precipitation / age hardening
what is precipitation hardening?
Precipitation hardening, also known as age hardening, is used to increase the yield strength of specific materials. Hardness of the materials is also increased. Use precipitation age hardening to harden maraging steel or other metals such as aluminum alloy.
Parts to be treated are initially machined in the soft (solution treated) condition before precipitation hardened at relatively low temperatures to produce the required properties. Different conditions can be achieved by changing the time and temperature of the precipitation hardening treatment.
precipitation hardening process
Precipitation hardening processes are critical to achieving the desired strength and hardness properties in metallic alloys. Each treatment step requires careful control to ensure precise control of material parameters.
1. solution annealing
The first step is solution annealing. It serves to reduce differences in the concentration of alloying elements through diffusion, in particular to reduce structural inhomogeneities within individual crystals (microsegregation).
The alloy is heated until all the elements required for precipitation are in solution. The temperature should not fall below a certain level, as otherwise coarse particles will remain, which are detrimental to the mechanical properties of the material. On the other hand, the eutectic temperature of the alloy must not be exceeded, as otherwise areas with accumulations of alloying elements could melt due to segregation.
2. quenching
Diffusion and thus the formation of precipitates can be prevented by quenching. The solid solution remains in the metastable supersaturated single-phase state. This is achieved by cooling at least at the critical speed. Cold water, tempered water, oil or compressed air can be used as the quenching medium.
Many nuclei are formed during quenching.
3. retirement
Subsequent aging at 150 °C to 190 °C (450 °C to 500 °C for maragin steel, the temperature depends on the alloy) accelerates diffusion and the supersaturated single-phase solid solution is transformed into a two-phase alloy through the formation of precipitates:
- the phase that is coherent in volume and generally occurs with a higher proportion is called matrix
- the newly formed phase is called precipitate
The many small precipitates are homogeneously distributed in the microstructure. This allows the properties of the workpiece to be specifically adjusted.
Like the diffusion rate, the type and speed of precipitation depend on the temperature. Nucleation, nucleus growth and precipitate maturation (ageing) can be adjusted.
advantages of precipitation hardening
There are various advantages associated with the use of precipitation hardening. It is important to note that the benefits of precipitation hardening are highly dependent on the specific alloy and process parameters.
Here are some of the benefits:
- Effective prevention of dislocation movement
- Significant increase in strength and yield strength
- Increased resistance to plastic deformation
suitable materials
Materials that are suitable for precipitation hardening are often specially developed – such as maraging steels or PH steel.
It is a process that is often used for aluminum, copper, nickel and some steel alloys.
precipitation hardening in various industries
Precipitation hardening is independent of a specific industry. The decisive factors are the material used and the mechanical properties to be achieved in the subsequent area of application of the component.
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frequently asked questions about precipitation hardening
Which materials are suitable for precipitation hardening?
Precipitation hardening is the most important way of increasing the strength of certain aluminum alloys (aluminum-copper alloys and aluminum-magnesium-silicon alloys), as these do not exhibit polymorphic transformation and therefore cannot be hardened by martensite formation.
What is a yield point?
The yield strength describes the special case of the elastic limit under uniaxial and moment-free tensile stress. The yield point must be determined by means of a tensile test. For many materials, the yield strength cannot be clearly identified in the tensile test or is not pronounced, which is why the 0.2% yield strength is used instead, for example.
How long does the precipitation hardening process take?
Like the rate of diffusion, the type and speed of excretion depend on the temperature. Germ formation, germ growth and excretion maturation (ageing) can be adjusted.
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