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
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 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 • High Productivity • Close Tolerances • Good Surface Finish • Economical Operation • Versatility
Feed Direction Broaching of Disks
A A Broached Grooves in Disk
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 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