The Science and Technology of Materials in Automotive Engines Part 12 doc

Appendix C: the phase diagramEquilibrium phase diagram: the crystal structure of a metal often changes with temperature.. A steel containing 0.8% carbon transforms from austenite into a

Malleability This term is used when plastic deformation occurs as the

result of applying a compressive load The plastic deformation under atensile load is referred to as ductility

Patenting An isothermal heat treatment applied to medium- and

high-carbon steel wire prior to its final drawing operation This process generatessteel wire having high tensile strength This produces strong wire such aspiano wire Patenting consists of passing the wire through tubes in afurnace at about 970 °C After austenitizing the wire at 970 °C and therapid cooling to 550–600 °C generate a very fine pearlitic microstructure.The resulting ferrite with a fine distribution of carbide has a very highductility and can be cold drawn with total reduction in diameter of 90%.The cold drawn wire may achieve tensile strength levels in excess of 1.6GPa without becoming brittle

Phase A homogeneous portion of a system that has uniform physical and

chemical characteristics

Precipitation A phenomenon in which a crystal of a different phase is

separated from a solid solution and grows

Quenching Operation which consists of cooling a ferrous product more

rapidly than in still air The use of the term specifying the cooling conditions

is recommended, for example air-blast quenching, water quenching, stepquenching, etc

Residual stress A stress that exists inside metal though no external force or

thermal gradient is acting When a heat treatment is carried out, thermalstress or transformation stress due to the difference of cooling rate isproduced inside and outside of the material and these combined remaininside the material as stress The residual stress is also produced by coldworking, welding, forging, etc

Segregation A phenomenon in which alloying elements or impurities are

unevenly distributed, or its state

Single crystal A crystalline solid for which the periodic and repeated

atomic pattern extends throughout the crystal A single crystal does notinclude grain boundaries

Solid solution Homogeneous, solid, crystalline phase formed by two ormore elements

Solution treatment Heat treatment intended to dissolve previouslyprecipitated constituents and retain them in the solid solution

Spheroidal (spheroidized) carbide or globular carbide Carbides in a

globular form

Spheroidizing Geometric development of the carbide particles, such asthe cementite platelets, toward a stable spherical form

Strain ageing An ageing occurring in cold worked materials.

Sub-zero treatment or deep freezing Heat treatment carried out to transform

the retained austenite into martensite after quenching, and consisting ofcooling to and soaking at a temperature below ambient

Science and technology of materials in automotive engines264

Superalloy Alloys capable of service at high temperatures, usually above

1,000 °C Ni and Co alloys are normally included

Supercooling (undercooling) An operation in which metals are cooled

down to the transformation temperature or the solubility line or lower sothat transformation and precipitation may be entirely or partly prevented

Temper embrittlement (brittleness) Brittleness which appears in a certain

quenched and tempered steel, after soaking at a certain temperature orduring slow cooling through these temperatures The primary temperembrittlement produced by tempering at about 500 °C and the secondarytemper embrittlement produced by slow cooling after tempering at evenhigher temperatures are called high-temperature temper embrittlement.The temper embrittlement in the case of tempering at temperatures around

300°C is called low-temperature temper embrittlement

Tempering Heat treatment applied to a ferrous product, generally after

quench hardening, or another heat treatment to bring the properties to therequired level, and consisting of heating to specific temperatures (<Ac1)and soaking one or more times, followed by cooling at an appropriaterate

Toughening An operation in which steel is turned into troostite or sorbite

structure by tempering at a comparatively high temperature (about 400 °C

or higher) after quench hardening This increases the ratio of the elasticlimit to the ultimate tensile strength (yield ratio) This is also referred to

as thermal refining

Transformation A crystal structure is changed into another crystal structure

by the rise or fall of temperature Temperature at which a change of phaseoccurs and, by extension, at which the transformation begins and endswhen the transformation occurs over a range of temperatures

