![]() 300-series stainless steel These sparks are not so dense as the carbon steel sparks, do not fork, and are orange to straw in color. High-speed steel High-speed steel has a faint red spark that sparks at the tip. Manganese steel Manganese steel has medium length sparks that fork twice before ending. The sparks are not as bright as the medium-carbon steel ones. High-carbon steel High-carbon steel has a bushy spark pattern (much forking) that starts at the grinding wheel. Medium-carbon steel This steel has more forking than mild steel and a wide variety of spark lengths, with more near the grinding wheel. ![]() Mild steel Mild steel sparks are similar to wrought iron's, except they will have tiny forks and their lengths will vary more. The tails of the sparks widen out near the end, similar to a leaf. (H) Special magnet steel Wrought iron Wrought iron sparks flow out in straight lines. It has been found that this system can determine the difference between two materials that give off sparks that are indistinguishable to the human eye. The system relies upon spectroscopy, spectrometry, and other methods to "observe" the spark pattern. Automated spark testing Īutomated spark testing has been developed to remove the reliance upon operator skill and experience, thereby increasing reliability. This makes observations of the spark length a much more reliable characteristic for comparison. The compressed air applies in essence the same "pressure" each time. This method is more accurate than using a grinder because it will always give off sparks of the same length for the same sample. The compressed air supplies enough oxygen to ignite the sample and give off sparks. Compressed air method Īnother less common method for creating sparks is heating up the sample to red heat and then applying compressed air to the sample. Also, the grinding wheel must be dressed frequently to remove metallic build-up. 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 the samples. The important spark characteristics are color, volume, nature of the spark, and length. The test sample is then touched lightly to the grinding wheel to produce the sparks. Moreover, the grinding wheel and surrounding area should be dark so that the sparks can be observed clearly. The test area should be in an area where there is no bright light shining directly into the observer's eyes. The wheel should be coarse and hard, therefore aluminium oxide or carborundum often are employed. In either case, the grinding wheel must have adequate surface velocity, at least 23 m/s (4500 surface feet per minute (sfpm)), but should be between 38 and 58 m/s (7500–11,500 sfpm). Ī bench grinder is usually used to create the sparks, but sometimes this is not convenient, so a portable grinder is used. Spark testing most often is used in tool rooms, machine shops, heat treating shops, and foundries. The spark comparison method also damages the material being tested, at least slightly. The main disadvantage to spark testing is its inability to identify a material positively if positive identification is required, chemical analysis must be used. Moreover, test samples do not have to be prepared in any way, so, often, a piece of scrap is used. Spark testing is used because it is quick, easy, and inexpensive. Spark testing also can be used to sort ferrous materials, establishing the difference from one another by noting whether the spark is the same or different. These sparks can be compared to a chart or to sparks from a known test sample to determine the classification. It normally entails taking a piece of metal, usually scrap, and applying it to a grinding wheel in order to observe the sparks emitted. Spark testing is a method of determining the general classification of ferrous materials.
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