DEVELOPMENT OF STAINLESS STEEL AND HIGH ALLOY POWDERS Christopher T. Schade Hoeganaes Corporation, Cinnaminson, NJ John Schaberl ANCOR Specialties, Ridgway, PA ABSTRACT 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. High Performance Atomizing High Performance Annealing AOD High Performance Atomizing High Performance Annealing AOD Figure 1. High Performance Stainless Processing Route
Transcript
DEVELOPMENT OF STAINLESS STEEL AND HIGH ALLOY POWDERS
Christopher T. Schade Hoeganaes Corporation, Cinnaminson, NJ
John Schaberl ANCOR Specialties, Ridgway, PA
ABSTRACT
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 paper1, 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.
High Performance Atomizing
High Performance Annealing
AOD High Performance Atomizing
High Performance Annealing
AOD
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 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.
Table I. Effects of pre-alloyed boron on the mechanical properties of 316L.
316L with B 50 690 6.54 3203 22.1 7.79 -5.90 183 1262 72.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC) in 100% Hydrogen.
Pressure Green Strength TRS
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 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 240 168.00 79.00316L with Cu and Sn 240 216.00 12.00
316Lwith Boron 240 NA1 0.20Salt Spray Testing per ASTM B117-02. Five TRS bars per alloy.1 At 240 hours, one bar had less than 1% red rust and 4 bars had no red rust
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.
P/M 316L with Boron 50 690 71 490 32 220 281 ASTM A479 (Specification for Annealed Bar)
UTS 0.20% OFFSETPressure
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 25 oF (14 oC), while at .25% boron the window is extended to 50 oF (28 oC). 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:
Km = 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 Km is now possible. By adjusting elements in real time in the AOD, a consistent value of Km 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 2300 oF [1260 oC] 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. Table IV. Chemistry of high strength-stainless grades.
Type C S O N P Si Cr Ni Cu Mn Mo Cb(%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%)
17-4PH 0.018 0.010 0.300 0.020 0.025 0.85 17.00 4.00 3.55 0.15 0.03 0.25 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.
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC) in 100% Hydrogen.Note: 17-4PH was heat treated at 900 oF (482 oC) for 30 minutes.
Pressure TRS UTS 0.20% OFFSET
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 Km 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.
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC) in 100% Hydrogen. Surface nitrided material was treated in 90% Nitrogen and 10% Hydrogen after sintering.
TRS UTS 0.20% OFFSETPressure
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
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 2205 0.009 0.004 0.23 0.076 0.028 1.00 22.20 5.50 0.04 0.16 3.43 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,
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.
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. 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.
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC).
Pressure TRS UTS 0.20% OFFSET
WEAR RESİSTANT ALLOYS One of the key benefits in the EAF-AOD processing is the ability to produce alloys with low inclusion contents. Both the EAF and AOD utilize a slag layer on top of the melt, that absorbs non-metallic inclusions and prevents re-oxidation of the metal. This slag layer is also used to remove certain harmful elements such as sulfur and phosphorus. The AOD includes a refining step where inert argon is bubbled through the melt. Inclusions attach to the bubbles and are carried to and captured by the slag. In the atomizing process physical shrouding and gas shrouding are used to prevent the metal from being exposed to air thus preventing re-oxidation. This leads to the ability to produce cleaner steels then currently produced by the induction furnace. One of the key groups of materials produced by this route in the wrought industry is wear resistant alloys. Wear resistant alloys such as tool steels and carbide containing stainless steels must have low levels of non-metallic inclusions in order to optimize performance. Inclusions reduce not only the wear resistance of the alloy but also reduce important mechanical properties such as impact toughness, fatigue and tensile strength. There currently is a renewed interest in wear resistant alloys for applications such as valve inserts, cam rings, and machining tools. The new high performance processing route allows for the economical production of such high quality steels. In order to cover the range of materials available experimental heats of an M2 tool steel and a less expensive carbide containing stainless steel (P/M 440C) were produced. The chemical compositions of each material is shown in Table IX. The powder was atomized using the high performance atomizing process and then screened to minus 100 mesh. Graphite was added to acheive the desired carbon levels and the materiasl were compacted and sintered at 2300 oF (1260 oC) in a hydrogen atmosphere. Table IX. Chemistry of Wear Resistant Alloys.
Type C S O N P Si Cr Ni Cu Mn Mo W V(%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%)
The physical properties of the two grades are shown in Table X. Most tool steel powders are gas atomized and hot isostatically pressed to form the final shape. This is done because the compressibility and oxygen content of water atomized powders has been relatively poor. The current high performance processing route can produce an M2 tool steel with green densities similar to that of stainless steels (Table X). The use of high temperature sintering in a hydrogen atmosphere can produce densities that are equivalent to those produced by vaccum sintering. High temperature sintering in P/M has become routine in the production of stainless parts. Like vacuum sintering, the sintering of tool steels requires precise control of temperature to produce the optimum distribution and size of carbides. Optimum sintering in a conventional high temperature
sintering furnace can lead to hardness in excess of 60 HRC. Due to the AOD refining process the relatively low initial oxygen contents lead to low sintered oxygen levels that are below 1000 ppm. Table X. Physical Properties of Wear Resistant alloys.
PM-440C 50 690 6.54 2254 15.5 7.15 -2.98 140 965 20M2 sintered at 2275oF (1246oC) and PM-440C sintered at 2300oF (1260oC). Both in 100% hydrogen.
