9-Alloying in Steel

Alloying in Steel

IntroductionLimitation of plain carbon steel

►They cannot be strengthened beyond 10.000 psi, without significant loss in toughness and ductility

► Large section can’t be made with martensitic structure throughout, and thus are not deep-hardenable

►Rapid quench rates necessary for full hardening in medium-carbon, plain carbon steels to produce a martensitic structure. This rapid quenching leads to shape distortion and cracking of heat-treated steel

►Plain carbon steel have poor impact resistance at low temperature

►Plain carbon steels have poor corrosion resistance for many engineering environments

►Plain carbon steels oxidize readily at elevated temperature

Objectives of Alloying

► Improving mechanical properties by increasing the depth to which a steel can be hardened (hardenability) allows advantage of tempered martensitic throughout allows slower quench

►To allow higher tempering temperatures while maintaining high strength and good ductility

► Improving strength at room temperature► Improving mechanical properties at high and low

temperatures► Improving corrosion resistance and elevated-

temperature oxidation► Improving special properties such as abrasion resistance

and fatigue behavior► Improving magnetic property

Alloying Elements Based on Their Function

►Ferrite stabilizer/former Stabilize ferrite so it can be stable at higher

temperatur Cr, Si, Mo, W dan Al

►Austenite stabilizer/former Stabilize austenite so that it can stable at lower

temperature even at room temperature Ni, Mn

►Nitrides former Forming nitrides layer during nitriding All elements of carbide former, Cr, W, Mo, V, Ti, Nb,

Ta, dan Zr

Alloying Elements Based on Their Function

►Carbide former Forming carbide phase in steels improves

hardness, wear resistance, Cr, W, Mo, V, Ti, Nb, Ta, and Zr

►Carbide stabilizer Stabilize carbide so that it can be stable at

particular temperature or particular phase, not easily dissolves in particular phase Co, Ni, W, Mo, Mn, Cr, V, Ti, Nb dan Ta

Behavior of Alloying ElementsDissolved in ferrite

►Improving hardness and strength by solid-solution hardening

►Elements: Cr, W, V, Mo, Ni, Mn, Si►In reality, the contribution of the elements

on the final hardness is relatively low

Behavior of Alloying ElementsForm carbides

► Improves hardness, strength and wear resistance. ►Complex carbides that is dissolved in

austenite(carbide forming elements) are strong deep hardening element

►Undissolved complex carbides reduce grain growth and decrease hardenability

►Hardness and wear resistance of an alloy are determined by number, size, and distribution of the carbide particles

►The effect of carbide former can be controlled via chemical composition, manufacturing methods, and heat treatment

Behavior of Alloying ElementsDistribution of alloying elements

Effect of Alloy Elements in SteelSpecific effects

►Boron, improves hardenability without loss of, or even with some improvement in, machinability and formability

►Calcium, deoxidizes steels, improves toughness, and may improve formability and machinability

►Carbon, improves hardenability, strength, hardness, and wear resistance. Reduces ductility, weldability, and toughness

►Cerium, controls the shape of inclusion and improves toughness in high strength. Low-alloy steels, deoxidizes steels

►Chromium, improves toughness, hardenability, wear and corrosion resistance, and high-temperature strength; increases depth of hardness penetration in heat treatment by promoting carburization

►Cobalt, improves strength and hardness at elevated temperatures


► Copper, normally added in amounts of 0.15 to 0.25% to improve resistance to atmospheric corrosion and to increase tensile and yield strengths with only a slight loss in ductility. Higher strength properties can be obtained by precipitation hardening copper-bearing steel, adversely affects hot working characteristics andsurface quality

► Lead, improves machinability. Causes liquid metal embrittlement. It is usually added in amounts from 0,15% to 0,35%.

► Magnesium, has the same effects as Cerium► Molybdenum, improves greatly to hardness and increases

toughness. Molybdenum tends to help steel resist softening at high temperatures and is an important means of assuring high creep strength. It is generally use in comparatively small quantities ranging from 0.10 to 0.40%.

