Mid term Study Guide

Five Main Strengthening Mechanism

• solution strengthening (solid solution strengthening)

• cold work strengthening (work hardening)

• Grain refinement

• precipitation strengthening (hardening)

• Transformation Strengthening (hardening)

How do each of the strengthening mechanisms make metal stronger?

They increase metal strength by obstructing dislocation motion—-atomic movement creates plastic deformation

Grain size

measure of the average dimension of representative grains in a test specimen

• size of individual grains is determined by how much they grow during solidification before meeting other growing grains

• fine grain size is desirable (smaller grain size) for toughness in quenched and tempered low-alloy steels

Grain Boundaries

• interface between grains that affect the behavior of metals

• most important part of the microstructure

• fine grain structure is better than course grain structure

• at grain structure boundaries the structure is disrupted

how does grain size affect toughness?

• warpage of the crystal structure at grain boundaries blocks the movement of dislocation which makes metal stronger and tougher

  • more of them= stronger tougher metals

How does welding affect grain size?

• the larger the grain size of the base metal the greater chance of HAZ cracking during welding

• HAZ= heat affected zone - describes the area base that change due to the grain growth of the weld

• grain size increase when welded

  • higher the heat input the greater the growth in grain size

Solution Strengthening

the simplest of all strengtheing mechanism

• occurs during manufacturing of the alloy

• no further processing or heat treatment is required

• welding has the least eggect on solution strengthening

• solution alloys are some of the most weldable alloys

solutes

elements being dissolved

solutes

elemnts doing the dissolving or the base alloy

solubility

when one material to dissolve into another

ex: salt in water, if you add a teaspoon of sale in water it will disappear

temperature and solubility

• higher temp increase solubility

• heating water that has crystals in the bottom, the rystals will dissolve and more salt can be added

• when cooled back down the salt crystals form and drop to the bottom

  • solid metals act the same

Solubility and metals

• metal alloying elements are dissolved into base alloys at high temperature

• good solubility=alloying element stays dissolved

• poor solubility= alloy element precipitate out as the metal cools

  • GOAL: in solution strengtheing is to stay below the saturation point

Saturation Point

the maximum amount of solute (such as carbon or alloy elements) that a metal solvent can hold in a solid solution at a specific temperature

Solute types

Substitional Solutes- replace solvent atoms in the atomic structure

interstitial solutes: replace solvent atoms in the atomic structure

How do each distort the atomic structure?

substitutional solutes distorts the crystal structure by the difference in size between the solute atom and the base material atom

Interstitial solutes- distorts the crystal structure by getting between atoms and pushing them apart

• pushing on small atoms in betwen other atoms to create distortion

What are some important substitutional and interstitial atoms in metallurgy?

Substitutional - larger atoms that are chemically similar to the base metal (chromuim, silicon, nickel, maganese)

Interstitial- tiny atoms that are chemically dissimilar to the base metal (boron, carbom, hydrogen, oxygen, nitrogen)

How does solution strengthening strengthen metals?

solution strengthening impedes the movement of dislocation by distorting the crystal structure

• substitutional solutes and interstitial solutes distort the crystal structure in different ways

  • the rougher and more distorted the crystal structure, the stronger the metal will be

Solution Strengthening Lab

tensile testing - most commonly used testing method

• often required by code for welding procedures qualification

• excellent way to determin weld quality

• only test that gives correct information

  • *tensile testing defined: a test piece is gripped at either end in a testing machine which slowly exerts an axial pull fore so the test piece is strecthed until it breaks

  • test provides ueild strength

  • ultimate tensile strength

  • eleongation

  • reduction of area

  • ductility

  • elastilc modules (young modules)

    • toughness

how does welding affect solution strengthening metals?

