Some effects of Alloying elements in Steel

Alloying additions are commonly added to steels to;

• increase hardenability,
• improve strength,
• improve mechanical properties (at operating temperature),
• improve toughness for a given strength or hardness,
• increase wear resitance,
• improve magnetic properties.

Increasing the hardenability means that pearlite transformation will be delayed to longer times. This means it is easier to obtain martensite or bainite on cooling, or by isothermal holding after cooling past the pearlite start temperature.

Classification of alloying elements by Bain in The Alloying Elements in Steel

Dissolved in Ferrite

Ni, Si, Al, Zr, Mn, Cr, W, Mo, V, Ti, P, S (?) Cu.

Nickel, silicon, aluminium, zirconia, manganese, chromium, tungsten, molybdenum, vanadium, titanium, phoshorous, sulphur and copper.

Combined in Carbide

Mn, Cr, W, Mo, Nb, V, Ti.

Manganese, chromium, tungsten, molybdenum, niobium, vanadium, titanium.

B

Boron can be present in borocarbides, or as borides.

In Nonmetallic Inclusions

SiO2, MxOy, Al202, etc

ZrO, MnS, MnFeO, MnO, SiO2, CrxOy

VxOy, TixOy, MnFeS, ZrS

Special Intermetallic Compounds

Ni-Si Compound (?), AlxNy, ZrxNy

VxNy, TixNyC2, TixNy

Elemental state

Cu above 0.8%

Pb (?)

The effects of common alloying elements in steel was summarised as follows (data from ‘Metals Handbook’ 1948, American Society for Metals, Metals Park, Ohio.

Al – Aluminium

Solid Solubility

In Gamma Iron (austenite)

1.1 % (increased by C)

In Alpha Iron (ferrite)

36 %

Influence on ferrite

Hardens considerably by solid solution.

Influence on austenite (hardenability)

Increases hardenability mildly, if dissolved in austenite.

Influence exerted through carbide

Carbide forming tendency

Negative (graphitizes).

Action during tempering

-

Principal functions

• Dexodises efficiently.
• Restricts grain growth (by forming dispersed oxides or nitrides).
• Alloying element in nitriding steel.

Cr – Chromium

Solid Solubility

In Gamma Iron (austenite)

12.8 % (20 % with 0.5 C)

In Alpha Iron (ferrite)

Unlimited

Influence on ferrite

Hardens slightly; increases corrosion resistance.

Influence on austenite (hardenability)

Increases hardenability moderately.

Influence exerted through carbide

Carbide forming tendency

Greater than Mn; less than W.

Action during tempering

Mildly resists softening.

Principal functions

• Increases resistance to corrosion and oxidation.
• Increases hardenability.
• Adds some strength at high temperatures.
• Resists abrasion and wear (with high carbon).

Co – Cobalt

Solid Solubility

In Gamma Iron (austenite)

Unlimited

In Alpha Iron (ferrite)

75 %

Influence on ferrite

Hardens considerably by solid solution.

Influence on austenite (hardenability)

Decreases hardenability as dissolved.

Influence exerted through carbide

Carbide forming tendency

Similar to Fe.

Action during tempering

Sustains hardness by solid solution.

Principal functions

• Contributed to red-hardness by hardening the ferrite.

Mn – Manganese

Solid Solubility

In Gamma Iron (austenite)

Unlimited

In Alpha Iron (ferrite)

3 %

Influence on ferrite

Hardens markedly; reduces plasticity somewhat.

Influence on austenite (hardenability)

Increases hardenability moderately.

Influence exerted through carbide

Carbide forming tendency

Greater than Fe; less than Cr.

Action during tempering

Very little in usual quantities.

Principal functions

• Counteracts brittleness from sulphur [by forming MnS sulphides).
• Increases hardenability inexpensively.

Mo – Molybdenum

Solid Solubility

In Gamma Iron (austenite)

~3% (8% with 0.3% C)

In Alpha Iron (ferrite)

37.5% (less with lowered temperature)

Influence on ferrite

Provides age hardening system in high Mo-Fe alloys.

Influence on austenite (hardenability)

Increases hardenability strongly (Mo > Cr).

Influence exerted through carbide

Carbide forming tendency

Strong; greater than Cr.

