Let’s continue our discussion of spark testing by providing characteristics and examples of spark patterns for multiple types of materials.
The important spark characteristics are color, volume, nature of the spark and length. Note that the length is dependent on the amount of pressure applied to the grinding wheel, so this can be a poor comparison tool if the pressure is not exactly the same for each sample. Also, the grinding wheel must be dressed frequently to remove buildup of unwanted metallic particles.
The steel sample to be tested should be of a convenient size and should be clean from scale and decarburization. Checking the spark on a decarburized and oxidized sample will produce an incorrect spark and distort the final spark observation.
The characterisitic pattern for selected materials is shown below (material followed by spark characteristics) and illustrated in Figs. 1 and 2.
- Cast iron – very short sparks that begin at the grinding wheel
- Cemented carbide – sparks under 3 inches; dark-red in color and do not fork
- High-temperature alloys, nickel and cobalt – thin and very short; dark-red in color and do not fork
- Stainless steel, 300-series – not so dense as carbon steel sparks; do not fork and are orange to straw in color
- Stainless steel, 310-series – much shorter and thinner than the 300-series sparks; red to orange in color and do not fork
- Stainless steel, 400-series – similar to 300-series sparks; slightly longer and have forks at the ends of the sparks
- Steel, manganese – medium-length sparks that fork twice before ending
- Steel, high carbon – bushy spark pattern (lots of forking) that starts at the grinding wheel; not as bright as medium-carbon steel
- High-speed steel – faint red spark that sparks at the tip
- Steel, medium carbon – more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel
- Steel, mild – similar to wrought iron except they will have tiny forks and their lengths will vary more; sparks will be white in color
- Titanium – gives off a great deal of sparks; easily distinguishable from ferrous metals as they are a very brilliant, blinding, white color
- Wrought iron – flow out in straight lines; the tails of the sparks widen out near the end, similar to a leaf
2. Pye, David, Spark Testing, Industrial Heating Experts Speak Blog, 25 June 2010