Transgranular fracture Fracture of polycrystalline materials by crack

propagation through the grains

T6 The temper designation system is used for all forms of wrought and cast

aluminum and aluminum alloys except ingot For heat-treatable alloys thefollowing designations are used T1: cooled from an elevated-temperatureshaping process and naturally aged to a substantially stable condition T2:cooled from an elevated-temperature shaping process, cold worked andnaturally aged to a substantially stable condition T3: solution heat-treated,cold worked and naturally aged to a substantially stable condition T4:solution heat-treated and naturally aged to a substantially stable condition.T5: T1 + artificial age T6: solution heat-treated and artificially aged(T4 + artificial age) T7: solution heat-treated and overaged/stabilized.T8: T3 + artificial age T9: T6 + artificial age T10: T2 + artificial age

Appendix A: international standards conversion table

for alloys

Table A.1 compares JIS and other alloy standards Blanks indicate no direct

comparison, although similarities in chemical composition among materialsmay be identified

Appendix B: function analysis table

The engine parts explained in this book have various functions, and thefunction analysis tables used in several chapters examine the function of apart and the associated requirements for materials and manufacture Forexample, the camshaft has to drive accurately to open and close the valves

while rotating at high velocity Figure B.1 analyses the three fundamental

functions required, which include: (i) the camshaft should drive the valveaccurately even at high rotational velocities; (ii) the camshaft itself shouldrotate at high speed without torsion and bending; (iii) camshaft manufacturerequires precision at low cost

The third column lists the means for meeting the requirements of eachfunction, and the fourth column lists the properties required of the materialsused The shape of the camshaft, which is another aspect that must be takeninto consideration in the design and manufacture of camshafts, is not included

in this table

The fifth column lists the materials and material technologies suitable formeeting the functional and property requirements of camshafts These areknown as technological seeds For instance, steel is preferred over aluminumfor the shaft portion because of its higher rigidity Various methods can beused to harden the cam lobe, such as quench-hardening of forged steel orcast iron, carburizing of forged steel, chilled cast iron, remelting of the camlobe portion of the cast iron camshaft or sintering Quench-hardening may

be used for forged steel or cast iron, but remelting is applicable only to castiron, so remelting cannot be used if the designer plans to use a lightweightsteel Generally, a part performs several functions simultaneously, and themechanical designer must choose the most suitable material and technology

on the basis of analysis and experience

Reference

1 Wright I.C., Design Methods in Engineering and Product Design, Berkshire,

McGraw-Hill Publishing Company, (1998) 221.

Required functions

Required functions for materials

Chosen material & technology

Generating accurate valve motion Operating at high rotational velocity Precise shape with less cost

High dimensional accuracy High rigidity to prevent abnormal torsion & bending W

Appendix C: the phase diagram

Equilibrium phase diagram: the crystal structure of a metal often changes

with temperature When a pure metal absorbs a certain amount of anotherelement, it becomes an alloy and the crystal structure will change The phasediagram is a map that shows the variations in crystal structure across a widetemperature range

Figure C.1 is the binary phase diagram of the alloy consisting of iron and

2.09 1420 4.35

γ + Fe3C (or graphite)

Fe3C (cementite) 1013

L + Fe3C 1525

L + graphite

α + Fe3C (or graphite)

C.1 Binary phase diagram consisting of iron and carbon A steel containing 0.8% carbon transforms from austenite into a mixture of ferrite and cementite This is called eutectoid transformation The 0.8% carbon steel is especially called eutectoid steel The

temperature at which the eutectoid transformation takes place is termed the eutectoid point.

The annealed eutectoid steel consists only of pearlite The steels having a higher carbon content above the eutectoid composition are called hyper-eutectoid steels The hyper-eutectoid steels comprise both cementite and pearlite The steels having a lower carbon content below the eutectoid composition are called hypo-eutectoid steels The hypo-eutectoid steels comprise both ferrite and pearlite.

