PRODUCTION OF STAINLESS STEEL POWDERS BY ADVANCED STEELMAKING TECHNOLOGY. Christopher T. Schade and John Schaberl

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1 PRODUCTION OF STAINLESS STEEL POWDERS BY ADVANCED STEELMAKING TECHNOLOGY ABSTRACT Christopher T. Schade and John Schaberl Hoeganaes Corporation 1001 Taylors Lane Cinnaminson, NJ USA Advanced melting technology is now being employed in the manufacture of stainless steel powders. The new process currently includes electric arc furnace (EAF) technology in concert with Argon Oxygen Decarburization (AOD), High Performance Atomizing (HPA) and hydrogen annealing. The new high performance processing route has allowed Hoeganaes Corporation to provide not only a more consistent product, but has allowed enhanced properties, such as improved green strength and green density. This paper will review the potential to use this processing route to provide products with improved properties and performance. INTRODUCTION The recent growth in stainless powder metallurgy has led Hoeganaes Corporation to develop a high volume, high performance, processing route. The new processing route is shown in Figure 1. A twenty-five ton electric arc furnace is used in conjunction with an argon-oxygen decarburization unit to melt and refine stainless steel to the required composition and temperature. The steel is then transferred by a bottom pour ladle to the High-Performance Atomizer were it is converted into powder. Ferritic grades are then annealed in the hydrogen atmosphere, annealing furnace. Powders are then screened and premixed according to customer specifications. As reviewed in a previous paper [1], this new processing route has led to the production of powders with improved chemistry control and enhanced mechanical properties. This paper explores new grades of stainless steel and high alloy powders that can be made economicaly through the new process. AOD High Performance Atomizing High Performance Annealing Figure 1. High Performance Stainless Processing Route STAINLESS STEEL POWDERS CONTAINING BORON The refining process in the AOD lends itself to the development of new grades. In wrought stainless production the AOD is used in producing stainless alloys with low levels of elements such as titanium, vanadium, tantalum and boron, many times consistently holding levels of

2 0.01%. to 0.10%. The use of argon stirring and real-time chemistry control give the AOD advantages over induction melting. Boron has long been of interest to producers of stainless P/M because of its ability to provide high densities through liquid phase sintering [2-3]. The need to control boron levels accurately is important because excessive boron leads to precipitates or excessive eutectic phase at the grain boundary, both of which embrittle and deteriorate performance. Previous work has suggested that boron levels of 0.25 weight percent can be used favorably in 316L to enhance mechanical properties [4] An experimental heat of 316L was made (Table I) to examine the effects of such an addition on physical properties. The microstructures show a nearly 100% dense structure (Figure 2). The use of the AOD in achieving tight boron levels is certainly advantageous for this grade. The proper level of Table I. Effects of pre-alloyed boron on the mechanical properties of 316L. Pressure Green Sintered Green Strength D.C. TRS Hardness Material (tsi) (MPa) (g/cm 3 ) (psi) (MPa) (g/cm 3 ) (%) (ksi) (MPa) (HRB) 316L L with B % Acrawax C used as lubricant and sintered at 2300 o F (1260 o C) in 100% Hydrogen. boron is needed. Insuffcient boron and sintering is not activated, whereas to high a level of boron reults in excessive precipitates which decrease ductility and corrosion resistance. Metallographic examination of the specimens showed no eutectic liquid around the pores and very few precipitates. (a) (b) Figure 2. Porosity distribution in (a) 316L with boron and (b) conventional 316L. The corrosion behavior of austenitic stainless steels with boron is dependent on the amount of the eutectic phase present and the final sintered density. If too much eutectic phase is present, it depletes the surrounding matrix of chromium and leads to sensitization. The lower the density of the material the more porosity exists and the corrosion resistance (especially pitting) of the material is reduced. Corrosion testing via salt spray (ASTM B117-02) is shown in Table II. The 316L with a boron addition is compared to conventional 316L and 316L with copper and tin (a grade developed for enhanced corrosion resistance). The 316L with the boron addition showed superior corrosion resistance. Table II. Salt Spray testing of 316L P/M materials. Material Test Duration Time for 1% Red Rust Rusted Area at Test Completion (Hours) (Hours) (%) Conventional 316L L with Cu and Sn Lwith Boron 240 NA Salt Spray Testing per ASTM B Five TRS bars per alloy. 1 At 240 hours, one bar had less than 1% red rust and 4 bars had no red rust

