Research of influence of heat treatment on metallophysical properties of metal of welded seams

Цель настоящего исследования - оценка влияния локальной термической обработки (нормализации и закалки с отпуском для продольных сварных соединений труб, сваренных контактной сваркой токами высокой частоты, а также высокотемпературного отпуска и термического отдыха для кольцевых стыков трубопроводов, сваренных дуговой сваркой) на показатели механических свойств и металлургического качества металла сварных соединений. Для проведения исследований в области продольных сварных соединений использовались трубы диаметром 530 мм с толщиной стенки 10 мм класса прочности К56, для оценки свойств металла кольцевых стыков были выбраны катушки длиной 500 мм с односторонней симметричной разделкой кромок с углом скоса 30°, изготовленные из труб диаметром 219-530 мм с толщиной стенки 8-15 мм из сталей класса прочности К42, К48, К56. В частности, показано, что высокий отпуск повышает пластичность, снижает уровень остаточных сварных напряжений (более чем на 30 %), твердость и прочность. Термический отдых сварных стыков способствует снижению уровня остаточного водорода в металле сварных соединений, что позволяет повысить свариваемость трубной стали при проведении ремонтных работ. В целом сделан вывод о том, что локальная термообработка позволяет оптимизировать показатели механических свойств металла сварных соединений и при необходимости получать требуемый уровень механических свойств. The purpose of this study is to assess the effect of local heat treatment (normalizing and quenching with tempering for longitudinal welded joints of pipes welded through contact welding with high frequency currents, and high-temperature tempering and cooling for arc welded circular joints of pipelines) on the mechanical properties and metallurgical quality of welded joint metal. To carry out research in the field of longitudinal welded joints, pipes with a diameter of 530 mm and a wall thickness of 10 mm with the K56 strength grade were used; to assess the metal properties of ring joints, 500 mm long spools with unilateral symmetrical beveling with a bevel angle of 30°, made of pipes with a diameter of 219-530 mm with wall thickness of 8-15 mm made from K42, K48, K56 strength grade steel, were used. In particular, it is shown that high tempering increases ductility, reduces the level of residual weld stresses (by more than 30 %), hardness and strength. Cooling of welded joints helps to reduce the level of residual hydrogen in the welded joint metal, which increases the weldability of pipe steel during repair works. In general, it is concluded that local heat treatment allows to optimize the indicators of mechanical properties of the welded joint metal and, if necessary, to obtain the required level of mechanical properties.

Underwater In Situ Local Heat Treatment by Additional Stitches for Improving the Weldability of Steel

In this paper the influence of in situ local heat treatment performed by additional stitches on the weldability of high-strength low-alloy (HSLA) S355J2C+N steel was tested. The investigated steel is characterized by high susceptibility to cold cracking. It is necessary to find a method to improve the quality of welded joints. The local heat treatment was applied as an effect of bead-on plate welding made on the face of a Tekken test joint. The specimens were made by the use of covered electrodes in the water environment. For testing weldability, Tekken test specimens were made. Then, the different number of the pad welds with different overlapping were laid on the face of the tested welds. Non-destructive (NDT) visual and penetrant tests were undertaken. During the NDT, imperfections like shape mistakes and spatters were found. Then, metallographic macro- and microscopic testing were performed. The macroscopic observations proved that water environment can generate imperfections like cracking and pores. However, for specimens with additional stitches the number of imperfections decreased. Microscopic tests proved that the proposed technique affected the structure of the heat-affected zone (HAZ). The specimens without the application of additional stitches are characterized by brittle bainitic and martensitic structure. Specimens, in which the additional stitches were applied, contain tempered martensite, fine ferrite and fine pearlite in their HAZ. It was also observed that the number of cracks decreased for in situ local heat-treatment specimens. The final step was Vickers HV10 hardness measurement. These measurements confirmed previous results. The heat from additional stitches affected the steel by significantly decreasing the hardness by 80–100 HV10. The results of experiments showed that the heat from pad welds provided microstructural changes in heat-affected zones and a decrease in the susceptibility to cold cracking, which results in improvement in the weldability of HSLA steel in wet welding conditions.