Piercing plug is a key deformation tool during manufacturing the seamless steel tubular product while oxidation is the most economical and practical method for the surface treatment of the piercing plug. The high-temperature oxidation behavior of piercing plug steel was investigated by employing the materials 20Cr2Ni3, 30Cr3NiMo2V and H13 under drop-feeding mixed H2O-C2H5OH atmosphere. A two-stage surface treatment process of first oxidation and then reduction reaction was designed by adjusting the volume ratio of alcohol to water, and thus a two-layer oxide scale structure where the external layer mainly containing FeO was obtained subsequently. Morphology, chemical composition and phase constituents of the oxide scale were studied by using SEM, EDS and XRD, while the microstructure and hardness distribution of decarburization layer were studied by using OM and microhardness tester. The results show that the thickness of external oxide scale decreases with the increase of chromium equivalent, while the thickness of the inner oxide scale keeps basically unchanged. In the process of high temperature oxidation, the vacancy in oxide scale accumulates into micro holes, where the volatile substances and gases were concentrated to elevate the internal pressure high enough that makes the oxide scale "protrude" outwards. The mass transfer in oxidation process varied for different alloy elements. Ni and Mo cannot be oxidized in the specific atmosphere at 950 ℃ according to the oxidation thermodynamics, but exist in elemental form. The oxidation of C determines the microstructure and mechanical properties of the decarburization layer, where the hardness curves of 20Cr2Ni3 and 30Cr3NiMo2V exhibit a characteristic of "double-platform", while the hardness of H13 increases first slowly, then rapidly, and then gradually flattens out. Finally, the material selection for piercing plug steel is suggested from the viewpoint of engineering application.
Table1 The chemical compositions of experimental materials
Fig.1 Decarburization layers of three materials
Fig.2
Fig.3 Cross-sectional oxidation morphologies of three materials (a~c) and magnifications of the interfaces between metal and oxide scale (d~f)(a, d) 20Cr2Ni3 (b, e) 30Cr3NiMo2V (c, f) H13
Fig.4
Fig.5 Line scan spectra of the oxide scale of 20Cr2Ni3 (a), 30Cr3NiMo2V (b) and H13 (c)
Fig.6 Morphology of oxide scale showing transmission "venation" in H13 and its line scan spectra from I to II in the inner oxide scale (a) and bright white substance in the external oxide scale (b)
Fig.7 EDS analyses of oxide scale for point 3 in Fig.6b (a), point 1 in Fig.3d (b) and point 2 in Fig.3e (c)
Fig.8
Fig.9
Reactant
C→CO
Si→SiO2
Mn→MnO
Cr→Cr2O3
Ni→NiO
Mo→MoO3
V→V2O3
Fe→FeO
H2O
-43927
-167544
-111405
-93619
59120
83466
-124066
-15062
FeO
-28865
-152482
-96342
-78556
74183
98529
-109004
-
Table 2 Standard Gibbs free energy () of different element oxides at the temperature of 950 ℃
Fig.10 Mechanism diagram of outer oxide scale gap formation
Fig.11 Magnification image of oxidation scale of H13 oxide scale
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