3 Since the density of the 316L with boron is 98% of theoretical, the mechanical properties of this grade can approach that of wrought materials. Table III shows tensile results compared to the ASTM standard for annealed bars. Table III. Tensile properties of P/M 316L with boron and wrought 316L. Pressure UTS 0.20% OFFSET Elongation Material (tsi) (MPa) (ksi) (MPa) (ksi) (MPa) (%) Wrought 316L P/M 316L with Boron ASTM A479 (Specification for Annealed Bar) Preliminary experiments with ferritic grades (409Cb and 410L) have shown similar response and densities in excess of 98% of the theoretical have been obtained. Work has shown that as the boron level is increased from.05% to.25% the sintering temperature over which densification occurs is widened. For example, at.15% boron addition the window for densification is 14 o C (25 o F), while at.25% boron the window is extended to 28 o C (50 o F). However, excessive amounts of boron lead to the rich eutectic phase, which degrades mechanical properties and corrosion resistance. DUAL PHASE STAINLESS STEELS Traditionally, when stainless parts producers have needed high strength and hardness, graphite has been added to a ferritic grade to promote martensite, or a more highly alloyed material, such as 17-4PH has been used. In applications that require abrasion resistance, hardness, ductility and high strength, wrought producers of stainless steel have developed a dual phase stainless steel consisting of a microstructure of martensite and ferrite. The level of martensite is controlled according to a chemical balance: K m = Cr + 6 x (Si) + 8 x (Ti) + 4 x (Mo) + 2 x (Al) 2 x (Mn) 4 x (Ni) 40 (C + N) 20 x (P) 5 x (Cu) In this equation, chromium, silicon, titanium, molybdenum, and aluminum are used to stabilize the ferrite. Manganese, nickel, carbon, nitrogen, phosphorus and copper promote formation of high temperature austenite, which transforms to martensite during cooling. Using AOD technology, the chemistry control to obtain an optimum K m is now possible. By adjusting elements in real time in the AOD, a consistent value of K m can be maintained. Also, with the use of hydrogen annealing, it is possible to increase the alloy content of certain grades without severely reducing the green density. Table IV shows the chemical composition of a newly developed dual phase stainless steel along with the two high strength options discussed earlier. The chemistry of the dual phase stainless steel has been optimized leading to a microstructure consisting of ferrite and martensite (Figure 3a). Table V shows the physical properties of the aforementioned grades. Using conventional sintering (typically 1260 o C [2300 o F] in hydrogen) 17-4PH is a poor option. The high alloy content is not only costly but leads to poor both green and sintered density. The lower sintered density leads to mechanical properties that are lower than expected for the level of alloy content. Vacuum sintering with long times at temperature may improve the properties of this alloy, but this is a costly option. The precipitation-hardening grade also requires secondary heat treatment to achieve its high strength.

4 Table IV. Chemistry of high strength-stainless grades. Type C S O N P Si Cr Ni Cu Mn Mo Cb (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) 410L + C Dual Phase PH The graphite addition to the 410L increases the formation of chromium carbides and leads to sensitization. The net effect is the depletion of chromium in solution and the lowering of corrosion resistance. Graphite also reduces the compressibility of the powder and requires strict atmosphere control in the sintering furnace for optimum carbon control. Even with the higher carbon levels, hardness and tensile properties of the 410L are inferior to the dual phase stainless steel. Table V. Mechanical propety data for high strength stainless steels. Pressure Sintered D.C. TRS Hardness UTS 0.20% OFFSET Elongation Material (tsi) (MPa) (g/cm 3 ) (%) (ksi) (MPa) (HRA) (ksi) (MPa) (ksi) (MPa) (%) 410L + C Dual Phase PH % Acrawax C used as lubricant and sintered at 2300 o F (1260 o C) in 100% Hydrogen. Note: 17-4PH was heat treated at 900 o F (482 o C) for 30 minutes. Another advantage of the Dual Phase stainless steel is that the amount of martensite, and hence hardness, can now be controlled by the elements in solution. Incorporating various sintering atmospheres the amount of nitrogen in the alloy can be used to alter the K m factor and change the amount of martensite present in the microstructure. Nitrogen can be used to increase strength, and since it is added during sintering, it does not impact the green density of the material. When this alloy is sintered in 90% hydrogen and 10% nitrogen, the level of marteniste in the microstructure is increased and the properties respond accordingly (Table VI). The use of nitrogen as an alloying element during sintering can be anticipated, and the overall alloy content in the base material can be reduced. Table VI. Effect of sintering atmosphere on the mechanical properties of dual phase stainless. Sintered Pressure D.C. TRS Hardness UTS 0.20% OFFSET Elongation Material (tsi) (MPa) (g/cm 3 ) (%) (ksi) (MPa) (HRA) (ksi) (MPa) (ksi) (MPa) (%) 100% Hydrogen % Hydrogen/ 10% Nitrogen Surface Nitrided % Acrawax C used as lubricant and sintered at 2300 o F (1260 o C) in 100% Hydrogen. Surface nitrided material was treated in 90% Nitrogen and 10% Hydrogen after sintering. Another advantage of the dual phase stainless steel is the ability to case harden with nitrogen. Similar to carburizing, a sintered P/M component can be treated in a nitrogen atmosphere to form a hard martensitic case (Figure 3c). Heating the material in the austenite/ferrite region allows nitrogen to diffuse into the austenite, and upon quenching, forms martensite. The amount of nitrogen diffusion controls the depth of the martensite formation. The solubility of nitrogen in austenite is greater than carbon and therefore the formation of chromium nitrides, which subsequently depletes the matrix of chromium and reduces the corrosion resistance, does not occur. This technique provides a material with high surface hardness and high matrix strength with reasonable ductility [5]

