1. Field of the Invention
This invention relates to molding compositions and forming processes for normally rust-prone iron-based metal alloy powders, and articles produced therefrom. Metal alloy systems that can be successfully formed using the processes of the invention, include elemental iron and iron alloys, including low and medium alloy steels, tool steels, and a number of specialty iron-base alloys.
2. Description of the Related Art
A widely used process for forming metal powders into complex three dimensional shapes is Metal Injection Molding (MIM). The steps of fabrication of metal or ceramic-metallic (CERMET) parts are the following:                i. Metal and/or ceramic powders are blended with a thermoplastic binder material to create an injection molding feedstock with thermoplastic properties.        ii. The thermoplastic feedstock is injection molded in a fluid state using methods and tools typical of conventional plastic injection molding, and removed from the mold in a solid state.        iii. The “green” state as-molded parts are subjected to thermal and/or chemical processes to remove the binder phase.        iv. The resulting “brown” state metal or CERMET parts are sintered at higher temperatures to effect consolidation and densification of the molded object.        
Several methods, processes, and binder systems have previously been described for fabrication of rust prone iron-based metal alloys and CERMET materials containing them. Each of these processes has one or more disadvantages that prevent important applications.
For example, commonly utilized polymer or wax binder MIM processes, such as the methods described by Achikita et al. in U.S. Pat. No. 5,250,254, while they work well with rusting iron alloys, are limited to small parts, weighing no more than a few hundred grams, and with maximum section thickness of less than 10 millimeters. These limitations are imposed by the difficulties associated with binder removal prior to sintering. The manufacture of larger parts is prevented or rendered uneconomical by dimensional instability, cracking, or simply the long times needed for binder removal from larger and thicker sections. In addition, great care must by taken when using wax or resin binders to avoid an undesirable out-of-specification increase in the carbon content of the alloy as a result of incomplete removal of the hydrocarbon binder phase.
Fanelli et al., in U.S. Pat. No. 4,734,237, disclose agaroid-based aqueous binders for molding of metal and ceramic powders. The development of aqueous-binder molding compositions, including those disclosed by Fanelli et al., has largely removed the part size restrictions imposed by wax and polymer binders, since the binder phase in these largely consists of water which is easily removed by evaporation under ambient conditions. In the special case of agar-based binders, the carbon content problem associated with wax and polymer binders is also reduced since the agar component of the binder is largely gasified at relatively low temperatures during the early stages of the sintering cycle. Further reduction in carbon content is easily achieved by employing an oxidizing atmosphere in the early stages of the sintering heat treatment as taught by Zedalis in U.S. Pat. No. 5,985,208. Carbon content can also be reduced by heat treatment in hydrogen as taught by Wu et al., “Effects of residual carbon content on sintering shrinkage, microstructure and mechanical properties of injection molded 17-4 PH stainless steel,: Journal of Materials Science, 37 (2002) pp. 3573–3583.
Zedalis et al., in U.S. Pat. No. 6,268,412, incorporated herein by reference to the extent not incompatible herewith, disclose molding compositions and processing steps for injection molding of non-rust-prone stainless steel articles using water-base agaroid binder systems. Stainless steels, a family of iron-based alloys containing between 10.5 and 28 atomic % chromium, are compatible with water-based binder systems, since the high chromium content confers great resistance to oxidation in the presence of water.
When rust-prone iron-base alloy powders are substituted for the stainless steel powders in the process taught by Zedalis, the resulting molding feedstock is chemically unstable and must be molded and dried within hours, or the water will react with the iron-base alloy powder to form rust, thereby substantially altering and degrading the Theological properties, as-molded strength, sintering, and shrinkage behavior of the feedstock.
It is commonly observed that ferrous alloys progressively oxidize or rust in the presence of air and moisture. The essential chemistry of rust formation, as described in The Metals Handbook, Volume 1, 8th Edition, published by the American Society for Metals (1961) p257, follows. In the first step of the reaction, iron reacts with water to form ferrous and hydroxyl ions and hydrogen:Fe+2H2O═Fe+++2OH−+H2  (1)
In a second step, oxygen, if present, reacts with the ferrous ions to produce ferric ions which precipitate out of solution as insoluble ferric hydroxide FeO(OH), otherwise known as rust. Since the rust deposit does not form a protective layer, reaction 1 is free to proceed until the metallic iron is consumed or equilibrium is reached.
