Here's the question: Does stainless steel produce sparks during grinding?
Yes, stainless steel produces sparks during grinding. Sparks result from small fragments of metal being heated to high temperatures by friction from the grinding process, which causes them to glow and appear as sparks. These sparks can vary in color and intensity, depending on the type and composition of the stainless steel to be ground.
1. Why do sparks appear when identifying steel sparks?
When steel is ground against a wheel with a certain pressure, it is reduced to fine grains by the abrasive action of the wheel. The particles heat up due to mechanical grinding and are ejected by the centrifugal force of the rotating wheel.
As these heated particles come into contact with air, they undergo oxidation, generating enough heat to bring the steel particles closer to their melting point and emitting light in the process. The trajectory of the bright spot follows a streamline.
During oxidation, a film of iron oxide (2Fe + O2 = 2FeO) is first formed on the surface layer of iron. Within the grains, steel contains carbon in the form of carbide, which decomposes at high temperatures, releasing carbon (as Fe3C – Fe + C in the presence of Fe3C carbon). The released carbon then reacts with iron oxide on the surface of the grain, forming carbon monoxide gas.
The carbon atoms reduce the surface iron oxide, allowing it to take oxygen from the air and be reoxidized. This process also triggers a reaction with the carbon within the particle, causing more carbon monoxide gas to accumulate within the grain.
When the internal pressure of the gas exceeds the surface tension of the outer layer of particles, an explosion occurs, which manifests itself as fireworks.
After a particle explosion, if there is carbon remaining that did not participate in the reaction within the finer particles, the oxidation reaction may occur again, followed by a second, third, and even fourth explosion, forming a dendritic pattern.
It is evident that the explosion is caused by the combustion of carbon.
Therefore, the amount of explosion in carbon steel is directly proportional to the amount of carbon present in the steel. The higher the carbon content in carbon steel, the more frequent explosions occur and vice versa. As dendritic bursts occur, the level of division and pollen increases with increasing carbon content in the steel.
2. What is the name of each part of the spark?
Spark patterns, various parts and shapes generally include:
1) Ray of fire
When the test sample is ground on the grinding wheel, all the sparks produced are collectively called fire beams.
The fire beam can be divided into three main parts:
The fire beam closest to the grindstone is called the root fire beam.
The middle part is called the intermediate fire beam.
The end part of the fire beam, which is furthest from the grinding wheel, is called the rear fire beam. See Figure 12-1.
2) Simplify
When grinding steel, grinding particles fly at high speed, creating shiny lines known as streamlines.
Based on the shape characteristics of streamlines, there are three common shapes: straight streamlines, wavy streamlines, and intermittent streamlines, as shown in Figure 12-2.
3) Fireworks
A burst pattern occurs in the middle of the streamline.
There are three common types of fireworks: dendritic fireworks, feathered fireworks, and bracts.
The branch-shaped fireworks resemble tree branches, with more or less branches, including two forks, three forks and many forks.
There are different levels of division, including primary division, secondary division and multiple division.
Feather-shaped blasts are a special form of steel blast with edges that resemble feathers. Bracts are special forms of expansion that occur in the middle of the streamline and include rupture before and after the expansion part.
See Figure 12-3. If the bract flower appears at the end of the chain line, it is also called the bract flower tail flower.
4) Knot
The point where the streamline breaks halfway is called the knot.
Some fireworks have shiny, plump knots, while some fireworks have no obvious knots.
5) Awning line
When the spark explodes, the coil is called the awn line.
Dendritic fireworks can be considered the collection form of most canopy lines.
6) Pollen
Sparks in the form of points between the explosion lines or close to the current line.
7) Tail flower
Cauliflower is an abnormal shape with a streamlined tail.
There are three types of common tailflowers: foxtail flower, spearhead tailflower, and magnolia tailflower. As shown in Figure 12-4.
Fig. 12-4 Shape of the tail flower
8) Color
The color and luminosity of the entire flame beam or part of the spark.
3. What is the equipment and operation for identifying sparks?
The main tool used for spark identification is a grinder.
Grinding machines can be tabletop or portable.
A bench grinder is suitable for inspecting steel samples and small-format parts.
A portable grinder can be used to identify batches of steel in workshops and warehouses.
The power of the motor used for a bench grinder is 0.5 KW and the rotational speed is about 3000 RPM.
The motor of a portable shredder has a power of 0.2 KW and a speed of 2,800 RPM.
Excessive power and speed can cause sparks to disperse, which is not conducive to identification.
If the power and speed are too low, it will be difficult to grind alloy tool steel and tungsten-containing high-speed steel, and may not even produce a flame beam.
The grinding wheel should have a grain size of 46# or 60# (preferably 60#) and an average hardness of 200mm, and the thickness should be 20~25mm.
The grinding wheel for a portable grinder can have a diameter of 9020 mm.
1) Be familiar with the performance of the tools
It is advisable not to frequently change tools such as grinding machine and grinding wheel grain.
