LECTURE 17

ENGR 151

Materials of Engineering

STEEL CLASSIFICATION

Four-digit number

First two digits indicate alloy content (Ni, Cr)

Last two digits give carbon concentration

1060 (plain carbon steel containing 0.60 wt%

C)

STEEL CLASSIFICATION

HARDENABILITY (HEAT TREATMENT OF STEELS)

Martensite: A microconstituent (phase) formed when austenized iron-carbon alloys are rapidly cooled (or quenched) to a low temperature relative to ambient.

The ability of a steel alloy to be hardened by the formation of martensite as a result of a given heat treatment

Quenching: continuous rapid cooling of an austenized specimen (iron in austenite state) in a quenching medium such as water, oil or air

Impossible to cool specimen at a uniform rate throughout

Surface always cools faster than interior regions

HARDENABILITY (HEAT TREATMENT OF STEELS)

Hardenability: Term used to describe the ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment

Hardenability is NOT “hardness,” rather it is the qualitative measure of the rate at which hardness drops off with distance into the interior of the material due to decreased martensite content

HARDENABILITY (HEAT TREATMENT OF STEELS)

Jominy end-quench test:

Determines hardenability

Cylindrical specimen austenized at specified temp and time

Remove from furnace and mount on fixture

Lower end is quenched by jet of water to room temp

Rockwell Hardness test (hardness v. position)

HARDENABILITY CURVES

Quenched end exhibits

maximum hardness

(100% martensite)

Cooling rates and

hardness decrease with

distance away from

quenched end

Hardness persists much

longer for 4340, 4140,

8640, 5140 steels than

for 1040 steel

SEVERITY OF QUENCH

Term associated with rate of cooling

The more rapid the quench, the more severe the quench

Water (most severe), oil, air

High-carbon steels cannot stand water quench (warping & cracking)

Air quench preferred (pearlitic structure)

Increasing velocity of medium across specimen surface enhances effectiveness of quench

QUENCH RATE

During quenching, heat must be transported from within material to surface to be dissipated into quenching medium

Cooling rate within an throughout interior of a steel varies with position and depends on geometry and size

Cooling rates are greater for locations within specimen closer to the surface

Cooling rate is decreased for oil quench

Smaller the diameter of specimen, higher the quenching rate

QUENCH RATE

Water Oil

RADIAL HARDNESS PROFILE

Water Oil

SPECIMEN SHAPE

Rate of cooling for a particular quenching

method depends on ratio of surface area to

mass of specimen

Larger surface area provides greater contact with

quenching medium

Larger the ratio = more rapid cooling rate = greater

hardening effect

Edges and corners increase ratio

EXAMPLE 11.1 (PG 433)

EXAMPLE 11.1 (PG 433)

EXAMPLE 11.1 (PG 433)

EXAMPLE 11.1 (PG 433)

PRECIPITATION HARDENING

Precipitation hardening: Enhance hardness of

metal alloys by forming small uniformly

dispersed particles of a second phase within

the original phase matrix

Examples: Al-Cu, Cu-Be, Cu-Sn, Mg-Al

(hardened by precipitation)

TYPES OF PRECIPITATION HARDENING

Heat Treatments:

Requirements within

phase diagram:

Require a maximum

solubility of one component

within the other (several

percent)

Solubility limit that rapidly

decreases in concentration

of major component with

temperature reduction

Example: both conditions

satisfied (point M & N)

OTHER REQUIREMENTS

Solution Heat Treating:

Solute atoms are dissolved

to form a single-phase solid

solution

Example:

Heat to To at Co (all β

dissolves) – no structure to β

Cool rapidly to T1 (prevents β

from forming)

Diffusion rates at T1 are slow

(retains alpha phase for long

period of time)

PRECIPITATION HEAT TREATMENT

Supersaturated α solid

solution heated to T2

within α+β region

(diffusion rates greater)

β begins to form in fine

particle evenly dispersed

throughout metal solid

Reduction in strength and

hardness after long

periods of time

(overaging)

