Optimization Of Low Temperature Impact Performance Of 3.5 Ni Steel Welded Joint

The impact and mechanical properties of 3.5Ni steel is good under low temperature conditions.It is widely used in the manufacture of equipment in the field of cryogenic separation,processing,air separation,and liquefied gas.In recent years,a large number of these materials,in domestic petrochemical projects,were used of low temperature equipment,such as pre demethane towers and methanol washing towers.3.5Ni steel welded joint should meet the requirements of-101 degrees centigrade.Ensuring the low temperature impact performance of the weld is the biggest difficulty in the manufacturing of 3.5Ni steel pressure vessel.The main factors affecting the impact performance of the welded joint are chemical composition and welding process.Some people improve the performance of the weld by increasing the content of Ni.But with the welding process to improve the content of Ni decreased.And there is a great increase in the brittle and hot tendencies of the weld and the tendencies of hot cracks.Therefore,this paper improves the low temperature impact performance of the weld by process.In order to optimize the low temperature impact properties of 3.5Ni steel welded joints from welding process,this paper systematically studies the influence of welding parameters on low temperature impact properties.A series of welding joints are obtained by controlling the main welding process parameters that affect the impact performance of the weld,the welding heat input,interpass temperature and the current characteristics.The influence of welding technology on the low temperature impact properties of welded joints is analyzed.It is concluded that the temperature between the electrodesand the arc weld is 150 degrees Celsius,and the heat input is 16 ~45KJ/cm.With the increase of heat input,the low temperature impact property of 3.5Ni steel weld metal decreases.The temperature of the 16KJ/cm inter channel temperature increases with the interpass temperature in the range of 150~350 degrees Celsius,and the low temperature impact performance decreases.The influence of the heat input and the interpass temperature on the submerged arc welding is the same as that of the arc welding.Under the same heat input and interpass temperature,the welding seam under the characteristic of alternating current is better than the welding seam under the characteristic of direct current.The influence of microstructure and distribution characteristics of each welding joint on low temperature impact performance is analyzed.It is concluded that the weld is composed of intercrystalline columnar grains and intergranular fine-grained zone.The ratio between fine grain zone and intergranular original columnar crystal zone is the main reason that influences the low temperature impact property of weld.In this paper,good low-temperature impact properties of welded joints were obtained by controlling the process technology and weld bead layout,and optimizing the proportion of the original columnar grains between the weld bead and the weld bead.