PRODUCTION OF IRON AND STEEL POWDERS

At present there are two basically different production methods which together account for

more than 90% of the world production of iron and steel powders, viz. The Höganäs sponge-

iron process and the water-atomizing process. The former process is based on reduction of

iron ore, yielding a highly porous sponge-iron which subsequently is comminuted to powder.

The latter process is based on atomization of a stream of liquid iron (or steel) by means of a

jet of pressurized water.

In the manufacturing of sintered parts, iron powders are always used admixed with a small

amount of lubricant in powder form in order to minimize the friction in the compacting tool. In

many cases, they are also blended with alloying elements in powder form, like graphite,

copper, nickel, molybdenum and others (in order to achieve increased strength properties).

The Höganäs Sponge Iron Process

The Höganäs sponge iron process, is essentially a chemical process in which finely divided

iron ore is being reduced with coke breeze yielding a spongy mass of solid metallic iron,

which can readily be comminuted to iron powder. The iron ore used at Höganäs is a

magnetite (powder Fe3O4) obtained from selected sources. This magnetite, which by nature

contains only very small amounts of gang and has extremely low contents of sulfur and

phosphorus, is being dressed and concentrated while still at the mining location and then

delivered to Höganäs in a highly pure state.

The Höganäs Water-Atomizing Process

The raw material for this process is a carefully selected iron scrap and sponge iron from the

process described in the preceding paragraph. This raw material (1) is melted down in an

electric arc furnace of 50 tons capacity (2) where, if desired, alloying elements can be added.

The melt is teemed slag-free through a bottom hole into a ladle (3) where it is refined with an

oxygen lance (4). The ladle is then transferred to the atomizing station (5), and the liquid iron

(or steel) is teemed slag-free through a bottom hole in the ladle into a specially designed

tundish (A).

From there, the liquid iron (or steel) flows in a well controlled stream (B) through the center of

a ring-shaped nozzle (D) where it is hit by jets of highly pressurized water (C). The stream of

liquid iron (or steel) explodes into fine droplets (E). Some of these droplets freeze immediately

to small spheres, others unite in small irregularly shaped agglomerates while freezing. Air,

swept along by the water jet, and water vapor arising in the atomizing process, cause

superficial oxidation of the small droplets. The solidified droplets and the atomizing water are

collected in a huge container, where they are settling as a mud. This powder mud is de-

watered (6) and dried (7). The dry powder is magnetically separated from slag particles (8),

screened (9) and homogenized (10), and eventually transported in special containers (11) to

the works at Höganäs for further processing.

In the state as leaving the atomizing plant, the atomized powder particles are not only

superficially oxidized but also very hard because, due to the extremely high cooling rates

residing in the atomizing process, they have solidified in the martensitic state – despite their

low carbon content.

The powder is, therefore, soft-annealed, and its surface oxides and residual carbon are

reduced in belt furnaces. Routines for homogenizing, quality checking, packaging and storing

are the same as for sponge iron powders.

Alloying Methods

In order to achieve hardenable sintered ferrous materials, carbon and other suitable alloying

elements, like e.g. copper, nickel, and molybdenum, have to be introduced.

While carbon is normally admixed to the iron powder in the form of graphite, metallic alloying

elements are commonly introduced by either of the following two methods:

Method 1: Water atomization of the liquid iron alloy, resulting in a homogeneously alloyed

powder.

Method 2: Mechanically blending plain iron powder with the respective alloying elements in

powder form, and letting the actual alloying process take place during sintering of the parts

compacted from the powder mix. Both methods have their advantages and disadvantages:

Homogeneously alloyed powders:

Advantages:

• Alloying elements do not segregate when the powder is handled .

• Yield fully homogeneously alloyed sintered parts.

Disadvantages:

• Have low compressibility, because their particles are solution-hardened.

• In order to change or correct the composition of a fully alloyed powder, if ever so little, a new

melt (usually 50 tons at time) will have to be atomized.

Powder mixes.

Advantages:

• Have higher compressibility.

• No additional mixing operation is required as the powder has to be admixed with a lubricant

anyway.

• The composition of a powder mix can very easily be changed or corrected by re-mixing it

with additional amounts of either iron powder or alloying elements.

