A method in which steel is brought into contact with a high-speed rotating grinding wheel to approximately determine chemical composition of steel according to shape and color of spark generated by grinding, which is called a spark discrimination method.

When steel sample is ground on grinding wheel, high-temperature fine metal particles are ejected in tangential direction of rotation of grinding wheel, then rubbed against air, temperature continues to rise, particles undergo intense oxidation and melting, thus presenting a streamlined streamline during operation.
Abrasive grains are in a high temperature state, surface is strongly oxidized to form a film of FeO. Carbon in the steel reacts easily with oxygen at high temperature, FeO+C→Fe+CO, which reduces FeO; reduced Fe will be oxidized again and then reduced again; this oxidation-reduction reaction will continue When CO gas is generated. When iron oxide film on the surface of particle cannot control generated CO gas, a burst phenomenon occurs to form a spark.
If cracked granules still leave FeO and C which are not involved in the reaction, reaction will continue to occur, two, three or more bursting sparks will occur.
Carbon in steel is a basic element in the formation of sparks. When steel contains elements such as manganese, silicon, tungsten, chromium, molybdenum, etc., their oxides will affect lines, colors and states of spark. According to characteristics of spark, carbon content of steel and content of other elements can be roughly judged.

Spark produced by grinding of steel on grinding wheel consists of a root spark, a middle spark and a tail spark. Line-like trajectory formed by high-temperature grinding particles is called a streamline.

 

Bright and thick points on the streamline are called nodes. When spark bursts, several short lines are called awn line. Spark composed of awn line is called knot flower.

 

As carbon content increases, continuous explosion on awning line produces secondary sparks and three sparks. Bright dots that appear near awn line are called pollen.

 

Due to different chemical composition of steel materials, sparks with different shapes at the end of streamline are called tail spark. Tail flower has a braided tail spark, a foxtail tail spark, a garland tail spark and a pinnate tail spark.

Braided spark

 

Foxtail spark

 

 

Chrysanthemum spark

 

Pintail spark

Carbon is basic element of sparks in steel materials and is main component of spark discrimination. Spark shape is different due to difference in carbon content.
1 Low carbon steel has thick and thin streamlines, less explosive spark and more first sparks. Awnings are thick, long and have bright nodes. Sparks are yellow with a dark red color.
20# steel
 
2 Medium carbon steel has a slender and long streamline, and there are nodes in the tail and middle of streamline. Explosion spark is more than that of low carbon steel. Spark is large, there is a first spark and a secondary spark, with a small amount of spark powder attached. Spark color is yellow.
45 # steel
 
3 Flow line of high carbon steel is thin, short, straight, and dense. More explosion spark, sparks are small, and there are many secondary sparks, three sparks or multiple sparks. Awn line is thin and sparse, spark powder is more, and spark color is bright yellow.
T10 steel

 

Spark bouquet of cast iron is very thick and there are many streamlines. Generally, it is a secondary spark. There are many spark powder and explosion spark. Tail is gradually thicker and drooping into an arc, and color is mostly orange-red. When spark test is performed, hand feel is soft.

Spark characteristics of alloy steels are related to alloying elements they contain. Under normal circumstances, elements such as nickel, silicon, molybdenum, and tungsten suppress spark burst, while elements such as manganese and vanadium chromium can promote spark burst. Therefore, identification of alloy steel is difficult to master.
Generally, spark bouquet of chrome steel is white and bright, streamline is slightly thick and long. Bursting is mostly a first spark, spark is large like a big star-shaped with many thin fork and broken spark powder attached. Sparking heart is brighter.
Nickel-chromium stainless steel has a fine spark bouquet, and light is dark, bursting into a first spark, five or six forks, star-shaped, and tip has a slight burst.
Spark bouquet of high-speed steel is slender, number of streamlines is small, there is no spark burst, color is dark red, roots and middle are intermittent flow lines, and tail sparks are arc-shaped.

 

Carbon steel spark characteristics table

 

Effect of alloying elements on sparks