Action during tempering

Opposes softening, by secondary hardening.

Principal functions

• Raises grain-coarsening temperature of austenite.
• Deepens hardening.
• Counteracts tendency toward temper brittleness.
• Raises hot and creep strength, red-hardness.
• Enhances corrosion resistance in stainless steel.
• Forms abrasion resisting particles.

Ni – Nickel

Solid Solubility

In Gamma Iron (austenite)

Unlimited

In Alpha Iron (ferrite)

10% (irrespective of carbon content)

Influence on ferrite

Strengthens and toughens by solid solution.

Influence on austenite (hardenability)

Increases hardenability mildly, but tends to retain austenite at higher carbon contents.

Influence exerted through carbide

Carbide forming tendency

Negative (graphitizes).

Action during tempering

Very little in small percentages.

Principal functions

• Strengthens unquenched or annealed steels.
• Toughens pearlitic-ferritic steels (especially at low temperature).
• Renders high-chromium iron alloys austenitic.

P – Phosphorus

Solid Solubility

In Gamma Iron (austenite)

0.5%

In Alpha Iron (ferrite)

2.8% (irrespective of carbon content)

Influence on ferrite

Hardens strongly by solid solution.

Influence on austenite (hardenability)

Increases hardenability.

Influence exerted through carbide

Carbide forming tendency

.

Nil

Action during tempering

-

Principal functions

• Strengthens low-carbon steel.
• Increases resistance to corrosion.
• Improves machinability in free-cutting steels.

Si – Silicon

Solid Solubility

In Gamma Iron (austenite)

~2% (9% with 0.35% C)

In Alpha Iron (ferrite)

18.5% (not much changed by carbon).

Influence on ferrite

Hardens with loss in plasticity (Mn Influence on austenite (hardenability)
Increases hardenability moderately.

Influence exerted through carbide

Carbide forming tendency

Negative (graphitizes).

Action during tempering

Sustains hardness by solid solution.

Principal functions

• Used as a general purpose deoxidiser.
• Alloying element for electrical and magnetic sheet.
• Improve oxidation resistance.
• Increase hardenability of steels carrying non-graphitising elements.
• Strengthens low-alloy steels.

Ti – Titanium

Solid Solubility

In Gamma Iron (austenite)

0.75% (1% with 0.2 % C)

In Alpha Iron (ferrite)

~6% (less with lowered temperature)

Influence on ferrite

Provides age hardening system in high Ti-Fe alloys.

Influence on austenite (hardenability)

Probably increases hardenability very strongly as dissolved, the carbide effects reduce hardenability.

Influence exerted through carbide

Carbide forming tendency

Greatest known (2% Ti renders 0.5% carbon steel unhardenable).

Action during tempering

Persistent carbides probably unaffected. Some secondary hardening.

Principal functions

• Fixes carbon in inert particles;
• reduces martensitic hardness and hardenability in medium Cr steels.
• prevents formation of austenite in high Cr steels.
• prevents localised depletion of chromium in stainless steel during long heating.

W – Tungsten

Solid Solubility

In Gamma Iron (austenite)

6% (11% with 0.25C)

In Alpha Iron (ferrite)

33% (less with lowered temperature)

Influence on ferrite

Provides age hardening system in high W-Fe alloys.

Influence on austenite (hardenability)

Increases hardenability strongly in small amounts.

Influence exerted through carbide

Carbide forming tendency

Strong.

Action during tempering

Opposes softening by secondary hardening.

Principal functions

.

• Forms hard, abrasion resistant particles in tool steels.
• Promotes hardness and strength at elevated temperature.

V – Vanadium

1 (4% with 0.2% C)

Solid Solubility

In Gamma Iron (austenite)

Unlimited.

In Alpha Iron (ferrite)

Hardens moderately by solid solution.

Influence on ferrite

Unlimited.

Influence on austenite (hardenability)

Increases hardenability very strongly as dissolved.

Influence exerted through carbide

Carbide forming tendency

Very strong

Action during tempering

Maximum for secondary hardening.

Principal functions

• Elevates coarsening temperature of austenite (promotes fine grain).
• Increases hardenability (when dissolved).
• Resists tempering and causes marked secondary hardening.