Weldability of Niobium-Containing High-Strength Steel for Pipelines The investigated steels showed no tendency to cold cracking in the heat-affected WELDING JOURNAL 23-s WELDING RESEARCH Introduction The high working pressure of modern gas pipelines up to 100–120 MPa require high-impact toughness Charpy V-notch (CVN) of the material (at least 180–250 J/cm2) at relatively low temperatures down to –20° to –40°C, depending on the specifications for the pipelines. Designed steel grades actually have higher CVN values; however, the most critical area of pipelines is the weld heat-affected zone (HAZ). The HAZ undergoes recrystallization, grain growth, followed by (at cooling) a large scope of austenite transformation, thus destroying the attractive thermomechanical-controlled processing (TMCP) microstructure, and often is the site of the lowest fracture resistance. The microstructure of high-strength low-alloy (HSLA) steels depends on the steel composition and thermomechanical processing route. With the recent trend toward lower carbon (C) contents, niobium’s (Nb) effect on transformation behavior has been noted with the emergence of acicular or bainitic steels. Under certain conditions, such as utilizing low interstitial contents and high austenitizing temperatures, small Nb additions increase hardenability by depressing the Ar3 transformation temperature. Microalloying with Nb is an integral part of the composition of modern high-strength steels for pipelines because of its significant and simultaneous effects on retardation of recrystallizaton, precipitation hardening, and hardenability of austenite facilitating the formation of a grain-refined structure of favorable acicular ferrite/bainitic ferrite and contributing substantially to the strength of low-C steels (Ref. 1). At the same time, there is considerable disagreement on the effect of Nb on HAZ toughness. Some controversy exists in the literature concerning the influence of Nb on HAZ properties under certain conditions that is discussed by pipeline construction companies and steel producers. In the study of the effect of Nb in the presence of nickel (Ni), chromium (Cr), vanadium (V), and molybdenum (Mo), it has been shown that the microalloying Nb steels with V and Mo leads to embrittlement of the HAZ (Ref. 2). Negative effect of joint microalloying pipeline steels by V and Nb was also noted in other studies (Refs. 3, 4). In a study performed at heat inputs ranging from 1.5 to 6 kJ/mm using steel with various C contents, it was shown that Nb additions can have a detrimental or beneficial effect at low heat inputs, depending on the C level (Ref. 5). Investigating HAZ embrittlement in Nb-containing C-Mn steels, it was shown that 1) C content dominates in the control of the toughness properties and is particularly detrimental to HAZ toughness at higher C levels (0.19% C) in combination with Nb; 2) Nb does not have a significant effect on HAZ toughness at low C levels (0.06% C) at high welding heat inputs up to 6 kJ/mm; 3) good toughness properties can be obtained at intermediate C levels of 0.12% with intermediate to high Nb additions at lower heat inputs in the range 1.5 to 3 kJ/mm; 4) high C levels (0.19% C) combined with a low heat input result in the formation of untempered brittle martensite and lower bainite with poor toughness properties regardless of Nb content. Numerous publications have discussed the effect of Nb addition on the properties and microstructure of the HAZ in low-C microalloyed steels. Niobium is reported to be beneficial as it expands the nonrecrystallization temperature range, which is useful not only for plate rolling, but because it increases hardenability, which, in turn, leads to retardation of the grain boundary ferrite network, thus enhancing intragranular ferrite formation in lowheat input HAZ (e.g., Ref. 6). The positive effect of Nb was found in another study, where it was noted that at higher C contents Nb facilitates the formation of carbides, decreasing the martensiteaustenite (MA) fraction (Ref. 7). Other works reported that the increase in the hardenability by Nb enhances not only the Widmanstätten ferrite and upper bainite but also MA formation in the re- zone, even with low heat input BY I. I. FRANTOV, A. A. VELICHKO, A. N. BORTSOV, AND I. Y. UTKIN ABSTRACT The presented study contains an assessment of weldability based on careful investigation of two niobium-containing industrial steel grades X70 and X80, with 0.056 and 0.094% Nb, respectively. Characteristics of their resistance to brittle fracture in the heat-affected zone (HAZ) were evaluated on simulated samples after rapid heating to high temperature followed by cooling at various rates corresponding to different heat inputs. As shown, the HAZ of both investigated steels ensure performance down to –30°C in submerged arc welded thick-walled pipes welded with high heat input. Investigations of phase transformations at cooling from 1300°C and microhardness measurements have shown that investigated steels with Nb content up to ~0.1% do not have a tendency for cold cracking in the HAZ during welding, even with very low heat input. KEYWORDS Weld Process Simulation Weldability Testing High-Strength Steels Piping/Tubing C-Mn Steels Submerged Arc Welding Shielded Metal Arc Welding I. I. FRANTOV (ifrantov@mail.ru), A. N. BORTSOV (alnicbortsov@gmail.com), and I. Y. UTKIN are with I. P. Bardin Central Research Center for Ferrous Metallurgy, Moscow, Russia. A. A. VELICHKO is with Izorsky Pipe Plant, Kolpino, Russia.
Welding Journal | January 2014
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