Geicasting of 316L stainless steel

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JournalofUniversityofScienceandTechnologyBeijing Volume14,Number反December2007,Page507 

Geicasting of 316L stainless steel Yah Li,Zhimeng Guo,andJunjie Hao Materials Science and Engineering School,University of Science and Technology Beijing,Beijing 100083,China (Received 2006一t2-t5) 

Materials 

Abstract:A novel near-net process,gelcasting,was successfully used to prepare larger size 3 16L stainless steel parts with complex shape.In this study,the effects of process parameters on the viscosity of me slurry and me dry green strength were investigated.The results show that gas atomization(GA)powder is more suitable for gelcasting compared with water atomization(WA)powder.The maximum solid loading is 55vol%for bal1.milled slurry with GA powders.And the optimum amounts of monomers(acrylamide (AM)+methylenebisacrylamide(MBAM);the mass ratio,30:1)and initiator in the AM system are 1.8%(based on the weight of metal powder)and 0.8%一1.4%(based on the weight of monomers),respectively,at which,the maximum green strength obmined is 33.7 MPa.The mechanical properties of the sintered specimen are as follows:ultimate tensile strength,493 MPa;yield strength,162 MPa;and HRB,72. 

Key words:gelcasting;3 16L stainless steel;complex—shape;slurry;green strength 

1.Introduction Sintered stainless steels with good oxidation resls— tance,corrosion resistance,and high mechanical prop— erty are widely used in mechanical and chemical in— dustries,such as shipbuilding,instrumentation,and motordom[1—2】.However,the present processes used for the production of sintered stainless steel,including traditional powder metallurgy process and metal in— jection molding(MIM),have some disadvantages.For example,the parts with complex shape are difficult to be formed using powder metallurgy process,and it is also difficult for MIM to produce large—size parts with complex shape.Additionally,slip—casting has the ad— vantage of forming complex shape parts,but it re— quires a rather long time for the cast to dry before de— moulding.And generally green bodies produced using slip—casting have low strength[3】. 

During the past decade,gelcasting has been consid— ered as one of the methods for near-net shapes process and still under development[3—4】,which is based on the in situ polymerization of organic monomer binder 

through which a macropolymeric network is formed to hold powder particles together[5—8】.Compared with the slip—casting and the injection molding,gelcasting can be used to fabricate complex shape parts with a higher strength and more homogeneous with only small quantities of organic binder and without the need of expensive moulds or special binder removal step.So,gelcasting is thought to be a better process to produce sintered stainless steel with large size and complex shape.In this study,gelcasting of stainless steel powder in the presence of water-soluble monomer acrylamide was investigated to find opti— mum process parameters. 

2.Experimental procedures 2.1.Raw materials Two kinds of 3 1 6L stainless steel powder commer— cially available,whose characteristics,chemical com— position,and morphologies are listed in Table 1 and Fig.1,respectively,were used in this study to discuss the effect of particle properties on the process. 

Table 1.Characteristics and chemical composition of powders 

The monomer used in this study was acrylamide (AM,C2H3CONH2,assay purity),and methylene— Corresponding author:Yan Li,E-mail:yanli79@sohu.com AIso available online at www.sciencedirecLcom 

维普资讯 http://www.cqvip.com bisacrylamide(MBAM,(C2H3CONH)2CH2,assay pu. rity)was used as the cross-linker.Ammonium persu1. fate((NH4hS2Os,assay purity)and ,|7、,,. tetramethylethylenediamine(TEMED,C6H16N2,assay 

2.2.Procedures 

J・Univ.sd.Techno1.Beijing,Vo1.14,No.6,Dec2007 purity)were used as an initiator and a catalyst,re. 

spectively,for the free-radical polymerization reaction 

of monomers.The dispersant,DP-1,was produced by 

US. 

Fig・1.SEM micrographs of the 316L stainless steel powders. 

The flow chart of stainless steel powder gelcasting process in the AM system is given in Fig.2.Initially. the monomer and the cross-linker at a fixed mass ratio 

of 30:1.which is one of the representative ratios sug. gested in previous work【3,9】,were mixed to make gel former.Then,at room temperature,the gel forliler was dissolved in deionized water to make a premix solution.In addition,an appropriate suspending agent was used in the premix to prevent settling of the large and high—density steel particles.And then stainless steel powder and the defoamer were added into the premix solution.The resulting slurry was bal1.milled for 24 h in N2 atmosphere to obtain homogeneous slurry with a lower viscosity.After degassing in vac- uum,the slurry with some initiator and catalyst were poured into a nonporous mould,and the temperature was held at 60。C for 2 h to ensure that gelatin reaction processed sufficiently and the slurry was solidified into the parts with required shape and size.To avoid inhomogeneous temperature distribution within the body,the mould was previously warmed at 60。C.Af- ter demoulding,the wet green body was dried in vac— uum at room temperature and then sintered under a typical sintering condition for the 3 1 6L stainless steel powder,f.e,1350。C for 2 h in H,atmosphere.