The residual stresses induced in weldments during welding may be relieved by applying heat at temperatures much lower than ordinarily used for stress relieving. The most satisfactory method for relieving stresses in carbon and low alloy steels is to treat the entire weldment in a furnace at a temperature between 540 to 650°C (1000 to 1200°F). Where the structure is too large to make this method practical, stresses due to

welding have been satisfactorily relieved by applying low-temperature heat to the weldment, the heat-affected zone, and adjacent base metal.

In carbon and low-alloy steels, low-temperature stress relief is accomplished by heating the weldment and adjacent base metal to not more than 315°C (600”F), although some low-alloy steels should not be heated above 205°C (400°F).

Heating Methods

Various heating methods used to accomplish low temperature stress relief of weldments include (1) oxyfuel gas torch heating with acetylene, propane, or butane, (2) electric resistance heating; and (3) induction heating. Each method has advantages and

limitations.

Torch Heating. For stress relieving, uniform heating can be accomplished with single burner torches to bring the temperature to 3 15°C (600°F). The “softer” flames produced by propane or butane torches are usually preferred over oxyacetylene torches.

Resistance Heating. Electric resistance heating is a widely used method for low-temperature stress relieving, especially at field erection sites, and also for applications requiring prolonged application of heat, such as weldments in thick sections. Resistance heating is usually accomplished with heating blankets that have nichrome wires in a pattern throughout the blanket. The high-resistance nichrome wires are heated by

passing an electric current through them.

 

Thermocouples are attached to the base metal and the weldment to be heated. The thermocouple wires are connected to control equipment that will automatically control the time-temperature cycle to ensure uniform heating of the weldment and adjacent base metal.

Induction Heating. Induction heating is extensively used at field erection sites for heat treatment of welds, and is particularly suitable for welds in pipes with wall thickness above 50 mm (2 in.), or for structural members on which it is possible to wrap the copper

conductors around the weldment and base metal to be heated. Relatively low frequencies, 25, 60, and 400Hz, are used with 60 Hz input equipment. On thick material, especially heavy-wall pipe, this method has the advantage of providing uniform heat throughout the material with a smaller temperature difference between the outside and inside surfaces of the base metal and weldment. The electric field is usually obtained by wrapping copper conductors around the weldment and adjacent base metal to be heated. Special fixtures are available to facilitate the rapid attachment and removal of the induction heating coils.

Temperature Control

Temperature control in heat treating operations is especially important and frequently influences equipment selection.

For torch heating, temperature-indicating crayons are widely used. They should be free of sulfur and lead. Stress relieving temperatures up to 370°C (700°F) may also be monitored with direct reading, magnetically attached surface thermometers.

In electric resistance heating, surface thermometers or electrically operated pyrometers are used to control the current flow automatically in the heating units.

For induction heating, thermocouples are usually attached to the surfaces of metals to be heated. The thermocouple wires are connected to a pyrometer to control the temperature of the weldment and adjacent base metal during heat treating.

 

 

 

 

 

 

 

 

 

 

 

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