Disadvantages:

• Yield less homogeneously alloyed sintered parts, because the admixed alloying elements

(except carbon) diffuse very slowly in solid iron.

• Alloying elements tend to segregate when the powder mix is transported and handled.

Distaloy and Starmix

In order to eliminate the segregation problem with powder mixes, Höganäs AB has developed

two special processes for the production of segregation-proof iron powder mixes. A large

variety of standard and tailor-made powder mixes produced according to these processes are

offered under the trade-names Distaloy™ and Starmix.

The Distaloy™ process can be described as follows:

Alloying elements used in the Distaloy™ process are mainly copper, nickel and molybdenum

(but not graphite!) in the form of very finely grained powders. The process starts with

weighing-in a production lot of 30 tons of iron powder and alloying powders in exactly

controlled proportions. This lot is mixed in a double-cone mixer. Special precautions are taken

to prevent segregation of the mix when discharged from the mixer.

The so produced powder mix is heat-treated in a continuous furnace under a reducing

atmosphere at a temperature somewhat below the melting point of the lowest melting alloying

element. During this heat-treatment, the fine particles of the added alloying elements are

safely bonded to the surfaces of the coarser iron powder particles.

Due to inter-diffusion between the diffusion bonded alloying particles and the iron particles,

the latter become, to a certain extend, locally pre-alloyed. The so treated powder mix contains

the alloying additives as finely and evenly dispersed as possible . The fact that the iron

powder particles are locally pre-alloyed has practically no negative effect on the

compressibility of the mix.

Graphite and lubricants have to be excluded from the Distaloy™ process because, during

heat-treatment of the powder mix, graphite would carbonize the iron particles and spoil the

compressibility of the powder mix, and lubricants would burn-off.

The Starmix process uses special types of organic binders to glue graphite and lubricants to

the iron powder particles during the mixing procedure.

General aspects

Iron and steel powders - as well as other metal powders - used in the production of sintered

parts can be characterized by three categories of properties:

1. Metallurgical properties

chemical composition and impurities⇒

microstructure⇒

microhardness⇒

2. Geometrical properties

particle size distribution⇒

external particle shape⇒

internal particle structure (particle porosity)⇒

3. Mechanical properties

flow rate⇒

bulk density⇒

compressibility, green strength and spring-back⇒

All these powder properties are inherited from and specific to the process by which the

powder was produced. Some of them are interrelated with each other. For instance:

microstructure and microhardness are depending on chemical composition;

• compressibility decreases with increasing microhardness, increasing particle porosity and

decreasing particle size;

• coarser powders and powders of regular particle shape flow better than fine powders and

powders of irregular particle shape;

• powders of irregular particle shape have better green strength after compacting than

powders of regular particle shape.

Metallurgical properties are determined by chemical analysis and metallographic procedures.

The chemical composition of a ferrous powder has a great influence upon the final strength

properties of the sintered parts. Non-metallic impurities may have an adverse effect upon

compressibility and upon the life of compacting tools.

Geometrical properties viz. particle size distribution, particle shape and particle porosity,

determine the powder’s specific surface which is the driving force of the sintering process

Particle size distribution is determined by sieve analysis if particle sizes are above 45 μm

(minimum screen aperture). Finer powders are suspended in water and analyzed by means of

laser diffraction methods

External particle shapeis analyzed by means of scanning electron microscopy

Internal particle structure is analyzed by means of metallographic techniques

Flow rate or rather its reciprocal value, is the time in seconds which an amount of 50 g dry

powder needs to pass the aperture of a standardized funnel. Flow rate is influenced by type

and amount of lubricant admixed to the powder. The flow rate decides about how fast a

compacting tool can be filled with powder, and thus is a limiting factor in the compacting cycle

of the powder press.

Bulk density (Apparent density) is determined by filling the powder through a standardized

funnel into a small cup, leveling-off the surplus powder on top of the cup, and dividing the

weight of powder contained in the cup by the cup volume (25 cm3). Apparent density is

influenced by type and amount of lubricant admixed to the powder. The apparent density of

the powder decides about the required filling depth of the compacting tool.

