A brief summary on Stress-relief Annealing

-> here is a more detailed write-up on stress-relief annealing

Whenever work pieces distort during heat treatment or when cracks/fissures appear, we might face the result of internal material stress.

Internal material stress may be due to:

• uneven cooling-down after casting (over the cross-section of the part)
• mechanical processing
• cold forming
• hot forming
• heat treatment
• welding

Internal stress can be reduced by annealing
At 450 °C already internal stress is approx. halved Customary are temperatures between 550 °C and 650 °C. The annealing temperature needs to be sufficiently low to avoid material structure changes but at the same time needs to be below the temperature used during any quenching that might have taken place.
It is of paramount importance that the work piece obtains the very same temperature throughout and cools down slowly in the furnace avoiding significant cross-sectoral temperature variations.

Stress-relief annealing always takes place shortly before final machining operations.
Internal material stress
Internal material stress exist in the work piece totally without external influences. Consequently there are sectors within the work piece with internal tensile stress that are in balance with sectors that have compressive internal stress.

Internal material stress may build up during any time of the work piece manufacturing.
Temperature differences and varying forming grades are the reason for the stress.
High cool-down rates with castings during shrink- and during size-reduction processes, during heat treatment and while welding unavoidably lead to internal stress. The stress increases with the wall thickness variation and the work piece complexity.
Varying forming grades during cold working as well as during hot working cause more or less strong internal stress. Mechanical machining too, causes stress at least in the outer regions of the work pieces.


More Details on Stress-relief Annealing

Stress-relief annealing is defined as an annealing method at temperatures below Ac1 followed by slow cooling-down with the aim to reduce material stress. Material structure changes are not intended.
The characteristics of the material treated will not be altered significantly.

Inner material stress superimposed by externally applied loads may lead to unwanted changes of the work piece shape (distortion) or may even result in broken parts.
In case such difficulties need to be expected - after cooling-down of castings or after hardening - we recommend that stress-relief annealing takes place soon after the stress has built up. Even more so if cracks/fissures are to be expected.

With most materials both the strength and the maximum material yield point are reduced with increasing temperatures. Consequently stress-relief annealing always means a thorough exposure to sufficiently high temperatures. The annealing duration ought to be 1 to 2 hours measured from the point when the parts have obtained the furnace temperature.
To play it safe the maximum stress-relief annealing temperature needs to be 20 °C to 30 °C below the annealing temperature.

Slow warming-up, an even temperature saturation, slow cooling-down and keeping the optimized annealing temperature are the criteria for good stress-relief annealing.
Warming-up to the stress-relief annealing temperature ought to be that slow that the yield point at elevated temperatures is evenly obtained right across the entire cross-section (depending on the type of steel and its composition). That is especially true for the most endangered temperature region of up to 200 °C and for work pieces with a low ductility or with relatively brittle zones, such as cast iron, welded parts, hardened - and here surface hardened in particular - work pieces.

Cooling-down from the stress-relief annealing temperature is the most important process stage because any deviation from the required procedure might not only lead to a lessened stress-relief but might even intensify the stress compared with the original state. That even holds true when the other steps - such as warming-up, letting the work pieces saturate evenly with the heat, holding the temperature - have been adhered to.
A cooling-down speed of 50 to 100 K/h is to be recommended..

Work pieces made of roller bearing steel after hardening are commonly annealed at a temperature of 150 °C to 180 °C for 1 to 2 hours. The aim is to reduce stress, to improve the ductility and to stabilize the material structure while keeping dimensional stability.
Higher annealing temperatures or longer annealing times would result in a unacceptable drop in hardness and thereby in a reduced load support ability of the bearing parts.

Nitrion GmbH does stress-relief annealing under vacuum if requested. That way no scaling or grain boundary oxidation needs to be expected.

Here is a longer and more detailed version of the above summary on stress-relief annealing

Whenever work pieces distort during heat treatment or when cracks/fissures appear, we might face the result of internal material stress.

Internal material stress may be due to:

• uneven cooling-down after casting
  (over the cross-section of the part)
  
• mechanical processing
  
• cold forming
  
• hot forming
  
• heat treatment
  
• welding
  

Internal stress can be reduced by annealing
At 450 °C already internal stress is approx. .halved Customary are temperatures between 550 °C and 650 °C. Even at a temperature of 450°C, the internal stress is halved. The stress-relief temperature needs to be far enough under Ac1, so that there are no changes to the matrix. Quenched parts must always be annealed at a temperature below that employed in quenching. The annealing temperature needs to be sufficiently low to avoid material structure changes but at the same time needs to be below the temperature used during quenching that might have taken place.
It is of paramount importance that the work piece obtains the very same temperature throughout and cools down slowly in the furnace avoiding significant cross-sectoral temperature variations.

Stress-relief annealing always takes place shortly before final machining operations so that there will be no fresh stress being brought into the work piece..

