Steels

Which steel has the following composition: 0.4%C 0.15%Si 0.02%S 1.3%P 0.9%Cr 0.2%Mo?

Low alloy steel or HSLA

Why is the Mn:S ratio important?

Manganese needs to bind with sulphur

What would the fracture surface of B look like at 50°C?

Shiny, reflective, flat as it would fail in a brittle manner.

Why is martensite tempered?

To increase ductility

Pearlite is made of

ferrite and carbide

You are designing some stamping platens to form hot aluminium and you need a tool steel to make the platens. You would choose

0.6%C, 3%Cr, 0.4%V, 0.4%W,

Difference between 1040 Q&T 350 C and 400 C steels

400 C steel will be tougher (absorb more energy)

Sample: Tensile strength: Elongation to failure A: 896: 16% B: 590: 18% C: 1350: 2% D: 519: 30%

A: Water quenched then tempered B: Normalised C: Water quenched D: Annealed

Why are normalised steels stronger than annealed steels?

Air cooled, therefore have finer pearlite.

(S/S curve) Curve B has higher strain than A, A has larger area under curve. Curve C is a linear line. What material would be the most impact resistant?

Annealed steel (A) Ductility

(S/S curve) Curve B has higher strain than A, A has larger area under curve. Which is normalised and which is annealed steel?

B - normalised (stronger) A - annealed (ductile)

Why is fracture stress not considered a material property?

Because it is determined by the fracture toughness (material property) and the flaw size.

(S/S curve) Curve B has higher strain than A, A has larger area under curve. Curve C is a linear line. Which material is the hardest?

C

Choose an appropriate steel for: - Door hinge - Landing gear - Pump used with sulphuric acid - Equipment used to form gears at high temps

Door hinge: plain carbon steel Landing gear: HSLA steel Pump: Stainless Gears: Tool

(Energy/Temp curve or brittle-ductile transition) Steel A: flat until -25°C, rises until 25°C then flat. Steel B: flat until 50°C, rises to 150°C then flat. Explain graph.

Each steel exhibits a brittle-ductile transition. Below this temp, material is very brittle, as dislocations don't have enough energy to move, therefore absorbing impact.

Manganese is added to steels to combine with the carbon and make the steel more ductile (T/F)

F

Normalised steels have thicker pearlite layers than annealed steels (T/F)

F

Steels which contain very few alloying elements are called tool steels (T/F)

F

The thicker the pearlite, the stronger the steeL (T/F)

F

How are martensitic steels formed?

Fast cooling austenite in oil or water

The main difference between a plain carbon steel and a high strength low alloy (HSLA) steel is:

HSLA steel has extra alloying elements in it, making it more corrosion resistant and to increase the strength

Why must martensite be tempered?

Martensite is too brittle to be used in its quenched state. Tempering regains ductility, reduces residual stresses.

Would you expect steel with composition 0.4%C 0.15%Si 0.02%S 1.3%P 0.9%Cr 0.2%Mo to be ductile?

No. Contains a lot of sulphur. No manganese. Manganese is added to steel to stop sulphur forming a brittle phase (FeS) along grain boundaries.

(Energy/Temp curve or brittle-ductile transition) Steel A: flat until -25°C, rises until 25°C then flat. Steel B: flat until 50°C, rises to 150°C then flat. For Material A, if it was tested at 50°C, what would its fracture surface look like?

Non-reflective, wrinkled, dull as it would fail in a ductile manner.

What form of steel do you need to maximise the strength of a steel?

Normalised Faster cooling rate produces finer pearlite Making the steel stronger

What microstructural feature controls the strength of pearlitic steels and what is this relationship?

Pearlite spacing/thickness. Strength reduces as pearlite gets thicker.

(S/S curve) Curve B has higher strain than A, A has larger area under curve. Curve C is a linear line: what kind of heat treatment has it had?

Quenched.

Why is manganese added to steels?

So iron-sulphur doesn't form

(S/S curve) Steel 1: linear then drop off. Steel 2: short linear. Steel 3: long linear. Which was martensitic steel tempered at 350?

Steel 2. Very little area under curve, and is likely to be very brittle. Tempering at 350 embrittles steel.

(S/S curve) Curve B is a linear line, curve A is linear then drops off. Which is most representative of tempered martensitic steel?

Steel A, as it shows some ductility.

(Energy/Temp curve or brittle-ductile transition) Steel A: flat until -25°C, rises until 25°C then flat. Steel B: flat until 50°C, rises to 150°C then flat. Which steel would be best to resist impact at 0°C?

