ME215 Exam 2

metallic

high strength, luster, good electrical conductivity, luster, high deformability without fracture, high density

nonmetallic

weaker, less ductule, less dense

physical properties

density, melting point, optical properties, thermal properties (specific heat, thermal conductivity, coeff thermal expansion), electrical conductivity and magnetic properties

mechanical properties

response of material to applied forces or loads

static loads

do not vary or vary negligibly

static properties

determined by standardized tests under static loads

proportional limit

initial response is leinear
below this limit the strain is directly proportional to stress

Young's Modulus

ratio of stress to strain (modulus of elasticity, E)

engineering stress

the load divided by original cross section area and elongation divided by original gauge length
S=F/(A0)

Hooke's Law

strain is directly proportional to stress

elastic limit

almost the same as proportional limit, sometimes higher

yield point

beyond elastic limit no proportionality between stress and strain

ultimate strength

maximum load

toughness

work per unit volume to fracture
total area under stress-strain curve
product of yield strength and uniform elongation

ductility

the degree of material deformation without the failure
= percent reduction of area

brittleness

failure with little or no ductility

true stress

sigma=F/A

true strain

natural or logarithmic
epsilon=2ln[(D0)/D]
For cylinder: [L/(L0)]=[(A0)/A]=[(D0^2)/(D^2)]

engineering strain

elongation divided by initial gage length
e=(delta L)/L

strain hardening

When metals are plastically deformed, they become harder and stronger.

compression strength

compression test with is similar to tensile test behavior but more difficult to conduct

damping capacity

ability of the material to absorb mechanical vibrations or damp them out quickly

hardness

ability of the material to resist to plastic deformation
testing: brinell , rockwell, vickers (diamond pyramid), knoop microhardness

Brinell Hardness Test

penetrator (ball of D=10 mm) made of tungsten carbide or hardened steel ball of D=10 mm, load time 10-15 sec

Rockwell Test

two stages of penetration, minor and major

Knoop Test

small diamond penetrator, good for thin samples

Jominy Test

Quenching one end of a one-inch diameter steel bar and then taking hardness measurements along its length

dynamic properties

sudden loads or impacts, repeated cycles of loading and unloading, changes in mode of loading (tension --> compression)

Dynamic tests

bending impacts
tension impacts
fatigue and endurance limit

Charpy test

impact at the center

izod test

impact at the end

fatigue

components fail at less than ultimate tensile strength

endurance limit

(strength)
stress below which material is safe from failing due to cyclic load

creep

failure due to long term exposure to increased temperature

ductile-to-brittle transition temperature (DBTT)

the temperature at which the response of the material goes from high energy absorption to low energy absorption

machinibility

depends not only on worked
material but on applied machining process
-The ease with which a metal can be
machined to an acceptable surface finish
-Require little power to cut, can be cut
quickly, easily obtain a good finish, and do
not wear the tooling much

formability

materials suitability for plastic deformation

weldability

Depends on the specific welding of joining process being considered.

fracture toughness

A quantity that describes the resistance of a material to fracture or a crack to grow.
high value - ductile
low value - brittle

material defects

pores, cracks, inclusions

fracture dynamics

dormant - don't change
dynamic - crack growth rate

phase

a form of material having characteristic structure and properties
-identifiable composition
-definable structure
-distinctive boundaries (interfaces)

continuous phase

ex: air in a room
mixed media

discontinuous phase

ex: salt grains in a shaker
-clear boundary between grains

Solubility I

Two metals completely soluble in each
other

Solubility II

Two metals soluble in liquid state and
insoluble in solid state

Solubility III

Two metals soluble in liquid state and partially soluble in solid state

Austenite

FCC, high formability, high solubility of C, over 2%C can be dissolved in it, most of heat treatments begin with this single phase

Ferrite

BCC, stable form of iron below 912 deg.C, only up to 0.02 wt% C in solid solution and leads to two phase mixture in most of steels

Cementite

(iron-carbide)
stoichiometric intermetalic compound, hard, brittle, exact melting point unknown

Currie point

770 C, atomic level nonmagnetic to magnetic transition

Peritectic

at 1495 deg.C, with low wt% C alloys

Eutectic

at 1148 deg.C, with 4.3wt% C, happens to all alloys of more than 2.11wt% C and they are called cast irons

pearlite

two-phased, lamellar (or layered) structure composed of alternating layers of alpha-ferrite(88 wt%) and cementite(12%) that occurs in some steels and cast irons

heat treatment

controlled heating and cooling of materials for the purpose of altering their structures and properties

annealing

reduce strength and hardness, remove residual stresses, improve toughness, restores ductility, refines grain size, reduces segregation, and/or alters electrical or magnetic properties

full annealing

- controlled cooling in furnace
- time and energy consuming process
- result in identical structures and properties

heated to just above the upper critical temperature, held for a sufficient length of time to fully austenitize the material structure, then allowed to cool at a slow, controlled rate in the furnace. A full annealing provides a relatively soft, ductile material free of internal stresses

hypoeutectoid steels are heated to convert the grain structure to homogeneous single phase austenite, then control cooled resulting in coarse pearlite with excess ferrite giving soft and ductile steel

hypertectoid steels undergo similar process but structure is coarse pearlite with excess cementite

undesirable features of annealing

time consuming, energy requirements, prep time between cycles for reheating

spheroidizing annealing

for high carbon steel > 0.6%C
- all of the cementite is in the form of small spheroids dispersed throughout a ferrite matrix

produces a structure of small globules of cementite dispersed in a ferrite matrix that enhances machinability or formability of high carbon steels

process annealing

for low carbon steel

normalizing

- cooling in air
- cost effective process than annealing
- Properties will vary between the surface and interior

heating steel to a temperature higher than in annealing, held to convert the structure to austenite, then air cooled

used when maximum softness is not required

Solid Solution Strengthening

Base metal dissolves other atoms in solid solution, either a substitutional solution, or as interstitial solutions

