Topics:

POWDER PRODUCTION

Q: What is the difference between sponge and atomized powders?

A: Sponge powder is produced from magnetite iron ore that is directly reduced at elevated temperatures to obtain sponge iron. The material is then disintegrated into powder and annealed to obtain the desired properties. Sponge iron has a very high surface area and exhibits high green strength. It is used for low and medium density ferrous PM parts, approximately 5.4 g/cm³ to 6.7 g/cm³.

To produce atomized powders, molten steel is atomized into irregular and homogeneous particles which are then annealed. The melt stock and the subsequent processing are carefully controlled to produce uniform steel powders designed for PM parts requiring densities over 6.7 g/cm³.

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ALLOYING METHODS

Q: What are the different alloying methods for ferrous metal powders?

A: The following are the different alloying methods:

ADMIXED: Iron powders are admixed with other elemental powders to produce the desired composition.
Examples:

Fe + 2 w/o Cu + 0.8 w/o graphite + 0.75 w/o lubricant (FC-0208)
Fe + 2 w/o Cu + 2 w/o Ni + 0.6 w/o graphite + 0.75 w/o lubricant (FN-0205) 

PARTIALLY ALLOYED (DIFFUSION ALLOYED): A powder in which the alloy addition or additions are metallurgically bonded to an elemental or pre-alloyed powder. Diffusion alloyed powders are partially alloyed by means of a diffusion anneal.

PRE-ALLOYED: Powders composed of two or more elements that are alloyed in the powder manufacturing process, in which the particles are of the same nominal composition throughout.

HYBRID ALLOY: Pre-alloyed or partially alloyed powder with elemental or ferroalloy additions admixed to produce the desired composition.

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MATERIAL PROPERTIES

Q: What affects the mechanical properties of ferrous PM materials?

A: Density: Mechanical properties improve as the amount of porosity is reduced. Tensile strength is almost a linear function of density. Ductility, toughness, and fatigue strength are even more dependent on density; they increase significantly at low levels of porosity.

Composition: Different alloying methods (admixing, partially alloying, pre-alloying, and hybrid mixes) are used to achieve the desired chemistry of a material. The composition and alloying method determine the hardenability for a given consolidation and sintering route. Constituents not in solution do not contribute to hardenability.

Microstructure: The microstructure depends on the chemistry of the powder, the alloying method used, the sintering time and temperature, the sintering atmosphere, and the cooling rate after sintering.

Q: Why is the apparent density so much lower in sponge products than atomized steel products?

A: Apparent density measurements are an indicator of how close particles pack together when not in a stressed mode. Sponge iron powder exhibits a lower apparent density (AD) than steel atomized powder due the different particle shapes each type of material possesses.

To produce sponge iron, magnetite ore is directly reduced at elevated temperatures. The resulting powder contains internal porosity which is not present in steel atomized powder. So, the sponge particles tend to pack together with more unfilled space than the atomized particles, resulting in a lower apparent density.

Q: Why is the apparent density of a powder mix or premix so important?

A: The apparent density of a powder or premix is used by part designers to design tooling that will produce parts with the desired dimensions. If the apparent density of the material changes, problems may result in terms of meeting part specifications, particularly with a fixed-fill tooling or shelf die. Also, the apparent density is an indication of the green strength of the material, in general, as green strength increases with decreasing apparent density.

Q: What is ‘apparent’ hardness?

A: Apparent hardness is a macrohardness (hardness testing in the macro scale). Due to the fact the PM parts are composites of metal and pores, the apparent hardness values obtained, since taken on the macro scale, is the hardness of the composite. Hardness is a measure of the resistance of a metal to permanent (plastic) deformation.

Q: What is the effect of density on Young’s Modulus?

A: Young’s Modulus increases linearly as a function of density in the range of 6.5 to 7.4 g/cm³ from 15 to 25.

Q: What is Poisson’s ratio for ferrous PM materials?

A: The value for Poisson’s Ratio of ferrous PM materials may be taken as 0.27+0.02. Poisson’s Ratio is a weak function of density.

Q: What are the coefficients of thermal expansion of PM materials?

A: The thermal expansion of a PM material is a weak function of density. A rough estimate can be obtained by taking the cube root of the relative density of the PM part and multiplying that value by the bulk thermal expansion coefficient.

Q: What is meant by the expression ‘pore-free’ density of a green compact?

A: Pore-free density is the density of a green compact if ALL of the porosity could be removed. Pore-free density is dependent on the density and percentage of each premix constituent addition. Graphite and lubricant have significant negative effects on pore-free density.