Steel A.

Which microstructural feature controls the strength of pearlitic steels? What is the relationship between strength and this feature?

Strength is controlled by pearlite thickness. The finer the pearlite, the stronger the steel.

Austenitising is performed to make the composition more homogenous (T/F)

T

Austenitising is performed to make the grain size homogenous (T/F)

T

Cementite and pearlite can exist in the same steel, as separate phases (T/F)

T

Ferrite and pearlite can exist in the same steel, as separate phases (T/F)

T

Martensite is tempered to improve its ductility (T/F)

T

Normalised steels are stronger than annealed steels (T/F)

T

Steels which contain very few alloying elements are called plain carbon steels (T/F)

T

Sulphur makes steel brittle (T/F)

T

(S/S curve) Curve B has higher strain than A, A has larger area under curve. Curve C is a linear line. What heat treatment could be further applied to C to improve its ductility? Describe this heat treatment in terms of temperature and the number of times the heat treatment should be performed. Also state if there are any temperatures which should be avoided when doing this heat treatment and why these temperatures need to be avoided.

Temper. Temps 200-600 (Avoid 350 - embrittles steel) 1-3 times

The tubes for a heat exchanger are made out of a pearlitic steel. During operation, the heat exchanger temperature rises to above 900°C in one section of one tube due to fouling. To quickly reduce the temperature, cold water is pushed through the tube. At a later time, the same tube and the ones nearby suffer an impact event, but only the tube which overheated broke on impact. How could the overheated pipe be heat treated to regain some ductility?

Tempered.

Difference between normalising and annealing?

The cooling rate. Annealed steels are cooled more slowly.

The tubes for a heat exchanger are made out of a pearlitic steel. During operation, the heat exchanger temperature rises to above 900°C in one section of one tube due to fouling. To quickly reduce the temperature, cold water is pushed through the tube. At a later time, the same tube and the ones nearby suffer an impact event, but only the tube which overheated broke on impact. Why did the overheated pipe break and not all the other pipes?

The sudden quenching made the pipe very brittle, thus only the affected pipe was broken.

Why are martensitic steels are double tempered?

To make sure all residual stresses are relieved

If an untempered martensitic steel was tested in impact by using the Charpy Test, describe the fracture surface it would exhibit. If a more ductile normalised steel of the same composition was also tested using the Charpy test, how would its fracture surface be different?

Untempered martensitic fracture surface would be flat and shiny. Normalised steel would have a rough, dull surface.

What is double tempering and why is it done twice?

When the tempering process is applied twice. It's done to ensure that any brittle martensite left on the first temper becomes tempered.

The tubes for a heat exchanger are made out of a pearlitic steel. During operation, the heat exchanger temperature rises to above 900°C in one section of one tube due to fouling. To quickly reduce the temperature, cold water is pushed through the tube. At a later time, the same tube and the ones nearby suffer an impact event, but only the tube which overheated broke on impact. What effect would the sudden heating and cooling have on the pearlitic microstructure?

Would transform the pearlite to maertensite.

Explain the difference between strength and toughness.

Yield strength is the maximum stress the material can carry before plastic deformation occurs. Toughness is the ability of the material to absorb energy.

A stainless steel is a steel with:

at least 12% Cr

More carbon in martensite the _______ the steel

harder

Austenitising is used to ensure the composition and grain size of a steel are both __________

homogenous

A 1040 steel is quenched and then tempered at 500 C. The cooling rate from the quenching temperature must be controlled because otherwise:

impurities will migrate to the grain boundaries and embrittle the steel

The carbon content of plain carbon steels is typically:

less than 0.8%

Annealing is a heat treatment where the steel is left in the furnace to cool. This means that the cooling rate in the furnace is ___________ than in the air and therefore annealed steels have _______ pearlite than normalised steels

slower, coarser

Two 1040 steels are austenitised. One is quenched, other is air cooled. Comparing yield strengths, the quenched steel would be _________ then the air cooled steel

stronger

A steel with the following composition has been rolled: C:0.2, Mn:0.55, P:0.012; S:0.037; Si:0.007, Cu:0.01, O:0.079; N:0.032 and it is found to contain MnS "stringers" stretched out in the direction of rolling. When a tension test is done, you would expect:

the elongation to break to be larger when the crack path is perpendicular to the MnS stringers

Two 1040 pearlitic steels. One, normalised, other annealed. You could tell the difference between them as the annealed steel will be _________

tougher