Green StrengthPressure TRS
P/M wear/corrosion resistant materials can be produced via the same route. The microstructure of P/M 440C stainless steel contains chromium carbides that promote higher hardness (HRC of 20) but provide relatively good corrosion resistance due to its high chromium level. The wear resistance and corrosion resistance of grades can be altered by changing the levels of chromium and carbide formers. A good example of this balancing between wear and corrosion is found in D2 which has an 11.5% chromium level and .8% vanadium.
(a) (b) Figure 5. Distribution of carbides in (a) M2 tool steel and (b) P/M 440C stainless. OXİDATİON RESİSTANT ALLOYS High temperature alloys which provide strength and oxidation resistance at elevated temperatures (300 oC to1200 oC) 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 6 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.10
2020 Powerplant Mix
Diesel15%
Hybrid/ Electric/ Fuel Cell
31%Alcohol/ Nat.Gas/ Propane
5%
Gasoline49%
Gasoline98%
2000 Powerplant Mix
Gasoline98%
Diesel2%
2020 Powerplant Mix
Diesel15%
Hybrid/ Electric/ Fuel Cell
31%Alcohol/ Nat.Gas/ Propane
5%
Gasoline49%
Gasoline98%
2000 Powerplant Mix
Gasoline98%
Diesel2%
Figure 6. Projected growth of diesel and fuel cell components. 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 XI. Chemistry of oxidation resistant alloys.
T ype S i C r N i M o C b W C o(% ) (% ) (% ) (% ) (% ) (% ) (% )
The alloy selection must be based on both cost and performance. Certainly high levels of nickel and chromium will aid in performace (Table XI) but the cost must be justfied. Figure 7 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 oC. 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.
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800 850 900 950 1000 1050 1100 1150 1200
Temperature (°C)
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Hastelloy X Super Ferritic 310L 18%Cr- 1%Cb 409Cb
Figure 7. Oxidation rate of alloys measured in air. MAGNETİC AND ELECTRİCAL ALLOYS Processing of high nickel alloys used in magnetic and electrical applications can benefit from the processing cycle in the AOD. The oxidation step in the AOD reduces the levels of elements such as manganese, chromium and carbon. Manganese and chromium are converted to oxides which are trapped in the refining slag. This slag is removed prior to the final refining step. Carbon is oxidized to carbon monoxide and escapes from the melt through the offgas system. Control of these elements is important in such alloys as 36 Ni / 64 Fe and 50 Ni / 50 Fe. 36 Ni / 64 Fe, known as Invar, has a thermal expansion coefficient that is nearly zero. Invar finds uses in applications where precision components are needed such as mechanical clocks, scientific instruments,microwave components and mechanical thermostats. The addition of other elements to Invar significantly changes the coefficient of thermal expansion (CTE) (Figure 8).11 Lower carbon levels produced in AOD refining are beneficial for Invar, since the solubility of carbon in the austentic matrix changes with temperature and lead to instability in the CTE. Typical mechanical properties of Invar are shown in Table XII.
Figure 8. Effect of residuals on the coefficient of thermal expansion of Invar.11
The preceding discussion on controlling tramp elements also pertains to magnetic alloys such as 50 Ni / 50 Fe. Elements such as carbon, sulfur, and nitrogen, all of which are reduced with AOD processing, tend to be located in the interstitial sites and can interfere with the movement of magnetic domains. Prealloyed 50 Ni / 50 Fe is selected for a magnetic core material when permeability needs to be maximized or coercive force needs to be minimized. Values of permeability routinely exceed 10,000 with coercive force less than 0.5 Oersteds. These values represent a material that has extremely good magnetic response. Specifically, small current inputs will create a relative large change in magnetic response. Thus applications that need to detect minor changes in current are satisfied with this material. A typical example of a P/M application are flux return paths for dot-matrix printers. In this application, low current is desirable to minimize heating of the circuit. Inductors sensing small changes in current that need to react rapidly are ideal candidates. Other potential applications are used for ground fault circuits to detect shorts. The other attractive aspect of 50 Ni / 50 Fe is the low coercive force. This implies that once magnetized, the amount of current to demagnetize is low. Applications that need to return to zero magnetism with low input current are well suited for this material. Two relatively minor disadvantages of the 50 Ni / 50 Fe system are: a low saturation induction level and susceptibility to a reduction in magnetic properties resulting from cold working. This alloy in the wrought condition has a maximum saturation induction of ~14 kGauss; whereas, iron is on the order of ~22 kGauss. This implies more material is needed for the 50 Ni / 50 Fe alloy to act as a flux return path. In regard to cold working, this material shows rapid loss of magnetic properties with coining, machining, etc. If sizing of the part is necessary then an anneal cycle will be required. Physical properties of this P/M alloy are shown in Table XII. Table XII. Mechanical properties of 36 Ni / 64 Fe and 50 Ni / 50 Fe.
.75% Acrawax C used as lubricant and sintered at 2300oF (1260oC) in 100% Hydrogen.
Pressure Green Strength TRS UTS 0.20% OFFSET
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.
• 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.
• Wear resistant alloys can be manufactured which can be processed by conventional
stainless steel P/M routes such as conventional compaction and high temperature sintering. Different combinations of wear resistance and corrosion resistance can be achieved by adjusting the chromium and carbide forming elements.
• 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.
• Magnetic and electrical alloys with low amounts of residual elements can be
produced. This reduction in residual elements improves the performance of the materials.
• 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.
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