We can roughly judge the carbon content in a steel through

observing its microstructure.

Transformation temperatures change with carbon content Each boundary line at which the crystal structure changes has a particular name A 1 : The horizontal line at 723 ° C (1000 K) Acm: the oblique line between γ and γ + Fe 3 C A 3 : the oblique line between γ and γ + α Also, the transformation temperature shifts a little either in cooling

or in heating To distinguish it, a suffix c is attached in heating, while

r in cooling These are indicated such as Ar or Ac

Science and technology of materials in automotive engines270

carbon The carbon content is shown on the horizontal axis and temperature

on the vertical axis Pure iron is represented on the left (carbon content =0%), and carbon content increases to a maximum of 7% on the right-handside of the diagram

The phase diagram indicates the equilibrium states at various compositionsand temperatures, and is also referred to as an equilibrium phase diagram Inthermo-dynamics, the state of equilibrium is reached when there is no netheat exchange between an object and its surroundings For instance, when aglass of water at 10 °C is placed in a room at 30 °C, the temperature of thewater will rise until it is the same as that of the room This is the equilibriumstate, and it will remain stable unless the temperature of the room is changed.The phase diagram displays equilibrium states on a temperature vs.composition plane For instance, in Fig C.1, an iron containing 4.3% carbon

is liquid at 1,450 °C (indicated by *) Below 1,154 °C, it is solid for allcompositions Crystal structures change in the solid state The boundarylines in the phase diagram separate the different crystal structures The areaenclosed by a boundary line has the same crystal structure throughout

Crystal structures given by equilibrium transformations: Table C.1

summarizes the characteristics of the typical crystal structures shown in Fig

C.1 Ferrite (Fig C.2 (a)) exists in the narrow portion on the left side in Fig.

C.1 Ferrite (α-iron) has a bcc structure where iron atoms (white circles) arearranged as shown schematically in Fig C.2 (a)

Table C.1 Characteristics of typical crystal structures 1

content below 2.06%.This transforms to pearlite below 723 ° C Alloys having this structure are tough, corrosion resistive and paramagnetic.

of carbon (0.02% at 723 ° C, and 0.006% at room temperature) This phase is soft, ductile and ferromagnetic.

Cementite Iron carbide (Fe3C) Hard and brittle iron-compound containing

6.67%C This phase is ferromagnetic at room temperature, while ferrimagnetic above A0 transformation (215 ° C).

precipitates of transformation, comprising ferrite and

There is a vertical line at 6.67%C, which represents the composition of acarbide called cementite Since the ratio FeC of iron to carbon does not

change up to the melting temperature, it is shown by a straight line Cementite

is very hard It raises hardness and strength when dispersed finely in the ironmatrix

The state γ + Fe3C, where austenite and cementite coexist, is stable below1,147 °C The mixed state α + Fe3C, consisting of ferrite and cementite,appears below 723 °C A steel of 0.8% carbon is austenite at 900 °C Itchanges to a mixed state comprising ferrite and cementite below 723 °C.This mixed state is called pearlite

Changes in crystal structure are referred to as transformation Thetransformation of 0.8% carbon steel from austenite to a mixture of ferriteand cementite is referred to as the eutectoid transformation, and 0.8% carbonsteel is frequently called eutectoid steel The temperature at which the eutectoidsteel transforms is termed the eutectoid point Steels with a carbon contentabove eutectoid steel are called hyper-eutectoid steels, whereas steels with alower carbon content are called hypo-eutectoid steels

From the phase diagram, it can be seen that pure iron transforms fromferrite to austenite at 910 °C (allotropic transformation) Figure C.3 showsthe microstructures of irons of various compositions, obtained by etchingpolished iron alloys with acids and viewed under 100 times magnification.Figure C.3 (a) is a typical ferrite of a 0.01% carbon steel Only lineargrain boundaries are observable (see Appendix G) Each grain boundaryseparates single crystals Figure C.3 (b) shows the microstructure of a 0.35%carbon steel, comprising ferrite and pearlite Pearlite is a mixture of ferriteand cementite Pearlite displays a lamellar microstructure similar to aherringbone pattern under microscopy

Figure C.3 (c) shows the microstructure of a 0.8% carbon steel consisting

of pearlite In the region of the mixture of α + γ, the amount of cementite

2.87 nm

(a)

3.63 nm (b)

C.2 (a) Bcc structure of ferrite (b) Fcc structure of austenite.

