An Experimental heat treatment cycle for stress relieving exhaust of turbo compressor manufactured by AISI 321

Residual stress occurs because of temperature gradient due welding, is one of the difficulties in welding process. Heat from welding may cause localized expansion. When the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. This result distortion and fracture for cases residual stress is upper than yield stress and toughness reduction for cases residual stress is lower than yield stress. Because of this, stress relieving performs on parts after welding process to minimizing these stresses.Exhaust of turbo compressor, one of the gas turbine units, has high working temperature (about 500°). AISI 321 used as material of this unit to avoid oxidation. This unit has huge dimension and welding process is used to manufacturing that. Concentrating on welding in view point of metallurgical and mechanical properties is very important. AISI Type 321 is an austenitic standard grade stainless Steel, including high percent chrome and potential to galvanic corrosion defect in case of annealing temperature is not regarded. In slowly cooling or heating rate (lower than 200° per hour) between 400° to 500° because of chrome presence in this material, lead to precipitation in grain boundary and coarse grain of metallurgical construction and finally ductile material and cracking in grain boundary due to part service. Then for heat treatment after welding process in range of 400° to 500°, heating and cooling specially cooling should be done very fast to carbide does not have opportunity to precipitate on grain boundary. For this purpose the best cycle for heat treatment stress relief for the exhaust with material AISI 321 is:Heating rate: 3 to 4 hour to 860° Holding time: 90 minute Cooling agent: air After applying mechanical and metallurgical test (tensile, bending and impact) on specimen, all results has been approved and considering metallurgical images, there was no carbide and its precipitation on grain boundary. Micro hardness test results approved no increased hardness on welding area compared to base metal

Keywords:

AISI 321 , stress relief , welding