The hot workability of lnconel Alloy 706 (IN 706) extends over a temperature range of 1700°F to 21OO'F (925°C to 1150°C).The good hot workability of IN 706 in relatively large ingot sizes makes this alloy fabricable into large forgings utilized by important end markets including power generation, chemical processing, and others.Successful utilization of large IN 706 forgings in critical components requires control of important microstructural and mechanical property features.Grain size, grain boundary precipitates, and matrix precipitates vary widely in IN 706 as a result of fabrication history and post-fabrication thermal treatments (TMP).TMP conditions which favor heavy grain boundary precipitates, such as y, y and 6 or n phases, affect ductility (elongation or reduction in area [RA]) and impact toughness (Charpy V-notch [CVN]) properties.Coarse matrix precipitates may also be observed.While tensile yield and ultimate strengths may increase slightly due to within grain and/or grain boundary precipitates, thicker, more continuous grain boundary precipitates result in lower ductility and toughness.Presented are fractographic and microscopic evaluations of IN 706 forgings illustrating key relationships between microstructure and ductility and fracture-related properties.Fracture behavior changes as grain interior and/or grain boundary precipitate and morphological features are changed.Critical impact properties of IN 706 are rationalized by microstructural observations.
Room and elevated temperature ductility, strength, fracture toughness and fatigue (low and high cycle) properties are strongly related to the grain size of a fully processed superalloy product. Variations in grain size are normally observed in a heavy section forging weighing one half ton to several tons. These variations in grain size may lead to considerable scatter in the mechanical properties. Very large IN-706 disc forgings weighing up to 30,000 lb are currently fabricated for General Electric Power Systems Division large turbine engines. In order to control the variations in grain size and to create fine grained products (grain size ASTM-S or finer), thermomechanical processes (‘IMP) are being optimized using inputs from statistically designed experimental results and analysis. A large number of variables play a significant role in the development of the final grain size in a forged product. It is extremely difficult to identify the individual effect of these 25 or more processing, as well as material variables. The synergistic and interactive effects of these variables are, however, very important in controlling the final grain size. To quantitatively determine the effects of these variables and their interaction, statistical experimental designs were adopted using a 2@ fractional factorial design. Higher orders of fractional factorial design are not statistically suitable for a more precise analysis. Several fractional factorial designs were overlapped to cover the effect of all significant processing variables. Results indicate that an ASTM-5 or finer grain size without mixed fine and coarse grains can be developed in large IN-706 disc forgings through TMP optimization; whereas, ASTM-2 or finer grain size is developed through the current conventional processing conditions. Results also indicate that thermal treatments and grain size exhibit significant influence on some of the mechanical properties.
Room and elevated temperature ductility, strength, fracture toughness and fatigue (low and high cycle) properties are strongly related to the grain size of a fully processed superalloy product.Variations in grain size are normally observed in a heavy section forging weighing one half ton to several tons.These variations in grain size may lead to considerable scatter in the mechanical properties.Very large IN-706 disc forgings weighing up to 30,000 lb are currently fabricated for General Electric Power Systems Division large turbine engines.In order to control the variations in grain size and to create fine grained products (grain size ASTM-S or finer), thermomechanical processes ('IMP) are being optimized using inputs from statistically designed experimental results and analysis.A large number of variables play a significant role in the development of the final grain size in a forged product.It is extremely difficult to identify the individual effect of these 25 or more processing, as well as material variables.The synergistic and interactive effects of these variables are, however, very important in controlling the final grain size.To quantitatively determine the effects of these variables and their interaction, statistical experimental designs were adopted using a 2@ fractional factorial design.Higher orders of fractional factorial design are not statistically suitable for a more precise analysis.Several fractional factorial designs were overlapped to cover the effect of all significant processing variables.Results indicate that an ASTM-5 or finer grain size without mixed fine and coarse grains can be developed in large IN-706 disc forgings through TMP optimization; whereas, ASTM-2 or finer grain size is developed through the current conventional processing conditions.Results also indicate that thermal treatments and grain size exhibit significant influence on some of the mechanical properties.
Triple-melted VIM-ESR-VAR large x 66100 in.height) are cast to fabricate very large forgings utilized for several end applications.Although macrosegregations, greater than l/16 in.dia, are not detected in most of these ingots, some microsegregations rich in hardening elements (Nb, Ti) are observed in ingot sections.A thermal treatment of 1600°F/1-2 hr followed by air cooling reveals these segregated areas which exhibit a high density of needle-type precipitates [possibly delta (s) or eta (?I) phase].Conventional homogenization of these ingots in the range of 2000" to 2200'F for 18 to 72 hr did not eliminate this type of microsegregation.Attempts were made to study the effect of mechanical deformation (hot working) and homogenization on microsegregation.Optical micrographs indicate that a 2: 1 upset with a redraw followed by a 2000" to 2200'F 5!5'F homogenization for 24 to 48 hr reduces the extent of microsegregation.However, electron microprobe line scan and spot scan analysis did not reveal a significant difference for Nb, Ti distribution between the conventionally homogenized and deformation assisted homogenized billet stock.Small scale pancake forgings (6 in.dia x 1 in.thick) fabricated from these billet stocks, however, indicate that the deformation assisted homogenized ingot sections develop a more uniform grain size and microstructure compared to those from a conventionally homogenized ingot section.
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