in Vol. 14 - September Issue - Year 2013
Assessing Peened Quality Surface Using Microscopy Techniques
Figure 1 - SAE 9254 Helical Spring failed prematurely in service due to poor quality peened surface. Arrow indicates start of failure region. No fatigue crack-growth visible. Magnification 5x. Courtesy: Testmat.
Figure 2 - SEM of 2mm diameter spring coil. (a) SEM of coil with no defects. Magn.:50x (b) SEM of coil with poor coverage showing wire drawing marks. Magn.:500x (c) SEM of coil with apparent good coverage using SE mode. Magn.:100x (d) SEM of same coil in (c) showing drawing marks revealed by BSE. Magn.:100x. Courtesy: Testmat.
Figure 3 - Metallography of bar surface. Material SAE 6150. Arrow indicates small overlap in a peened depression surface. Magn.: 500x. Courtesy: Testmat.
Figure 4 - Metallography of spring surface. Material SAE 9254. Arrow indicates surface crack of 8
Figure 5 - Metallography of spring surface. Material SAE 9254. Arrows indicate several surface overlaps of 12 to 15
Suspension components, powertrain items and other automotive components are worldwide shot-peened to obtain a longer life in service. Several controls from the peen media and the peening machine determine the quality of the superficial peening treatment. Also, there is an important observation from the peened surface that quantifies the surface area peened and is called coverage. This is measured as a percentage of the area that was deformed or received the impact of the peened media.
There is a theoretical limit of 99,8% of the achievable coverage area in industrial peening processes, and reaching this requires an intense peening process. Today, suspension products like springs have the highest coverage specification associated with high compression residual stresses. Non-covered areas are very critical and reduce the component life in service. Owing to this, the practice in the workshop is to expose the components to a maximum peening time to assure the best surface coverage. Nevertheless, there is a limit in the process and exposing the components to peening in excess results in over-peened areas.
Over-peening is not desired in a good quality peened surface and needs to be avoided. This operational limit is characterized by cracks and overlaps in the component surface. These typical defects from over-peening are 2-5 grains in size, reduce the compression tension in the surface, and are regions that nucleate fatigue cracks.
Components with high tension in service like helicoidal springs do not have a significative fatigue crack-growth and do break right after the crack nucleation. So, even with a peened surface, components can fail in service with a reduced life due to the surface quality.
Assessing Surface Quality
The defects that reduce quality surface like cracks, overlaps and poor coverage are too small to be inspected using only visual inspection. At least a stereo microscope with a good resolution and magnification should be used.
In a helical spring of 4mm diameter, for example, an inspection technique would need to identify areas of 100µm square to assure more than 90% of coverage. Normally, the not-peened areas tend to be very small and spread randomly along the part when the coverage reaches more than 90%. At this level, to measure the not-covered area is not an easy task.
For more precision in the measured area and in identifying the defects, it is possible to use microscopy techniques. For coverage measure near 90% a Scanning Electron Microscopy (SEM) might be very useful to identify the not-peened areas. In Figure 2, it is possible to verify the use of these technique by assessing the coverage in a 2mm diameter spring. The excellent depth of field (DOF) of this kind of microscope allows magnification of the interested area and very sharp observation of how all the surface was treated. Another resource with this microscope is to use the Back Scattered Electrons (BSE) image that allows clear identification in the image chemical surface modifications. Different compositions will have different shades of grey and this allows identification of areas with oxides that are present at the original surface and were not removed during the peening treatment.
Overlaps and cracks might be identified also using SEM, but optical microscopy will be more cost-effective. A simple metallographic observation will help to inspect the surface for overlaps and cracks.
The sample needs to be prepared with a longitudinal or a cross-sectional cut, compression-mounted in resin with good specimen edge retention, then ground and polished according to common metallographic practice. The samples shall not be etched.
With magnifications from 200 - 500x, it is possible to identify all the defects that occur in the over-peened state.
Figures 3 - 5 show different defects identified by this technique.
Using scanning electron microscopy with back-scattered electrons is a valid technique to identify the coverage area after the peening treatment. This technique could be used at series start-up to determine the correct process parameters that will be controlled along the serial production.
The metallographic technique is able to identify several surface defects, including the ones generated by the peening process. Because of the low cost of this analysis, it is possible to be done within a determined frequency in the serial production.