Austenite ( γ -iron) has a fcc structure The interaction between atoms determines metal structures A metal includes countless crystal lattices comprising such atomic arrangements One lattice has a size

of 3–4 nm The difference in the crystal structure corresponds to the difference in the atomic arrangement.

Science and technology of materials in automotive engines272

increases with increasing carbon content The amount of ferrite inverselydecreases

Coarse grain size in steel lowers impact strength considerably, so it is veryimportant to measure and control the grain size The grain size is adjusted byheating the steel in the austenite temperature region However, since austenitetransforms to ferrite and cementite below 723 °C (Fig C.1), the originalaustenite grain boundary is not observable at room temperature and a differenttechnique is needed to see the austenite grain boundary at high temperatures

50 µ m (a)

C.3 (a) Microstructure of a 0.01% carbon steel Grain boundaries are observable.

(b)

C.3 (b) Microstructure of a 0.35% carbon steel (hypo-eutectoid steel) The white portions are ferrite Pearlite is gray because it is fine.

50 µ m

50 µ m (c)

C.3 (c) Microstructure of a 0.8% carbon steel (eutectoid steel).

Pearlite showing a herringbone appearance containing white ferrite and gray cementite.

The specimen is quenched from the austenite state and etched by a picricreagent Figure C.3 (d) indicates the austenite grains thus revealed

(d)

C.3 (d) Austenite grains.

Cast iron: Figure C.3 (e) is a micrograph of 3% C cast iron (Cast iron

refers to an iron containing carbon content above 2%.) Crystallized graphiteflakes are observable When the carbon content is as high as this, the carbon(see Appendix D) appears as graphite The specimen is not etched, so themicrostructure of the matrix is not observable

Martensite: under slow cooling, austenite transforms into ferrite at 910

°C as shown in the phase diagram The crystal structure changes from that ofFig C.2 (b) to that of Fig C.2 (a) The atoms rearrange into a dissimilar

50 µ m

Science and technology of materials in automotive engines274

configuration through atomic diffusion However, if austenite is cooled rapidlydown to room temperature, the rearrangement of atoms is restricted In thiscase, the transformation illustrated in the phase diagram does not take place.Slow cooling to temperatures below 723 °C transforms austenite into amixture of ferrite and cementite as indicated in Fig C.1 However, rapidquenching using water or oil transforms austenite into a crystal structurecalled martensite, and the transformation shown in the phase diagram doesnot take place Figure C.3 (f) indicates a micrograph of martensite, showing

a needle-like shape Martensite is not shown in the phase diagram because itdoes not appear as an equilibrium phase, it is a crystal structure of anonequilibrium state Martensite is hard and greatly strengthens steels Thisheat treatment is known as quench-hardening (see Appendix F)

Metastable equilibrium phase diagram: the iron-carbon system phase

diagram is sometimes called a double phase diagram because it indicates twotransformations in one diagram One is the equilibrium iron-graphite systemand the other is the metastable iron-cementite system In the equilibriumstate, carbon exists in iron as graphite However, the iron combines withcarbon to form cementite during cooling after solidification It is predictedfrom the iron-graphite system that cementite decomposes into the equilibriumstate of iron and graphite during prolonged heating However, this takesplace very sparingly, and so the cementite phase can exist as an almost stablephase, known as a metastable phase