An Examination of Metallurgical Features in Broached Turbine Disks Ernesto Gutierrez-Miravete (with material from C. Cook – CCAT) Rensselaer at Hartford.

An Examination of Metallurgical Features in Broached Turbine Disks

Ernesto Gutierrez-Miravete(with material from C. Cook – CCAT)

Rensselaer at Hartford - CCATPresented at CINVESTAV-Queretaro

December 2010

Broaching• Broaching is an orthogonal machining process

consisting in the unidirectional travel of a specially designed tool to create the desired machined shape in a single pass of the tool.

• The broaching action is obtained by designed the tool as a series of increasingly larger, specially designed teeth

• The process can be used for internal or external machining and for flat, round or contoured surfaces

• View BCI Video

Advantages and Limitations

Advantages

• High Productivity• Close Tolerances• Good Surface Finish• Economical Operation• Versatility

Limitations• Workpiece must be suitable

for broaching• No obstructions along the

direction of tool motion must be present

• Starting hole required (internal broaching)

• Material must withstand high thrust forces and be firmly supported

• Rigid machines and workholders required

Broaching of Disks

Feed Direction

Broached Grooves in Disk

A

A

BOSS Modeling

4140 As Broached Microstructure

4140 As Broached Microstructure

4140 As Broached Microstructure

4140 As Broached Microstructure

As-Broached MicrostructureSummary

• Tempered Martensite Microstructure• Smooth, good quality machined surface• Highly deformed zone near machined

surface (thickness ~ 15 micrometer; shear strain > 2), undeformed structure below

• White Layer ( < 5 micrometer thick). This may result in premature fatigue failure!

Shot Peening• Shot Peening is a mechanical surface

treatment process designed to induce compressive stresses near the surface of materials.

• The process directs a swarm of high velocity small shots against the surface

• Fatigue life improvements have been documented to result

• See related video and Howard’s Thesis and Anason’s Project

As-Peened Microstructure of Broached Sample

As-Peened Microstructure of Broached Sample

As-Peened Microstructure of Broached Sample

As-Peened MicrostructureSummary

• Tempered Martensite Microstructure• Surface somewhat rough but still as

satisfactory as the machined surface• Compressive deformed zone under peened

surface (thickness ~ 50 micrometer)• White Layer still there but its potential

deleterious effect may well be fully eliminated by the residual stress from shot peening.

Conclusion

• Broaching is a fast, efficient machining process• Broached microstructures in steel exhibit

white layers and highly deformed near surface regions

• Shot peening of broached surfaces produces thick, compressively loaded near surface regions