Embrittlement of Intercritically Reheated Coarse Grain Heat-Affected Zone of ASTM4130 Steel
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sing concerns on the shortage of oil–gas resources on land and offshore, oil exploitation and development in deep sea have become globally attractive. Thus the study on deep-sea oil–gas drilling technology and equipment has become a hot topic. The semi-submersible drilling platform adopted as deep-sea oil–gas drilling equipment has gained great attention due to its unique advantages of high storm-resistance ability, wide range of operating depths, and the capability of accommodating variable deck loads, and so on. The operating depth of the newest deep-water semi-submersible drilling platform is as high as 3000 m, where the working pressure is up to 70 MPa. Hence, there is a high demand on the strength and pressure resistance of the materials used for high-pressure pipelines. In recent years, ASTM4130 steel, a heat-treatable low-alloy (HTLA) steel, has been adopted as the material for high-pressure pipelines of the mud-cement systems of
LIYING LI, TAO HAN, and BIN HAN are with College of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, P. R. China. Contact e-mail: llying3456@163. com Manuscript submitted February 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the semi-submersible drilling platform due to its remarkable merits of high strength, excellent low temperature toughness, and low price.[1] However, its weldability is not desirable due to the presence of both cold cracks and the embrittlement in the weld heat-affected zone (HAZ) resulting from high carbon equivalent. It is well known that the balance of strength and toughness in HAZ is usually upset due to the effect of welding thermal cycle, hence causing the degradation of the mechanical properties. For the single-pass welding, the coarse-grained HAZ (CGHAZ) generally possesses the lowest toughness due to the unfavorable formation of coarse-grained microstructures, such as untempered martensite, upper bainite, and martensite–austenite (M–A) constituents.[2–5] Generally, during multi-pass welding, a CGHAZ can be roughly categorized into four zones according to the peak temperature of subsequent thermal cycles: the unaltered CGHAZ (UA CGHAZ, 1350 ~ 1100 C), the supercritically reheated CGHAZ (SCR CGHAZ, 1100 C ~ Ac3), the intercritically reheated CGHAZ (IR CGHAZ, Ac1 ~ Ac3), and the subcritically reheated CGHAZ (SR CGHAZ, < Ac1).[6–8] However, the toughness of the whole CGHAZ cannot be improved effectively in the following passes. Extensive studies have shown that IR CGHAZ,
whose toughness is even lower than the CGHAZ in low-carbon structural steels, has the lowest toughness in the reheated CGHAZ.[6–10] The main factors underlying the loss in IR CGHAZ toughness are the presence of M–A constituents[6,11–14] and structural heredity.[15,16] Under most conditions, M–A constituents are high carbon martensite with some retained austenite.[14] It has been reported that the toughness of the IR CGHAZ is closely related to the size, volume fraction, and shape of M–A constituents.[17–20] Generally, larger size and higher vol
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