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Powder Metallurgy of Stainless Steel

Isfahan University of Technology

Department of Material Science and Engineering

Dr. Salimi

Masoud Kasrayi

Amirhasan Akbarian

Vahid Badiei

Mahsa Ghodsi

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Index

Introduction to Powder Metallurgy……………………………………………………...3

Introduction to Stainless Steel……………………………………………………………..3

Powder Production by Advanced Steelmaking Technology…………………..4

Stainless Steel – Tungsten Carbide……………………………………………………….4

Effect of Boron……………………………………………………………………………………..6

Precipitation Hardening……………………………………………………………………….7

Dual Phases Stainless Steels…………………………………………………………………7

Duplex stainless steels………………………………………………………………………….9

Effect of Magnetic Permeability………………….……………….………………………9

Effect of Sintering Atmosphere…………………………………………………………..10

Effect of Various Additives……………………………………………………….…………11

Recycling ……………………………………………………………………………………………12

References …………………………………………………………………………………………12

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Introduction to Powder Metallurgy

Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). The powder metallurgy process generally consists of four basic steps: (1) powder manufacture, (2) powder blending,(3) compacting, (4) sintering. Compacting is generally performed at room temperature, and the elevated-temperature process of sintering is usually conducted at atmospheric pressure. Optional secondary processing often follows to obtain special properties or enhanced precision.

Introduction to Stainless Steel

The material we know as stainless steel is such a common feature of 21st century living that there can be few of us who have not seen or handled articles made from it. Like all types of steel, stainless steel is not a single metal but an alloy that is a material made from two or more separate elements alloyed or "melted" together. What all steels have in common is that their major "ingredient" (alloying element) is the metal iron, to which a small amount of carbon has been added. Stainless steel was invented early in the 20th century when it was discovered that a certain amount of the metal chromium (usually a minimum of 11 per cent) added to ordinary steel gave it a bright shiny gloss and made it highly resistant to tarnishing and rusting. This rust-resisting property which we call “corrosion resistance” is what sets stainless steel apart from most other forms of steel.

While the original form of stainless steel, (iron with around 12% Chromium) is still in widespread use, engineers now have a wide choice of different types (grades). In all, there are more than 100 different grades but these are usually sub-classified into distinct metallurgical “families” such as the austenitic, ferritic, martensitic and duplex families.

The proportions of iron to chromium may be varied and other elements such as nickel, molybdenum, manganese and nitrogen may be added to widen the range

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of capabilities. Each particular grade of stainless steel has its own unique mechanical and physical properties and will usually be produced in accordance with an established national or international specification or standard.

Powder Production by Advanced Steelmaking Technology

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 us to provide not only a more consistent product, but has allowed enhanced properties, such as improved green strength and green density. The most common method for producing stainless steel powders is water atomization.

Stainless Steel – Tungsten Carbide

In an experiment stainless steel (AISI 316L) powders were mixed with various amounts of tungsten carbide (WC) powders (at 5%, 10% or 15% by weight), Afterwards, the powder mixtures were uniaxially compacted at 300 MPa. Specimens were sintered in hydrogen atmosphere, at 1200 °C, 1250 °C or 1300 °C with a holding time of either 30, 45 or 60 minutes. Sintered density of the composites were measured and hardness testing was performed.

The microstructure of a sintered specimen at 15% wt WC, 1200 °C for 30 minutes is shown in Figure 1. It was found that 316L powders were sintered and WC powders were agglomerated to be larger particles (lighter phase). Increasing the WC content generally increased the hardness value of the specimens at all

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sintering temperatures and all times. Figures 3 (a) and (b) present the effect of WC content on hardness of specimens sintered at 1200 °C, 1250 °C and 1300 °C for 30 and 60 minutes, respectively. The higher sintering temperatures also led to higher hardness for all holding times. However, there is no statistically significant difference on the hardness values of composite specimens sintered with various holding times.

Figure 1: SEM image of a sintered specimen containing 15 wt% of WC

Powder metallurgy technique can be employed for a fabrication of stainless steel tungsten carbide metal matrix composites. However, the composite specimens had high porosity. Hardness was increased with increasing WC contents at all temperatures and times conducted in the present work. Higher sintering temperature led to an increase in hardness and a reduction of porosity. Nonetheless, the effect of holding time on the hardness value did not show any statistically significant difference. The highest value of hardness was gained from specimens, containing 15 wt% of WC, sintered at 1300 °C.

Figure 2: Hardness of specimens with various WC contents and sintering temperatures with holding times of (a) 30 minutes and (b) 60 minutes

Figure 3: Optical micrograph of a specimen, containing 15 wt% of WC, sintered at 1300 °C for 60 minutes

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Effect of Boron

Boron improves the degree of sinter in the P/M steels evaluated. Above the eutectic temperature of 1175°C (2150°F) liquid Fe2B forms which fills the pores. Above 0.05w/oB the boron segregates to the grain boundaries where Fe2B forms improving the degree of sinter. The mechanical properties are improved by the increase in sintered density; however, once grain boundary segregation occurs the steel becomes weak and brittle. Boron improves hardenability by inhibiting the nucleation of ferrite at the austenite grain boundaries , this allows bainite to form, increasing the depth to which the steel hardens.

Figure 4: Effect of boron and temperature on sintered density and elongation and tensile strength of some stainless steels.

Figure 5 : Porosity distribution in 316L with boron and conventional 316L.

