Seetharaman S. Fundamentals of metallurgy

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11.9 Uniaxial stress±strain curves for a 0.45% C steel (HC45). Test direction

parallell to r olling direct ion (RD). Tests before and after each stand in the

tandem mill. Double tests. Fitted Ludwik model.

11.10 Flow stress in plane strain compression for a 0.45% C steel (HC45) and a

0.75% C steel (HC75). Symbols: experimental points from tensile and

compression tests in different directions and before and after each stand in the

tandem mill.

466 Fundamentals of metallurgy

turned out that the data could be well described by applying two sets of the

classic Lud wik equation. At low strains we expect this behaviour as we have a

two constituent microstructure, ferrite plus pearlite, and the creation of

geometrically necessary dislocations should prevail. At higher strains the

statistically stored dislocations will dominate, giving the lower hardenin g rate.

An interesting feature is that the work hardening is almost identical for both

steels at high strains. The logical explanation is that in both cases deformation

by statistical dislocations in ferrite dominates.

11.5.3 Skin-pass rolling

Speed changes are frequent in skin-pass mills, both in standalone mills and mills

incorporated in continuous lines. A speed change will, in addition to the effect

on the oil film in the roll bearings, also lead to a significant change in the flow

stress of a ferritic steel. One typical example is given in Fig. 11.11. Here it is

very clear that speed changes causes concurrent changes in the roll force. A

comprehensive descrip tion of this can be found in Wiklund and Sandberg

(2002).

11.5.4 Levelling

Roll levelling of sheet subjects the outer fibres of the material to a series of

tensile and compressive strains in the levelling direction of the sheet (often the

rolling direction). The first cycles are used to get sufficient plastic deformation

11.11 Logged data from a skin-pass mill in a continuous annealing line.

Analysing metal working processes 467

into the material in order to decrease variations in the incoming material. The

subsequent cycles, with successively decreasing strains, are then used to

redistribute the internal stresses to a low level across the thickness of the

material. For a high strength steel like DP 600 (dual phase with R

minimum

600 MPa) the strains typically start around 1%. Tests have been performed at

SIMR with fully reversed tensile/compressive testing under strain control. Some

results are shown in Fig. 11.12. As is seen the Bauschinge r effect is significant.

The usual way to model this is to impose a back stress due to the deformation

history. Domkin (2003) has applied one such model combined with a dislocation

based model for work hardening and strain rate and the results are also shown in

the figure.

11.6 Development trends

The material behaviour during an d after metal wo rking processes are

incorporated into process control systems in more detail. Prediction of product

properties can be used to change the set-up of the process, as has been utilised by

VAI-Q (Andorfer et al. 2000) to change coiling temperatures during hot strip

rolling. This is achieved by using the actual temperature after roughing and

calculating the expec ted properties using the initial set-up. Deviations from

11.12 Experimental hysteresis loops and model results for DP 600 at different

strain amplitudes.

468 Fundamentals of metallurgy

target properties are then used to adjust the coiling temper ature. The core of

their system is a set of phy sical±metallurgica l models that depi ct the

microstructural evolution during hot rolling and the relation of microstructure

t o mechanical properties using information of the time±temperature±

deformation sequence (Andorfer et al. 1999).

In summary, the geometry, time and initial temperature together with the

material composition are fed into the model (knowledge-based, physical) and

results in predictions of rolling load and power, shape, microstructure and

mechanical properties. For future uses it has been suggested (Sellars 2000) that

the sequence is reversed, meaning that desired properties (mechanical ,

geometrical, etc.) are used as input and results in a specification of the material,

time, initial temperature, geometry, load and power of the process.

In FE modelling of metal working processes the description of the material

behaviour plays an increasingly important role and more complex material

models are used in simulations (for example, crash simulations, see Engberg and

Carlsson (2002)).

11.7 References

Andorfer J, Auzinger D and Hubmer G (1999) `Two years experience with VAI-Q ± an

online system for controlling the mechanical properties of hot rolled strip', AISE.

Andorfer J, Hribernig G, Luger A and Samoilov A (2000) `Operational experience with

the metallurgical control of the mechanical properties of hot rolled strip', in

THERMEC 2000, Las Vegas, USA.

Ashby MF (1971) `The deformation of plastically non-homogeneous alloys', in Kelly A

and Nicholson RB, Strengthening Methods in Crystals, London, Applied Science

Publ., 137±190.

BergstroÈm Y (1983) `The plastic deformation of metals ± a dislocation model and its

applicability', Reviews on Powder Metallurgy and Physical Ceramics, 2, 79±265.

Bodin A (2002) Intercritical deformation of low-alloy steels, Dissertation, Delft

University, ISBN 90-805661-2-8.

