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A Research on Selective Carburization Process of Low Carbon Steel

R. D. Han* and K. H. Hu
Research and Develop center of WISCO,
Wuhan 430080, Hubei, P.R. China
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*
E-mail: Rongdong.han@deakin.edu.au

B. Rolfe and E. Pavlina

Institute for Frontier Materials, Deakin University, Waurn Ponds Campus,
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VIC 3216, Australia

E-mail: bernard.rolfe@deakin.edu.au

Selective carburization of steel components is often used to case harden specific areas without
altering the properties. The present work is concerned with Selective carburization treatment on low
carbon steel by fluidized-bed furnaces. Selective carburizing was achieved by the use of a water-
based ceramic stop-off slurry painted on the surface which formed an effective barrier to carbon
diffusion. After carburising the Specimens were quenched. The microstructure of carburization area
is martensite, and the hardness can reach 750~950 HV0.05. The microstructure of non-carburization
area is still ferrite, the hardness under 160 HV0.05 This type of microstructure is potentially used for
the manufacture of tailored properties blanks by hot stamping in the automotive industry.
Keywords: Selective carburization; Fluidized-bed furnace; Low carbon steel.

1. Introduction
Selective carburization is a process used to produce carbon enrichment and subsequent
hardening on quenching, only in certain selected areas of a steel component. This may be
necessary either because some machining is required after the carburizing treatment or
simply because the benefits of case hardening are only needed in a specific portion of the
surface of the component[1,2]. This type of microstructure is potentially used for the
manufacture of tailored properties blanks by hot stamping in the automotive industry[3].
The access of carbon into the steel is restricted by the use of mask, which is a layer of
foreign material[4]. A two-steps carburizing treatment is described in the gas carburizing
article in the Metals Handbook. The diffusion treatment helps develop a relatively flat
carbon distribution near the surface. Although there have been many studies of case
hardening, little has been done to understand the selective carburization process. The
objective of this paper is to report results of research on the metallurgical condition of
steel that has been selectively carburized. The main focus of the research has been the
analysis of the carbon content, microstructure, and hardness of the steel in the vicinity of
the carburizing mask.
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2. Experiment Methods
The chemical composition of the 1mm thickness steel used in this study was 0.054C,
0.005Si, 0.20Mn, 0.017Cr, 0.011Ni, 0.17P and 0.12S (wt.%). Selective carburizing area
was achieved by the use of a water-based ceramic stop-off slurry painted on both sides
which formed an effective barrier to carbon diffusion as shown in Figure 1.
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Fig. 1. The strip painted with water-based ceramic stop-off slurry.

Table 1 outlines the experimental conditions used in this study. Specimens were
heated in a fluid bed furnace to a temperature of 925oC using a bed of aluminum oxide
sand fluidized with high purity nitrogen. Carburising was performed using an atmosphere
of 10 vol.% propane, balance nitrogen. After carburising the strips were quenched by
dropping vertically into a bath of oil at ambient temperature immediately.
Table 1. The carburizing and quenching method.

Carburizing condition
Sample No. Quenching method
10 vol.% propane+ nitrogen purity nitrogen
1 45min 30min oil
2 60min 30min oil
3 60min 50min oil

The total depth of the carburised case was determined by hardness profiles
(FutureTech microVickers, 50 gf, 10 s dwell time). Cross-sectional optical micrographs
were prepared by standard metallographic techniques and then etched with 3% nital
solution. The carbon concentration profile in the total depth of the carburised reign was
determined by Glow Discharge Optical Emission Spectroscopy (GDOES).

3. The Experimental Results

The carbon content distribution with depth from the carburized surface was analyzed by
GDOES, as shown in Figure 2, indicating that the diffusion treatment helps develop a
relatively flat carbon distribution from surface to center.
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Fig. 2. Carbon content distribution with depth from the carburized surface.

Microstructural analysis showed that uniform martensite can be obtained with long
time diffusion (Fig. 3). If the diffusion time is not enough, the surface layer get more
retained austenite and center get more pearlite.

Fig. 3. Optical micrographs of the carburized section (a) No. 1 surface (b) No. 1 center (c) No. 2 surface
(d) No. 2 center (e) No. 3 surface (f) No. 3 center.
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Fig. 3. (Continued)

The hardness curves of the carburized section under different carburizing conditions
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are shown in Fig. 4. The No. 3 sample with long time diffusion get more flat hardness
profile. The higher hardness of the surface layer is attributed to the more carbon content
and less retained austenite[5].

Fig. 4. Hardness curves of the samples with different conditions.

Fig. 5 shows the microstructure near the carburizing. The left part is masked, it’s still
ferrite and a few pearlite with the hardness 160 HV0.05. The right part is unmasked, it’s
microstructure is martensite.

Fig. 5. Optical micrographs in the vicinity of the carburizing mask of No. 3.

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4. Conclusions
Selective carburization process of low carbon steel has been research in this paper.
Martensite can be obtained with carbon content above 0.5% wt by oil quenching. With
long time diffusion, the carbon content curve from surface to center tend to be flat. The
hardness of carburized area can reach to 750~950HV0.05, and the uncarburized area is
still under 160HV0.05.

References
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2. J.I. Goldstein and A.E. Moren, Diffusion modeling of the carburization process.
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