Having knowledge and familiarity with tool performance is an essential aspect of identifying sparks.
The spark shape may vary due to changes in grinding wheel speed and particle size of different grinders.
2) Maintain the sharpness and roundness of the grinding wheel friction surface
The sharpness and roundness of the grinding wheel's friction surface must be maintained regularly to ensure consistent projection force.
If the grinding wheel is not sharp, it can reduce aerodynamics, while if roundness is not maintained, the steel can bounce when rubbing against it. Therefore, the circularity of the grinding wheel should not be too small.
3) Use standard blocks to correct the impact of the environment
Before starting work, it is important to identify a standard sample to correct for the potential influence of the objective environment.
The brightness of the work environment can significantly affect the observation of sparks.
4) Choose a good workplace
The identification location should not be excessively bright, but it does not need to be completely dark. It is important to maintain a consistent brightness to ensure accurate identification.
It is generally not advisable to operate outdoors. However, if outdoor operation is necessary, a mobile tarpaulin covered with black cloth should be used to avoid interference from strong light, such as that from rabbits.
5) Standard steel sample of self-made steel type
A set of standard steel samples with known steel grades should be provided for comparison in learning and identification. The more comprehensive the steel samples, the better.
To determine the correct content of each element, standard steel samples must undergo chemical analysis.
4. What is the significance of spark identification?
As a technical worker in the machinery manufacturing industry, you will encounter challenges in metal material selection and heat treatment.
Improper selection of materials or mixing of different types of steel during parts processing may cause parts to fail to meet processing and usage requirements, leading to economic losses or serious accidents.
To ensure the correct use of steel, it is crucial to understand the variety and performance of the steel used. Different types of steel have unique numbers and marks, and steel type identification is an indispensable and crucial link in machine manufacturing.
Steel identification methods can be divided into chemical and physical methods. Although chemical analysis is reliable, it is only applicable for sampling inspection of steel and can be time-consuming and expensive, making it unsuitable for on-site work.
Physical identification methods may not be as accurate as chemical analysis, but accumulated experience makes them useful for preliminary on-site analysis. Spark identification and Taoist analysis methods are the most practical and straightforward among physical identification methods.
The spark identification method is widely used because it is quick, convenient and does not damage the steel.
Steel is a significant raw material in machine manufacturing.
When a large amount of steel products enter the factory, they may become mixed due to repeated transportation and storage rotations.
In the production process, quality must have priority before steel supply.
Before heat treating steel parts, it is necessary to double-check and confirm the quality of the steel.
Different types of steel require different heat treatment conditions and process specifications, so it is important not to confuse the heat treatment process.
When parts need to be discarded, the steel types of materials used in the parts must be identified to ensure proper disposal.
5. What are the effects of alloying elements on spark changes?
Carbon is the main element in steel and its activation form changes with increasing carbon content.
6. What is the commonly used steel spark pattern?
The spark patterns of common steels are as follows:
Fig. 12-6 30 Steel
The beam of flames appears entirely yellow, featuring a thick line in the middle, with slightly thinner ones at the root, and slightly larger fireworks at the tail. Additionally, there are long aerodynamic lines that hang down slightly.
In the case of the secondary explosion, it has multiple branches with bright explosion nodes.
Fig. 12-7 Steel 40
The length of the fireworks beam has increased slightly. All fireworks are now secondary bursts and the fuse line is long and thick. Additionally, there are now more fireworks throughout the beam and some pollen is starting to appear. The tail of the fireworks beam is also larger and the color is bright yellow.
Fig. 12-8 45 Steel
The length of the fire beam is longer than that of 40 steel. The shape of the fireworks is larger, and the number of chain lines and fireworks has increased. The chain lines are thicker and the awning line is longer. There is an adequate amount of pollen between the streamlines and they emit with force, resulting in a greater degree of explosion. The knots are bright, and the number of fireworks on the tail is significantly greater than 40 steels. Furthermore, the color is bright yellow.
Figure 12-9 Steel 50
The length of the flame beam is equivalent to that of a 45 steel beam.
The burst pattern is significant, with an increase in the number of streamlines and flares. The streamlines are thick, with long lines of wind and pollen between them, making the explosion powerful. The nodes are bright, and the number of explosions in the tail is noticeably greater than that of a 45 steel beam. The color of the flame is bright yellow.
The entire flame is yellow, with a slightly thick and long streamline and straight shape. The central portion towards the tail is slightly dropped.
A single burst pattern with multiple branches, made of carbon structural steel with the same carbon content, is slightly more regular than the burst pattern. The degree of explosion is large and the nodes are brighter.
The presence of chromium at this stage demonstrates its role in stretching and cracking.
The spark beam is bright yellow and has many streamlined lines. The secondary burst of the composite flower is large, organized and regular, with a significant number of fireworks. The awning line is long and thick, and the flower angle is clear and well separated.
There is adequate amount of pollen, and the degree of explosion is high, with thick aerodynamic lines, falling slightly from the middle to the tail. The explosion degree of the large chrysanthemum branch is even more intense.