5 a.) b.) c.) Figure 3. Etched optical photomicrographs of dual phase stainless: a.) Sintered in 100% hydrogen, b.) Sintered in 90% hydrogen and 10% nitrogen, and c.) surface-nitrided using a second treatment in 90% nitrogen and 10% hydrogen (note: nitrided surface is at the bottom of the photomicrograph). DUPLEX STAINLESS STEELS While dual phase stainless steels provide high strength and corrosion resistance slightly better than typical ferritic stainless steels, its use in highly corrosive environments is somewhat limited. Duplex stainless steels display a corrosion resistance equal to or better than that of the austenitic grades while their mechanical properties range between ferritic and austenitic. A typical chemistry for 2205 (duplex stainless steel) is shown in Table VII. Table VII. Chemistry of Duplex Stainless Steel Type C S O N P Si Cr Ni Cu Mn Mo (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Duplex The microstructure of a duplex stainless steel is a mixture of austenite and ferrite with optimum properties normally achieved within a range of 30% to 70% ferrite. Wrought producers of stainless steel have found that the use of the AOD has been important in limiting the amount of inter-metallic phases [6]. It has been shown that for optimum corrosion resistance and to avoid sigma and chi phases the chemistry has to be controlled precisely, with the chromium and molybdenum kept in the high end of their ranges. An advantage of P/M duplex stainless steels is the availability of nitrogen during sintering as an alloy element. Nitrogen is a strong austenite former and can replace nickel, thus lowering the cost. Figure 4 shows the microstructure of the experimental heat sintered in atmospheres ranging from pure hydrogen to nearly pure nitrogen. The nitrogen content varied from.076% to 1.2%. Table VIII shows the corresponding mechanical properties, Generally when nitrogen is introduced into stainless steels there is a concern for the formation of chromium nitrides, which may adversely affect the corrosion resistance of the alloy. However, in duplex stainless steels, the high nitrogen content promotes austenite which has a high solubility for nitrogen and significantly reduces the formation of chromium nitrides. In this sense, the use of nitrogen in duplex stainless steels increases pitting and crevice corrosion resistance.

6 a.) b.) c.) Figure 4. Etched optical photomicrographs of duplex stainless steel: a.) Sintered in 100% hydrogen, b.) Sintered in 50% hydrogen and 50% nitrogen, and c.) Sintered in 90% nitrogen and 10% hydrogen. Since nitrogen is a very effective solid-solution strengthening element, duplex stainless steels have excellent strength. However, the presense of austentite in the microstructure allows the alloy to have good toughness and ductility. Table VIII. Mechanical property data for varoius sintering atmospheres of dulpex stainless. Sintered Pressure D.C. TRS Hardness UTS 0.20% OFFSET Elongation Material (tsi) (MPa) (g/cm 3 ) (%) (ksi) (MPa) (HRA) (ksi) (MPa) (ksi) (MPa) (%) 100% Hydrogen % Hydrogen/ 50% Nitrogen % Hydrogen/ 90% Nitrogen % Acrawax C used as lubricant and sintered at 2300 o F (1260 o C). OXİDATİON RESİSTANT ALLOYS High temperature alloys which provide strength and oxidation resistance at elevated temperatures (300 o C to1200 o C) will be critical in new applications such as fuel cell technology and diesel engines. These materials are generally used in the presence of combustion gases that may be generated from exhaust and pollution control equipment. The opportunities for these materials are on the rise. As Figure 5 shows the gasoline engine will decline in deference to the use of new technology such as hybrids and diesel engines. The fuel cells used in the hybrid vehicles offer new oportunities for P/M materials [7] Powerplant Mix 2020 Powerplant Mix Gasoline Gasoline 98% 98% Diesel 15% Gasoline 49% Diesel 2% Hybrid/ Electric/ Fuel Cell 31% Alcohol/ Nat.Gas/ Propane 5% Figure 5. Projected growth of diesel and fuel cell components.

