A Study On CGHAZ Precipitates And Toughness Of Underwater Welds On Pipeline Steels

The precipitates, toughness and other mechanical properties were investigated in the CGHAZ (coarse-grained heat-affected-zone) of underwater welds in different condition and surface welds on three kinds of pipeline steels for transference of oil and gas. The results were compared with those of the baseplates. What was discussed principally in this thesis was particle dissolution, precipitation, coarsening in the weld thermal cycles, and the effects of these behaviors on the CGHAZ toughness were also discussed.A technique named carbon extraction replica was used to take precipitates from the CGHAZ. Particle shape, distribution, size, and density were surveyed by employing transmission electron microscopy (TEM), and precipitate composition was examined by energy dispersive X-ray (EDX) analysis units. It was found that there are two kinds of precipitate in the steels used: one is Ti-rich cuboidal precipitate; another is V-rich spheroidal precipitate. Ti-rich cuboidal precipitates hardly dissolved in the welding heat cycles, while V-rich spheroidal precipitates dissolved. The above phenomenon was consistent with the calculated results of maximum precipitate diameters which could dissolve in the weld thermal cycles. Changing welding conditions had great effect on the precipitate behaviors such as dissolution, reprecipitation, and coarsening.The CGHAZ microstructures of underwater wet welds were principally martensite and a fraction of bainite. It was inclined to reduce martensite frequencies when the heat input reduced. The bainite frequencies in the CGHAZ of locally dry underwater welds was greater than those of underwater wet welds. The CGHAZ microstructures of surface welds were bainite and acicular ferrite.For X60 and X70 steel, the CGHAZ toughness of underwater wet welds...