structural steel Flashcards
what type of construction material is steel made of
- alloy between carbon and iron
- can be alloyed with other metal to obtain special properties
- macroscopic properties based on carbon content
why is structural steel one of the most used construction materials across the world
- high tensile strength
- range of application
- wide availability and economy
what are the components during manufacturing process? what does the limestone do?
- iron ore, coal
- limestone acts as flux to lower the temperature of steel
describe basic smelting
- 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
what is coke
high temperature highly pure carbon source in oxygen deprived zone
what are the two primary phases at microstructural level of steel
- ferrite and cementite
- pearlite is a combination of the two
ferrite crystals description
- 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
cementite/ iron carbide crystal description
- high carbon content (6.67% at room temperature)
- due to high carbon content, extremely strong hard and brittle material
- low ductility
pearlite crystal description
- 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
which are the stable crystals at low temperatures
ferrite and cementite
what is austenite
a high carbon soluble phase that occurs above 725ºC with a carbon content as high as 2%
the microscopic grain structure of steel is dependent on the carbon content but also what else
the cooling process: grain growth takes time
name the different steel grades
- hypoeutectic steel (low carbon)
- eutectic steel (eutecttic point)
- hypereutectic steel (high carbon)
describe hypoeutectic steel
- primary phases are pearlite ‘islands’ in a ferrite matrix
- highly ductile, low strength, good workability, thoughness
- increasing carbon content: increased formation of pearlite
describe eutectic steel
- poin at which ferrite stops being primary phase and cementite begins to appear in bulk, pearlite is the main crystal
describe hypereutectic steel
- primary phases are pearlite and cementite
- low ductility, high strength
- usually requires additional heat treatment
why do we add alloys (additional metals during smelting process)
to give additional properties
which alloys are added to add strength to the steel
- 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
what is heat treatment
manipulating steel temperature ranges and controlling the cooling to produce desirable microstructural properties
what is full annealing
the temperature is in austenite range, slow cooling in furnce
what happens to mechanical properties when in full annealing
- large crystal growth
- ductility increased, strength decreased
what is normalizing during heat treatment
temperature in austenite range, slow cooling in room temperature
what is hardening during heat treatment
- temperature in austenite range, extremely fast cooling
- austenite doesn’t have time to diffuse, results in small crystals
what happens to the mechanical properties when in the normalizing phase
- slightly less ductility
- slightly more strength (vs. full annealing)
what it process annealing during heat treatment
- temperature in lower austenite range, slow cooling
- promotes ferrite growth
what is the mechanism of strain hardening
- mechanical work applied to the steel
- applies localized stresses that exceed yield point and enter plastic range (stress-strain curve)
strain hardening function
- involves plastic deformation on level of individual steel grains (ferrite and pearlite)
- makes grains align with loading
- increases maximum yield point, decreases ductility
what is welding and how does it work
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
what is the relation between carbon content and weld
lower carbon content = better weld
what is the heat affected zone (HAZ_
area of reduced mechanical properties
what affects the durability of steel?
electrochemical corrosion
explain the mechanism of electrochemical corrosion
- 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
what are the different types of corrosion
- general
- pitting
- galvanic
- crevice
- stress
what is general corrosion
- electrochemical cells form uniformly in area, causing general corrosion
what is pitting corrosion
localized are of deep corrosion, usually caused by exposure to specific stimulus (chlorides, arc strikes)
what is galvanic corrosion
electrochemical cell between metals of different potentials.
- iron can be protected if it is the cathode (sacrificial zinc anodes)
what is crevice corrosion
- water and oxygen are trapped due to member geometry
- causes localized corrosion where water and oxygen are present
what is stress corrosion
corrosion process accelerated in areas of high stress
where is fatigue a concern in steel
- 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
steps of fatigue
- Crack initiation: forms from flaw
- crack propagation: repeated cycles make crack wider, creating beach marks
- Failure: member resistance below loads and brittle failure occurs
what are the components in steel other than iron
- carbon (comes from coal)
- coal
what name is it given when there is too much carbon (fusion)
cast iron
why would we not use cast iron in construction
- very brittle
- weak in tension
what are the three ways to strengthen steel
- heat treatment
- alloying
- strain hardening
what is the main downside of strengthening steel in connections besides the fact that is has reduced ductility
reduced weldability (really hard to weld when the strength is increased)
what is the problem with using the alternative connection other than welding and what is it?
- bolting
- consumes resources so more expensive
- more time consuming
what is toughness
- ability to absorb energy before failure
name the most practical practice to measure toughness in steel
- 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
explain how strain hardening decreases toughness
because toughness is area under the curve and when you increase strain hardening you decrease the area under the curve
- show picture
what is hardness
- resistance to strength
which is the most to least variable and why ? concrete, timebr, steel
- timber: grows naturally; has macroscopic defects in it randomly so very variable
- concrete: made in the field with unskilled labor, uncontrolled conditions
- steel- made in factory with more consistent conditions (very controlled and precise)
how do we account for variability in structural design?
- the phi factor for concrete and steel
- for timber: other design adjustment factor (like 5% inclusion)
what is the difference between steel and concrete for the phi factor
- phi concrete = 0.6 or 0.65
- phi steel = 0.85 or 0.9
comment on why the phi factor is higher for steel than it is for concrete
steel is less variable
lets say we are loading a piece of steel in tension: what is the max permanent strain in the linear elastic region
its 0 - no permanent strains since it is elastic
what mechanical property can be calculated in this region and how (elastic range(
- elastic modulus by take the slope = stress/strain
what is unique about steel in elastic range compared to other materials
- very linear
- the proportional limit (where it stops being linear) is the same as the yielding stress
why is ductility preferred in construction materials like steel
warning before failure
what is the mechanical property used to indicate failure
yielidng
why is yielding as failure
- building starts to collapse when you get past yield point (permanent deformation)
what is the physical difference between the uniform pastic deformation and localized plastic deformation
- 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!
what is the difference in the failure planes of a ductile steel coupon vs brittle cast iron coupon?
- brittle: 90 so perpendicular from the axis of laoding - fails instantaneously
- ductile:yields - goes to 45º stretching occurs (sloping) and then deforms
- diagram
what is the difference in the corrosion reaction if the steel is submerged vs exposed
- submerged: only undergoes the first stage of corrosion
- exposed: all 6 stages of corrosion
what reactant is limited in submerged conditions
oxygen is limited when underwater
which submerged or exposed has more expansion
exposed becasue reaction progressives further
what does this mean for the concrete surrounding steel in exposed/submerged
- 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
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
- 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 …
why are welded connections the worst are for damage to occur
- high stress regions means more corrosion
- more crevice corrosion (water between small gaps between metals)
- low fatigue strength at the heat affected zone
name three components of the microstructure at a weld
- heat affected zone
- weld metal
-base metal
heat affected zone
decreases the further you are from weled metal bc change in microstructure?
