1) It is necessary to control the phase ratio The most appropriate ratio is that the ferrite phase and the austenite phase represent approximately half, and the number of a phase cannot exceed 65%, to ensure the best overall performance. Currently, regardless of ASTM A789, A790, A240 or national standard GB1220, although duplex stainless steel grades are listed, there is no arrangement for two-phase ratios, which is detrimental to the use of duplex stainless steel. If the two-phase ratios are unbalanced, for example, the number of ferrite phases is too much, it is easy to form single-phase ferrite in the welded ZAT and is sensitive to stress corrosion cracking in some media.
(2) It is necessary to master the law of transformation of the structure of duplex stainless steel and be familiar with the TTT and CCT transformation curves of each steel grade. This is the key to correctly orienting the formulation of heat treatment processes and hot forming for duplex. Stainless steel The precipitation of the fragile phase of duplex stainless steel Much more sensitive than austenitic stainless steel.
(3) The continuous-use temperature range of duplex stainless steel is -50 x 250C, the lower limit depends on the fragile transition temperature of the steel, the upper limit is limited by the 475C fragility and the upper limit temperature cannot exceed 300oC.
(4) Duplex stainless steel requires rapid cooling after treatment of the solution. Slow cooling will cause precipitation of fragile phases, leading to decreased steel toughness, especially local corrosion resistance. Currently, ASTM A789, A790, A240 and GB1220 are not included in the requirement of "intermetallic phase precipitation is not allowed", which is unfavorable for use.
(5) The hot-working and hot-forming lower limit temperature of high chromium-high molybdenum duplex stainless steel should not be less than 950oC, and low chromium duplex stainless steel should not be less than 900oC to prevent surface cracks caused by precipitation of fragile phases during processing.
(6) Tension relief treatment at 650 x 800oC commonly used in austenitic stainless steel cannot be used, and annealing treatment is generally used in solution above 950oC. After the duplex stainless steel surface on the low alloy steel surface, when general stress relief treatment is required at 600 x 650oC, the toughness and corrosion resistance caused by fragile phase precipitation, especially local corrosion resistance, should be considered. To reduce the problem of reducing heating time as much as possible in this temperature range, heat treatment of low alloy steel and duplex stainless steel composite plates should also be considered at the same time. It is a difficult problem to eliminate tension after hardening of thick plates of low alloy steel that need a tempering and tempering treatment after the surface of the duplex stainless steel surface layer, because the tempering temperature of low alloy steel is located right in the precipitation zone of the Fragile Phase of Duplex Stainless Steel.