The pore-free density of a premix can be calculated from the premix composition:

Density of Various Materials as Measured by Pycnometry 

Material	Density (g/cm³)
Fe	7.84 to 7.90
Copper (atomized)	8.05
Nickel (Inco 123)	8.85
Graphite	2.30
Lubricants	0.90 to 1.15
Pore-Free Density Example
Fe + 2 w/o Ni + 0.6 w/o graphite + 0.6 w/o lubricant
Additive	Mass%		Density	Volume
Fe	96.80	÷	7.84	12.35
Nickel	2.00	÷	8.85	0.23
Graphite	0.60	÷	2.30	0.26
Lubricant	0.60	÷	1.00	0.60

Total Mass (g) = 100.00 Total Volume (cm³) = 13.44

Pore-Free Density = Mass ÷ Volume = 100.00 ÷ 13.44 = 7.44 g/cm³ 98%

Pore-Free Density* = 7.29 g/cm³

*98% Pore-Free Density is what is seen typically in practice

Q:  What kinds of data are included within MPIF Material Standards 35?

A:  MPIF Material Standard 35 for Structural Components includes the following characteristic data for commonly used PM materials:

Chemistry
Tensile Properties

Yield Strength

Ultimate

Elongation

Elastic Constants

Young's Modulus

Poisson's ratio

Unotched Charpy Impact Energy

Transverse Rupture Strength

Compressive Yield Strength

Macro (apparent) Hardness

Micro (connected) Hardness

Fatigue limit (90% survival)

Density

A limited number of alloys have data for:

Fatigue Strength (10⁷cycles)

Magnetic Response

Maximum Induction

Residual Induction

Coercive Field

Hardenability Data exists on many alloys as does comparative Machinability Data.

There is also limited data on Coefficient of Thermal Expansion.

Fracture Toughness (over a range of densities) and a Corrosion Resistance Rating

(based on 2% H₂SO₄).

Units are available in English or SI units.

Standards Data has also been developed for:

Metal Injection Molding Materials

Powder Forged

PM Bearing Materials

MPIF Standard 35 for Metal Injection Molding includes the following characteristic data for commonly used MIM materials:

Chemistry

Tensile Properties

Ultimate Tensile Strength

Yield Strength @0.2% offset

Elongation (%inches per inch)

Density
Young's Modulus
Unnotched Charpy Impact Energy
Hardness

Macro (apparent)

Micro (connected)

Corrosion Resistance (for Stainless Steels) using H₂SO₄, CuSO₄ and Boil Test.

Magnetic Properties (for soft magnetic alloys—ASTM-A-773)

Magnetizing Field

Induction (max.)

Permeability (max.)

Coercive Field

Residual Induction

MPIF Standard 35 for Powder Forged Steel Parts includes the following characteristic data:

Chemistry

Mininum density based on grade

Heat Treat Condition (normalized or ground and tempered)

Tensile Properties

Ultimate Strength

Yield Strength @ 0.2% offset

Elongation

Reduction of Area (%)

Hardness

Impact Energy

Compressive Yield Strength @ 0.1% offset

Mean Fatigue Limit

There are also Hardenability Curves using standard Jominy Test Procedure (ASTM A-255)

MPIF Standard 35 for Self-Lubricating Bearing (1998 edition) contains the following characteristic data:

Chemistry

Strength Constants (radial crushing force)

Oil Content (volume%)

Min. & Max. density

Typical loads

Recommended press fit

Running Clearances

Recommended Tolerances

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SINTER HARDENING

Q: What is sinter hardening?

A: Sinter hardening refers to a process where the cooling rate experienced in the cooling zone of the sintering furnace is fast enough that a significant portion of the steel matrix is transformed into martensite. The benefits of sinter hardening are:

Eliminates the need for secondary heat treatment
Improved part dimensional control (reduced distortion)
Easier tempering in air because of the elimination of oil quenching
Sinter-hardened parts do not need oil removal prior to finishing operations such as plating

*See MPIF Standard 35 for Structural Components Rev.2000, p. 18, for chemistries and mechanical property details. 

Q: What are the typical cooling rates required?

A: Cooling rate depends upon the material composition, but, in general, it is recommended to have a cooling rate of 0.9°F/sec (0.5°C/sec) or greater between 1200°F (649°C) and 380°F (193°C).

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ELECTROMAGNETIC APPLICATIONS

Q: What PM materials are available for electromagnetic applications?