Compressibility is the name of a curve obtained by plotting the compact densities of a series

of small cylindrical powder compacts (Ø 25 mm), over the applied pressures. Compact

density is the weight of a powder compact divided by its bulk volume. Compact density is

influenced by type and amount of lubricant admixed to the powder. Green density is the

compact density of a small cylindrical powder compact (Ø 25 mm) pressed with a

standardized pressure (either 4,2 tons/cm2 or 600 N/mm2). The compressibility of the powder

decides about how high a compacting pressure is needed to achieve a desired compact

density.

Green strength is the bending strength of a green (i.e. compacted but not sintered)

rectangular test bar. Green strength increases with increasing compact density and is

influenced by type and amount of lubricant admixed to the powder. Sufficient green strength is

required to prevent compacts from cracking during ejection from the compacting tool and

prevent them from getting damaged during handling and transport between press and

sintering furnace. The more complex and delicate the shape of a compact, the higher its

green strength should be. If the green strength of compacts is high enough, they may even be

machined prior to sintering (e.g. undercuts, traverse slots and holes).

Spring-back is the elastic expansion of a cylindrical powder compact (Ø 25 mm) after

ejection from the compacting die. Its value is expressed as the difference between the OD of

the compact and the ID of the (empty) die divided by the ID of the die. Spring-back increases

with increasing compacting pressure and is influenced by type and amount of lubricant

admixed to the powder, and by the elasticity coefficient of the die material in which the powder

is compacted. The spring-back value is important for calculating the exact dimensions of the

compacting tool in relation to the required dimensions of the compact.

Sintering

Sintering is the process by which metal powder compacts (or loose metal powders) are

transformed into coherent solids at temperatures below their melting point. During sintering,

the powder particles are bonded together by diffusion and other atomic transport

mechanisms, and the resulting somewhat porous body acquires a certain mechanical

strength.

The sintering process is governed by the following parameters:

• temperature and time,

• geometrical structure of the powder particles,

• composition of the powder mix,

• density of the powder compact,

• composition of the protective atmosphere in the sintering furnace.

Temperature and time.

The higher the sintering temperature, the shorter is the sintering time required to achieve a

desired degree of bonding between the powder particles in a powder compact (specified e.g.

in terms of mechanical strength). This constitutes a dilemma: From the view point of

production efficiency, shorter sintering times would be preferable; but the correspondingly

higher sintering temperatures are less economical because of higher maintenance costs for

the sintering furnace.

In iron powder metallurgy, common sintering conditions are: 15 - 60 min at 1120 – 1150°C.

Geometrical structure of the powder particles.

At given sintering conditions, powders consisting of fine particles or particles of high internal

porosity (large specific surface), sinter faster than powders consisting of coarse compact

particles. Again, we have a dilemma: Fine powders are usually more difficult to compact than

coarse powders, and compacts made from fine powder shrink more during sintering than

compacts made from coarse powder. Particles of commercial iron powders (spongy or

compact types) for structural parts are usually ≤ 150 μm

Composition of the powder mix.

The components of powder mixes are selected and proportioned with a view to achieving

desired physical properties and controlling dimensional changes during sintering (ref. Chapter

3). When mixes of two or more different metal powders (e.g. iron, nickel and molybdenum)

are sintered, alloying between the components takes place simultaneously with the bonding

process.

At common sintering temperatures (1120 - 1150°C), alloying processes are slow (except

between iron and carbon), and a complete homogenization of the metallic alloying elements is

not achievable. If the powder mix contains a component that forms a liquid phase at sintering

temperature (e.g. copper in iron powder mixes), bonding between particles as well as alloying

processes are accelerated.

Density of the powder compact.

The greater the density of a powder compact, the larger is the total contact area between

powder particles, and the more efficient are bonding and alloying processes during sintering.

Furthermore, these processes are enhanced by the disturbances in the particles’ crystal

lattice caused by plastic deformation during compaction

Composition of the protective atmosphere in the sintering furnace.

The protective atmosphere has to fulfill several functions during sintering which in some

respects are contradictory. On the one hand, the atmosphere is to protect the sinter goods

from oxidation and reduce possibly present residual oxides; on the other hand, it is to prevent

decarbonization of carbon-containing material and, vice versa, prevent carbonization of

carbon-free material.