• welding has the least effect on solution strengthening than any other strengthening mechanisms

• any loss will be due to change in the grain structure or segragation of solutue atoms

• stress concentration from weld bed may reduce strength

• strength loss typically be less than 10%

Structure of Metals

atoms- all mattes is made up of atoms

• smallest unit that can be considered a specific material

• bonds with other atoms that create a molecule

  • atoms are made up of 3 basic subatomic particles: protons, neutrons, and electrons

Molecules

• when atoms combine with one another they form molecules

• can be very simple such as a water molecule

• can be complex containing several atoms

  • can also be extremely complex containing millions or billions of atoms of different kinds such as himan DNA molecules (204 atoms)

Type of atomic bonding

covalent Bonds- atoms share electrons locally (water)

ionic bonds - atoms transfer of electron from one atom to another (salt)

metallic bond - atoms shared in sea of electrons with the whole (iron, aluminum, copper, gold, etc)

Dislocation

• crystals that have defects

  • dislocation reduce the amount of force required to plactically deform a metal the easier to move dislocation the softer and more ductile the metal

  • you can make a metal stronger by impeding movement of dislocation

    • every strengthening mechanism works by impeding the movements of dislocation

Crystal Structure of Metals

• most metals exist as a crystalline structure or lattice at room temp.

• metal atoms are bonded with metallic bonds

• Pure iron at room temp. exist as (bcc) body center cubic

  • cubic face centered - soft, ductile, stretchable heat iron, it changes structure

State of Matter (4 Types)

Solid - bound toher by strong forces (tight/dosen’t change shape

liquid- bound togehter by weaker forces (takes shape of contour)

gas- molecules bound together with very weakforces (moves in all directions, tough to control)

Plasma- inoized gas (increase or decrease in electrons) (welded using oxygen)

• all metals exist in solid state at room temp

• mercury is the only metal that exist as liquid at room temp

  • some rare metals melt at slightly above room temp

Metal Solidifcation

Metal grain formation

• neutleation- as liquid metal solidifies small crystals (dendrties) begin to grow in the liquid

Dentrite Growth

• liquid materials continues to solidify

• dendrites grow larger

Grain Boundaries

• are the interface between grains; at grain boundries the structure is disrupted

• most important part of the mircostructure

• in almost every situation fine grain structures is better than coarse grain structure

eventually the dendrites grow together to form crystals

*grain boundaries begin to form

Metal solidification process

• when completely solidified, all dentrites have grown together to make continuous grain structure

• entire process can take a fraction of a second in a weld to hours in a large ingot

castings

generally have large grains which varies in size from one area of casting to another

Dentrites

tree like structure of crystals grow as liquiid metal solidifies (also called necleation)

Dentrites growth: liquid material continues to solidify, dentrites grow larger

Weld Grain Structure

Welds simply small casting

• welds exhibits a dendritic mircostructure like castings

  • the base metal is relatively cool, grains start to grow there and grow toward the center of the weld until they meet near the middle called Columnar grains

Equiaxed or Equal Axis - Grain Shape

grains that are approximately the smae dimension in all directions are called equiaxed or equal axis

Elongated or non-equiaxed Grain shape

grains that are not the same dimension in all directions

• metal forms that have had a large amount of deformation after solidification such as sheet, plate extrusions, forging will have non- equiaxed grains

Grain Isotropy

Equiaxed grains have similar properties in all directions (isotropy)

non- equiaxed grains have different poperties in different directions (anisotrophy)

Grain Anisotrophy

produces property anisotrophy

• property anistrophy means that the material will have different properties such as strength, toughness, and corrosion resistance depending on the direction you test it

  • metals have the greatest strength and toughness when tested so the stress is going with the grain

Grain direction

are determined by the elongation of the grains due to rolling, forging, stretching etc.

Longitudinal grain direction

strongest, best, corrosion resistance

Long Transverse

2nd strongest, 2nd best corrosion resistance

Short Transverse

weakest, worst, corrosion resistanve

Cold Work strengthening

• work hardening is the oldest strengthening mechanism

• metals get stronger as you work them relativelt low temperatures

• work hardening occurs during plastic deformation

• plastic deformation occures when atom slips over each other on specific planes

  • these planes are called slip planes

Elastic Deformation

is not permanent

• the part will return to it’s original shape after the load is removed

• stong brittle metals deform more elastically

  • knife balde

  • springs

  • titanium

  • tool steel

Plastic Deformation

Is permanent

• the part will not return to its original shape after the load is removed

• plastic defromation is called “worl"k”

