Effects of Silicon on the Microstructure and Propertiesof Cast Duplex Stainless Steel with Ultra-HighChromium and High Carbon
WANG Guiqin,WANG Qin,CHE Honglong,LI Yajun,LEI Mingkai()
Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
Cite this article:
WANG Guiqin,WANG Qin,CHE Honglong,LI Yajun,LEI Mingkai. Effects of Silicon on the Microstructure and Propertiesof Cast Duplex Stainless Steel with Ultra-HighChromium and High Carbon. Acta Metall Sin, 2020, 56(3): 278-290.
Duplex stainless steels have excellent corrosion resistance, but its insufficient wear resistance limits its application scope. Therefore, alloying of duplex stainless steels to form hard phases, such as carbides, becomes one of the important research directions to enhance their wear resistance keeping their good corrosion resistance. Specifically, carbides are considered as an ideal hard phase to strengthen Fe-Cr-C alloys, where their types, hardness, volume fraction, morphology, size, spacing and interconnecting feature are the important factors affecting the wear resistance of the alloys. It has been shown that, Si as an alloying addition can modify the carbide precipitates in type, phase morphology and distribution in Fe-Cr-C alloys, having an obvious influence on their wear and corrosion resistance as well as their mechanical properties. In this work, the microstructure and properties of two types of ultra-high chromium (40%, mass fraction) and high carbon (1.5%) duplex stainless steels with different concentrations of Si (0.46% and 1.36%) are investigated. The composition, as-cast microstructure and solidification process as well as the changes in the microstructure, phase structure after solution treatment were studied by chemical analysis, OM, SEM, EPMA and XRD. The mechanical properties including hardness, tensile strength, fracture toughness, corrosion resistance and wear resistance have been tested correspondingly. It is revealed that, both the two kinds of duplex stainless steels have a constitution of three phases in as-cast state, i.e. γ phase, σ phase and M23C6. During the solidification process of the steel with 0.46%Si, δ ferrite dendrite forms at the beginning, followed by eutectic (δ+M23C6), peritectic γ, and finally eutectic (γ+M23C6), in which the δ phase transformed into eutectoid (γ2+σ) in the subsequent cooling process. For the duplex stainless steel with 1.36%Si, the increase of δ ferrite amount is observed leading to obviously increased content of σ phase, the morphology of peritectic γ becomes intermittent and irregular shape, and no eutectic (γ+M23C6) forms. After solution treatment, the two kinds of steels are composed of ferrite, austenite and M23C6 type carbide. Note that, the volume fraction and continuity of α ferrite are promoted obviously by increasing Si content from 0.46% to 1.36%. The Si addition slightly improves the hardness, tensile strength and fracture toughness of the duplex stainless steel, while has little effect on the corrosion resistance and slightly reduces the wear resistance.
Table 1 Chemical compositions of two kinds of ultra-high chromium and high carbon steels (mass fraction / %)
Fig.1 XRD spectra of casting samples of ultra-high chromium and high carbon steels
Fig.2 As-cast metallographs of ultra-high chromium and high carbon steel with 0.46%Si etched by KMnO4 solution (a) and V2A-Beize solution (b, c)
Fig.3 As-cast metallographs of ultra-high chromium and high carbon steel with 1.36%Si etched by KMnO4 solution (a) and V2A-Beize solution (b)
Fig.4 Backscattered electron images of as-cast ultra-high chromium and high carbon steel(a) full view of 0.46%Si (b) eutectoid structure in the core of area A of 0.46%Si(c) full view of 1.36%Si (d) eutectoid structure in the core of area A of 1.36%Si
