NED University of Engineering And Technology

Aging or Precipitation Hardening By

Uzair Bin Asim

Age hardeningPrecipitation hardening, also called Age hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some stainless steels. In superalloys, it is known for providing excellent high temperature strength.

•A process in which small particles of a new phase precipitate in matrix which harden material.

• Also known as “Age Hardening” because hardness often increases with time (even at room temperature!)

The Method

• The precipitation hardening heat treatment involves the following stages:

1.Solution treatment2.Quenching3.Aging

Solution treatment• During solution treatment a part is heated to a

temperature above the solvus temperature in order to dissolve the second phase in the solid solution.

• All the solute atoms dissolves to form a single phase solid solution.

• The part is held at this temperature for a time varying from 1hour to 20 hrs. until the dissolving has been accomplished.

• The temperature and the soaking time of solution treatment should not be too high to prevent excessive growth of the grains.

QUENCHING

Quenching• Quenching is carried out in water, water-air

mixture or sometimes in air. • Object of the quenching operation is obtaining

super saturation solid solution at room temperature.

• Since the second phase retains dissolved at this stage, hardness of the quenched alloy is lower than after age precipitation, however higher than hardness of the alloy in annealed state.

Precipitation Heat Treatment (Aging)

• Depending on the temperature at which this operation is carried out aging may be artificial or natural.

• Artificial aging.• At this stage the part is heated up to a temperature below the solvus

temperature, followed by soaking for a time varying between 2 to 20 hours.

• The soaking time depends on the aging temperature (the higher the temperature, the lower the soaking time).

• The aging temperature and the soaking time are also determined by the desired resulted combination of the strength and ductility of the alloy.

• Too high aging temperature and time result in overaging – decrease of the strength and increase of ductility.

• Natural aging.• Natural aging is conducted at room temperature and it takes a relatively

long period of time (from several days to several weeks).

Let’s age it !!!!

overaging

• The precipitates, however, continue to grow. The fine precipitates disappear. They have grown larger, and as a result the tensile strength of the material decreases. This is called overaging.

The conditions for an alloy to be age hardened

Effect of age hardening on physical properties

Hardness vs. TimeThe hardness and tensile strength vary during aging and overaging.

Effects of Temperature

Characteristics of a 2014 aluminum alloy (0.9 wt% Si, 4.4 wt% Cu, 0.8 wt% Mn, 0.5 wt% Mg) at 4 different aging temperatures.

Alloys • Precipitation hardening heat treatment is commonly used

for the following alloys: • Aluminum alloys: Al-Cu, Al-Mg-Si, Al-Cu-Mg, Al-Zn.• Copper alloys: beryllium bronze,aluminum bronze,

aluminum-nickel bronze, chromium copper.• Stainless steels: iron-chromium-nickel alloys with additions

of copper, aluminum, titanium and niobium. The precipitation hardening stainless steels are either austenitic or martensitic.

• Nickel alloys: nickel-copper alloys with additions of titanium, aluminum and iron.

• Titanium alloys: titanium-aluminum alloys, titanium-vanadium alloys.

Applications

Aluminum-2014

Aluminum-2024

Aluminum-7075

Copper

Nickel

Steel

Aluminum rivets Alloys that experience significant

precipitation hardening at room temp, after short periods must be quenched to and stored under refrigerated conditions.

Several aluminum alloys that are used for rivets exhibit this behavior. They are driven while still soft, then allowed to age harden at the normal room temperature.

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Reference• Heat Treatment Technologies by Dr. Dmitri Kopeliovich

• Phase transformations in metals and alloys, D.A.Porter, & K.E. Easterling, Chapman & Hall.

• Materials Principles & Practice, Butterworth Heinemann, Edited by C. Newey & G. Weaver.

• Mechanical Metallurgy, McGrawHill, G.E. Dieter,3rd Ed.

• Hull, D. and D. J. Bacon (1984). Introduction to Dislocations. Oxford, UK, Pergamon.

• Courtney, T. H. (2000). Mechanical Behavior of Materials. Boston, McGraw-Hill.

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