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Acta Metall Sin  2018, Vol. 54 Issue (10): 1350-1358    DOI: 10.11900/0412.1961.2017.00558
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Effect of H2O(g) on Decarburization of 55SiCr Spring Steel During the Heating Process
Kai ZHANG1, Yinli CHEN1(), Yanhui SUN1, Zhijun XU2
1 Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing,Beijing 100083, China
2 Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China
Cite this article: 

Kai ZHANG, Yinli CHEN, Yanhui SUN, Zhijun XU. Effect of H2O(g) on Decarburization of 55SiCr Spring Steel During the Heating Process. Acta Metall Sin, 2018, 54(10): 1350-1358.

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Abstract  

Spring steel is an important steel widely used in the national economic construction. Its application environment is quite harsh, so it strongly demands for a high quality of the surface. However, the decarburization behavior on the surface seriously affects the surface quality and reduces the fatigue performance of materials. Decarburization is caused by chemical reaction between oxidizing atmosphere and steel surface. Hydrosphere, as a component of the oxidizing atmosphere, could lead to a significant influence on decarburization process. Our experimental materials, taken from continuous casting billet, were polished carefully by abrasive paper in order to remove stains, original scale and decarburization layer. Components of furnace gas were controlled by mixing units designed independently. Spring steel samples were heated by a tube vacuum furnace in 600~950 ℃. The mixed atmosphere contains (15%~20%)CO2, (2%~4%)O2, hydrosphere of different contents and N2 in balance. With the condition of the mixed atmosphere, influence of hydrosphere on surface decarburization of spring steel 55SiCr was investigated by 3D measuring laser microscope. The results show that, with the influence of the mixed atmosphere, decarburization is able to happen during low temperature interval, 650 ℃~Ac1 (Ac1: starting temperature of austenitization during slow heating). The thickness of total decarburized layer increases correspondingly with the temperature. In the mixed atmosphere, slight decarburization occurs at 650 ℃ and obvious ferrite decarburization layer can be detected within a temperature range of 700~950 ℃, which is more serious than that in the air. The peak temperature of ferrite decarburization is 850 ℃ under both mixed atmosphere and air. The surface decarburization in low temperature region is related to the dissolution of cementite in pearlite lamellae driven by carbon concentration gradient. When the decarburization degree deepens as time goes, partial pearlite colony begins to shrink, much line form cementite dissolves gradually, and the line cementite becomes short and punctate. The grain morphology of ferrite decarburization layer is different from that who generates within α+γ phase region, which is small in size and without a strong orientation. Hydrosphere in mixed atmosphere could increase porosity of oxidation layer and destroy the important protection mechanism of preventing decarburization by compact scale. It is deduced that existing of hydrosphere offers a chance for low temperature decarburization process occurring and hydrosphere plays an important role in deepening the decarburization degree of samples.

Key words:  spring steel      surface decarburization      hydrosphere      heating temperature      oxide scale     
Received:  26 December 2017     
ZTFLH:  TG142.1  

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00558     OR     https://www.ams.org.cn/EN/Y2018/V54/I10/1350

Fig.1  Schematic of equipment on controlling gas component (PLC—programmable logic controller)
No. H2O(g) content Holding time Heating procedure
gm-3 min
1 5.77 30 Vacuum protection heating at 600~950 ℃, heat
preservation in mixed atmosphere, air cooling
2 - 30 Heating and holding at 700~950 ℃,
muffle furnace, air cooling
3 12.09 60 Vacuum protection heating at 650 ℃,
heat preservation in mixed atmosphere, air cooling
4 12.09 60 Vacuum protection heating at 650 ℃, heat preservation
in nitrogen atmosphere, air cooling
5 - 15 Vacuum protection heating at 850 ℃, heat preservation
in nitrogen atmosphere, air cooling
Table 1  Experiment conditions of surface decarburization
Fig.2  Microstructures of surface decarburization on 55SiCr spring steel with 5.77 g/m3 H2O(g) (No.1) at 600 ℃ (a), 650 ℃ (b), 700 ℃ (c), 750 ℃ (d), 800 ℃ (e), 850 ℃ (f), 900 ℃ (g) and 950 ℃ (h) for 30 min
Fig.3  Decarburization layer thicknesses of 55SiCr spring steel varies with temperature when holding 30 min in different atmosphere
Fig.4  Microstructures of surface decarburization on 55SiCr spring steel with 12.09 g/m3 H2O(g) at 650 ℃ for 60 min (No.3 and No.4) in mixed atmosphere (a) and pure nitrogen (b)
Fig.5  Microstructure of surface decarburization on 55SiCr spring steel at 850 ℃ for 15 min with pure nitrogen protection (No.5)
Fig.6  Microstructures of different positions on 55SiCr spring steel decarburized with 5.77 g/m3 H2O(g) at 700 ℃ for 60 min in mixed atmosphere
(a) matrix
(b) light decarburization region (Regions I and II show the pearlite colony shrinking region and short line and punctate cementite region, respectively)
(c) far-gone decarburization region (Regions III and IV show the ferrite decarburization layer and tran-sitional region, respectively. Arrow shows the direction of decreasing C concentration)
Fig.7  Schematics of surface decarburization mechanism in low temperature region
(a) iron-carbon equilibrium diagram for decarburization (Cs—surface equilibrium carbon concentration, Cα/cem—ferrite saturated carbon concentration, C0—initial carbon concentration, T—temperature)
(b) carbon profile evolution corresponding to time (Curve 1 shows preliminary carbon concentration field during decarburization; curve 2 shows later carbon concentration field during decarburization)
Fig.8  Equilibrium C concentration in 0.001 mol mixed atmosphere (a) and distribution of solid phase products in 0.01 mol mixed atmosphere (b)
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