Original Date: 01/26/1998
Revision Date: 01/18/2003

(equipment: Shot Peening Machine, I/O Cards for Motor Controls and MagnaValve Controllers, Incremental Encoder, Proximity Sensors, Analog Air Pressure Transducer, Digital Air Quality Transducer, Touch Screen Monitor, Gateway 2000 PC, Hewlett Packard Laser Printer, SyQuest 1.5G Internal SCCI Removable Drive, Windows NT 4.0 Workstation, Office Professional 97 Software, Texas Instruments TiSoft 505 Program, National Instruments Lookout Software, Norton PC Anywhere )
Shot peening is a technique that propels small diameter shot at the surface of metal parts to induce a residual compressive stress on the surface to enhance fatigue resistance and reduce susceptibility to stress corrosion cracking. In order for shot peening to conform to prescribed Aerospace Material Specification 2432B, a large number of conditions and parameters must be maintained and controlled during the process. Key parameters of a peening operation include shot size and condition, shot flow, air pressure, angle of attack, table indexing, part speed, and peening cycle time. Shot peening produces material performance characteristics that no other processes (e.g., heat treating, annealing, powder coating) can accomplish. Typical materials include titanium, magnesium, steel, aluminum, nickel, and cobalt alloys used in the aerospace industry. At Corpus Christi Army Depot (CCAD), a reliable process can now be performed on parts which rely on the benefits of peening in order to attempt to restore the part to its original component design characteristics.

Prior to July 1996, the peening operation was performed to Mil-S-13165C, which required no process control documentation. Over time, because of operator and system changes and the lack of a formalized operating procedure, the conditions and parameters of the process deteriorated. The equipment was not capable of monitoring and recording key parameters such as table indexing, part speed, and shot flow which were necessary to prevent substandard shot peening of parts. The process was misunderstood, and operators were not familiar with the mechanical effects of shot peening, many thinking it was a cleaning process instead of a beneficial stress and fatigue life enhancement process. Operators did not have the most efficient setup parameters. Maintenance personnel were not familiar with the operation of the equipment; therefore, no preventive maintenance schedule was in place. As a result, customers had little confidence in the process.

In July 1996, the Depot had an opportunity to acquire additional workload from the Navy. The Navy demanded complete process control and conformance with AMS 2432B. This led to the Depot taking steps to change its shot peening operations. The first step was to define the customer’s current and future requirements. The shot peening machine was modified with a complete new programmable logic controller, new input/output cards for motor controls and magna valve controllers, incremental encoder for the table, new wheels for the proximity sensors, analog air pressure transducer for the pressure system, digital air quality transducer to control environment, and a touch screen operator interface. Other equipment included a Gateway 2000PC, Hewlett Packard laser printer, SyQuest 1.5GB internal removable drive, Windows NT 4.0 workstation operating system, Office Professional 97 software package, Texas Instruments TiSoft 505 program for the programmable logic controller, National Instruments Lookout Software for the control screens, and Norton’s PC Anywhere for remote monitoring by a client computer located in the laboratory. A fully equipped laboratory performs a full set of acceptance tests on all shot prior to use in the system. Statistical process control was applied to track performance and assure process repeatability. Hard copy documentation was made available to the customer.

These improvements have added efficiency and capability to the shot peening operation. The programmer can streamline some of the original program logic to the current system, avoiding the loss of older programs. New code can be written more efficiently. The additional axis of motion and indexing capabilities allow peening parts with complex geometries such as inside surfaces of slots. Programming nozzle and stand off distances are easier due to nozzle location being displayed on a video screen. The shot peening operation is now in a position to comply with the latest standards in accordance with AMS 2432B which should open the doors for new business opportunities, both internal and external.

For more information see the Point of Contact for this survey.