Study of Effect of Long Cycle Cryogenic Treatment on Mechanical Properties of Steel Weldments

Cryogenic treatment is a process of treating materials, at temperatures well below 0°C to achieve improved mechanical properties. Cryogenic treatment was observed to enhance the mechanical properties of various materials like tool steels, bearing materials, carbon fiber composites and copper resistance welding electrodes. Application of cryogenic treatment as a post weld treatment to weldments is a new area to be studied. In the present investigation, 2.25 Cr 1 Mo low carbon low alloy steel weldments were cryogenically treated and their mechanical behavior evaluated. Changes in microhardness, Charpy V notch toughness and changes in the microstructure were studied. The performance of as welded, tempered, long cycle cryogenic treated and tempered and long cycle cryogenic treated weldments were compared. J-integral as a measure of fracture toughness of the weldments was computed and compared. The long cycle cryogenic treatment was found to result is substantial increase in impact toughness with the marginal reduction in micro hardness. At the micro structural level, that formation of acicular ferrite, grain refinement and precipitation of alloying elements were found to be the basic mechanisms responsible for the increased impact toughness.