How to relieve stress by heating
Every step in the manufacturing process adds residual stress to the metal. Heating is a common procedure to relieve stress in the workpiece and obtain the desired mechanical properties. There are several heat treatment processes to achieve the desired result, which are differentiated by the induced temperature.
The importance of stress relieving in metal
In the process of fabrication, metal parts go through numerous heat treatments. Every stage adds stress to the workpiece and thus can leave the metal with a shorter fatigue life. Furthermore, cutting, machining and welding in particular induce internal stresses in the material which can lead to distortions and dimension changes. Although it is not possible to reduce the internal stress to zero, there are numerous ways to minimize the amount of stress in the metal. Especially stress relieving by heat treatment is a common procedure to reduce stress in a workpiece.
The process of stress relieving by heat
Stress relieving by heat can be applied to a variety of metals such as steel, but also to non-ferrous materials like aluminum and copper alloys. Steel parts are normally heated to a temperature range between 540 and 700 °C for 1 hour per 25 mm of thickness. It should be noted that the temperature ranges below the point at which the austenite phase begins to form. However, if the goal is to reduce the stress to the lowest possible amount, the stress relieving should be completed at or near 700 °C, the point where austenite begin to form. This process is called sub-critical annealing. Stress relieving by heating does cause surface oxidation when it is performed in an open fire furnace. This can have several effects on the material, which ranges from discoloration at low temperature to a fine scale at sub-critical temperatures. This circumstance can be avoided by performing the process in a nitrogen atmosphere or in a vacuum. However, the second option can be far more expensive.
Stress relieving in stainless steels
Stainless steel can undergo various heat treatments. The procedures are differentiated by the induced temperature.
Stress-relief annealing at 290 to 425 °C
The procedure of stress relieving at low temperatures of 290 to 425 °C results in a redistribution of peak stress. Further, the treatment increases tensile strength and yield strength in the workpiece. The fact that the maximum temperature is held below 425 °C has some advantages, such as the process can be applied to a wide range of higher carbon steel grades since carbide precipitation and sensitization to intergranular attacks are not an issue.
Heat treating at 425 to 595 °C
Higher carbon grades should not be treated with temperature above 425 °C since these grades are sensitized to intergranular attack. Therefore, stress relieve at 425 to 595 °C should only be performed on the stabilized 321 and 347 grades or on low carbon “L” grades. Temperatures between 425 and 595 °C are normally sufficient to prevent undesirable distortions in the materials.
Heat treating at 815 to 870 °C
Heat treatments with such high temperatures are only used when a full stress relieved assembly is required. This method can only be applied to low carbon “L” grades and 321 or 347 steel grades. Even then, it is advisable to test for sensitization to intergranular attack.