OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 changes in the amount of martensite, bainite and acicular ferrite [30]. Figure 2 shows that a high scatter band is observed when high-strength weld metals are cooled for diff erent times in the temperature range 800–500 °C [4]. Standards [5–10] allow a wider range of alloying and micro-alloying elements, and therefore each manufacturer off ers its own chemistry to achieve qualifi cation requirements. Carbon equivalent (Ceq) was included in the standard [5] because it is generally related to hardenability. Limits for Ceq were calculated based on the minimum and maximum alloying element contents. Therefore, a lower Ceq value is always preferable, indicating good weldability. The American Petroleum Institute has adopted two formulas (CEIIW and CE Pcm) [5] to determine the carbon equivalent limit for API PSL 2 pipe steel. The CEIIW formula is provided by the International Welding Institute and is commonly used for carbon and carbon-manganese steels. In Europe Pcm is the critical parameter of the metal. CE Pcm is taken from the documents of the Japan Society of Welding Engineers. CE Pcm was proposed specifi cally for testing the weldability of high-strength steels. The balance of superior strength and toughness can be disrupted following thermal cycling that occurs during welding, causing poor toughness in the heat-aff ected zone (HAZ) [11–19]. General welding issues Modern steels with high strength and high impact toughness are widely used in pipelines, shipbuilding and various manufacturing industries [2, 3]. Changes in steel production technology and the steel rolling process pose a challenge to the production of welding consumables and joining technology. It is important to note that, in contrast to the production of wrought steel, the strength and toughness of weld metals, as a rule, should be achieved through alloying [2–4]. As a consequence, due to the complexity of welding processes and the limitation of heat input and, consequently, cooling rates, the toughness of the weld metal at low temperatures is lower than that of the base metal [3, 4]. In addition [2–4], the microstructure of weld metals with a yield strength of 600 MPa and above consists mainly of bainite and martensite, rather than predominantly acicular ferrite. Therefore, the calculation of the basic composition of the weld metal should a b Fig. 1. The eff ect of the carbon equivalent on the ultimate tensile strength (a) and impact strength at 20 °C of weld metals (b) [32] Fig. 2. The eff ect of weld metal cooling rate (Δt8/5) on the ultimate tensile strength of high-strength pipeline steels [4]
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