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Precipitation Hardening

Applications requiring high strength stainless steels are growing rapidly. Precipitation- hardening stainless steels have seen limited use in powder metallurgy despite their high strength. Strengthening of these alloys is achieved by adding elements such as copper and niobium, which form intermetallic precipitates during aging. The precipitation-hardening grades exhibit corrosion resistance levels comparable with those of the chromium-nickel (300 series) grades. For example 17-4 PH, a high-chromium, martensitic precipitation hardening stainless steel, has been optimized for use in PM applications, and a new low chromium (12 w/o) alloy that utilizes copper in the precipitation reaction.

In an experiment based on UNS J91151 alloy, a series of PM copper containing stainless steel alloys was fabricated. All the powders were produced by water atomization with a typical particle size <150 μm (– 100 mesh) with 38 to 48 w/o <45 μm (-325 mesh). All the alloying elements were prealloyed into the melt prior to atomization.

The mechanical properties of PM 17-4 PH can be improved by utilizing a finer particle size distribution, higher sintering temperatures and longer sintering times. The corrosion resistance of PM 17-4 PH is similar to that of 304L, when processed under similar conditions.

Dual Phases 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

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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 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.

Figure 6 : Effect of Km on ferrite phases

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Duplex stainless steels

Duplex stainless steels are the newest in the stainless steels family. They are characterized by a two-phase structure consisting of approximately equal amounts of ferrite and austenite. Ferritic phase is responsible of the strength increase while austenite ensures the toughness and better corrosion resistance. Properties make the duplex stainless steels very attractive for numerous applications. Sintered stainless steels are used in many industrial branches due to their high mechanical properties and good corrosion resistance.

These Properties change by changing sintering method on duplex stainless steels. For example sintering cycle with fast cooling directly form sintering temperature proves its advantage in case of mechanical properties and additionally it seem to be very promising for obtaining a balanced duplex structure, also working with cycles easy to be introduced in industries. Introduction of pressurized nitrogen to achieve fast cooling rate directly from sintering temperature enables precipitates free microstructure of sintered duplex steels in one sintering cycle

Effect of Magnetic Permeability

Magnetic permeability response of the pressed and sintered component should be nearly zero. This prevents flow restrictions when used in conjunction with a magneto-rheological fluid.

Figure 7: Fracture surface of composition sintered with fast cooling rate

Figure 8: Fracture surface of composition sintered with slow cooling rate

Figure 9: Magnetic response at sintering temperature of various alloys.

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Effect of Sintering Atmosphere

The use of atmospheres other than pure hydrogen or vacuum in sintering high quality stainless steel parts has been examined in depth. In processing for optimal magnetic performance, use of atmospheres other than hydrogen or vacuum has been shown to be inferior. However, market pricing pressures and the need for cost controls warrant the investigation and comparison of alternative atmospheres and their effects on the magnetic properties of 410L. The synthetic dissociated ammonia (75% H2/25% N2) atmosphere produced a slightly lower density value than pure hydrogen. However, the 10% H2/90% N2 atmosphere proved to be far worse, with significantly lower sintered density and extremely low maximum induction. Even the synthetic D.A. maximum induction value represents a serious decline relative to that of the hydrogen sintered material. These results are not surprising, since previous work in this area generated similar results, virtually eliminating the use of nitrogen bearing atmospheres for the sintering of magnetic stainless steel parts. Coercive force and permeability values also indicate a significant reduction in magnetic performance when sintering in nitrogen-containing atmospheres. Sintered chemistry results explain the cause of the property degradation to be retained nitrogen. A six-fold increase in nitrogen content (hydrogen versus D.A.) results in almost a four-fold decrease in permeability and a near doubling of coercive force at 30 oersteds. Nitrogen, oxygen and carbon levels determined in the 10% H2/90% N2 sintered samples are unacceptably high as reflected by the poor magnetic properties.

Figure 10 : Effect of various atmosphere on sintered density.

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Effect of Various Additives

Figure 11 : Effect of various additives on stainless steels

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Recycling

Some experiments reveal that the sintering between stainless steel powder and scraps of the same material is possible. The volumetric fraction of emptiness is about 12%. It is an acceptable value for sintered metallic materials. Moreover, this procedure involves a reduction of energy use, therefore the scrap doesn’t pass for casting processes.

References 1-Papers: Alan Lawley Emeritus Professor Department of Materials Science and Engineering Drexel University Philadelphia, “DEVELOPMENT OF A HIGH-STRENGTH-DUAL-PHASE P/M STAINLESS STEEL”

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

Sanjay N. Thakur and Vaidehi C. Dongre Hazentec, L.L.C., “PRE-ALLOYED BORON IN POWDERED METAL (P/M) STAINLESS STEELS”

John Schaberl Ancor Specialties Ridgway, “STAINLESS STEEL AISI GRADES FOR PM APPLICATIONS”

Michael Marucci, Alan Lawley, Robert Causton, and Suleyman Saritas, “Effect of Small Additions of Boron on the Mechanical Properties and Hardenability of Sintered P/M Steels”

lan Lawley Emeritus Professor Department of Materials Science and Engineering Drexel University Philadelphia, “DEVELOPMENT OF A HIGH-STRENGTH-DUAL-PHASE P/M STAINLESS STEEL”

Tom Stuart, Delphi Corporation Chris Schade, Hoeganaes Corporation Francis Hanejko, Hoeganaes Corporation , “Unique Stainless Steel Materials and Processing For High Strength With Reduced Magnetic Performance”

Alan Lawley, Roger Doherty Drexel University Philadelphia, “PRECIPITATION HARDENING P/M STAINLESS STEELS”

2-Websites

Wikipedia.com

Grundfos.com