Brown LM and Ham RK (1971) `The deformation of plastically non-homogeneous

alloys', in Kelly A and Nicholson RB, Strengthening Methods in Crystals, London,

Applied Science Publ., 10±135.

Chandrasekaran D (2003) Grain Size and Solid Solution Strengthening in Metals,

Dissertation, KTH, Stockholm, ISBN 91-7283-604-0.

Domkin K (2003) `Physically Based Models of Metal Plasticity', Licentiate thesis, Lulea

University of Technology, 2003: 30.

Engberg G and Carlsson B (2002) `The strain-rate sensitivity and deformation hardening

of ferritic steels', in Int. Conf. New Developments in Sheet Metal Forming

Technology, Fellbach.

Friedel J (1964) Dislocations, Oxford, Pergamon Press.

Frost HJ and Ashby MF (1982) Deformation-mechanism Maps, Oxford, Pergamon Press.

Granlund J (1997) `Structural Steel Plasticity', Dissertation, Lulea

University of

Technology, 1997: 24.

Humphreys FJ (1997) `A unified theory of recovery, recrystallization and grain growth,

Analysing metal working processes 469

based on the stability and growth of cellular microstructures ± I. The basic model',

Acta Mater., 45, 4231±4240.

Karjalainen LP and Perttula J (1996) `Characteristics of static and metadynamic

recrystallization and strain accumulation in hot-deformed austenite as revealed by

the stress relaxation method', ISIJ International, 36(6), 729±736.

Kocks UF, TomeÂ CN and Wenk H-R (1998) Texture and Anisotropy ± Preferred

orientations in polycrystals and their effect on materials properties, Cambridge

University Press.

KoÈmi J, Ruha P, Engberg G, Borggren U, Karlberg M, Abajo Martinez N, LopeÂz J and

Karjalainen P (2004) `The prediction of the mechanical properties of hot-rolled strip

products by means of hybrid methods', Luxemburg, EUR 20939 EN.

Sandberg A and Siwecki T (1985) `Evaluation of microstructure development and rolling

loads in connection with hot strip rolling of CMn-steels and Nb-microalloyed steels',

Stockholm, Swedish Institute for Metals Research, IM-2060.

Sellars M (1997) `Microstructure Modelling in Hot Deformation', in Hutchinson B,

Andersson M, Eng berg G, Karlsson B and Siwecki T, Thermo-Mechanical

Processing in Theory, Modelling and Practice, [TMP]

, Stockholm, The Swedish

Society for Materials Technology, 35±51.

Sellars CM (2000) `State of the art of microstructural modelling', in VAI-Q Symposium,

Linz.

Siwecki T and Engberg G (1997) `Recrystallization Controlled Rolling of Steels', in

Hutchinson B, Andersson M, Engberg G, Karlsson B and Siwecki T, Thermo-

Mechanical Processing in Theory, Modelling and Practice, [TMP]

, Stockholm, The

Swedish Society for Materials Technology, 121±144.

Vanel L, Bourdon G, Depreitere, Uijtdebroeks H, Ablewhite J, DeLaRue T, Rudkins N,

Engberg G and Pettersson K (2004) `Heavy reductions on first stands of a tandem

cold mill', Luxembourg, EUR 20929 EN.

Wang X, Siwecki T and Engberg G (2003) `A Physical Model for Prediction of

Microstructure Evolution during Thermomechanical Processing', in `Thermec

2003', Madrid, part 5, 3801±3806.

Wiklund O and Sandberg F (2002) `Modelling and Control of Temper Rolling and Skin

Pass Rolling', in Lenard JG, Metal Forming Science and Practice, Oxford, Elsevier

Science, 313±343.

Yoshitake A, Sato K and Okita T (1996) `Impact absorbed energy of hat square column in

high strength steels', SAE SP-1172, 960020, 7.

470 Fundamentals of metallurgy

12.1 Introduction

Powder metallurgy (PM) represents a market of about eigh t billion dollars/year

and a production exceeding one million tons/year. Powder metallurgy allows the

production of near-net-shape components, implying material saving, combined

with better properties, which can make PM processes competitive compared to

the conventional casting, forging or machining route.