MECHANISM OF HARDENING

Precipitation hardening: used with high

strength aluminum alloys (Cu-Al)

96 wt% Al-4 wt% Cu:

Copper atom clusters or “zones” form in small, thin discs

throughout α (1-2 atoms thick, 25 atoms in diameter)

Clusters are not particles since they are so small

Zones pass through two transition phases

Zones become particles as they increase in size

CU-AL PHASE DIAGRAM

CU-AL PHASE DIAGRAM

Desired phase is θ” (overaging associated with

θ’ and θ phases)

Notice the stronger tension bonds created by θ”

presence

CU-AL PHASE DIAGRAM

Notice changes

in yield strength

and ductility

with duration of

heat treatment

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CHAPTER 12: STRUCTURES & PROPERTIES OF

CERAMICS

ISSUES TO ADDRESS...

• How do the crystal structures of ceramic materials

differ from those for metals?

• How do point defects in ceramics differ from those

defects found in metals?

• How are impurities accommodated in the ceramic lattice?

• How are the mechanical properties of ceramics

measured, and how do they differ from those for metals?

• In what ways are ceramic phase diagrams different from

phase diagrams for metals?

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• Bonding: -- Can be ionic and/or covalent in character.

-- % ionic character increases with difference in

electronegativity of atoms.

• Degree of ionic character may be large or small:

ATOMIC BONDING IN CERAMICS

SiC: small

CaF2: large

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CERAMIC CRYSTAL STRUCTURES

Cation – positively charged metal ion

Anion – negatively charged ion – oxide,

halide, etc.

Crystal as a whole must be electrically neutral

Positive and negative charges balanced out

Oxide structures oxygen anions larger than metal cations

Oxygen anions have accepted electrons, metal cations

have given up electrons

close packed oxygen in a lattice (usually FCC)

cations fit into interstitial sites among oxygen ions

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FACTORS THAT DETERMINE CRYSTAL

STRUCTURE 1. Relative sizes of ions – Formation of stable structures: --maximize the # of oppositely charged ion neighbors.

Adapted from Fig. 12.1,

Callister & Rethwisch 9e.

- -

- - +

unstable

- -

- - +

stable

- -

- - +

stable

2. Maintenance of

Charge Neutrality : --Net charge in ceramic

should be zero.

--Reflected in chemical

formula:

CaF 2 : Ca 2+

cation

F -

F -

anions +

A m X p

m, p values to achieve

charge neutrality

• Cations

touching

anions give

stability

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• Coordination Number increases with

COORDINATION NUMBER AND IONIC RADII

Adapted from Table 12.2,

Callister & Rethwisch 9e.

2

r cation r anion

Coord.

Number

< 0.155

0.155 - 0.225

0.225 - 0.414

0.414 - 0.732

0.732 - 1.0

3

4

6

8

linear

triangular

tetrahedral

octahedral

cubic

Adapted from Fig. 12.2,

Callister & Rethwisch 9e.

Adapted from Fig. 12.3,

Callister & Rethwisch 9e.

Adapted from Fig. 12.4,

Callister & Rethwisch 9e.

ZnS

(zinc blende)

NaCl (sodium

chloride)

CsCl (cesium chloride)

r cation r anion

To form a stable structure, how many anions can

surround around a cation?

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COORDINATION NUMBER AND IONIC RADII

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COMPUTATION OF MINIMUM CATION-ANION

RADIUS RATIO Determine minimum rcation/ranion for an octahedral site

(C.N. = 6)

a = 2ranion

 

2ranion + 2rcation = 2 2ranion

 

ranion + rcation = 2ranion anioncation )12( rr =

414.012anion

cation ==r

r

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COMPUTATION OF MINIMUM CATION-ANION

RADIUS RATIO

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COMPUTATION OF MINIMUM CATION-ANION

RADIUS RATIO

34

COMPUTATION OF MINIMUM CATION-ANION

RADIUS RATIO

HOMEWORK

HW (Due Monday, May 1st )

10.6, 10.8, 10.10, 11.28, 11.29