Heat-Affected Zone Toughness of a TMCP Steel Designed for Low-Temperature Applications

Abstract

The objective of this investigation was to provide a detailed evaluation of the heat-affected zone (HAZ) toughness of a high-strength TMCP steel designed for low-temperature applications. The results from both Charpy-vee notch (CVN) and cracktip-opening displacement (CTOD) tests conducted on two straight-walled narrow groove welds, produced at energy inputs of 1.5 and 3.0 kJ/mm, show that significantly lower toughness was exhibited by the grain-coarsened HAZ (GCHAZ) compared with the intercritical HAZ (ICHAZ) region. This is explained based on the overall GCHAZ microstructure, and the initiation mechanism which caused failure. For the particular TMCP steel investigated in this study very good ICHAZ toughness properties were recorded using both HAZ Charpy and CTOD tests. In general, this was attributable to the low hardness, relatively fine ferrite microstructure, and the formation of secondary microphases that were not overly detrimental to the toughness. The lower-bound GCHAZ CTOD results obtained for both welds (KA W-L and KA W-H) did not meet the targeted requirement of δ = 0.07 mm at −50°C. It was found in both welds that low CTOD toughness was associated with the initiation of fracture from nonmetallic inclusions, which were complex oxides containing Ce, La, and S. The sites were located in the subcritical GCHAZ (SCGCHAZ) region in the case of the 1.5 kJ/mm weld and in the GCHAZ for the 3.0 kJ/mm weld. Some variation in CVN toughness was observed at different through-thickness locations. Toughness was lowest for the GCHAZ of the weld deposited at 3.0 kJ/mm and was related to the proportion of GCHAZ being sampled, which was ~55 percent for the bottom compared to 25–30 percent for that of the top location. Recommendations are proposed on the preferred practices and criteria that should be used in establishing guidelines and specifications for evaluating the HAZ toughness of candidate steels for construction of Arctic class ships.

The Influence of Thermal Cycles on the Microstructure of Grade 92 Steel

“…[5,30] It is worth noting that the conventional definitions of the HAZ microstructure, primarily in low alloy steel welds, involves a further classification of the regions with a peak temperature above the Ac 3 temperature into the so-called coarse-grain and fine-grain regions, related to whether the grain structure was significantly influenced by second-phase particles. [1,2,14] However, it was observed in this case that the rapid weld thermal cycle results in the Ac 3 temperature being close to the temperature at which the pre-existing secondary precipitates are completely dissolved. Thus, although some variation in the grain structure in the regions that were heated above the Ac 3 temperature was observed (Figures 20 and 21), this was primarily a function of distance from the weld line, rather than being caused by the presence of undissolved secondary precipitates.…”

“…Compared with the existing definition of the HAZ microstructure in 9 wt pct Cr steel welds, [1,2,14] the ranges of the corresponding peak temperatures of these two regions are close to the so-called intercritical and so-called overtempered (OT) regions, respectively. However, the terminology intercritical does not clearly indicate the fact that not only the martensitic matrix was partially reaustenitized but also the secondary precipitates in the original matrix were affected by weld thermal cycles.…”

“…[1,2] These descriptions have continued to be used for steels that are within the family of 9 to 12 wt pct Cr, tempered martensitic steels. The classical description for the HAZ that is commonly applied to most steel grades considers that it is divided into four regions [3] :…”

Role of tandem submerged arc welding thermal cycles on properties of the heat affected zone in X80 microalloyed pipe line steel

“…5, plays an important role in regards to the HAZ Charpy impact testing. According to Gianetto et al (1997) as the width of the HAZ is growing, with increased heat input, the impact energy for the fusion line is reduced According to Fig. 11(b) at the fusion line region, the highest absorbed energy of about 40 J was obtained in specimen 7, which was welded with a total heat input of 6 kJ/mm.…”

“…The austenite grain in the HAZ is considerably coarser when compared to the grain size of the parent metal. Gianetto et al (1997) have reported the extent of the austenite grain growth is dependent upon the weld thermal cycle, the steel composition and the precipitate chemistry. In order to prevent brittle fracture, excellent low-temperature toughness is required in the base metal and the HAZ of a pipeline's welded joint.…”