The equilibrium constant for reaction 1 is:K=[Fe++][OH−]2PH2  (2)where the square brackets indicate the concentration of the species and PH2 is the partial pressure of hydrogen.
Equation 2 suggests that the equilibrium concentration of Fe++ can be suppressed by increasing the hydroxyl ion concentration, equivalent to increasing the pH, and/or increasing the hydrogen partial pressure.
Rusting can be further inhibited by passivation of the exposed ferrous alloy surface. Typically, passivation involves a thin but impervious layer of iron oxide formed, in-situ, by reaction of the iron with oxidizing ions. Pourbaix, in Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, New York (1966) p. 312 states that passivation of iron is difficult at a pH below 8, relatively easy at a pH above 8 and very easy between pH 10 and 12. Above pH 13, according to Pourbaix, iron will corrode by hyperferrate ion formation. Passivation of ferrous alloy surfaces is typically effected by the addition of oxidizers to aqueous environments. For example, nitrite and nitrate salts have been used in this manner as rust-inhibiting additives in cooling water and other process water applications. pH buffers, salt solutions formed by reaction of weak acids with strong bases, are frequently employed with nitrites and nitrates to maintain pH in the proper range. The Metals Handbook, Vol. 1, 8th Ed., American Society for Metals, P. 279, 1961 states that sodium nitrate-borate combinations have been used to inhibit corrosion in diesel engine cooling systems and in low pressure, hot water recirculating systems. In this case, sodium borate, a salt formed by reaction of the weak acid H3BO3 with the strong base NaOH, supposedly functions as a pH buffer. In a similar fashion, calcium nitrite is frequently added to concrete formulations to inhibit rusting of embedded steel reinforcing bars. In this case, the desired alkaline environment is synergistically provided by the calcium oxide component of the Portland cement concrete.
Interestingly, various metal borate additives to enhance the gel strength and viscosity of polysacharide-based aqueous binders for molding have been disclosed by Sekido et al. in U.S. Pat. No. 5,258,155, and Fanelli et al. in U.S. Pat. No. 5,746,957. Anions of boric acid, acting in concert with metal cations are thought to induce crosslinking of the agar polysaccharide molecules, thereby substantially increasing the viscosity of the agar-water sol and the strength of the gel. Fanelli et al. teaches that calcium borate, magnesium borate, zinc borate, ammonium borate, tetraethyl ammonium borate, tetramethyl ammonium borate, and boric acid are preferred gel strengthening additives for agaroid binder powder injection molding of ceramic and/or metal powders.
Sekido et al., broadly teach the use of sodium borate for gel strengthening but, Sekido does not define sodium borate in useful chemical terms. That is, neither the preferred concentration range nor the preferred stoichiometry range (i.e., the preferred atomic fractions of sodium and boron) are specified.
