Evaluation of Fracture toughness properties of Inconel grade 625
The fracture hardness of Inconel alloy 625 in the cast and welded conditions has been evaluated as a part of complete properties of alloy for advanced steam turbine applications. The fracture properties of solution treated alloy vary because of prolong aging at 650oC, with metallurgical precipitation causing changes to critical toughness and fracture resistance profiles.
Finding toughness properties of the cast nickel base alloy introduced several challenges. At high temperatures, specifically 750oC, there were test measurement issues to handle. In the solution treated condition, the noticed fracture resistance of Inconel 625 wire could not be stated following the standard practice. Variability in fracture characteristics arising from the large grain size of the cast alloy. The type of these challenges, the ways in which they were stated and practical implications are considered.
The high temperature fracture toughness of Inconel alloy 625 in the cast and weld conditions has been evaluated to use it in the development of new generation of pulverized coal fired power plants featuring latest steam conditions at temperatures up to 700 to 720oC.
Fracture hardness describes the resistance to fracture. In plane strain deformation conditions, fracture toughness parameters like Kic, Jic and Kjic are geometry independent and feature the beginning of fracture instability or slow crack extension. At low temperatures, these factors are insensitive to loading time. With increase in temperature, loading time creates wide effect.
High temperature fracture toughness is not a regularly determined property for newly made alloys. In low alloy ferritic steels beyond their brittle to ductile fracture transformation temperature, fracture hardness is very high that becomes a fact of concern in fitness for purpose defect evaluation, however it can turn to lower values at high application temperatures.
Austenitic stainless steels and nickel base alloys have high toughness at room temperatures, and do not attain a fracture mode transition, and are hence not anticipated to attain a considerable reduction in virgin toughness with increase in temperature.
Material high temperature fracture toughness after long durations of service duty causes embrittlement due to ageing. The mechanism of embrittlement and temperature at which it happens varies.
High temperature fracture toughness properties are found for cast alloy 625 in solution treated, long time aged and welded forms. Testing at 750oC needed a use of specialized displacement measuring equipment and a non-standard consideration of the test outcomes in specific situations.
At temperatures lower than 550oC, the fracture toughness and tearing resistance of cast Nickel base alloy in the solution processes, HAZ and weld metal conditions is very high that blunting resistance properties are attained. In this temperature limit, ageing of solution processed alloy for 1kh at 650oC results into considerable reduction in toughness and tearing resistance, however absolute value minimum Kjic are still high in range of 170 to 200 MPa√m. At 750oC, the fracture toughness of cast Inconel 625 subsequent to aging is similar to alloy has in the virgin solution treated form.