Spark Testing

One recurring problem in the steel industry, which is often revealed during hardness testing after a heat treat operation or later as the result of a failure, is the problem of mixed material.

A bar of high carbon steel may be identical in appearance to a bar of mild steel . Widely differing grades of tool steel (once any identifying colours are removed) may be indistinguishable and the result, too often, is that a mix occurs, and within a batch of machined parts there are some which will have insufficient strength or hardness or other significant physical property and this will render them worthless, or if undetected perhaps dangerous.

Conventional methods of sorting mixed steels by analysing their chemical compositions are accurate, but may also be expensive and slow and are not alwavs readily available.

An alternative method which works well when the steels are very different from each other (and with practice, when the differences are not so great) is spark testing. This method makes use of the differences in the appearance of a stream of sparks generated when steel is applied to a grinding wheel.
The most obvious features to look for are the brightness and colour of the spark stream and the number of spark bursts.

The sketches give a better idea of what to look for. To see how the method works, take a sample of high carbon steel and a sample of low carbon steel and apply them alternately to a grinding wheel so that the spark stream can be easily observed. The differences in the quantity and intensity of the spark bursts should be readily apparent.
Remember that this is a comparative method and as such it is very helpful if a collection of steel samples is available to compare with an unknown sample.
 

MILD STEEL Approx. .1% Carbon Few small bursts.	HIGH CARBON STEEL Approx. .8% Carbon Increasing numbers of larger bursts.
01 GRADE TOOL STEEL Distinctly red lines which end in "commas" and star bursts.	D2 TOOL STEEL Much leaner spark stream, fewer bursts, uniform colour.