In the development of duplex and super duplex stainless steel, people pay attention to some physical and metallurgical problems. For example, the content of Cr%+Mo% in steel should be greater than or equal to 2 1%, so as to prevent martensitic transformation and steel performance degradation caused by cold forming. The effect of cold deformation on martensite formation and corrosion resistance of improved 18-5 duplex stainless steel (when Cr+Mo content is 2 1%, N content is 0.07% and 0. 10%) was studied in China. It shows that the formation and increase of martensite will reduce the stress corrosion resistance and pitting corrosion resistance of this steel. For another example, in order to improve the corrosion resistance of duplex stainless steel, when increasing the content of chromium and molybdenum in steel, necessary preventive measures should be taken to prevent the thermal stability of duplex stainless steel structure from decreasing and intermetallic precipitation after Cr%+Mo% is greater than or equal to 35%, resulting in the deterioration of plasticity, toughness, hot workability, welding and other manufacturability and corrosion resistance. For another example, nitrogen is a very beneficial and important element, but the addition of a large amount of nitrogen will cause the formation of chromium nitride and precipitate along the grain boundary, thus causing corrosion damage such as intergranular corrosion and reducing toughness. The nitrogen content in steel exceeds the solid solution content of steel, which will cause a lot of waste products during the solidification of steel. If a large amount of manganese is added to improve the solid solubility of nitrogen in steel, the harm caused by the formation of manganese sulfide should also be evaluated.
At present, the main brand of super dual-phase steel in China is 2507(S32750).
2507(UNS S32750) duplex stainless steel has strong strength and corrosion resistance, and is mainly used in chemical processing, petrochemical industry and submarine equipment. It has strong chloride corrosion resistance, high thermal conductivity and low thermal expansion coefficient. The content of chromium, molybdenum and nitrogen is high, and it has high resistance to pitting corrosion, crack corrosion and general corrosion. Its impact strength is also very high, and 2507 alloy is not suitable for use in the temperature environment above 570℃, and its toughness will be reduced if it is kept in such an environment for a long time. Its chemical composition and mechanical properties are shown in table 1 and table 2.
Table 1 chemical composition (%) Steel grade C ≤ Mn ≤ Si ≤ S ≤ P ≤ Cr Ni Mo Cu ≤ N S32750 (SAF 2507)
00 Cr 25 ni 7 mo 4n 0.03 1.2 0.8 0.02 0.035 24/
26 6.0/
8.0 3.0/
5.0 0.5 0.24/
0.32s 3 1803(SAF 2205)00 Cr 22 ni 5 mo 3n 0.03 2.0 1.0 0.02 0.03 2 1/
23 4.5/
6.5 2.5/
3.50.08/
0.20 S3 1500(3RE60)
00cr 18 ni 5 mo 3si 2 0.03 1.2/
2.0 1.4/
2.0 0.03 0.03 18/
19 4.25
5.25 2.5/
3.00.05/
0. 10 Table 2. Heat treatment system of mechanical properties steel grades
Celsius σb (MPa)
≥ σs (MPa)
≥ δ%
≥ hardness ≤ Brinell hardness S32750(SAF2507)
00 Cr 25 ni 7 mo 4n 1025- 1 125
Water quenching 8005501531032s31803 (SAF 2205)
00 Cr 22 ni 5 mo 3n 1020- 1 100
Water quenching 620 450 25 290 30.5S31500 (3re60)
00cr 18 ni 5 mo 3si 2 980- 1040
Water quenching 630 440 30 290 30.5