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202 FINAL > structural steel > Flashcards

structural steel Flashcards

1
Q

what type of construction material is steel made of

A
  • alloy between carbon and iron
  • can be alloyed with other metal to obtain special properties
  • macroscopic properties based on carbon content
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2
Q

why is structural steel one of the most used construction materials across the world

A
  • high tensile strength
  • range of application
  • wide availability and economy
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3
Q

what are the components during manufacturing process? what does the limestone do?

A
  • iron ore, coal
  • limestone acts as flux to lower the temperature of steel
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4
Q

describe basic smelting

A
  • coal is burned to create “coke”: high temperature highly pure carbon source in oxygen deprived zone
  • iron ore is melted using coke as heat source
  • purer combination of iron and carbon sinks to the bottom
  • slags and impurities are lighter, can be separated
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5
Q

what is coke

A

high temperature highly pure carbon source in oxygen deprived zone

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6
Q

what are the two primary phases at microstructural level of steel

A
  • ferrite and cementite
  • pearlite is a combination of the two
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7
Q

ferrite crystals description

A
  • extremely low carbon content ( less than 0.008%) at room temperature
  • essentially pure iron arranged in crystal form
  • Ductility is primarily derived from this phase
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8
Q

cementite/ iron carbide crystal description

A
  • high carbon content (6.67% at room temperature)
  • due to high carbon content, extremely strong hard and brittle material
  • low ductility
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9
Q

pearlite crystal description

A
  • combination of both cementite and ferrite in composite material
  • primarily consists of alternating thin bands of cementite and ferrite
  • resutls in bi-crystalline phase which has properties of both constituent phases
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10
Q

which are the stable crystals at low temperatures

A

ferrite and cementite

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11
Q

what is austenite

A

a high carbon soluble phase that occurs above 725ºC with a carbon content as high as 2%

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12
Q

the microscopic grain structure of steel is dependent on the carbon content but also what else

A

the cooling process: grain growth takes time

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13
Q

name the different steel grades

A
  • hypoeutectic steel (low carbon)
  • eutectic steel (eutecttic point)
  • hypereutectic steel (high carbon)
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14
Q

describe hypoeutectic steel

A
  • primary phases are pearlite ‘islands’ in a ferrite matrix
  • highly ductile, low strength, good workability, thoughness
  • increasing carbon content: increased formation of pearlite
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15
Q

describe eutectic steel

A
  • poin at which ferrite stops being primary phase and cementite begins to appear in bulk, pearlite is the main crystal
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16
Q

describe hypereutectic steel

A
  • primary phases are pearlite and cementite
  • low ductility, high strength
  • usually requires additional heat treatment
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17
Q

why do we add alloys (additional metals during smelting process)

A

to give additional properties

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18
Q

which alloys are added to add strength to the steel

A
  • alloys combine with carbon to produce additional metal carbides (Tungsten, titanium)
  • alloys combine with iron in solid solution (silicate, cobalt, nickel, chromium) - may offer corrosion resistance
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19
Q

what is heat treatment

A

manipulating steel temperature ranges and controlling the cooling to produce desirable microstructural properties

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20
Q

what is full annealing

A

the temperature is in austenite range, slow cooling in furnce

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21
Q

what happens to mechanical properties when in full annealing

A
  • large crystal growth
  • ductility increased, strength decreased
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22
Q

what is normalizing during heat treatment

A

temperature in austenite range, slow cooling in room temperature

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23
Q

what is hardening during heat treatment

A
  • temperature in austenite range, extremely fast cooling
  • austenite doesn’t have time to diffuse, results in small crystals
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24
Q

what happens to the mechanical properties when in the normalizing phase

A
  • slightly less ductility
  • slightly more strength (vs. full annealing)
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25
Q

what it process annealing during heat treatment

A
  • temperature in lower austenite range, slow cooling
  • promotes ferrite growth
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26
Q

what is the mechanism of strain hardening

A
  • mechanical work applied to the steel
  • applies localized stresses that exceed yield point and enter plastic range (stress-strain curve)
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27
Q

strain hardening function

A
  • involves plastic deformation on level of individual steel grains (ferrite and pearlite)
  • makes grains align with loading
  • increases maximum yield point, decreases ductility
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28
Q

what is welding and how does it work

A

common method of joining structural members
- locally brings steel temperature above melting point and allowing the two metals (base metal and weld metal) to fuse on a microscopic scale
- thermal gradient is established due to the localized heating
- gradient causes variation on microstructural level in the heat affected zone (HAZ)
- low carbon content = better weld

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29
Q

what is the relation between carbon content and weld

A

lower carbon content = better weld

30
Q

what is the heat affected zone (HAZ_

A

area of reduced mechanical properties

31
Q

what affects the durability of steel?