This illustrates the problem of choosing the right atmosphere for each particular type of sinter

goods. In iron powder metallurgy, the following sintering atmospheres are common :

• reducing-decarbonizing type: hydrogen (H2), cracked ammonia (75% H2, 25% N2),

• reducing-carbonizing type: endogas (32% H2, 23% CO, 0-0.2% CO2, 0-0.5% CH4, bal. N2),

• neutral type: cryogenic nitrogen (N2), if desirable with minor additions of H2 (to take care of

residual oxides) or of methane or propane (to restore carbon losses).

6.2 Basic Mechanisms of Sintering

6.2.1 Solid state sintering of homogeneous material

Judging by the changing shape of the interspace between sintering particles, the sintering

process passes through two different stages: 1) an early stage with local bonding (neck

formation) between adjacent particles, and 2) a late stage with pore-rounding and pore

shrinkage. In both stages, the bulk volume of the sintering particles shrinks – in the early

stage, the center distance between adjacent particles decreases, in the late stage, the total

pore volume shrinks.

The driving force behind these sintering phenomena is minimization of the free surface

energy (ΔGsurface< 0) of the particle agglomerate

In the absence of a liquid phase, five different transport mechanisms are possible:

• volume diffusion (migration of vacancies),

• grain-boundary diffusion,

• surface diffusion,

• viscous or plastic flow (caused by surface tension or internal stresses),

• evaporation/condensation of atoms on surfaces.

6.2.2 Solid state sintering of heterogeneous material

When a mixture of particles of two different metals is being sintered, alloying takes place at

locations where necks are formed between particles of different metallic identity.

These two processes interact with one another: On the one hand, the growth rate of the neck

now depends not only on the diffusion rates in the two pure metals but also on the different

diffusion rates in the various alloy phases being formed in and on either side of the neck. On

the other hand, the neck width controls the rate of alloy formation.

The outcome of this interaction varies with the chemical identity of the two metals: it may have

an accelerating, a delaying or no effect at all on the growth rate of the neck.

6.2.3 Sintering in presence of a transient liquid phase

Consider a compact made from a mixture of particles of two different metals. If one

component of the mixture melts at sintering temperature, the arising liquid phase is first being

pulled by capillary forces into the narrow gaps between the particles of the solid component,

creating the largest possible contact area between liquid and solid phase.

Then, alloying takes place and, if the initial proportion of the liquid phase is smaller than its

solubility in the solid phase, the liquid phase eventually disappears. The bulk volume of the

compact swells because the melting particles leave behind large pores, while the framework

of solid particles increases in volume corresponding to the amount of dissolved liquid phase.

See schematic illustration at Fig. 6.11.

It can be seen that the liquid copper not only infiltrates the gaps between the iron powder

particles but also penetrates their grain boundaries.

If, in the example above, the pure iron particles are substituted with carbonized iron particles

having a pearlitic microstructure, the liquid copper penetrates the interfaces between ferrite

and cementite lamellae. This leads eventually to a partial disintegration of the pearlitic

particles.

Consequently, the initially rigid framework of solid particles collapses locally, and the bulk

volume of the compact shrinks. The micrograph at Fig. 6.13 shows beginning disintegration of

pearlitic iron particles under the influence of liquid copper.

These examples explain why additions of copper to iron powder mixes result in less shrinkage

or produce growth during sintering of structural parts, and why additions of carbon (graphite)

to iron-copper powder mixes compensate the growth-producing effect of copper

6.2.4 Activated sintering

A special kind of sintering with a transient liquid phase is often referred to as activated

sintering. Here, a base powder is admixed with a small amount of a metal or metal compound

which, although having a melting point above sintering temperature, forms a low-melting

eutectic together with the base metal. See Fig. 6.14.

The added metal or metal compound is called the activator. During sintering, atoms from the

activator diffuse into the particles of the base metal until the latter begin to melt superficially.

This superficial melting enhances the formation of necks between adjacent particles of the

base metal. As the activator continues to diffuse deeper into the particles of the base metal,

the liquid phase (eutectic) disappears again. Activated sintering is utilized e.g. in the

manufacturing of so called heavy metals.

