Guide 4 Some Important Final Things

8/8/2019 Guide 4 Some Important Final Things

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GUIDE 4 - Some final things

TOPIC 1 MORE ON HEAT TREATED STEELS

1. NORMALISING

Temp°C

50

At 950 °C

Weight % Carbon

910

727

2

0.4 0.8 1.00.1

α-Fe + γ – FeFerrite +

γ – Fe(Austenite)

α-Fe + Fe3C(Ferrite +

α-Fe + γ – Fe(Austenite+cementite)

γ – Fe(Austenite)solidsolutiongrains

A B D50 μm CAt 20 °C

α – Fe(Ferrite)solidsolutiongrains

Coarseparticles of Fe3C (bigger

than 3μm)Pearlite

(layers of α-Fe and Fe3C)

Coarse(excess)

Fe3C

Evolution of steel microstructures

Slow cooling from austenite range

50 μm

SLOW COOLING IN AIR

“Normalising”

Samples A, B & C were tested during LAB 2.

1



2. HARDENING & TEMPERING

50

At 950

γ – Fe(Austenite)solidsolutiongrains

Evolution of steel microstructures“Hardening and tempering”

50 μm

Temp°C

Wei ht %

910

727

200.4 0.8 1.0

0.1

α-Fe + γ – Fe

γ – Fe(Austenite)

α-Fe + Fe3C(Ferrite +

α-Fe + γ – Fe(Austenite+ cementite)

RAPID COOLING (QUENCHING IN OIL

OR WATER)

(“Hardening”)

α’ – FeMARTENSITE – Fesupersaturated with C(microstructure sameregardless of carbon content)

At 20 °C

Tempered at 400°C

Very fine α – Fe (Ferrite) solid solution grains (~1um)

Very fine particles of Fe3C (smaller than 1μm)

A B D C

1 μm

Tempered at 600°C

1 μm

Slightly coarser α – Fe (Ferrite) solid solution grains ~3 um

Slightly coarser and fewer – more widely spaced - particles of Fe3C (smaller

than 2μm)

B* - LAB 2

2



UNDERSTANDING fracture toughnessCONTRAST the energy absorbed from a crack growing through:

(i) Pearlite (layered Fe3C) containing steels– refer to your NOTES !

Sample B - LAB 2

(ii) Hardened and tempered (particle Fe3C particle containing ) steels – refer toyour NOTES ! B* - LAB 2

Sample B* - LAB 2

3



HENCE – WE ACHIEVE THE BEST TOUGHNESS (ENERGY

ABSORBTION – AFTER TEMPERING AT 600°C – SEE LAB 2

DATA FOR SAMPLE B* & page 5)

For all steels – having more carbon in the steel microstructure means:-

1. For Normalised steels more pearlite (Fe3C palates)

2. For Hardened & tempered steels More Fe3C particles

In both cases Yield strength/proof strength, hardness & UTS (σu)

increase as the quantity of Fe3C increases. For normalised steels strength

increasing due to more Fe3C plate formation (pearlite) & ductility

(measured by the strain to fracture – εf ) deceases:- see Drawing 1 below)– also refer to Guide 1. Pure Fe3C & pure solid solution (α-Fe, called

ferrite) also shown here for reference.

1. For Normalised steels

Stress (σ)

Strain (ε)

DRAWING 1

PURE SOLIDSOLUTION α-Fe(C)

PURE COMPOUND, Fe3C

0.4% C steel

0.3% C steel

0.1% C steel

Increasing Fe 3 C plate content & increasing yield strength

Increasing α-Fe (C) content & increasing ductility (strain to fracture)

σy

σy

σy

σy

σy

εf εf εf εf εf

4



For Hardened & tempered steels

l hardened and tempered at different

fracture TOUGHNESS INCREASES (for the reasonsdicated on p.3).

nship/rule

.mm-2)-2

= locking parameter of grain boundaries (N.mm-3/2

)

CONSIDER A 0.4%C stee

temperatures…DRAWING 2

A range of strength, ductility & toughness can be achieved FOR THE

SAME CARBON CONTENT. In this example, the amount (volume) of

Fe3C particles does NOT change, but their size & spacing increases (seep.2) with increasing tempering temperature. Similarly, the ferrite grain

size (in H & T steels), which is always small (a few micrometres)

increases with increasing tempering temperature. Hence strength

decreases (Hall-Petch Relationship/rule) with increasing tempering

temperature, whilein

Hall-Petch Relatio

σ = yield strength of polycrystalline material (MPa/Ny

σ = yield strength of a single crysta0 l (MPa/N.mm )

d = grain diameter (size) - (m/mm)

k

Stress σ

Strain (ε)