Internal Material Stress
Internal material stress exists in the work piece totally without external influences. Consequently there are sectors within the work piece with internal tensile stress that are in balance with sectors that have compressive internal stress.

Internal material stress may build up during any time of work piece manufacturing.
Temperature differences and varying forming grades are the reason for the stress.
High cool-down rates with castings during shrink- and during size-reduction processes, during heat treatment and while welding unavoidably lead to internal stress. The stress increases with the wall thickness variation and the work piece complexity.
Varying forming grades during cold working as well as during hot working cause more or less strong internal stress. Mechanical machining too, causes stress at least in the outer regions of the work pieces.

More Details on Stress-relief Annealing
Stress-relief annealing is defined as an annealing method at temperatures below Ac1 followed by slow cooling-down with the aim to reduce material stress. Material structure changes are not intended.
he characteristics of the material treated will not be altered significantly.

Inner material stress superimposed by externally applied loads may lead to unwanted changes of the work piece shape (distortion) or may even result in broken parts.
In case such difficulties need to be expected - after cooling-down of castings or after hardening - we recommend that stress-relief annealing takes place soon after the stress has built up. Even more so if cracks/fissures are to be expected.

Stress within the work piece can only be released by a plastic deformation in the micro sector. That requires the material yield point to be lowered below the stress total.
The further the material yield point is reduced below the stress level the higher the plastic deformation level gets with the consequence of stress-relief.
With most materials both the strength and the maximum material yield point are reduced with increasing temperatures. Consequently stress-relief annealing always means a thorough exposure to sufficiently high temperatures. The annealing duration ought to be 1 to 2 hours measured from the point when the parts have obtained the furnace temperature.

There is a restriction with quenched and tempered steel, however: the stress-relief annealing temperature is limited in high by the annealing temperature that has been applied.

To play it safe the maximum stress-relief annealing temperature needs to be 20 °C to 30 °C below the annealing temperature. After such a treatment one has to assume a high remaining stress level whenever a relatively low annealing temperature had to be used. Should the remaining stress level exceed a certain limit, then only a steel selected for a higher hardening level can be the remedy due to the ability to operate with a higher annealing temperature. Other steels also do not permit a maximum temperature to be exceeded if the material strength is not to be endangered. Cast iron for example ought not to be annealed at above 550 °C. Generally speaking the lower the annealing temperature the longer one needs to anneal. Steels that increase in strength by precipitation, such as micro-alloyed fine grain steels of higher basic strength also require careful strees-relief annealing. The temperature range of 530 °C to 580 °C ought not fallen short of ( insufficient stress-relief) nor should it be exceeded (worsening of mechanical properties by the precipitation process being influenced).

Even with such sensitive steels sufficient stress-relief without the appearance of cracks can be obtained by observing the correct annealing technology. The technology is based on the height of the remaining stress. The annealing duration and the temperature is to be adjusted according to the Hollomon parameter P.

For micro alloyed fine grain steels:

Permitted remaining stress in %	Annealing time in h at
	530 °C	550 °C	580 °C
40 30 20 10	0,5 6 63	2 18 160	3 28

The lower the permitted remaining material stress is meant to be, the higher the recommended stress-relief annealing temperature within the optimized range. Generally speaking: a stress-relief annealing at below 400 °C does not make sense due to the yield point being quite high with the materials in question.
There is an exception with surface hardened work pieces. Here stress-relief annealing at 150 °C to 200 °C with consideration to a limited loss of surface hardness one may ignore the demand for a maximum stress-relief. However, the stress-relief temperature always ought to be above the maximum ambient temperature that the work piece might later be exposed to.

Work pieces made of roller bearing steel after hardening are commonly annealed at temperature of 150 °C to 180 °C for 1 to 2 hours. The aim is to reduce stress, to improve the ductility and to stabilize the material structure while keeping dimensional stability.
Higher annealing temperatures or longer annealing times would result in a unacceptable drop in hardness and thereby in reduced load support ability of the bearing parts.

Slow warming-up, an even temperature saturation, slow cooling-down and keeping the optimized annealing temperature are the criteria for good stress-relief annealing.
Warming-up to the stress-relief annealing temperature ought to be that slow that the yield point at elevated temperatures is evenly obtained right across the entire cross-section (depending on the type of steel and its composition). That is especially true for the most endangered temperature region of up to 200 °C and for work pieces with a low ductility or with relatively brittle zones, such as cast iron, welded parts, hardened - and here surface hardened in particular - work pieces.

Cooling-down from the stress-relief annealing temperature is the most important process stage because any deviation from the required procedure might not only lead to a lessened stress-relief but might even intensify the stress compared with the original state. That even holds true when the other steps - such as warming-up, letting the work pieces saturate evenly with the heat, holding the temperature - have been adhered to.
A cooling-down speed of 50 to 100 K/h is to be recommended..

Nitrion GmbH does stress-relief annealing under vacuum if requested. That way no scaling or grain boundary oxidation needs to be expected.