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Copy of Precipitation Hardening
Transcript of Copy of Precipitation Hardening
What is it?
Precicipitation hardening of Alloys
1. Solution Treatment:Heated upto 982 ̊C to 1066 ̊C.
Supersaturated solid solution.
2. Quenching: Cooled using air, water or oil.
Faster the cooling rate the finer the grain size and better the mechanical properties.
3. Aging:alloying elements form small precipitate clusters. Metal resists deformation and becomes harder
Precipitation hardening, also called Age hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminum, magnesium, nickel, titanium and some stainless steels.
Factors affecting aging process
Process starts by heating the metal to a very high temperature in order to dissolve the precipitate. It takes anywherew from 1 hr to 20 hrs for the precipitate to dissolve. The new mixture then becomes supersaturated and is ready to be treated further.
Supersaturation of the solution is achieved through quenching. Quenching can be completed in water, air, or some mixture of a air and water. As an important step in solid solution strengthening, it leaves the material softer and more prepared for the next phase of precipitation hardening.
Artificial aging occurs by heating the solution to above room temperature and then allowing it to soak for 2 to 20 hours.
Aging temperature and soaking time depends on required strength of the final product.
Too high of a temperature and too much soaking leads to less precipitates which decreases strength & increases ductility.
Natural aging occurs at room temperature and it takes a relatively long period of time (from several days to several weeks).
Precipitation hardening stainless steels are used in aerospace, defence and offshore oil & gas industries.
For missile components, motor shafts, valve stems, gears and other mechanical components.
They are used for making parts of pressure vessel or a turbocharger.
Silicon is used to manufacture semiconductor materials
Copper is used for a variety of purposes for ship building to roofing.
2. Cold work on a'
3. Alloy composition
4. Impurities and Homogeneity
The 2 main characteristics are high strength and high corrosion resistance.
There are two equations to describe the two mechanisms for precipitation hardening:
Dislocations cutting through particles:
T= ∏ (y) r/ b l
This governing equation shows that the strength is proportional to the radius of the precipitate particles.
Dislocations bowing around particle
T= Gb / (L – 2r)
where T is the material strength
G is the shear modulus
b is the magnitude of the Burgers vector
L is the distance between pinning points
r is the second phase particle radius
They are characterized into one of 3 groups based on final microstructure after HT: martensitic, semi-austenitic & austenitic.
The solubility of CuAl2 in aluminum increased with increasing temperature which is the hardening constituent.
The strongest aluminum alloys are produced by age hardening.
A German engineer, Alfred Wilm began a study to replace the heavy brass alloy with an aluminum alloy for weight savings.
Wilm was the first person to combine both alloying and heat treating in his research thus calling it age hardening.
Paul Merica, Howard Scott and R.G. Waltenberg found that there existed this method of hardening metals, and that Wilm’s alloy was just a single example of this universal behavior.