotion of weld mechanical properties as well as welding procedures.