Steel
Point
Fe
Cr
C
Ni
Mo
Mn
Cu
Si
N
0.46%Si
1#
59.186
26.884
-
9.410
1.166
0.681
1.677
0.461
0.535
2#
59.951
26.314
0.054
9.269
1.256
0.670
1.494
0.488
0.504
3#
20.769
69.377
4.389
1.649
3.182
0.501
0.132
0.001
-
4#
54.602
36.311
-
4.272
2.900
0.583
0.528
0.692
0.113
5#
62.014
26.973
-
7.160
1.372
0.647
1.364
0.420
0.049
1.36%Si
1*
56.497
28.070
-
10.067
1.375
0.597
1.755
1.263
0.376
2*
19.832
69.498
4.465
1.642
3.711
0.528
0.151
0.022
0.151
3*
50.661
37.148
-
5.579
3.831
0.561
0.534
2.032
-
4*
63.970
22.784
-
8.510
0.309
0.588
2.483
0.980
0.376
Table 2 Microzonal composition analyses in Fig.4 of as-cast ultra-high chromium and high carbon steel (mass fraction / %)
Fig.5 SEM image of as-cast ultra high chromium and high carbon steel(a) full view of 0.46%Si (b) details of σ in the area A for 0.46%Si(c) full view of 1.36%Si (d) details of σ in the area A for 1.36%Si
Steel
Point
Fe
Cr
Mo
Ni
Si
0.46%Si
1#
50.75
40.70
3.54
4.17
0.84
2#
19.92
72.93
5.99
1.16
-
1.36%Si
1*
48.48
37.56
4.55
6.55
2.86
2*
19.67
71.28
6.76
2.21
0.08
Table 3 EDS analyses of as-cast ultra high chromium and high carbon steel in Fig.5 (mass fraction / %)
Fig.6 XRD spectra of ultra-high chromium and high carbon steel after solution treatment
Fig.7 Metallographs of ultra-high chromium and high carbon steels with 0.46%Si (a~c) and 1.36%Si (d~f) after solution treatment etched by KMnO4 (a, d), V2A-Beize (b, e) and KOH (c, f), respectively
Fig.8 Backscattered electron images of ultra-high chromium and high carbon steel with 0.46%Si (a) and 1.36%Si (b) after solution treatment
Steel
Point
Fe
Cr
C
Ni
Mo
Mn
Cu
Si
N
0.46%Si
1#
58.856
26.540
0.158
10.632
1.266
0.748
1.264
0.279
0.258
2#
58.858
33.639
-
5.498
1.887
0.581
1.063
0.620
0.045
3#
18.500
69.414
5.447
1.965
4.030
0.485
0.082
0.001
0.076
1.36%Si
1*
56.544
28.002
-
9.449
1.362
0.517
1.601
1.355
0.126
2*
55.649
32.219
-
6.877
2.047
0.321
1.102
1.786
0.000
3*
18.540
69.713
5.582
1.707
4.089
0.148
0.197
0.024
0.000
Table 4 Microzonal composition analyses of ultra-high chromium and high carbon steel after solution treatment in Fig.8 (mass fraction / %)
Steel
As-cast
Solution treatment
HRC
HRC
σb / MPa
KIC / (MPa·m1/2)
0.46%Si
52.2
41.5
793
28.321
1.36%Si
56.5
42.1
835
32.678
Table 5 Mechanical properties of ultra-high chromium and high carbon steel
Fig.9 Engineering stress-strain curves of ultra-high chromium and high carbon steel after solution treatment
Fig.10 Tensile fracture morphologies of ultra-high chromium and high carbon steel with 0.46%Si (a) and 1.36%Si (b) after solution treatment
Fig.11 Friction coefficient curves of ultra-high chromium and high carbon steel after solution treatment
Fig.12 Surface morphologies of worn surfaces of ultra-high chromium and high carbon steels with 0.46%Si (a) and 1.36%Si (b) after solution treatment
Fig.13 Anodic polarization curves of ultra-high chromium and high carbon steel in boric acid solution after solution treatment
Fig.14 Mott-Schottky plots of the passive films formed on ultra-high chromium and high carbon steel in boric acid solution after solution treatment (C—capacitance of electronic double-layer)
Steel
ND
NA
Efb (vs SCE)
1020 cm-3
1020 cm-3
mV
0.46%Si
3.3
6.6
-439
1.36%Si
3.4
6.9
-430
Table 6 Carrier density and flat band potentials (Efb) of the passive films formed on ultra high chromium and high carbon steel in boric acid solution after solution treatment
Fig.15 Schematic of liquidus projection diagram of Fe-Cr-C alloy (w—mass fraction of element)
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