Technical Paper: PIM International, Vol.5 No.1 March 2011, pages 60-67, 3328 words

Authors: Hidefumi Nakamura[1], Hisataka Toyoshima[1], Hidenori Otsu[1], Akihiko Chiba[2], Koetsu Abe[3]

[1] EPSON ATMIX Co., 4-44 Kaigan, Kawaragi, Hachinohe 039-1161, Japan
[2] Institute for Material Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai  980-8577, Japan
[3] Intelligent Plaza Hachinohe, Inc., 1-4-43 Kita-Inter Kogyodanchi, Hachinohe 039-3345, Japan

  

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Abstract

The main objective of the present work is to evaluate the effects of Zr addition on various properties of SUS316L stainless steel powders produced by water-atomisation of liquid metals. It can be thought that an addition of Zr leads to an improvement in the sintering characteristics and mechanical properties by contributing to the refinement of the powder structure and Si oxide in addition to the inhibition of Si oxide formation on the powder surface.

KEYWORDS
Metal injection moulding, water atomised powder, Zr, Si, sintered density

Introduction

In general, powder can be sintered by either solid-phase sintering or liquid-phase sintering. In solid-phase sintering, surface diffusion acts as the driving force in the initial sintering stages; however, because of the low energy of surface diffusion, it is volume diffusion that promotes densification. The speed of volume diffusion varies depending on crystal structure, inhibiting densification in some types of steel and resulting in a deterioration of mechanical properties. This problem can be prevented by allowing for a second phase of high diffusion velocity [1-2], or generating a liquid phase by the addition of B or the like [3-4] to achieve densification.   

The metal injection moulding (MIM) method is often applied to austenitic stainless steels. Some types of austenitic stainless steels are designed to form a specific alloy composition so that the δ ferrite phase, which has a higher diffusion velocity, precipitates out in order to promote sintering. However, even if the structure is fully austenitic around the sintering temperature, if the rate of cooling is slow, the δ ferrite phase will be precipitated, which may result in a deterioration in corrosion resistance.

Furthermore, for water-atomised powders, oxide film forms on the powder surface, delaying surface diffusion which in turn hampers densification.
Sawai et al. [5] report that when Zr has been added after the deoxidation of molten, low-sulfur steel with Mn-Si, the Mn-Si oxide and Zr-O2 coexist with each other in cases of small amounts of Zr addition. Also, Fujikawa et al. [6] report that when a stabilising element such as Zr has been added to ferritic stainless steel, oxidation resistance at high temperatures is improved. In addition, it has been reported that adding Zr to stainless steel results in a decrease in grain size and an improvement in mechanical properties.........

Further sections of this paper include:

• Experiment
  - Powder fabrication
  - Preparation of sintered compact
  - Evaluation
• Experimental Results and Discussion
  - Powders
  - Sintered compacts
  - Analysis of the sintering process
  - Analyses of powders and sintered compacts
• Conclusions
• References
  

Figures and Tables:

Fig. 1 Specimen shape
Fig. 2 Auger analysis of 316L and 316L-Zr powders
Fig. 3 XPS analysis of 316L-Zr powder
Fig. 4 Optical microstructures of 316L and 316L-Zr powders
Fig. 5 Mapping analysis of 316L and 316L-Zr powders
Fig. 6 XRD patterns of 316L and 316L-Zr powders
Fig. 7 Optical microstructures of sintered compacts
Fig. 8 Specimens after salt sprays
Fig. 9 Relation between sintering temperature and relative density
Fig. 10 Micrographs of sintered compacts by SEM
Fig. 11 Fracture surface of sintered compacts
Fig. 12 Oxide discovered in SPEED-etched 316L-Zr sintered compact
Fig. 13 Surface observation of sintered powders by SEM
Fig. 14 Oxide discovered on the boundaries of a 316L-Zr sintered compact
Fig. 15 Mapping analysis of Si, Zr and O in 316L-Zr powder
Fig. 16 Proposed sintering mechanism for 316L and 316L-Zr powders

Table 1 Chemical compositions and physical properties of powders
Table 2 State analysis result of Zr in powder
Table 3 Mechanical properties of sintered compacts

 

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