For the purposes of the present invention, it is important to clearly distinguish between the different borate salts of sodium and potassium. According to the CRC Handbook of Chemistry and Physics 56th edition, CRC Press, Cleveland Ohio (1974), two crystalline sodium borate salts are known, sodium tetraborate (Na2B4O7) and sodium metaborate (NaBO2). Moreover, both of these may occur as anhydrous or hydrated salts. The most familiar sodium borate salt is the mineral borax, or sodium tetraborate decahydrate (Na2B4O7.10H2O). In aqueous sodium borate solutions, of course, one is not limited to these fixed stoichiometry compounds and a continuous range of boron to sodium ratios can be obtained between the endpoints NaOH and H3BO3. Similarly, potassium tetraborate, potassium tetraborate tetrahydrate, potassium metaborate and other fixed stoichiometry crystalline potassium borate compounds are known, but any boron to potassium ratio can be obtained in solution. The various sodium and potassium borate salts are formed by reaction of the weak acid H3BO3 with the strong bases NaOH and KOH. For example:H3BO3+NaOH═NaBO2+2H2O  (3)
For clarity in describing various borate salts herein, the molar ratio of H3BO3 to NaOH will be used to specify the stoichiometry of sodium borate salt solutions, and the molar ratio of H3BO3 to KOH will be used to specify the stoichiometry of potassium borate salt solutions. These ratios are the same as the atomic ratios of boron to sodium and boron to potassium. Thus, Na2B4O7 has a B:Na ratio of 2:1 while NaBO2 has a B:Na ratio of 1:1. We will also at times use the mole fraction of H3BO3 used to make the salt solution, defined as (moles H3BO3)/(moles H3BO3+moles (Na,K)OH). Thus, a solution of NaBO2 has a mole fraction of H3BO3 of 0.5 or 50%, and a solution of Na2B4O7 has a mole fraction of H3BO3 of 0.66 or 66%. These conventions are convenient for the synthesis of various sodium and potassium borate salt solutions from boric acid (H3BO3), which is available as a crystalline solid, and the respective sodium and potassium hydroxides, which are readily available as solutions of specified molar concentration.
Thus, the concentration and stoichiometry of a solution of any sodium or potassium borate salt can be fully described by specifying the equivalent molar concentrations of H3BO3 and NaOH or KOH in the solution.
For example, Sekido, in his Example 1, used a combination of agar and an aqueous solution of sodium borate as a binder for 316 stainless steel powder. The concentration of sodium borate in the water was about 0.3 wt. %. Presumably the sodium borate used was common borax (sodium tetraborate decahydrate). The molar concentration of Na2B4O7.10H2O was therefore 0.0079 moles/liter, the equivalent molar concentration of H3BO3 was four times this or 0.0316, and the equivalent molar concentration of NaOH was twice that of Na2B4O7.10H2O or 0.0158.
Behi et al. in U.S. Pat. No. 6,261,336, specifically addressed the problem of rust formation in aqueous agar binder injection molding feedstocks containing rust-prone ferrous alloy powders, and taught that these materials can be stabilized against rust formation by the addition of alkaline sodium silicate to the aqueous binder. It was shown by Behi that carbonyl iron powder feedstocks containing appropriate amounts of sodium silicate are somewhat stable against rust formation and attendant hydrogen evolution, and that the stability is further enhanced by the addition of potassium borate. The sodium silicate is thought to function by reacting with the iron surface to form a barrier layer of iron silicate and the potassium borate in this application apparently serves as a pH buffer similar to the use of the sodium borate/nitrite combination discussed above. Behi cites potassium tetraborate and potassium tetraborate tetrahydrate as preferred potassium borate compounds and gives a preferred borate concentration range of from about 0.01 to about 0.2 weight % of the composition (which would correspond to about 0.125–2.5% weight % relative to the aqueous solvent at a typical moisture content of 8 wt. %). While Behi's sodium silicate/potassium borate stabilized feedstocks certainly represent an improvement over unstabilized iron-based aqueous binder feedstocks, experience with the sodium silicate stabilized feedstocks has revealed that the long term chemical stability is marginal, and that the sodium silicate addition renders the feedstock pellets somewhat tacky and difficult to feed through the hopper of an injection molding machine. Moreover, residual SiO2 and/or iron silicate inclusions, resulting from decomposition of higher loadings of the sodium silicate during sintering, may be undesirable for applications requiring maximum ductility and fatigue resistance in the final sintered steel part.
More recently, Morris, in U.S. Pat. No. 6,689,184, has disclosed stabilization of rusting iron aqueous molding feedstocks using a combination of borate and nitrate/nitrite salts. One disadvantage of this system is that the nitrate and nitrite salts serve as nutrients for a range of micro-organisms. Another disadvantage is that the nitrate and nitrite salts may tend to oxidize minor alloy components such as silicon and chromium during the elevated temperature sintering process.
Thus, a need remains for new materials and methods enabling molding of rust prone iron-based alloys that avoid the size limitations of the prior art wax and polymer based binders, and the processing and ductility limitations of sodium silicate and nitrite/nitrate stabilized aqueous binders.