Grain size refinement

metals with smaller grains tend to be stronger

precipitation hardening

strength is obtained from a nonequilibrium structure that is produced by a three-step heat treatment

age hardening

• produces coherent precipitate cluster (Solute atoms continue to occupy lattice sites within the
parent structure )
• increase strength and hardness

overaging

• coherent precipitate clusters revert to discrete second phase precipitate particles (solute atoms
have their own crystal structure and distinct interphase boundaries )
• decrease strength and hardness

natural aging

aging at room temp

artificial aging

aging with an elevated temperature

aging

sacrifice toughness and ductility for strength
opposite tempering

tempering

sacrifice strength for ductility and toughness
opposite aging

hardenability

a measure of the depth to which full hardness can be obtained under a normal hardening cycle and is related primarily to the amounts and types of alloying elements

quenchant

medium performing quenching

oil vs water quenching

oil cools slower, reduces quench cracking
rate: brine>water>oil>salt bath
all may cause corrosion except oil

dispersion hardening

Dispersing second-phase particles through a base material

phase transformation

heated to form a single phase at an elevated temperature and subsequently
transform to one or more low temperature phases upon cooling

heat treatment of nonferrous metals

provide a uniform and homogeneous structure stress relief, and induce recrystallization
precipitation hardening is most effective, alloys most become less soluble as temperature decreases

coherency

solute atoms distort or strain lattices

tempering

reheating quenched steel to a temperature bellow the eutectoid temperature then cooling
restores ductility and reduces hardness

Austempering

Quench and hold 15 °C above martensite start (Ms)
temperature until uniform temperature
• at enough time: austenite → bainite

Martempering

Quench and hold 15 °C above martensite start (Ms)
temperature until uniform temperature
• slow cooling through martensite transformation
austenite → martensite

Ausforming

Ausforming is thermomechanical processes in which
deformation and heat treatment are intimately combined
• slowly cool to produce bainite
• rapidly quench to martensite

Martensite transfornmation

Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature

maraging steels

iron alloys known for superior strength and toughness without losing malleability

amorphous metals

- for electrical and magnetic application
• No crystal structure, grains, or grain boundaries
• Magnetic domains can move freely
• Properties are the same in all directions
• Corrosion resistance is improved
requires high cooling rates

Stress-relief anneal

Reduces residual stresses in casting, welded assemblies, and cold-formed products

Materials are heated and then slow cooled

stages of quenching

-formation of vapor jacket (thin gaseous layer between the metal and the liquid during cooling)
-nucleate boiling phase (produces rapid rates of cooling down to the boiling point of the quenchant)
-conduction and convection (slower cooling from the boiling point to room temperature

flame hardening

-Uses an oxy-acetylene flame to raise the surface temperature to reform austenite
-Surface is then water quenched to form martensite
-Tempered to a desired hardness

induction hardening

-Steel part is placed inside a conductor coil and alternating current is used to change the surface of the steel
-Rate and depth of heating can be controlled
-Ideal for round bars and cylindrical parts

laser beam hardening

-Absorptive coatings (zinc or manganese phosphate) are applied to the steel to increase efficiency
- Beam size, beam intensity, and scanning speed are adjustable to affect the depth of heating

electron beam hardening

similar to laser beam hardening, different heat source

Carburizing

diffusion of carbon into FCC austenite steel at elevated temperatures
-gas: hot gas containing carbon surrounds the part
-pack: steel is surrounded by solid part containing carbon
-liquid: steel is placed on molten bath containing carbon

nitriding

hardens surfaces by producing alloy nitrides in certain materials that contain nitride forming elements such as aluminum, chromium, molybdenum, and vanadium

ionitriding

plasma process that places parts in an evacuated furnace and treats them with a direct current potential, nitrogen is then introduced at a low pressure and becomes ionized

ion carburizing

similar to ionitriding except methane is used

plasma process that places parts in an evacuated furnace and treats them with a direct current potential, methane is then introduced at a low pressure and becomes ionized

carbonitriding

introduces carbon and nitrogen to the part surface for surface hardening

deoxidation

Oxygen reacts with deoxidizers and produce solid metal oxide
that are removed from the molten metal or become dispersed throughout the structure

Degassification

reduces amount of dissolved gases in a steel
− vacuum degassing
− Consumable-electrode remelting process
− Electroslag remelting (ESR)

iron

4th most plentiful element in earth's crust
metallic iron must be made my processing raw ore by breaking iron-oxygen bonds resulting in pig iron

steel

strong, rigid, durable
construction and automotive industries use the most
oxidation process removes undesirable elements from pig iron

plain carbon steel

alloy of iron and carbon, other elements present but not in regulated ammounts
strength is function of carbon content