► Manganese, deoxidizer; improves hardenability, strength,abrasionresistance and machinability. Reduce hot shortness; decreases weldability. Manganese is usually present in quantities from 0.5% to 2%, but certain special steels are made in the range of 10% to 15%.


► Nickel improves strength, toughness, and corrosion resistance; but is one of the least effective elements for increasing hardenability.

► Niobium (Columbium) imparts fine grain size, improves strength and impact toughness; lowers transition temperatures; may decrease hardenability

► Phosphorus improves strength, hardenability, corrosion resistance, and machinability; severely reduces ductility and toughness

► Selenium improves machinability► Sulphur, an important element in steel because when present

in relatively large quantities or combined with manganese, it increases machinability. Lowers impact strength, impairs surface quality and weldability. The amount generally used for this purpose is from .06 to .30%. Sulphur is detrimental to the hot forming properties.


►Silicon improves strength, hardness, corrosion resistance, and electrical conductivity; decreases magnetic hysterisisloss, machinability, and cold formability. Silicon is one of the common deoxidizers used during the process of manufacture. It also may be present in varying quantities up to 1% in the finished steel and has a beneficial effect on certain properties such as tensile strength. It is also used in special steels in the rage of 1.5% to 2.5% silicon to improve the hardenability. In higher percentages, silicon is added as an alloy to produce certain electrical characteristics in the so called silicon electrical steels and also finds certain applications in some tool steels where it seems to have a hardening and toughening effect.

►Tantalum has effects similar to those of niobium►Tellurium, improves machinability; formability; and

toughness


►Titanium improves hardenability; deoxidizes steels. Titanium is added to 18-8 stainless steels to make them immune to harmful carbide precipitation. It is sometimes added to low carbon sheets to make them more suitable for porcelain enameling.

►Tungsten used as an alloying element in tool steel and tends to produce a fine, dense grain and keen cutting edge when used in relatively small quantities. When used in larger quantities of 17 to 20% and in combination with other alloys, it produces a high speed steel which retains its hardness at the high temperatures developed in high speed cutting. Tungsten is also used in certain heat resisting steel where the retention of strength at high temperatures is important. It is usually used in combination with chrome or other alloying elements.


►Vanadium improves strength, toughness, abrasion resistance and hardness at elevated temperaturres.Vanadium, usually in quantities from 0.15 to 0.20% retards grain growth, even after hardening from high temperatures or after periods of extended heating. Tool steel containing vanadium seem to resist shock better than those which do not contain this element do.

►Zirconium has the same effects as Cerium

Effect of Alloy Elements in SteelOn phase diagrams

►Austenite stabilizer elements (Ni, Mn) decrease eutectoid temperature

►Ferrite stabilizer elements (Cr, Si, Mo, Al, W) increase the eutectoid temperature

►Consequently those elements above will modify the phase diagram (area of austenite & ferrite)

Effect of Alloy Elements in SteelOn phase diagrams

Effect of Alloy Elements in SteelOn ferrite phase

►Elements that are dissolved in ferrite will improve its hardness and strength

Effect of Alloy Elements in SteelOn transformation diagram

►Alloying elements tend to displaced the position of transformation curve to the right

►Some elements also tend to lower the position of transformation curve

►Separating pearlite and bainite nose►Lowering Ms and Mf temperature►All the effects mentioned above improve

hardenability►Creating “knee” area on CTT diagram, which

support bainitic transformation►Elements: Ni, Cr, Mn, Mo, V

Effect of Alloy Elements in SteelOn transformation diagram

Effect of Alloy Elements in SteelOn grain size

►Fine carbide or nitride particles that are dispersed uniformly in the grain will impede grain growth smaller grain size improved hardness and strength

►Elements: V, Ti, Nb, Al.

y = 335 MPa y = 540 MPa

Effect of Alloy Elements in SteelOn tempering

►Hindering softening (decrease in hardness) rate during tempering

►High amount of carbide complex elements (Cr, W, Mo, V) will increase hardness at specific tempering temperature (secondary hardening)

9-Alloying in Steel

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