DRAWING 2

TENSILE DATA –

Tempered at 400°C

σy

σy

σy

Tempered at 600°Csample B*

Tempered at 700°C

Quench hardenedσy

0.4%C STEELS, TEMPERED AT VARIOUSTEMPERATURES

Increasing STRENGTH

ε ε ε ε

Normalised Sam le B

Increasing α-Fe (C) grain size,increasing spacing between Fe 3 C particles and increasing ductility

σy

2 / 1−.+= d k o y σ σ

5



TOPIC 2 CREEP Deformation in brief

Creep is a concern for materials subjected to constant stress (below the yield strength)

at application temperatures more than about 0.3 times the absolute melting

temperature (>0.3 Tm). Window frames made of plastic deform over a couple of years unless they are reinforced with glass fibres, old fashioned lead lined window

frames and lead tiles show the same effect. More elegant examples include the creep

of Ni and Ti alloy turbine blades and fibre reinforced wind turbine blades.

There are 3 regimes of creep:-

failure

Strain

time

Drawing 1

I II

III

I – primary creep

II – secondary creep

III – tertiary creep

Unless you have designed things completely wrong, most materials find

themselves somewhere in the secondary creep regime.

6



Strain

time

Drawing 2

The arrow shown in drawing 2 indicates the effect on creep rates when

temperature and/or the applied force is increased.

Limiting creep deformation is based on controlling material

micro/macrostructutes.

For alloys this means:

• Maximising the grain size – possibly use single crystals, e.g., some

gas turbine blades

• Controlling the grain orientation to prevent plastic flow

• Increase the stress needed to cause creep deformation by increasing

the compund content of the alloy

• Preventing grain boundary sliding/rotation by causing very fine

(nanometre sized) compound formation on the solid solution grain

boundaries – this causes a type of grain boundary “pinning”.

Also refer to the handout (see pdf file) – circulated earlier this year - on

some research work aimed at developing Mg alloys for creep

applications.

7



TOPIC 3 MORE ON POLYMERS

Here’s a reminder of what happens to the amorphous chain

structure of a simple polymer during tensile testing (no

crystallites in these materials).

POLYMER CHAINS REARRANGED

IN DIRECTION OF APPLIED

STRESS

σσ

RANDOMLY

ARRANGED CHAINS

(NON‐DEFORMED PART OF

TEST‐PIECE)

8



Now consider the same material again taken to the point of

fracture and let’s contrast this with what happens to a semi-

crystalline polymer. Semi-crystalline polymers contain (i) the

usual amorphous chains and (ii) “crystallites”. The latter are

tightly folded chains with a high Young’s modulus (maybe

220GPa) compared to about 1 GPa for the amorphous material.

Here’s what happens to both structures during plastic deformation

(of course this is above Tg when plastic deformation is possible).

Applied shear

Chains & crystallites rotate

in direction of shear Randomly oriented chains &

crystallites

Crystallites

vertically align

to form

“micro‐fibrils”

Fracture

Chains & un‐tangle

in direction of shear

Amorphous polymers

Semi‐crystalline

Applied shear

REGARDLESS OF THE POLYMER TYPE a critical chain length is

required to enable their structures to “hang together” during plastic

deformation – remember the demo/ with the string !

The Young’s moduli of Semi-crystalline polymers can be modelled using

composite rules. In fact they follow ISOSTRESS behaviour (see Guide3):-

⎥⎦

⎤⎢⎣

⎡+⎥

⎦

⎤⎢⎣

⎡=

⎥⎥⎦

⎤

⎢⎢⎣

⎡

2

2

1

11

P

V

P

V

PComposite

Here, P = property or parameter (e.g., hardness, Young’s modulus etc) and the subscripts 1 and 2

refer to phases 1 & 2 or components 1 & 2. So we can rewrite this as:-

⎥⎥⎦

⎤

⎢⎢⎣

⎡+

⎥⎦

⎤

⎢⎣

⎡=

⎥⎥⎦

⎤

⎢⎢⎣

⎡

− cryst

cryst

amor

amor

polcryst semi E

V

E

V

E .

1

9



TOPIC 4 - MORE ON INTERFACES AND

BOUNDARIES

MANY TIMES WE HAVE STATED:

1. Solid solutions are TOUGH

2. Compounds are BRITTLE

FURTHERMORE THE BUILDING BLOCKS OF AN ALLOY ARE:

MIXES of solid solutions and compounds.

WE DISCOVERED IN THE LECTURES THAT

3. Phase boundaries and grain boundaries are very important too !

SUMMARY of the “LATEST” NEWS

Grain boundaries in pure metals and pure (clean) alloys

ARE TOUGH

PHASE boundaries between

(i) Ceramics and metals

(ii) solid solutions and compounds

ARE VERY BRITTLE

This explains WHY compound SHAPE and SIZE is so important in influencing

FRACTURE TOUGHNESS !

