STEELSTEELSTEELSTEEL

Question 1

What is the use of a rebar in a concrete structural member?

Answer 1

it provides tensile support to the concrete, which is strong in compression byut quite weak in tension.

Question 2

What is the difference between cast iron and steel? compare their properties

Answer 2

Steel has a lower carbon concentration (<1.7%) and cast iron has a higher carbon content. Steel is ductile and has more tensile strength than cast iron, while the latter is more brittle. It is also weak in tension, hence it has little use in construction.

Question 3

What are the effects of carbon concentration on steel?

Answer 3

High carbon concentration provides higher strength and hardness, but is not very weldable. Steel can have low, medium, or high concentration. The medium concentration is still strong but whether it will be ductile or hard will be determined by the heat treatment.

Question 4

name the two types of cast iron and their properties.

Answer 4

Gray cast Iron: slow cooling, excess carbon (free carbon still available)
White cast Iron: Faster cooling, carbon reacts with iron to form more iron carbide (cementite).

Cast iron in general is weak and brittle, not much use in construction

Question 5

name and describe the main elements of the microstructure of steel.

Answer 5

ferrite, pearlite and cementite. Ferrite is the main component of low-carbon steels, it is responsible for the softness and ductility of steel. Pearlite is a mixture of ferrite and cementite. it starts being high in concentration when steel approaches eutectic state. Cementite is high in carbon, it is responsible for the strength and brittleness of the the steel

Question 6

what are ways to strengthen structural steel?

Answer 6

Alloying; introduction of interstitial and substitutionnal atoms in the microstructure.
Strain Hardening: Applying stress/causing strain to increase the yield point of the material.
Heat Treatment: Formation of additional grain boundaries.

Question 7

what are three main goals of alloying?

Answer 7

-strengthening and increasing corrosion resistance by forming solid solutions with iron( carbon)
- additionnal hardness and temperature strength by forming more carbides (titanium)
-increasing machinability by forming an undissolved second phase in steel. (lead)

Question 8

what is smelting?

Answer 8

iron ore is molten under the burnt coal and sinks combined with carbon, making steel. This is also when alloys are added to the mixture.

Question 9

what is heat treatment. describe the different methods

Answer 9

manipulating steel temperature ranges and letting it cool at different rates to achieve desired microstructure and properties.
Full Annealing: Temp in autentite range, slow cooling in furnace - this gives a weaker and more ductile steel due to increased crystal growth.
Normalizing: temp in austentite range but slow cooling in room temp - a bit less ductile compared to full annealing
Hardening: austentite range bit fast cooling. AUstentite doesnt have time to diffuse, therefore smaller crystals and more brittle/strong
Process annealing: lower austentite range, slow cooling - promotes ferrite growth, therefore give a ductile material.

Question 10

what is welding? describe its shortcomings

Answer 10

Welding is a way of joining steel members at a temperature above melting point. This method works best in low carbon steels and leaves a zone affected zone (HAZ), which is cause by variation in microstructure. The HAZ has weaker mechanical properties than the rest of the beam.

Question 11

what is cold-working

Answer 11

its strain hardening in a room temperature (the recrystallization temperature of the steel is way too high). Strain hardening works is that increased amount of dislocations increases the resistance for deformation. This does decrease the ductility and toughness of the material.

Question 12

name all the states of steel throughout the stress-strain curve

Answer 12

1. elastic region
   yield point (proportional limit)
2. uniform plastic deformation
   ultimate stress
3. localized plastic deforemation (necking)
   failure

Question 13

what are the main causes for failure in steel?

Answer 13

fracture, corrosion, fatigue, environmentally-assisted cracking

Question 14

what is a fracture in steel, whats the difference between a ductile and brittle fracture

Answer 14

physical separation of the piece due to an applied. A ductile fracture is cup shaped due to the necking process, brittle steels fail in a straight plane

Question 15

what is toughness?

Answer 15

the ability to absorb energy by deforming plastically before failure, often upon impact

Question 16

what is electrochemical corrosion and give a common example.

Answer 16

electrochemical corrosion is when electrons from atoms at the surface of the steel are transferred to a suitable electron acceptor or depolarizer. A common example is iron (anode) and copper (cathode), reacting with water and oxygen

Question 17

what are three main conditions/ parameters that affect steel’s brittle to ductile behaviour?

Answer 17

temperature (increases -> more ductile), strain rate (increases -> more brittle) and degree of triaxiality (increases -> more brittle) .

Question 18

Why does welding have a strong effect on fatigue strength?

Answer 18

There are heat affected zones that have variation in the microstructure, making it weaker than the rest of the beam. Corrosion processes also occurs way faster when steel is under stress. The geometry and type of the weld lead to stress concentrations and influence the mechanical properties of the weld.

Question 19

when two identical steel members meet at a welded joint, there are three distinct regions within the microstructure. Name and describe them.