Microstructure, properties and welding technology of 2507 super duplex stainless steel
Duplex stainless steel has become an important engineering material, widely used in petrochemical, offshore and coastal facilities, oilfield equipment, papermaking, shipbuilding, and environmental protection. 2507 super duplex stainless steel is developed on the basis of the second-generation duplex stainless steel 2205. At present, there are SAF2507, UR52N+, Zeron100, S32750, 00Cr25Ni7Mo4N, and other grades. The 2507 structure is composed of austenite and ferrite, and both austenite The dual characteristics of stainless steel and ferritic stainless steel have a lower thermal expansion coefficient and higher thermal conductivity than austenitic stainless steel.
Its pitting corrosion coefficient (PREN) is greater than 40, and it has high resistance to pitting and gaps. Corrosion, chloride stress corrosion cracking resistance, high strength, high fatigue strength, low temperature, and high toughness at the same time, is a widely used duplex stainless steel. In recent years, with the continuous expansion of the application fields of duplex stainless steel, the demand for welding technology has increased, which has accelerated the development of welding technology. Therefore, summarizing and discussing the research results on the weldability of 2507 super duplex stainless steel at home and abroad has important engineering practical significance for the application of 2507 super duplex stainless steel.
The very low C content in the chemical composition of 2507 duplex stainless steel can improve the weldability of the steel and reduce the precipitation tendency of carbides at the grain boundary during heat treatment, increase the intergranular corrosion resistance, high chromium, high molybdenum, and higher nitrogen content, It can improve the corrosion resistance, so that it has good resistance to uniform corrosion such as formic acid, acetic acid, nitride, etc., resistance to pitting corrosion, and resistance to stress corrosion.
Nitrogen is added as an alloying element to stainless steel, which can improve the stability of austenite, balance the phase ratio of dual-phase steel, increase the strength of steel without affecting the plasticity and toughness of the steel, and can partially replace Ni in stainless steel and reduce Cost, N in the duplex stainless steel has the effect of delaying the dispersion and precipitation of intermetallic compounds and stabilizing austenite.
The structure of 2507 super duplex stainless steel is composed of ferrite and austenite. The austenite is distributed on the ferrite matrix in strips. At higher magnifications, the interface between austenite and ferrite is not smooth and appears to be jagged. Which shows that during the cooling process after rolling, austenite is formed by nucleation and growth at the ferrite interface. The presence of austenite in the structure of duplex stainless steel can reduce the brittleness and grain growth tendency of high chromium ferrite, improve weldability and toughness, and chromium-rich ferrite can increase the yield strength of austenite in stainless steel.
Resistance to intergranular corrosion and stress corrosion, that is, the ferrite dual-phase structure has high strength and high toughness, but also maintains high resistance to stress cracking, pitting, and crevice corrosion, especially chloride and sulfide It has high resistance to stress corrosion cracking, so it can effectively solve the long-standing failure problem of austenitic stainless steel caused by local corrosion.
The 2507 super duplex stainless steel welding method has a wide range of applicability. It can be welded in a variety of ways. The welding heat input and cooling rate affect the phase balance of ferrite and austenite and the performance of the welded joint. In order to ensure that the weld has a suitable structure Comparative example and good mechanical properties and corrosion properties.
Too small or too large heat input should be avoided during welding, and the heat input should be controlled within 5～20kJ/cm. The lower limit should be removed when welding thin-walled parts, and the heat should be increased appropriately when welding thick-walled parts. Enter, the temperature between tracks should not exceed 100°C.