• weak soft metals deform more plastically

  • copper plumbing

  • lead

  • gold

  • carbon steel

  • hot metals

Hot Work vs Cold Work

• work can be done “hot” in hot forging or hot rolling

• Work can be done “cold” as in cold forging or cold rolling

• work hardening or strengthening occurs only during cold working

Strengthening Mechanism

• in perfect crystals slip occurs along a plane over the entire crystal simultaneously

• as metal plastically derforms, each plane slider over another

• as they slide they rotate so the planes are in the direction of stress

Dislocation

• typically crystals are not perfect

• crystals have defect called dislocation

• dislocation reduce the amount of force requried to plastically deform a metal

• the easier it is to move dislocation the softer and more ductile the metal

• you can make a metal stronger by impeding the movement of dislocation

• every strengthening mechanism works by impeding the movement of dislocation

Too Many Dislocations leads to:

• as you plastically deform a metal you move the dislocation and also create dislocation

• this increases dislocation density

• the more plastic deformation the higher the dislocation density

  • at some point dislocation stop making it easier to plastically derform metal and start making it harder

Dislocation Pinning

dislocation distort the slip planes

• as dislocation density increses, the crystal structure becomes so distorted the dislocation can no longer move

How does cold work strengthening strengthen metals?

• as dislocation get pinned, plastic deformation becomes more difficult

• the more you plastically deform, the more dislocation you create and the stronger the material gets

• work hardened materials can be much stronger than annealed materials

Specifying Cold Worked Materials

Austenitic stainless steels

non-heated treatable aluminum

copper alloys

nickel alloys

many solution strengthened alloys are also cold work strengthened

• work hardening is often listed as a fraction of full hard or as % cold work

• As strength goes up, ductility goes down ( on exam)

Annealing

• many alloys derive their strength from work hardening

• however, in some instances work hardening is not desirable

• if parts are heated to a high enough temperature, the atoms will rearrange and eliminate the dislocations

  • this effectively wipes the slate clean

Grain Refining

• grain size has many effects on a metal properties

• as grain size gets smaller, strength, toughness, and weldability increase

• therefore, small grain size is desirable

What are the three ways to refine grain size?

chemically- grain refining additions during manufacture of the metal

heat treatment- heating to just above the recrystallization temperature in materials that undergo a phase transformation or have cold work

cold work- cold rolling during processing of the metal into shapes

Chemical Grain Refinfig

• oxides of titanium, aluminum, niobium, vanadium and boron are added to produce and/or retain small grains

• they produce small grains during solidification by many small particles for grains to nucleate on

  • the more nucleation sites the finer the grains

• they keep grains small by physically pinning grains and keeping them from moving at high temps

Heat Treatment

• grains in a metal can be refined by heating the metal to just above the recrystallization temperature

• if a metal undergoes a crystal structure change or has cold work in it, the grains recrystallize and get smaller

  • the grains are refined as the metal cools

• if metal are heated to a considerably higher temperature than the recrystallization tempertaure grains grow large

  • the longer the metal is above this temp. the larger the grains grow

Cold Work Refining

• cold working metal causes grains to get smaller

• grains break up and form new grain boundaries

• this happens primarily in the first few percent of cold work

What are the effects of grain size on strength?

• grain boundaries distort the crystal structure

• the dislocation impedes the movement of dislocation

• the more grain boundaries you have the stonger the material will be

What are the effect of grain size on toughness?

• cracks propagate along preferred crystallographic planes

• these planes are not align betwen grains

• every time a crack crosses a grain boundary it has to change direction

• it takes more force to make a crack change direction

• the more grain boundaries you have in a crack’s way the harder it is for that crack to propagate

What are other effects of grain size on touchess?

• cracks that have a long straight paths along preferred planes run easily

• cracks that have to cross many grain boundaries and change direction require alot of force to propagate

• fine grains increases strength and toughness

How does welding affect cold work strengthening metals?

• the heat from weld effectively eliminates all work hardening and grain refining in the weld and heat affected zone

• work hardened and grain refined metals are softened by weling

• failure will occur in the heat affected zone

• the metal exhibits full soft properties in the weld and heat affected zone

• the strength will gradually ramp up to full base metal properties

• the higher the heat input the greater the distance it takes to get to full properties