Introduction to types, grades, standards and uses of duplex stainless steel

Introduction to types, grades, standards and uses of duplex stainless steel
Classification of duplex stainless steel:
The first category is low alloy type, with the representative grade UNS S32304 (23Cr-4Ni-0.1N). The steel does not contain molybdenum, with a PREN value of 24-25. It can replace AISI304 or 316 in terms of stress corrosion resistance.
The second category is medium alloy type, with the representative grade UNS S31803 (22Cr-5Ni-3Mo-0.15N), with a PREN value of 32-33. Its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel. The
third category is high alloy type, generally containing 25%Cr, molybdenum and nitrogen, and some also contain copper and tungsten. The standard grade is UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), with a PREN value of 38-39. The corrosion resistance of this type of steel is higher than that of 22%Cr duplex stainless steel.
The fourth category is super duplex stainless steel, containing high molybdenum and nitrogen, standard grade UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), some also contain tungsten and copper, PREN value is greater than 40, can be used in harsh media conditions, has good corrosion resistance and mechanical comprehensive properties, comparable to super austenitic stainless steel.
Main grades or materials of duplex stainless steel:
SAF2205(S31803/1.4462/022Cr22Ni5Mo3N/F51/DP8);
S32205(F60/022Cr23Ni5Mo3N);
SAF2507(S32750/1.4410/F53/022Cr25Ni7Mo4N);
S32760(1.4501/F55/022Cr25Ni7Mo4WCuN);
S32550(255/1.4507/F61/03Cr25Ni6Mo3Cu2N/329J2L);
SAF2304(S32304/1.4362/00Cr23Ni4N/DP11);
S31500 (022Cr19Ni5Mo3Si2/3RE60)
duplex stainless steel is standardized as follows: ASTM A276, A182, A479, GB1220;
many duplex stainless steel grades have been added to China's new standard GB/T 20878-2007 "Stainless Steel and Heat Resistant Steel Grades and Chemical Composition". For example: 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti.
Duplex stainless steel has the following performance characteristics:
(1) Molybdenum-containing duplex stainless steel has good resistance to chloride stress corrosion under low stress. Generally, 18-8 type austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solution above 60°C. Heat exchangers, evaporators and other equipment made of this type of stainless steel in trace chloride and hydrogen sulfide industrial media have a tendency to produce stress corrosion cracking, while duplex stainless steel has good resistance.
(2) Molybdenum-containing duplex stainless steel has good resistance to pitting corrosion. When having the same pitting resistance equivalent value (PRE=Cr%+3.3Mo%+16N%), the critical pitting potential of duplex stainless steel and austenitic stainless steel is similar. The pitting corrosion resistance of duplex stainless steel and austenitic stainless steel is equivalent to that of AISI 316L. The pitting and crevice corrosion resistance of high chromium duplex stainless steel containing 25% Cr, especially nitrogen, exceeds that of AISI 316L.
(3) It has good corrosion fatigue and wear corrosion resistance. Under certain corrosive media conditions, it is suitable for making power equipment such as pumps and valves.
(4) It has good comprehensive mechanical properties. It has high strength and fatigue strength, and the yield strength is twice that of 18-8 austenitic stainless steel. The elongation in the solid solution state reaches 25%, and the toughness value AK (V-notch) is above 100J. (5) It has
good weldability and low thermal cracking tendency. Generally, no preheating is required before welding, and no heat treatment is required after welding. It can be welded with dissimilar materials such as 18-8 austenitic stainless steel or carbon steel.
(6) The hot working temperature range of duplex stainless steel containing low chromium (18%Cr) is wider than that of 18-8 austenitic stainless steel, and its resistance is low. It can be directly rolled into billets to produce steel plates without forging. The hot working of duplex stainless steel containing high chromium (25%Cr) is slightly more difficult than that of austenitic stainless steel, and it can produce products such as plates, tubes and wires.
(7) The work hardening effect during cold working is greater than that of 18-8 austenitic stainless steel. In the early stage of deformation of tubes and plates, a large stress needs to be applied to deform.
(8) Compared with austenitic stainless steel, it has a large thermal conductivity and a small linear expansion coefficient, and is suitable for use as equipment lining and production of composite plates. It is also suitable for making the core of heat exchangers, and its heat transfer efficiency is higher than that of austenitic stainless steel.
(9) It still has various brittle tendencies of high chromium ferritic stainless steel and is not suitable for use under working conditions above 300°C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phases such as σ are.
Main applications of duplex stainless steel: Duplex steel products have been used in industries such as petrochemicals, refining and shipbuilding, onshore and offshore oil and gas, pulp and paper, fertilizers and chemical fibers, machinery and equipment manufacturing, and in products such as heat exchangers.