Stabilization treatments - relaxation
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
The existence of elastic stresses in a metallic structure is subject to dimensional instability as a result of the evolution of these stresses due to temperature variations and/or by the intervention of operating stresses. The mechanical part thus endowed with residual stresses will be poorly controlled on a dimensional level and will see its operating stress regime modified.
These treatments will therefore be intended to :
Eliminate residual stresses that may arise from:
Machining conditions
Shaping by deformation (folding, stamping, extrusion, spinning, etc.)
Welding assembly
The cooling of the solidification of a cast part.
The manufacturing conditions of the semi-finished product (including for example a straightening)
To guarantee good dimensional stability during use or before subsequent heat treatment (nitriding).
To improve fatigue life by eliminating residual tensile stresses.
How do you do it?
By heating, holding and slow cooling, the temperature is selected according to the material and the holding time is shorter the higher the temperature.
Example:
steels and cast iron : 550 to 650°C / 3 hours minimum
light alloys : 150 to 200 °C / 6 Hours
austenitic stainless steels : 400 to 450°C / 3 Hours
The choice of temperature is generally such that it does not cause any metallurgical transformation. In the case of a stabilization treatment before nitriding, temperature and time must take into account the nitriding conditions temperature and time (calculation of equivalence t - T°).
The treatment medium is chosen according to the nature of the material, the temperature and the desired surface condition after treatment.
It can be air, neutral atmospheres, vacuum.
The thermal relaxation of stresses is due to the weakening of the mechanical resistance characteristics: modulus of elasticity, elastic limit with the rise in temperature. The stresses cease to be elastic as soon as their value becomes higher than the characteristics of the material then a plastic flow occurs relaxing these stresses.
APPLICATIONS
Mechanically welded equipment :
Welding usually induces stresses due to the thermal effects of the welding cycle, which causes alternating expansions and contractions that can be accompanied by the development of stresses if certain welded elements are clamped. The existence of these stresses can contribute to an increase in operating stresses and thus affect fatigue life.
On safety equipment, such as pressure vessels, there are codes specifying the conditions for carrying out these treatments: temperature and time (depending on thickness)
Examples: public works equipment (booms, balancers, etc.), tanks, machine tool frames, gearbox casings.
Castings in graphite iron:
The cooling of the mold must be such that the thermal gradient of cooling is homogeneous. If this is not the case due to too rapid a pickling, unfavorable environmental conditions after pickling, unfavorable geometry, it may be necessary to anneal in order to eliminate residual stress.
Examples: ventilated brake discs, pump bodies, frames.
Relaxation between machining operations:
The search for good mechanical precision can be improved by stabilization annealing between different machining or shaping operations.
The application of a nitriding treatment with temperatures between 500 and 590°C must necessarily be preceded by a treatment of total elimination of the stresses which could be released during the holding for nitriding, before the last machining operation before nitriding. It is advisable, if compatible, to carry out this treatment at a temperature at least 50°C above the planned nitriding temperature. In all cases, it is prudent to estimate the equivalent time-temperature, especially for very long nitriding times.
Implementation
Main equipment (furnace, reactor, line, machine...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
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