SOMETHING NEW Grain boundaries in pure ceramics

ARE ALSO BRITTLE

10



TOPIC 4 MORE ON CERAMICS

CERAMICS AT

ROOM TEMPERATURE BRITTLE

Obey the Griffith’s Equation

GRIFFITH’S

EQUATION

σf = FRACTURE

STRESS (M.Pa)

E = YOUNG'S

MODULUS (G.Pa)

γ = WORK TO

FRACTURE (J/m2

)

c = DEFECT RADIUS (m)

FLAWS/DEFECTS ORIGINATE FROM

PROCESSING

NOTE: WHEN CERAMICS ARE DEFECT FREE σy ~ σf

[σy = YIELD STRENGTH]

11



γ (WORK TO FRACTURE)

FOR CERAMICS γ ~ 10 J/m2

SODA GLASS γ ~ 5 J/m2

FOR METALS γ ~ 103

TO 106

J/m2

(Compare these figures with your own data obtained

from Labs 1 and 2 !)CRACK TIP (METALS)

PLASTIC ZONE ABSORBS ENERGY OF

ADVANCING CRACK

CRACK TIP (CERAMICS)

LITTLE OR NO

PLASTIC ZONE LITTLE ENERGY

ABSORPTION

12



FROM THE VIEWPOINT OF DESIGNING WITH

CERAMICS – MODIFY THE GRIFFITH’S EQ:-

FOR COMPRESSION

FOR TENSION

HISTOGRAM REPRESENTATION – statistics

of fracture for ceramics

Numberof tests

Fracture stress, σf

σf - tension

σf - compression

σf com ression ~ 15 σf tension

13



IN COMPRESSION CRACKS PROPAGATE

STABLY

ON APPLICATION OF

STRESS CRACKS "TWIST-

OUT" OF ORIGINAL

ORIENTATION -

ALIGN PARALLEL TO

AXIS

OF APPLIED STRESS

IN COMPRESSION

FAILURE

PROCEEDS VIA

PROGRESSIVECRUSHING

"CRUSHED

ZONE” FORMS

FOLLOWED BY

COLLAPSE

[NOTE: "c" IN EQUATION 1, WILL BE AVERAGE VALUE

IN THIS CASE]

HENCE, THERE WILL BE A DEFINITE "WARNING”

OF FAILURE

14



IN TENSION FAILURE IS SUDDEN

AND CATASTROPHIC (NO

WARNING)

[NOTE: "c" IN EQUATION 1, WILL BE MAXIMUM VALUE IN THIS

CASE]

15



THE MINIMISATION OF DEFECTS AND

PORES IN CERAMICS

TYPICAL MICROSTRUCTURE OF A CERAMIC

SHOWING GRAINS AND DEFECTS

TECHNICALCERAMICS PRODUCED FROM

POWDERS

TO ACHIEVE ~95-99% DENSITY – TWO MAIN STEPS:

(A) COLD PRESSING

MIX POWDER WITH WAX

COLD PRESS POWDER IN RIGID DIE

(B) HIGH TEMPERATURE DIFFUSION BONDING

"FIRE" OR "SINTER" >1500ºC

16



MINIMISE “c” INCLUDE:-

(1) ONLY ULTRA-FINE (<1µm) POWDERS

(2) "CLEAN ROOM" CONDITIONS

(3) APPLY PRESSURE DURING SEOND STAGE (HIGH

TEMPERATURE)

(a) HOT PRESSING (~ 30 M.Pa)

17





Here is a plot using Equation 2 for three typical Engineering

ceramic materials. As the AMOUNT of porosity decreases E

proportionately decreases…

FROM EQUATIONS 1 AND 2

We can deduce that if we want MAXIMISE the fracture

strength (σf ) of a ceramic we must:

MAXIMISE E

MINIMISE c

MINIMISE P

19



Here’s a typical numerical problem based on the Griffith’s equations

and other relationships given in this short guide on ceramics

An Al2O3 ceramic contains defects (pores) with an average diameter0.96μm and with a density that is 97.5% of the theoretical value, i.e., the

ceramic contains 2.5 volume % porosity. Based on this data and by

applying the appropriate equation(s) determine the fracture strength of

the ceramic in (i) tension and; (ii) compression. (The theoretical value

of the Young’s modulus for Al2O3 ceramic is 400 GPa. Assume the

work to fracture is 10 J/m2.)

Strategy:

FIRST Determine the Young’s modulus (E) of the porous ceramic via

equation 2

E = E0 (1 - 1.9P)

obviously here P=0.025 & E0=400GPa

SECOND Determine the fracture strength of the ceramic in compression.

We are given the average pore daimeter (2c = 0.96 x 10-6

m so

c=0.48x10-6

m). Next all we need do is to rearrange the appropriateversion of the Griffiths’s equation (for compression) and insert the value

of 10 J/m2 for the work to fracture. Take care here to keep the E units in

N/m2.

THIRD Convert your compressive fracture strength into a tensilestrength via the rule of thumb relationship:

σf compression ~ 15 σf tension

SIMPLE !

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Guide 4 Some Important Final Things