EVALUATION OF THE WELDABILITY OF STEEL PIPES

SCIENCE AND PEDAGOGY IN THE MODERN WORLD: PROBLEMS AND SOLUTIONS Vol. 3. Issue 9. 53 EVALUATION OF THE WELDABILITY OF STEEL PIPES O. Sh. Ochilov [email protected] Welded metal structures are widely used in all branches of mechanical engineering, including oil and gas industry machinery. In natural gas transmission systems, steel pipes are mainly used, and their reliability and long service life largely depend on the strength of the welded joints employed and the safety of pipeline operation. These factors are closely related to the quality of the weld seam and the defects that may arise in and around the welded joint. Normative documents establish dimensional parameters and tolerances for welded joints, and any deviations from these norms are considered defects. Such defects can arise during the welding process itself, during preparation of the components for welding, or in the course of assembling the structure into a single unit. For various reasons, damage that affects the strength characteristics of welded joints may occur. These defects are generally classified into three groups: external, internal, and cavity (pore or hole-type) defects. One of the most dangerous defects occurring in welded joints is cracking — including both macroand microcracks. In weld seams, these defects can appear in the form of longitudinal, transverse, radial, crater, branched, or scattered cracks. To detect such defects, non-destructive testing methods such as ultrasonic, magnetic, capillary, and radiographic inspection are used. For detecting microcracks, methods such as eddy current testing, infrared radiation, and diffraction–time analysis are applied. Naturally, the use of such diagnostic methods requires expensive equipment. One of the main causes of the formation of cracks and microcracks in weld seams and materials is the composition of the welded material — specifically, the SCIENCE AND PEDAGOGY IN THE MODERN WORLD: PROBLEMS AND SOLUTIONS Vol. 3. Issue 9. 54 amount of alloying elements and carbon — as well as the structural and phase transformations occurring in the heat-affected zone (HAZ). Therefore, before welding operations are carried out, it is extremely important to consider the chemical composition of the steel to be welded, the carbon and alloying element contents, and the structural–phase changes that may develop in the HAZ. The weldability of steel refers to its ability to achieve the required quality parameters during welding and service, ensuring a strong, integral joint that possesses the necessary physical, mechanical, and functional properties. The main indicators characterizing steel weldability are the tendency for cold cracking in welded joints and the change in their mechanical properties. Cold cracks in the HAZ occur under the combined influence of three main factors: (1) formation of martensite; (2) the presence of diffusible hydrogen; and (3) the existence of tensile stresses. Therefore, when assessing the “weldability of steels,” the carbon equivalent (Ceq) value is often used to evaluate the material’s susceptibility to cold cracking. The carbon equivalent is determined mathematically, based on the critical cooling time required for 100% martensitic transformation in the weld metal. The shorter the time required for complete martensite formation (i.e., the higher the critical cooling rate), the better the weldability of the steel and the greater its resistance to cold cracking. The formation of cracks and microcracks is of a diffusion nature and is directly related to the redistribution of hydrogen within the weld metal. When the time for martensite formation is very short, hydrogen concentration in the weld metal remains low, and the cooling rate is so high that local cold cracks are unlikely to form. However, as the time for martensitic transformation increases, the amount of residual austenite also increases. At the martensite–austenite interface, the accumulation of hydrogen concentration leads to the appearance of boundary cracks and the onset of embrittlement in the weld and base metal. If a martensitic structure is obtained in the weld metal and the joint is then subjected to prolonged holding or high-temperature service conditions, the slow SCIENCE AND PEDAGOGY IN THE MODERN WORLD: PROBLEMS AND SOLUTIONS Vol. 3. Issue 9. 55 diffusion of hydrogen through the weld can, over time, cause degradation and eventual failure of the welded metal. In summary, the shorter the critical cooling time required to achieve 100% martensitic structure in steel, the lower the carbon equivalent value. Consequently, the weldability of the steel increases, and the likelihood of cold crack formation in carbon and alloy steels and their welded joints decreases. References 1. Ющенко К. А. Свариваемость и перспективные процессы сварки материалов. // Автоматическая сварка. – 2004. – № 9. – С. 40–45. 2. Костин В. А. Математические описание углеродного эквивалента как критерия оценки свариваемости сталей. // Автоматическая сварка. –2012. –№ 8. – С. 12–17.