A B the role of Nb in specific steel compositions on parameters of phase transformation of overheated austenite at specified cooling conditions, defined by specific heat input. In fact, there is a lack of data correlating the thermal conditions of the HAZ, in particular for multipass welding, with Nb effect on phase transformation at corresponding cooling rate. Thus, the presented study of two high- Nb-containing pipeline steels aims to characterize not only the impact toughness of the simulated HAZ as a function of temperature and a wide range of heat inputs, including two-pass and multipass welding, but also to investigate phase transformations of coarse-grained austenite at various cooling rates as well as the type/microhardness of the obtained structure. Materials and Methods of Investigation Material. Investigation of weldability in the current study was carried out on samples of steels with strength of the X70 to X80 classes corresponding to the requirements of Russian and international standards, whose compositions and tensile properties are shown in Tables 1 and 2. Sample thicknesses for steel grades X70 and X80 were, respectively, 25.4 and 16.4 mm. The low-C steel investigated contained 1.62–1.75% Mn, no V or Mo, and Nb microalloyed in the range of 0.06 to 0.10%. Sulfur (S), and phosphorus (P), aluminum (Al), and titanium (Ti), as well as calcium (Ca) and trace elements, are not significantly different in those two grades: 0.0007 –0.001% S; 0.006–0.0013% P; 0.02–0.04% Al; 0.012–0.026% Ti; 0.004–0.006% N2; 0.0012–0.0015% Ca; 0.0002% boron (B); 0.004–0.005% tin (Sn); 0.000% arsenic (As); 0.05–0.10% copper (Cu); 0.001% cobalt (Co); and 0.003% lead (Pb). Figure 1 shows an example of the grade X80 steel base metal microstructure. Simulation of welding.With all existing varieties of evaluation of weldability, the final assessment of the suitability of pipe steels for use in specific conditions is accomplished by testing the impact toughness of the welds. As is well known, the coarse-grained HAZ (CGHAZ) undergoes heating to 1300°–1320°C and therefore has the most reduced, in comparison with the base metal, impact toughness, but a direct investigation of its properties with the necessary localization of fracture in the site of the HAZ is difficult. Simulation of various heat inputs in the current study was implemented by varying the applied cooling rates to samples heated at high heating rates up to 1300°–1320°C, as is widely used in modern studies (Refs. 3, 14). In comparison with those studies, where a Gleeble was used, the authors of this work applied contactless induction heating to samples with the same capability to simulate a real welding process and obtain dilatometric data at cooling. This method allowing the assessment of weldability criteria and investigations of phase transformation in the HAZ based on simulation of thermal welding processes within tubular steels has been developed by the I. P. Bardin Central Research Institute of Ferrous Metals and actively used for more than two decades.The samples for subsequent mechanical testing were subjected to heating and cooling, using thermal cycles that corresponded to typical welding conditions adopted during the manufacture of pipes, as well as in the construction of pipelines. For simulation of the submerged arc welding (SAW) process, when the cooling rate is less than 10°C/s, 10 × 10-mm samples were used. For multipass welding with low heat input and therefore high cooling rates, 5 × 10- mm samples were applied to reduce the temperature gradient over the cross section of the blanks. For normalizing Charpy toughness values, the converting factor of 0.65 was used for smaller samples, which has been established by comparing the experimental results of the impact tests of subsized and traditional full-size Charpy samples of compared steels. Thermal simulation facilitates not only the investigation of impact toughness and hardness of the HAZ, but also the morphology of microstructures corresponding to specific welding conditions. In the process of manufacturing pipelines, various types of welding are used including two-pass SAW during pipe production and multipass shielded metal arc (SMA) or other welding processes during the construction of gas pipelines. These welding processes are fundamentally different in terms of the welding heat input and the character of the thermal fields. Calculations of thermal fields are made using two-dimensional field equations, applied to the factory mode of welding pipes with large heat inputs, and threedimensional ones for multipass welding of butt joints in pipes at low heat-input values. Calculation of thermal fields and determination of cooling rates for multipass welding and two-pass SAW. Based on the theory of thermal processes (Ref. 16), the WELDING JOURNAL 25-s WELDING RESEARCH Fig. 4 — The impact of toughness of steel in the HAZ of A — X70; B — X80 at different temperatures of testing vs. the applied cooling rate (W800/500): 2 — the line of the average brittleness threshold (T50 CVN); 3 — the line of specified minimum toughness (here 70 J/cm2). Table 2 — Tensile Properties of Investigated Steels Grade Tensile Properties η YS0.5 UTS TE YS0.5/UTS E (MPa) (MPa) (%) x-70* 551 631 32.2 0.87 x-80* 614 715 33 0.86
Welding Journal | January 2014
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