Nickel is a very important material, yet the resources are deficient. 08Cr19Ni10 (S30408) steel is expensive with containing 8% (mass fraction) nickel and has a low strength, while low nickel austenitic stainless steel has poor corrosion resistance property.In order to save nickel resources, the strength of austenitic stainless steel was improved by partly replacing Ni with Mn and N on the basis of ensuring that the corrosion is as well as S30408, 08Cr19Mn6Ni3Cu2N (QN1803) high strength nitrogen alloyed low nickel austenitic stainless steel was designed by Thermo-Calc software in place of S30408 steel. Microstructures, mechanical and corrosion resistant properties of QN1803 steel were investigated by means of OM, SEM, electrochemistry workstation and other methods. The results reveal the grain size of QN1803 steel is smaller than that of S30408, and difference of average grain size is increased from 1.8 μm to 16.27 μm with temperature rising from 1040 ℃ to 1120 ℃. Yield strength of QN1803 steel is increased to more than 400 MPa, and is 1.3 times than that of S30408 steel for nitrogen playing a role of grains refining and solution reinforcing. The impact energy of QN1803 steel is significantly lower than that of S30408 steel for nitrogen atoms reducing low temperature toughness of nitrogen alloyed austenitic stainless steel below -60 ℃. After 600~900 ℃ temperature ageing, chromium-rich carbideparticles first occur in grain boundaries, nose temperature of precipitation phase is 800 ℃; the inter-granular corrosion of QN1803 steel need more ageing time than S30408 steel, because nitrogen atoms can impede nucleation and growth of carbides, inter-granular corrosion of QN1803 steel is occured with double ageing time of S30408 steel at ageing temperature 700 ℃. Compared with S30408 steel, the passivation film depth of QN1803 steel has higher content of nitrogen and chromium; QN1803 steel has similar pitting corrosion rate (4.72 g/(m2·h)) and more stable austenitic microstructure and higher corrosion potential (327 mV); the pitting resistance of QN1803 steel is 1.15 times than that of S30408 steel with 60% cold reduction, and products have lower risk of stress cracking than S30408 steel. Due to addition of 1.65%Cu element improving corrosion resistance capability in dilute sulfuric acid solution, the surface of QN1803 steel can be enriched with a layer of copper-rich film protecting substrate, as a result, its corrosion resistance reaches 6.6 times than that of S30408 steel in 5% dilute sulfuric acid solution.
Table 1 Chemical compositions of 08Cr19Mn6Ni3Cu2N and 06Cr19Ni10 austenitic stainless steels
Fig.1 Phase diagrams of QN1803 (a) and S30408 (b) steels calculated by Thermo-Calc software
Fig.2 Positions of QN1803 and S30408 steels in Schaeffler-Delong diagram (A—austenite, F—ferrite, M—martensite, Creq—chromium equivalent, Nieq—nickel equivalent )
Fig.3 Effects of solution temperature on grain size of QN1803 and S30408 steels(a) grain size (b) difference of grain size between QN1803 steel and S30408 steel
Steel
Hardness
HV
Tensile strength
MPa
Yield strength
MPa
Elongation
%
Yield ratio
QN1803
215
755
425
51
0.56
S30408
165
680
285
58
0.42
Table 2 Mechanical properties of QN1803 and S30408 steels at room temperature
Fig.4 Effects of nitrogen content on yield strength and elongation of austenitic stainless steel
Fig.5 Comparisons of mechanical properties of QN1803 and S30408 steels at different temperatures(a) yield strength (b) tensile strength
Fig.6 Comparisons of impact energy of QN1803 and S30408 steels
Fig.7 Effects of cold deformation on magnetic phase content (a) and relative permeability (b)
Fig.8 Effects of ageing treatment on the microstructure of QN1803 steel
Fig.9 Precipitation curve of carbide in QN1803 steel
Fig.10 EPMA (a) and EDS analysis (b) of ageing precipitated phase in QN1803 steel heat treated at 900 ℃ for 5 h
Temperature
Ageing time / min
℃
5
10
30
60
120
300
600
-
0.08
0.12
0.13
0.30
0.80
650
-
0.13
0.77
2.12
2.31
4.69
700
0.36
0.75
2.56
3.69
4.27
5.36
750
-
0.51
2.44
3.45
5.02
4.07
800
-
0.12
0.25
0.35
0.40
0.31
Table 3 Current ratio Ra (Ir/Ia) of QN1803 steel measured under different sensitization conditions
Fig.11 Comparisons of temperature-time-sensitization (TTS) curve between QN1803 and S30408 steels
Fig.12 Comparisons of pitting potentials of austenitic stainless steel with different pitting resistance equivalent numbers
Steel
Self-corrosive
potential / mV
Pitting potential
mV
Corrosion rate / (g·m-2·h-1)
In 6%FeCl3+0.16%HCl
In 5%H2SO4
In 5%HCl
QN1803
-88
327
4.72
48.75
0.39
S30408
-110
298
4.96
321.64
1.98
Table 4 Comparisons of corrosion resistant property between QN1803 and S30408 stainless steels
Fig.13 Distributions of the alloying elements in the surface of QN1803 (a) and S30408 (b) steels
Fig.14 Distributions of Cr (a) and N (b) elements in the surface of QN1803 and S30408 steels
Fig.15 Comparisons of polarization curves between QN1803 and S30408 steels
Fig.16 EPMA images of surface of QN1803 (a) and S30408 (b) steels corroded in 5%H2SO4 for 6 h
Fig.17 Macrostructures of deep drawing products with height 40 mm and diameter 50 mm immersed in 0.16%HCl+6%FeCl3 solution for 24 h
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