A: There are two families of materials for electromagnetic applications. The first are powders used in the sintered state. These materials include pure irons, such as iron-phosphorous alloys , iron-silicon alloys with silicon contents varying typically up to 6% and ferritic stainless steel powders. The second family of powders are the insulated powders.

Q: What differentiates the two families of powders and their application?

A: The first materials are compacted to net shape and sintered to obtain optimal mechanical and magnetic properties. Since these materials are utilized in their sintered state and are essentially large pieces of ferrous materials, they are typically used in applications where high permeability values and low coercive-force values are required and the presence of rapidly alternating magnetic fields are kept to minimum (DC applications).

Insulated powders are designed for electromagnetic applications requiring constant magnetic permeability and low core losses over a wider range of operating frequencies (AC applications). For this material, each powder particle is insulated from the other prior to the compaction step. The insulation is made up of either a combination of an oxide coating with a thermoplastic coating or a single thermoplastic coating. The materials are not sintered following compaction resulting in a high resistivity component. Despite the fact that the materials are not sintered, these highly engineered materials result in parts with tensile strengths in excess of 15,000 psi.

Q: What are typical magnetic properties of these materials?

A: Like all PM materials, the magnetic properties of these materials can be easily modified to meet specific application requirements. The chemistry, density and processing of the component can provide a wide range of magnetic and physical properties.

The following tables outline the range of performance possible. .

Typical Magnetic Properties for Sintered (DC) Materials Measured at a Maximum Drive Field of 15 Oe
Alloy System	Typical Density (g/cm³)	Maximum Permeability	Coercive Force (Oe)	Maximum Induction (kGauss)	Resistivity (µohm-cm)
Iron	6.8-7.2	1,800 / 3,500	1.5 / 2.5	10 / 13	10
Fe-P	6.7-7.4	2,500 / 6,000	1.2 / 2.0	10 / 14	30
Fe-Si	6.8-7.5	2,000 / 5,000	0.6 / 1.2	9 / 14	60
Ferritic Stainless Steels	5.9-7.2	5,00 / 1,500	2.0 / 4.0	6 / 8	50
50Ni / 50Fe	7.2-7.6	5,000 / 15,000	0.2 / 0.5	9 / 12	45
Magnetic Properties for Insulated (DC) Materials Measured at a Maximum Drive Field of 40 Oe Following Compaction at 50 tsi (690 MPa)
Material	Polymer Coating (%)	Oxide Coating?	Density (g/cm³)	Maximum Permeability	Coercive Force (Oe)	Maximum Induction (kGauss)
Example 1	0.60	No	7.45	425	4.7	11.2
Example 2	0.75	No	7.40	400	4.8	10.9
Example 3	0.75	Yes	7.20	210	4.7	7.7

Q: What are typical applications for the insulated particle material?

A: Typical applications are for AC applications that have an operating frequency above 300Hz. Below this operating frequency the lower permeability of the insulated materials will give inferior performance to laminations.

Q: Can  insulated powder (IP) be used in DC applications?

A: Yes, but the application will be better served by a sintered material. The reduced permeability for IP is just not a good match.

Q: What effects sintered magnetic properties?

A: Several material and processing parameters effect magnetic performance of PM materials.

The addition of certain alloying elements such as P and Si, improve the nearly all magnetic properties. In particular, these alloying elements tend to increase resistivity while decreasing (improving) coercive force. The presence of the interstitial elements, carbon and nitrogen, have a very potent detrimental effect on magnetic performance. Elevated levels of these elements are usually found as the result of their presence in the sintering atmosphere.

Magnetic performance is also enhance by the use of higher sintering temperatures. High temperature sintering results in higher density, more refined pore structure and larger grain size, each which will improve magnetic performance.

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SECONDARY TREATMENTS

Q: What is green machining?

A: Green machining is the machining of a part prior to the sintering process. In general, this is not done with conventionally processed PM parts, since they do not have sufficient green strength. With new powder formulation, green parts can be manufactured with green strengths in the range of 3500 psi to 6000 psi. These green strengths are sufficient to facilitate the thrusting and clamping forces realized in machining process.

Q: Can PM parts be heat treated?

A: Yes, heat treatment generally involves heating into the austenitic phase region followed by controlled cooling to achieve a desired microstructure. There are different types of heat treatment operations:

Normalizing and annealing utilizes slow cooling rates to
develop a ferrite/ pearlite microstructure well suited for
machining, welding, and cold working

Neutral hardening and case hardening utilize a rapid cooling or
quench in oil to develop a martensitic microstructure; the
process results in maximum strength and apparent hardness

The use of salt baths is avoided because of entrapment of the
compounds in the pores of the parts

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