Autofrettage is a metal fabrication method with a wide range of applications, from the manufacture of gun barrels on battleships and tanks to the manufacture of fuel injection systems for diesel engines. Autofrettage is similar in theory to shot blasting.
During metal fabrication, a pressure vessel is subjected to immense pressure, which causes the internal parts of the vessel to yield, thus resulting in internal compressive residual stresses. The purpose of using the autofrettage process is to enhance the durability of the final product, and also increase its resistance to stress corrosion cracking.
How Does Autofrettage Work?
A single steel tube with internal diameter lesser than the resultant diameter is used for performing the autofrettage method. Immense pressure or autofrettage pressure is used on the tube to expand the bore and to stretch the inner layers of the metal beyond their elastic limit such that the metal can no longer return to its original shape. Autofrettage pressure is much greater than working pressure.
Despite the extreme internal pressure, the outer layers are not stretched beyond their elastic limit. This is because the stress distribution via the walls of the tube is non-uniform. The change only occurs to the metal adjacent to the source of pressure. As the outer layers are still in their elastic state, they tend to return to their original shape, but this is prevented by the permanently stretched internal layers. Now the stretched internal layers are subjected to low temperature heat treatment causing the elastic limit to be further increased.
In case a bent pipe requires the effects of autofrettage, then the method has to be performed after the bending. Similarly, if an object has cross cutting, care should be taken while performing autofrettage process as portions around the drill hole will have stress concentration and tends to crack. However, the crack will only develop to a certain length and stop due to the internal stresses that are inside the wall as the internal stresses press the crack together. In order to receive just the right amount of internal stresses, the pressure of the autofrettage has to be held for a short time.
Two common autofrettage processes that are used are mentioned below:
- Peening of the surface with metallic or ceramic spheres
- Forcibly inserting a mandrel that is larger than the hole, thus straining the inner material layers.
Applications and Products of Autofrettage
The autofrettage process is widely used in various industries. For increasing operating pressures for advanced industrial, automotive, aerospace and defence systems, the application of autofrettage process is required to improve component fatigue life.
Autofrettage is also used for strengthening cannon barrels. Similarly, the process is also used in the expansion of tubular components down hole in oil and gas wells. Meta, a Norway-based oil service company, uses it to connect concentric tubular components with sealing and strength properties outlined above.
More specifically, in the vast field of engineering, the autofrettage process is required to change the inner surface of a container by inducing permanent compression to its inner layers. It is quite similar to pre-stressing concrete for building bridges.
Benefits of Autofrettage
Autofrettage provides the following benefits:
- The working pressure, the pressure that could be held by the autofrettaged components without failure, increases. e.g., with pipes up to 1.8 times higher and with components with cross boring up to 2.5 times higher
- The spreading of the resultant pressure changing numbers in the time solidity area of the “Wöhler line” decreases
- In tests, autofrettage components had a spreading of T<=1,1, while non-autofrettage parts in general have a spreading of T=1,3.
- The sensitivity of the notch is greatly reduced.
The following improvements are achieved due to the collective benefits of autofrettage:
- The working pressure remains the same - the thickness of wall is reduced
- When working pressure remains the same, inexpensive material can be used
- Overall there is the possibility to increase the working pressure
- The finishing of borings and surfaces can be eliminated
- The service life of a component can be increased at a corresponding working pressure.
Sources and Further Reading
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