Steels, heat treatment and alloying

Question 1

What is Hardenability

Answer 1

Hardenability is the ability of a steel to form martensite on quenching – the prerequisite for producing a tempered microstructure.

Question 2

What is the Critical cooling rate (CCR)?

Answer 2

Critical cooling rate (CCR) is a possible measure of hardenability – the lower the CCR, the higher the hardenability

Question 3

What is the Critical Diameter Do?

Answer 3

The diameter of the bar, quenched in a given medium, which forms 50% martensite at its centre

Question 4

Effect of Carbon content on hardenibility

Answer 4

• ferrite formation precedes the formation of pearlite.
• hardenability increases as C content increases (curves for bainite/pearlite move to the right)
• martensite start (Ms) and finish (Mf) temperatures fall with increasing C content (reflecting greater strain energy in higher C martensite)

Question 5

Effect of Austenite Grain size on hardenibility

Answer 5

The austenite grain size affects both the rate of nucleation of ferrite and pearlite, and the time taken for growth to complete the transformation.

The larger the austenite grains, the higher the hardenability.

Grain size can typically change the critical diameter by up to a factor of 10 –

Fine grain sizes are desirable in steels as fine-grained structures have higher hardness and toughness.

Hardenability is best obtained by other means (alloying).

Question 6

Effect f Alloying

Answer 6

Eutectoid steel only has sufficient hardenability to form martensite throughthickness In sections of diameter 20-30 mm.

To quench and temper larger sections, we need higher harden ability – this is achieved by alloying.

Substitutional alloying elements such as Mn, Ni, Cr, Mo diffuse slowly in iron, and delay the diffusional transformations from austenite to ferrite and pearlite.

Different solubilities in austenite compared to ferrite. need to redistribute

The CCT curves move to the right, enabling martensite to form with lower cooling rates.

Question 7

Explain Tempering in steels

Answer 7

Martensite is too brittle for use as a bulk microstructure in a component, so it is softened and toughened by tempering.

Tempering is reheating to a temperature below A1 to allow the supersaturated solution of carbon to precipitate as spheroidal Fe3C precipitates in a matrix of ferrite.

A wide range of combinations of strength and toughness can be achieved by varying the temper temperature and time.

In alloy steels, alloy carbides also form during tempering – this is called secondary hardening.

Question 8

What is the equaivalent diameter

Answer 8

The diameter of an infinitey long circular cylinder which, if subjected to the same cooling conditions as the component would have a cooling rate on its axis equal to that at the position of slowest cooling in the component.

Question 9

How does alloying increase cast properties?

Answer 9

Alloying is important in primary steelmaking before casting, to deal with residual impurities, e.g. adding Al to remove oxygen (preventing formation of porosity), or adding Mn to react with sulphur and prevent formation of brittle FeS

Cast irons are inherently castable due to their high carbon content (giving lower melting temperatures). Their as-cast properties are enhanced by alloying additions, e.g. adding Si to give “grey cast iron” (Si promotes the formation of free graphite in cast iron, giving lower strength and better machinability), or adding Ce to give “SG cast iron” (Ce makes the graphite form as spheres instead of flakes, improving toughness).

Question 10

What alloys increase hardenibility

Answer 10

Ni, Cr, Mo etc

Question 11

What alloys increase Weldability

Answer 11

Use 0.1% Ti, V, or Nb

Question 12

What alloying is exploted in tool steels and stainless steels?

Answer 12

(i) Cr, Mn, Mo, Ni, Co and W all dissolve substitutionally in γ and α, without special heat treatment. Solid solution hardening is retained at elevated temperature – this is exploited in tool steels and stainless steels

Question 13

What alloying is exploted in tool steels and low alloy steels?

Answer 13

Ti, Nb, V, Mo, W and Cr all strongly form carbide precipitates – this is exploited in low alloy steels and tool steels.

Metalworking temperatures are too high for plain carbon steels – the tools soften (“running the temper”). Temperatures can only be controlled using slow cutting speeds and a lot of coolant, at higher cost.

Question 14

How are high speed tools made?

Answer 14

High speed steels are alloy steels used for cutting tools which can run at high cutting speeds and temperatures. Typical composition: 1% C, 0.4% Si, 0.4% Mn, 4% Cr, 5% Mo, 6% W, 2% V, 5% Co.

High speed steels are quenched and tempered, with air cooling being sufficient, due to the high hardenability.

On tempering secondary hardening due to formation of alloy carbides occurs.

Question 15

How does alloying increase corrosion resistance in steels?

Answer 15

Stainless steels have high Cr content for corrosion resistance, in combination with other elements to give a wide range of mechanical properties –

Question 16

How does alloying increase steel machinability?

Answer 16

Free-machining steels contain elements such as S that enhance machinability, by promoting the formation of inclusions.

These are usually detrimental in steels due to their damaging effects on toughness and fatigue.

The inclusions promote weakness in the chip shear zone, reducing chip size and reducing cutting forces.

The alloy constituents may also transfer to the tool cutting edge, acting as a tool lubricant.

Question 17

Explain Austenitic Stainless steels?

Answer 17

The majority of stainless steels are austenitic – the commonest being grades 304 and 316 (18% Cr, 8% Ni, and ≤ 0.08% C).

They contain sufficient austenite-stabilising elements, such as Ni, to retain austenite down to room temperature. To minimise susceptibility to “sensitisation” during welding, “L-grade” alloys with especially low C contents are used, e.g. “304L”.

Austenitic stainless steels are not hardenable by quenching and tempering, but the high solute content gives a reasonable yield stress and strong work hardening, combined with very high ductility and toughness.

On cooling below room temperature, martensitic and ferritic steels are characterised by a transition in toughness, from tough to brittle behaviour.

The austenitic stainless steels do not exhibit a toughness transition – they retain toughness at all temperatures, and are the alloys of choice for cryogenic applications (storage of liquefied gas).