Kenji Iwai

PM nickel-base superalloy dual-property disks produced by superplastic forging

Abstract A manufacturing process for dual-structured turbine disks made from PM nickel-base superalloys has been specially developed. The disks are composed of two metals: AF115, which has superior stress rupture properties, was used in the rim region, and TMP-3, which has good tensile properties and a low cycle-fatigue life, was used in the bore region. A 400mm diameter dual-property disk was successfully produced through double-stepped HIP treatment and superplastic forging, in which gas atomized powders composed of the above alloys were consolidated into a well-designed preformed structure. After heat treatment, the microstructure of the product was very fine and homogeneous in the bore region, and coarse grains were obtained in the rim region. The mechanical properties were of such high quality as to enable practical use as a turbine disk.

The Relationship between Fatigue Crack Propagation Property and Fatigue Life at Elevated Temperatures on P/M Ni-base Superalloy, AF 115. Through Fatigue and Creep Strain Energy Parameters, .DELTA.Wf and .DELTA.Wc.

Low cycle fatigue properties under pure fatigue and creep/fatigue conditions were evaluated using a powder metallurgy superalloy, AF 115, containing a variety of defect-types and defect-sizes. The relationship between life, defect size, and loading condition were analyzed on the basis of crack growth rules and strain energy parameters. Following results were obtained ; (1) Inclusions, micro porosities and prior particle boundaries were observed at the origin of fatigue specimens. (2) By treating these defects as either cracks or notches, the lower or upper boundary of fatigue life can be obtained from the elastic-plastic strain energy parameter, lWf, and the fatigue crack growth rule. (3) The introduction of a tension hold period to the stress cycle or a decrease in the strain rate in the strain cycle significantly affects the fatigue life. The linear summation rule for partitioned lives, based on the elastic-plastic and creep strain energy parameters, lWf and lWc, and fatigue and creep crack growth rules, is suitable for life estimation under creep/fatigue conditions.

Welding residual stresses around nozzles in the thick-wall pressure vessels.

In order to make sound welds free from defects like weld cracking, characteristics of residual stresses around nozzle welds were experimentally investigated in an as welded condition for a thick-wall reactor vessel and an accompanying heat exchanger utilized in a petroleum refining process or a coal liquefaction process.The residual stresses were measured throughout the thickness of the weldments by applying the measuring methods that had been newly developed for axisymmetrical stress distributions by the authors.The main results obtained for set-in type nozzle, set-on type nozzle and simple butt welds of pipes are summarized as follows.(1) Stress parallel to the weld line-The stress indicates the same distribution in the thickness direction regardless of the joints. That is, the maximum value appears just behind the final layer of welds and with descending to the initial layer it decreases. The stress gradient, however, differs from one joint to another, depending on the self-restraint intensity of the joint. Moreover, the peak value of simple butt welds of pipes is about 100 MPa smaller than the others.(2) Stress perpendicular to the weld line-It indicates the similar distribution as the stress parallel to the weld line. The maximum value in the set-in type nozzle changes from 400 MPa to 600 MPa with the decrease in the diameter of welds from 600 mm to 300 mm. As for the set-on type nozzle, it reaches almost the same level due to the existence of channel as a stress raiser. The stress for the simple butt welds of pipes, however, indicates much smaller value of about 245 MPa and further the same order of tensile stress occurs in the inner surface as well as just behind the final layer of welds in the outer side.(3) The stress in the thikness direction shows the distribution where the maximum value appears in the center portion of the thickness and it is a low stress level of about 150 to 200 MPa, regardless of the joints.