Spark testing has become a lost art in the metallurgical community. There was a time when incoming material was sorted by spark testing and an unknown steel was identified by simply looking at the spark pattern. Metallurgists had their own grinding wheels used for no other purpose than to check metals and alloys.
Simply stated, spark testing is a method of determining the general classification of any ferrous material. It normally entails taking a piece of metal, usually scrap, and holding it on a spinning grinding wheel in order to observe the spark pattern emitted. (Special note: All appropriate personal protective equipment must be used). A bench grinder is usually used to create the sparks, but sometimes when this is not convenient, a portable grinder is used. In either case, the grinding wheel must have adequate surface velocity, at least 23 m/s, or 4,500 surface feet per minute (sfpm), but should be 38-58 m/s (7,500-11,500 sfpm). The wheel should be coarse and hard. Therefore, aluminum oxide or carborundum are often employed.
The test area should be in an area where there is no bright light shining directly into the observer's eyes. Moreover, the grinding wheel and surrounding area should be dark so that the sparks can be observed clearly. The test sample is then touched lightly to the grinding wheel to produce sparks.
The sparks created can be compared to a chart or to sparks from a known test sample to determine the material type. The writer has personally seen a skilled spark tester distinguish between SAE 1040 and 1045 steel. Spark testing can also be used to sort ferrous materials, establishing the difference from one another by noting whether the spark is the same or different.
Spark testing is used because it is quick, easy and inexpensive. Moreover, test samples do not have to be prepared in any special way. The main disadvantages of spark testing are its inability to positively identify a material and the fact that it is highly dependent on the skill of the individual performing the tests. The spark comparison method also damages the material being tested. If absolute identification of a material is required, portable X-ray analyzers (so-called alloy “guns”) can be used.
Part 2 will include characteristics and examples of spark patterns for multiple types of materials.