Precipitation hardening

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PRECIPITATION HARDENING

Precipitation hardening involves the formation of fine particles (precipitates) that act to block the movement of dislocations and thus strengthen and harden the metal.

It is the principal heat treatment for strengthening alloys of aluminum, copper, magnesium, nickel, and other nonferrous metals.

Precipitation hardening at times strengthen certain steel alloys.

Precipitation hardening
Precipitation hardening: phase diagram of an alloy system consisting of metals A and B that can be precipitation hardened.

When applied to steels, the method is termed maraging (an abbreviation of martensite and aging), and also the steels are called maraging steels.

The necessary condition that determines whether an alloy system are often strengthened by precipitation hardening is the presence of a sloping solvus line.

A composition which will be precipitation hardened is one that contains two phases at temperature, but which might be heated to a temperature that dissolves the second phase.

Heat  treatment
Heat treatment: (1) solution treatment, (2) quenching, and (3) precipitation treatment.

The heat treatment process consists of three steps.

(1) solution treatment, within which the alloy is heated to a temperature Ts above the solvus line into the alpha phase region and held for a period sufficient to dissolve the beta phase.

(2) quenching to a particular temperature to form a supersaturated primary solid solution.

(3) precipitation treatment, during which the alloy is heated to a temperature.

This third step is called aging, and for this reason the full heat treatment is usually called age hardening.

However, aging can occur in some alloys at temperature, then the term precipitation hardening seems more precise for the three-step heat treatment
process under discussion here.

When the aging step is performed at temperature, it’s called natural aging.

When it’s accomplished at an elevated temperature, the term artificial aging is commonly used.

It is during the aging step that top strength and hardness are achieved within the
alloy.

Effect of temperature and time on precipitation hardening
Effect of temperature and time during precipitation treatment (aging): (a) high precipitation temperature; and (b) lower precipitation temperature.

The mix of temperature and time during the precipitation treatment (aging) is critical in bringing out the specified properties within the alloy.

At higher precipitation treatment temperatures the hardness peaks in an exceedingly relatively short time; whereas at lower temperatures, more time is required to harden the alloy but its maximum hardness is probably going to be greater than within the first case.

Continuation of the aging process ends up in a discount in hardness and strength properties, called overaging.

Its overall effect is analogous to annealing.

ADVANTAGES OF PRECIPITATION HARDENING

Relatively low distortion during heat treatment.

Increases yield and tensile strength of most materials.

Increases through hardness.

Enables easier bulk machining to be carried out in the soft solution treated condition, followed by a relatively simple low temperature age/precipitation treatment.

DISADVANTAGES OF PRECIPITATION HARDENING

High capital investment is required.

Skilled manpower is necessary.

APPLICATIONS

Strengthening of metals like aluminum, nickel, stainless steel and titanium.

Hardening gate valves, engine parts, shafts, gears and plungers.

Strengthening balls, bushings, turbine blades, fasteners, molding dies and nuclear waste cracks.

Treating aircraft parts, processing equipment and valve stems.

CONCLUSION

We have covered all the important concepts related to precipitation hardening. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!

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