7 These components will be exposed to high tempertures and various service atmospheres such as superheated steam, hydrogen, and air. Particulate filters for capturing soot and ash from the exhaust of diesel engines are being used to meet the Eurpoean environmental regulations. The ease of which shapes can be made from P/M products make them extremely attractive for this application. Table IX. Chemistry of oxidation resistant alloys. Type Si Cr Ni Mo Cb W Co (%) (%) (%) (%) (%) (%) (%) 409Cb CB L Super Ferritic Hastelloy X The alloy selection must be based on both cost and performance. Certainly high levels of nickel and chromium will aid in performace (Table IX) but the cost must be justfied. Figure 6 shows the TGA results of several stainless compositions. The weight gain of various powder compositions was measured in air while increasing the temperature from ambient to 1200 o C. The alloys tested ranged from a 409Cb which is commonly used for exhaust flanges to Hastelloy X, which has been used for industrial applications (furnaces) because it has unusual resistance to oxidizing, reducing and neutral atmospheres. In general, as the alloy content increased (in particular nickel), the oxidation resistance increased. The super ferritic alloy, which contained 24% chromium, is especially resistant to oxidation. In the wrought industry, this grade is used in the manufacture of parts which must resist scaling at high temperatures. This AOD processing of this alloy, which produces low carbon and nitrogen levels, improves its relative corrosion resistance and mechanical properties. The high temperature-mechanical properties of this alloy are far less than Hastelloy X % of Initial Weight Temperature ( C) Hastelloy X Super Ferritic 310L 18%Cr- 1%Cb 409Cb Figure 6. Oxidation rate of alloys measured in air. CONCLUSIONS The high-performance processing route for stainless steel, which includes EAF/AOD refining, High Performance Atomizing and Hydrogen Annealing have led Hoeganaes to explore new grades of materials that can be economically manufactured. Advances in chemistry control, improved green strength and green density have allowed for the development of new stainless grades with improved performance.

8 Pre-alloyed boron containing stainless steel can achieve near full density leading to corrosion resistance and mechanical properties that are superior to conventional P/M materials and approach wrought performance. Low cost, dual phase stainless steels can be manufactured that have excellent green and sintered density while having strength that exceeds 17-4PH in the heat-treated condition. The dual phase microstructure also allows for surface nitriding, leading to improved wear resistance and corrosion resistance. Duplex stainless steel can be manufactured by the new processing route, which can be used in high strength applications where corrosion resistance of ferritics and dual phase stainless is not adequate. Super-Ferritic stainless steel shows promise in high temperature-oxidation resistant applications such as fuel cells and diesel exhaust filters. For those grades that need good mechanical properties at high temperatures, Hastelloy X is an excellent candidate. The relationship between processing and properties has allowed for the development of potential new grades for the P/M industry. Properties such as density, tensile strength, hardness, wear resistance, corrosion resistance, magnetic and electrical response can all be tailored for the application. REFERENCES 1. Causton, R.J., T. Cimino-Corey, and C.T. Schade Improved Stainless Steel Process Routes, Advances in Powder Metallurgy and Particulate Materials 2003, MPIF, Princeton, NJ, USA, Reen, O.R., Sintered Liquid Phase Stainless Steel, U.S. Patent No. 3,980, Reen, O.R., Sintered Liquid Phase Stainless Steel, U.S. Patent No. 4,032, Rajiv Tandon, Yixiong Liu and Randall German, High of Ferrous Alloys via Supersolidus Liquid Phase Sintering, Advances in Powder Metallurgy and Particulate Materials, Vol. 2 pp.5-27 to 5-36, 1995, MPIF- Princeton, NJ. 5. Hans Berns, Case Hardening of Stainless Steel Using Nitrogen, Industrial Heating Website, 3/25/ The International Molybdenum Association, Practical Guidelines for the Fabrication of Duplex Stainless Steels. Edited by Technical Marketing Resources, Inc. Pittsburgh, PA, G. Hoogers, Fuel Cell Technology Handbook, SAE International, Warrendale, PA, 2003, pp. 4-1 to 4-27.