Measuring casting residual stress with x-ray diffraction.

The residual stresses present in cast metal components can significantly improve or diminish a component's typical life cycle. Residual stresses are generated in nearly every step of the metalcasting process. Identifying and managing these stresses in castings and using this knowledge to implement tighter process controls can increase fatigue life and improve product performance significantly, while minimizing scrap rates, early field failures and warranty returns.

With an increased demand for quality and value at a competitive price, the pressure on process control has become challenging. It's no longer possible to rely on empirical data or subjective decision making. Engineering decisions require reliable quantitative data, and x-ray diffraction (XRD) residual stress measurement provides this data. XRD Residual Stress Measurement Systems from Proto Manufacturing Ltd., Oldcastle, Ontario, Canada, then can be used to measure stress during casting design or after heat treatment, machining and surface treatments.

Smaller castings (up to 2 ft. [61 cm]) can be loaded into the lab-based system (Fig. 1) for inspection. For larger castings, a portable system can be brought to the cast component (Fig. 2).

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The XRD method measures strain and converts it to stress. A low-energy x-ray beam diffracts off of the sample. Changes in strain in the sample translate into changes in the diffraction angle that are measured by two x-ray detectors and then processed by software into a stress value.

The method employs the use of fiber optic-based solid state detectors. The fiber optics allow the detector electronics to be remote from the sensing head, thus enabling configurations of the measurement head that are small enough to allow access to tight locations. A stress measurement can be performed in as little as a few minutes.

XRD residual stress measurement can be used to benchmark qualified processes. Components can be measured after heat treatment to ensure that the treatment effectively reduces the residual stresses. The residual stress measurement also can be used to eliminate problems, such as the distortion of parts during machining, which often is caused by redistribution of the residual stresses in the component after material removal or introduction of residual stresses by the machining process.

Further, this technology can be used for the automated collection of residual stress maps on areas of a component.

The latest generation of XRD residual stress measurement systems are lightweight, easy to set up and operate and incorporate many options for use in the lab or in the field.

Select No. 002 a www.moderncasting.com/info

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