Here, an addition of only a few percent of nickel powder to tungsten powder produces a

transient tungsten-rich eutectic at 1495°C which substantially accelerates the sintering

process. The sintering of iron powder can be activated through small additions (e.g. 3 wt.%)

of finely ground ferro-phosphorous (Fe3 P). As can be seen from the binary phase diagram

shown at Fig. 6.15, Fe and Fe3P form a eutectic at 1050°C.

During sintering at 1120°C, the phosphorous concentration at the surface of the iron powder

particles temporarily exceeds 2,6 wt.%, and the particles melt superficially. But as the

phosphorous diffuses deeper into the iron particles, its concentration at the surface drops

below 2,6 wt.% again, and the liquid phase disappears.

Then, a second benefit of phosphorous becomes effective: Surface regions of the iron

particles with phosphorous concentrations between 2,6 and 0,5 wt.% have changed from

austenite to ferrite. As will be seen in the next paragraph, the coefficient of selfdiffusion

(volume diffusion) for iron is approx. 300 times greater in ferrite than in austenite.

Consequently, at equal temperature, sintering proceeds faster in ferrite than in austenite.

• Tensile strength and elongation adopt noticeable values first at sintering temperatures above

650 and 750°C respectively. From there-on, they increase almost exponentially until reaching

an intermediate maximum at approx. 900°C. Just above 910°C, where the crystal structure of

iron changes from ferrite to austenite, the values of tensile strength and elongation suddenly

drop a little and then increase again, but more slowly than below 910°C.

All test bars begin to sinter already during the heating-up period, while still in the ferrite state,

and those which are heated up to higher temperatures have already acquired a certain level

of strength before they change from ferrite to austenite.

In order to utilize the advantage of a transient liquid phase during sintering and to achieve

higher strength properties, many commercial iron powder mixes contain copper. Copper

additions to iron powder can produce undesirable dimensional growth during sintering.

Graphite additions to iron-copper powder mixes counteract the dimensional growth caused by

the copper (see § 6.2.3). The carbonization of the iron caused by the graphite additions

boosts the mechanical strength of the sintered parts.

6.4 The sintering atmosphere

The main purpose of sintering atmospheres is to protect the powder compacts from oxidation

during sintering and to reduce residual surface oxides in order to improve the metallic contact

between adjacent powder particles. A further purpose of sintering atmospheres is to protect

carbon-containing compacts from decarbonization.

SINTERED IRON-BASED MATERIALS

There are several ways to achieve desired strength properties with iron-based sintered

materials. The most important parameters of influence are:

• Density

• Sintering conditions

• Alloying elements

• Heat-treating conditions

Density is of prime importance with respect to the physical properties of sintered structural

parts, because tensile strength and fatigue strength increase in approximate linear proportion,

elongation and impact strength exponentially, with sintered density.

With maximum pressing loads tolerable under mass production conditions (600 - 650

N/mm2), densities up to 7,1 - 7,2 g/cm3 are achievable. This density range can be extended

up to 7,3 - 7,4 g/cm3 when utilizing a warm-pressing technique developed by HÖGANÄS AB

[9.1]. Densities up to 7,5 - 7,6 g/cm3 can only be achieved by pre-sintering and re-pressing

the compacts before final sintering (chapter 7, § 7.2). Still higher densities, up to 7,7 - 7,8

g/cm3, can be achieved by means of hot-forging pre-pressed (and pre-sintered) compacts

[9.2].

Sintering conditions decide (1) how fast and efficient powder particles in the compact weld

together and pores get rounded, (2) how fast homogenization of alloying elements takes

place, and (3) whether sensitive alloying elements oxidize or not.

In iron powder metallurgy, sintering is most commonly carried out in continuous mesh-belt

furnaces operating at 1120 to max.1150°C. Sintering temperatures of 1250 -1350°C

accelerate the homogenization of alloying elements and allow the use of beneficial but

oxygen-sensitive alloying elements like chromium and manganese. With modern materials

and furnaces, chromium alloys can now be sintered at 1120° C. Meshbelt furnaces cannot

withstand temperatures above 1150°C.

Time at temperature is usually no longer than 20 to 30 minutes, since longer sintering times

yield only marginally improved properties which do not justify the increased sintering costs.