A

electrochemical corrosion

32
Q

explain the mechanism of electrochemical corrosion

A
  • chemical oxidation of metallic iron (ferrite) with water in presence of oxygen creating rust
  • rust causes reduction of cross-sectional are at anode (reduces structural capacity)
  • expansion, flaking off
  • different patterns of corrosion in different environments
33
Q

what are the different types of corrosion

A
  • general
  • pitting
  • galvanic
  • crevice
  • stress
34
Q

what is general corrosion

A
  • electrochemical cells form uniformly in area, causing general corrosion
35
Q

what is pitting corrosion

A

localized are of deep corrosion, usually caused by exposure to specific stimulus (chlorides, arc strikes)

36
Q

what is galvanic corrosion

A

electrochemical cell between metals of different potentials.
- iron can be protected if it is the cathode (sacrificial zinc anodes)

37
Q

what is crevice corrosion

A
  • water and oxygen are trapped due to member geometry
  • causes localized corrosion where water and oxygen are present
38
Q

what is stress corrosion

A

corrosion process accelerated in areas of high stress

39
Q

where is fatigue a concern in steel

A
  • flaws at the microstructural level are present in steel - grain boundaries, HAZ, arc stikes, abrasion, etc
    -in welds, with reduced properties as compared to base steel
40
Q

steps of fatigue

A
  1. Crack initiation: forms from flaw
  2. crack propagation: repeated cycles make crack wider, creating beach marks
  3. Failure: member resistance below loads and brittle failure occurs
41
Q

what are the components in steel other than iron

A
  • carbon (comes from coal)
  • coal
42
Q

what name is it given when there is too much carbon (fusion)

A

cast iron

43
Q

why would we not use cast iron in construction

A
  • very brittle
  • weak in tension
44
Q

what are the three ways to strengthen steel

A
  • heat treatment
  • alloying
  • strain hardening
45
Q

what is the main downside of strengthening steel in connections besides the fact that is has reduced ductility

A

reduced weldability (really hard to weld when the strength is increased)

46
Q

what is the problem with using the alternative connection other than welding and what is it?

A
  • bolting
  • consumes resources so more expensive
  • more time consuming
47
Q

what is toughness

A
  • ability to absorb energy before failure
48
Q

name the most practical practice to measure toughness in steel

A
  • area under the stress-strain curve (most efficient for structural steel)
  • not good for other steels bc structural steel is very ductile
  • cant take area under the curve for brittle material - do impact tests instead
49
Q

explain how strain hardening decreases toughness

A

because toughness is area under the curve and when you increase strain hardening you decrease the area under the curve
- show picture

50
Q

what is hardness

A
  • resistance to strength
51
Q

which is the most to least variable and why ? concrete, timebr, steel

A
  1. timber: grows naturally; has macroscopic defects in it randomly so very variable
  2. concrete: made in the field with unskilled labor, uncontrolled conditions
  3. steel- made in factory with more consistent conditions (very controlled and precise)
52
Q

how do we account for variability in structural design?

A
  • the phi factor for concrete and steel
  • for timber: other design adjustment factor (like 5% inclusion)
53
Q

what is the difference between steel and concrete for the phi factor

A
  • phi concrete = 0.6 or 0.65
  • phi steel = 0.85 or 0.9
54
Q

comment on why the phi factor is higher for steel than it is for concrete

A

steel is less variable

55
Q

lets say we are loading a piece of steel in tension: what is the max permanent strain in the linear elastic region

A

its 0 - no permanent strains since it is elastic

56
Q

what mechanical property can be calculated in this region and how (elastic range(

A
  • elastic modulus by take the slope = stress/strain
57
Q

what is unique about steel in elastic range compared to other materials

A
  • very linear
  • the proportional limit (where it stops being linear) is the same as the yielding stress
58
Q

why is ductility preferred in construction materials like steel

A

warning before failure

59
Q

what is the mechanical property used to indicate failure

A

yielidng

60
Q

why is yielding as failure

A
  • building starts to collapse when you get past yield point (permanent deformation)
61
Q

what is the physical difference between the uniform pastic deformation and localized plastic deformation

A
  • uniform: deforms uniformly: whole thing stretches out (reduced in cross sectional area bc of poisson effect)
  • localized: necking (reduction of cross-sectional area at one specific point which will be the weak point in the specimen)
  • diagram!
62
Q

what is the difference in the failure planes of a ductile steel coupon vs brittle cast iron coupon?

A
  • brittle: 90 so perpendicular from the axis of laoding - fails instantaneously
  • ductile:yields - goes to 45º stretching occurs (sloping) and then deforms
  • diagram
63
Q

what is the difference in the corrosion reaction if the steel is submerged vs exposed

A
  • submerged: only undergoes the first stage of corrosion
  • exposed: all 6 stages of corrosion
64
Q

what reactant is limited in submerged conditions

A

oxygen is limited when underwater

65
Q

which submerged or exposed has more expansion

A

exposed becasue reaction progressives further

66
Q

what does this mean for the concrete surrounding steel in exposed/submerged

A
  • for exposed: more expansion = more cracks called sprawling
  • for submerged: it will lose all its tensile capacities to the reinforced concrete(steel) but the concrete itself nothing will happen
67
Q

why is it a bad idea to use two different grades of steel for bolts and structural members? why does it occur? how can this concept be used to protect steel in concrete

A
  • you will get galvanic corrosion
  • it occurs bc difference in electrochemical potential (more reactive of two metals will act as anode and undergo corrosion)
  • sacrifitional anode approach: a piece of metal with higher chemical potential is attached …
68
Q

why are welded connections the worst are for damage to occur

A
  • high stress regions means more corrosion
  • more crevice corrosion (water between small gaps between metals)
  • low fatigue strength at the heat affected zone
69
Q

name three components of the microstructure at a weld

A
  • heat affected zone
  • weld metal
    -base metal
70
Q

heat affected zone

A

decreases the further you are from weled metal bc change in microstructure?

202 FINAL (7 decks)

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