PM compounds are used in many fields such as energy, automotive, aerospace,

medical indus tries, and for many applications such as electrodes for glass

production (e.g. Mo), wires of lamps (e.g. W), soft magnetic cores (e.g. Fe-Ni, Fe-

Si), tools (e.g. WC-Co), dies (e.g. high speed steel), bearings, gears, brakes,

injection moulds (e.g. stainless steel), gas turbine blades (e.g. superalloys),

medical implants (e.g. Ti-alloys), and aircraft wing weight (e.g. W, ODS). A wide

range of materials can be processed by powder metallurgy such as light alloys (Al,

Mg, Ti), stainless steels, high speed steels, Ni- and Co-based superalloys, ODS

materials, refractory metals, etc. For refractory metals (e.g. W, Mo, Ta) powder

metallurgy is the only economical way of production because of their very high

melting temperatures. Moreover, materials produced by powder metallurgy have a

much finer microstructure (down to nano-scale) and a micro-scale instead of a

macro-scale segregation provided by conventional metallurgy which highly

increases the properties of the PM component.

12.2 Production processes for powders

There are numerous processes, which allow the production of powders with a

wide range of characteristics (composition, size, size distribution, shape,

microstructure and purity). In fact, these characteristics have to be adapted to

subsequent production steps of the component and its application field. For

example, the required particle shape depends on the process used to obtain the

final product. Spherical atomised powders are needed when the powders are

used for thermal spraying, loose powder sintering or hot conso lidation

Understanding and improving powder

metallurgical processes

F LEMOISSON and L FROYEN,

Katholieke Universiteit Leuven, Belgium

(extrusion, isostatic pressing) rather than for cold pressing. In the last case

irregular powders are more suited in order to guarantee a sufficiently high green

strength for handling. Another example concerns mechanically alloyed (MA)

powders: these powders are smaller (particles of a few m with a grain size of a

few nm) than those (10 to 100m) for conventional powder metallurgy

processes and, therefore, special consolidation methods such as hot compaction

followed by hot extrusion, or direct hot extrusion have to be used.

Powder contamination is also an important criterion for many applications.

The amount and the type (inclusions, superficial oxidation, etc.) of con-

tamination depends on the powder production process. It can originate from

initial impurities of the powder, containers, fluids (e.g. solvent) or additives (e.g.

binders), atmosphere, etc.

Powder production processes can be classified in mechanical, atomisation,

physical, chemical and plasma routes.

12.2.1 Mechanical routes

Mechanical routes are mainly used for the production of hard metal and ceramic

powders. They deal with the production of powder by means of four different

forces (impaction, attrition, shear, and compression) involved separately or in

combination.

Impaction produces powder by fracturing the material subm itted

to a high and rapid load (striking of one object by another). Attrition is the

production of particles created by the rubbing action between two bodies. Shear

consists in fracturing particles along their sh ear planes ( e.g. cleava ge).

Compression involves fracture of the particle und er compressive forces

(crushing or squeezing of particulate material).

Under the influence of those forces the powders can undergo fracture,

deformation (work hardening), cold-welding in varying degrees or polymorphic

transformation. This results in particles size reduction (grinding or com-

minution), solid state all oying (mechanical all oying), polym orphic tran s-

formation (mechanical milling), shape change (flaking in the case of ductile

material), mixing or blending of two or more mater ials and contamination.

Grinding, mechanical alloying and mechanical milling will be discussed in

more detail followed by an overview of the different milling technologies and a

particular process called the `Coldstream' process.

Grinding

Particle size reduction by mechanical means is called grinding.

The objective is

to produce a powder with a specific particle size distribution adapted to the

subsequent process steps.

During grinding, particles fracture when the applied stresses exceed the

particle strength globally (massive fracture) or locally (fine wear debris is

472 Fundamentals of metallurgy

considered to be formed by attrition). The rate of particle size reduction in-

creases with frequency of stress application and magnitude of the stress. How-

ever, particle size reduction rate typically decreases during the grinding process,

for exampl e, due to the increase in fracture resistanc e of the smaller particles.

Impact energy depends on the specific mill design. It increases with mill

speed, density and size of milling media (ball, rod, etc.). High impact energy is

required to produce fine powders. However, very high mill speeds could lead to

high wear of the balls inducing high contam ination, excessive heating and lower

powder yields.

The size of the milling media (typically 20:1 for ball:powder diameter) also

influences size, morphology and microstructure of the powder. To produce fine

powders, it is recommend ed to mill in several steps while reducing successively

the milling media size, as one milling step usually induces a particle size

reduction of about factor 10.

An increase in ball to powder ratio increases the impact frequency and the

total energy consumption per second while the average impact energy per

collision decreases. Typical values of this ratio range from 5 to 30. For the

amorphisation of a powder this ratio may approach 100.

Additives (surface-active agents or process control agent (PCA) and

lubricant) are used to nullify autohesion (Van der Waals forces) and so inhibit

agglomeration, to reduce welding (atomic bonding) and/or to lower the surface

tension of the material (proportional to the energy required to create new

surfaces). Their aims are to shorten milling times and/or to produce finer

powders.