Alloying elements dissolved in the base metal, give rise to the formation of various

microstructures and increase the materials resistance to deformation. See Fig. 9.2a. Alloying

elements also influence the dimensional change of structural parts during sintering. Alloying

elements are indispensable with respect to the hardenability of conventional as well as

sintered steels. See Fig. 9.2b.

In principle, alloying elements have the same effect on sintered steels as on conventional

steels. However, not all alloying elements common in conventional steels can be utilized in

sintered steels because some of them, as e.g. Mn, and V, are too easily oxidized in

commercial sintering atmospheres (chapter 6, § 6.4). On the other hand, elements

undesirable in conventional steels, like e.g. phosphorous (”blue brittleness”), can have

beneficial effects on sintered steels (chapter 6, § 6.2.4).

Alloy compositions of sintered steels for structural parts have to be carefully selected not only

with respect to desired strength but also with respect to dimensional stability during sintering.

With alloy compositions yielding hardness levels above 150 - 180 HV, it is important that

dimensional changes of the structural parts during sintering are as small as possible and,

even more important, that the scatter of these dimensional changes is kept within the closest

possible limits.

Heat-treating conditions, when applied to sintered steel components, must be especially well

controlled to ensure the highest possible degree of dimensional stability of the component in

the hardening and tempering procedure. Asymmetric cooling during quenching of a sintered

component, especially when of complex shape, may lead to distortions so severe that the part

must either be rejected or subjected to expensive remachining which would wipe out the cost

advantage of P/M technology over conventional production methods.

Dimensional stability of the sintered parts depends on the accuracy with which the above

mentioned parameters can be controlled. Two examples shown at Fig. 9.3 illustrate the

influence of small variations in compact density, sintering conditions and powder composition

on the dimensional changes of powder compacts during sintering. It can be seen from these

examples that intelligently chosen types and amounts of alloying elements can make

dimensional changes of sintered compacts less sensitive to varying processing parameters.

9.2 Alloying Systems, Microstructures and Properties

In the majority of cases, sintered iron and steel components are today made of materials

based on one or the other of the following alloying systems:

• Plain Iron

• Iron - Carbon

• Iron - Copper

• Iron - Copper - Carbon

• Iron - Phosphorus - Carbon

• Iron - Copper - Nickel - Carbon

• Iron - Copper - Nickel - Molybdenum – Carbon

Normally, these materials are mechanical mixtures of plain iron powder with the respective

elements and some lubricant in powder form. Such mixtures can be compacted much more

easily than fully pre-alloyed powders. However, mechanical powder mixtures tend to

segregate when transported and handled. Therefore are many of these materials today

available in the form of partially pre-alloyed non-segregable pressready powder mixes known

under the trade-names Distaloy™ and Starmix (chapter 3).

Microstructures of sintered alloyed steels, produced from powder mixes, are typically much

more heterogeneous than those of conventionally alloyed steels. While carbon diffuses very

rapidly in the basic iron powder and soon reaches equilibrium during sintering, other alloying

elements like copper, nickel, and molybdenum diffuse much slower and would reach

equilibrium only after extremely long sintering times (chapter 6, Fig. 6.9). Hence, produced

under commercially acceptable sintering conditions, these materials will always exhibit a

certain degree of heterogeneity.

Grain size is a parameter which has an important influence upon the physical properties of

plain iron. With decreasing grain size, strength generally increases, but with increasing grain

size, ductility and soft-magnetic properties are improving. Finely dispersed pores have a

greater negative effect on soft-magnetic properties than coarser pores

Plain Iron

9.2.2 Iron - Carbon

A very efficient way to boost tensile strength and hardness of sintered iron is to alloy it with

carbon. Most conveniently, this is achieved by adding graphite powder to the iron powder

before compacting and sintering. Being an interstitial alloying element, carbon dissolves very

rapidly in the iron powder structure during sintering. However, successful sintering of carbon

containing materials requires a very carefully controlled nondecarbonizing sintering

atmosphere

Apart from the presence of pores, these microstructures are practically identical with

those of corresponding conventional plain carbon steels.

9.2.3 Iron - Copper, Iron - Copper - Carbon