1. An Examination of Metallurgical Features in Broached Turbine Disks
Ernesto Gutierrez-Miravete
(with material from C. Cook – CCAT)
Rensselaer at Hartford - CCAT
Presented at CINVESTAV-Queretaro
December 2010

2. 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

3. Advantages and Limitations
High Productivity
Close Tolerances
Good Surface Finish
Economical Operation
Versatility
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

4. Broaching of Disks
Feed Direction

5. Broached Grooves in Disk

6. BOSS Modeling

7. 4140 As Broached Microstructure

8. 4140 As Broached Microstructure

9. 4140 As Broached Microstructure

10. 4140 As Broached Microstructure

11. As-Broached Microstructure Summary
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!

12. 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

13. As-Peened Microstructure of Broached Sample

14. As-Peened Microstructure of Broached Sample

15. As-Peened Microstructure of Broached Sample

16. As-Peened Microstructure Summary
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.

17. 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