1,5

The most widely used PCAs are alcohols, stearic acid and ethyl

acetate.

Small amounts of these additives remain in the powder and are

responsible for contamination in C, H and/or O, which can reach 0.5 to 3 wt%.

Mechanical milling

Mechanical milling leads to polymorphic transformation of the powder. It

comprises high-energy ball milling of uniform (often stoichiometric)

composition powders, such as pure metals, intermetallics, or prealloyed powders

(stainless steel), where material transfer is not required for homogenisation,

contrary to mechanical alloying. For example, mechanical milling of ordered

intermetallics can induce disordering or amorphisation.

Mechanical alloying

Mechanical alloying (MA) is a dry, high-energy ball milling technique. MA can

be used for many different material systems such as metals, intermetallics,

ceramics and composites (cermets, and metal matrix composites).

During MA of powder particles from ductile±ductile or ductile±brittle

material systems, the impact with the milling balls repeatedly plastically deforms

Understanding and improving powder metallurgical processes 473

the powder particles (flattening), creates new surfaces (fracture) and enables the

particles to weld together (rewelding). A process control agent (PCA) (0.5 to 4

wt%) is usually added to allow fracture of the particle and to reduce but not to

eliminate cold welding, as it is one basic mechanism of mechanical alloying.

After a short time of MA, the particles have a characteristic layered structure

build-up from different starting powders (material transfer). With increasing MA

time, the partic les become work hardened and fracture by a fatigue failure

mechanism and/or by the fragmentation of fragile flakes. As MA proceeds, the

internal structure of the particles (grain size, lamellar spacing, etc.) is steadily

refined to nanometer dimensions even if the particle size becomes constant. For

britt le±britt le mate rial systems, smaller and smaller parts of the harder

constituent are incorporated into the less hard one as MA proceeds.

In the 1960s, MA was mainly used to produce ODS (oxide-dispersion

strengthened) materials. These have a high strength at room and elevated

temperatures, as well as excellent oxidation and hot corrosion resistance. The

elevated temperature strength of these materials is due to the uniform dispersion

(with a spacing of around 100 nm) of very fine (5 to 50 nm) oxide particles

, ThO

and La

) in a nickel- or iron-base superalloys.

These

dispersoids inhibit dislocation motion in the metal matrix. It also retards or

inhibits the recovery and recrystallisation processes and so increases the creep

resistance of the alloy.

Moreover, MA allows the production of metastab le phases, such as

supersaturated solid solutions (extension of solid solubility limits), non-

equilibrium crystalline or quasi-crystalline intermediate phases, amorphous

(glassy) alloys, alloying of difficult to alloy elements (e.g. elements which are

immiscible under equilibrium conditions).

5,7

For example solid solutions in the

systems Cu-Fe, AlSb-InSb, Cu-Co have been obtained. MA also favours

chemical reactions at lower temperature (chemical displacement).

The main applications concerns the ODS materials, which are used in the

glass, energy production, and aerospace industries. But as the producing costs

are high, the MA materials are niche materials.

Milling technologies

Different kind of mills are suitable for grinding, mechanical alloying and

mechanical milling such as horizontal mills (tumbler ball mill), stirred mill

(attritor, e.g. Szegvari attrition mill

), planetary ball mill, vibrating mill (tube

vibrating mill, Sweco vibrating mill and shaker vibrating mill (e.g. Spex is a lab-

scale mill

)). Their working princi ples and operating conditions are summarised

in Fig. 12.1 and Table 12.1. The classification on a scale of increasing mill

energy is: horizontal ball mill, attritor, planetary ball mill and vibrating ball mill.

For example, a process that takes only a few minutes in the SPEX mill can take

hours in an attritor and a few days in a commercial tumbler ball mill.

474 Fundamentals of metallurgy

The choice of a milling technique is determined by many parameters. For

example, attrition mills are more efficient than tumbler ball mills for mixing and

blending WC-Co cutting tool powders because of short milling time, production

of fine particle size (submicron sized) and enhanced smearing of Co onto

carbide particles. However, as the product output is relatively low with attrition

mills, tumbler ball mills are usually used for production runs of over 100 kg/day.

Moreover, powder contamination, which is an important criterion for many

applications, can be due to the initial purity of the powder, the milling equip-

ment (design), the milling operating conditions (mill speed, balls size and

material, atmosphere) and/or the use of process control agent. It increases with

milling time, milling intensity and with the reduction of the difference of

strength/hardness between the powder and the milling balls.

12.1 Principle of the different mills: (a) horizontal mills,

(b) Szegvari attrition

mill,

(d) shaker vibrating mill (e.g. SPEX is a lab-scale

mill).

Understanding and improving powder metallurgical processes 475