Currently, the inscription medium low carbon still serves to promote the explosion.
Figure 12-12 20CrMo Steel
The flame beam of the material is shorter than that of 20Cr steel. The aerodynamic line is a little thinner and there are multiple forks and a single explosion at the same time.
When compared with 20CrMo, the explosion pattern has decreased, the explosion degree has been weakened, the nodes are not very bright, and the color is yellow. Additionally, the streamliner's tail has flowers at the tip of the weapon.
Molybdenum has the inhibition property at this stage.
Although chromium is an explosive element, it coexists with molybdenum and its properties become subordinate.
Figure 12-13 40CrMo Steel
The flame color of 42CrMo steel is slightly darker than that of 40Cr steel and its aerodynamics are similar. It forms secondary burst compound flowers with adequate amount of pollen, and the nodes appear shiny. However, the explosion patterns are irregular and confusing, and the explosion degree is slightly weakened. On the tail there is a flower at the tip of the weapon, which is not seen on 20CrMo steel.
From this, it can be inferred that the carbon content has a certain impact on molybdenum.
Figure 12-14 60Si2Mn Steel
The fire beam is moderate in length and has reduced aerodynamics, as well as being slightly thick. Most of them burst twice, while some burst three times with a small flower type and an obvious silicon bud knot. These types have few, short awn lines, a slightly weaker blast rating, and no pollen. The spark color and explosion node are not very bright.
Fig. 12-15 GCr15 steel
The fire beam is moderate in length and features many simplified, triple-burst patterns. The streamlines are slightly thin and densely covered with branch-shaped fireworks.
The number of fireworks is large, the patterns are small, and the canopy line is thin and uneven. There is a significant amount of pollen between the awn lines and the nodes are not very distinct. The color of fireworks is orange.
The internal organization is pearlite troostite in the hot-rolled state. The fire beam is long and thick and has three bursts. The explosion intensity is strong, the awn line is long, and there is a significant amount of pollen between the awn lines. The popped knots are shiny and the tail pattern is long and located in the middle.
Fig. 12-16 Cr12MoV Steel
The fire beam is thin and extremely short, with an intermittent, wavy current line that appears to be numerous and slender.
The fireworks are exceptionally powerful, producing sparks that explode into three different multi-branched flowers and significant stars. The flowers contain numerous broken flowers and pollen and are filled with fire.
The end of the aerodynamic line has an obvious gun-shaped tail flower as a result of the molybdenum content. Additionally, the aerodynamic line on the tail is slightly thicker, giving the material a hard feel when rubbed.
Color: yellow to orange. Spark shape is no different from Cr12.
Fig. 12-17 5CrMnMo Steel
The fire beam is the thickest and longest, the current line is medium in thickness, and the explosion is the second strongest. They all burst three times, sometimes with a few flowers, and there are flowers on the molybdenum gun-tipped tail.
The flower shape is a three- or four-segmented star-shaped multi-branched flower with a lancet-shaped tail flower. The canopy line is dense, the flower distribution area accounts for 55-60% of the entire fire beam, the flower shape is large, and the flower angle is wide.
In terms of color, the fire beam is bright yellow and the nodes are yellow to white. The resistance is less strong during grinding.
Figure 12-18 3Cr2W8V Steel
The fire beam is relatively long and the current line is very thin, wavy and intermittent. The burst is weak, with only a small amount of flowers in the shape and size of a bald fox tail.
Body color: fuchsia.
Bald and lonely, light cherry red.
It seems very resistant to sanding.
Figure 12-19 Steel W6Mo5Cr4V2
The flame beam appears as a short, bright orange-yellow color with a dark red tinge at the base.
There are some irregular streamlines along with some wave-like patterns.
The streamlines are not very thick and are medium in length.
The streamlined tail is thicker and resembles a willow leaf with flowers on the tail, and the tip is slightly bald.
There are few fireworks, but they are large in size.
There are only a few awn lines, which are also bald.
The aerodynamic line of the tail drops downwards.
7. What are the precautions for identifying sparks?
Judging the steel type of the tested sample based on the observed spark pattern can be challenging.
This is because spark patterns can have subtle differences that are difficult to describe and express accurately. It takes a qualified professional with extensive experience and knowledge to accurately discriminate between these subtle differences in spark patterns.
1) Identify and verify the types of steel required from known parts.
Currently, it is necessary to use spark identification only to confirm whether a material belongs to the intended steel type.
When identifying a batch of parts, the first part must be carefully observed and analyzed. Once it is confirmed that #1 steel is being used correctly, the part should be lightly ground to observe the basic spark characteristics when it is less worn.
This feature must be taken into account, and the remaining parts can be ground with light pressure. This approach not only helps with identification, but also minimizes wear on parts, thus avoiding any negative impact on their appearance or functionality.
2) It is known that two types of steel are mixed for identification.
At this point, it is important to focus on the fundamental differences between the two types of steel in their spark patterns. Once you have a clear understanding of their respective main characteristics and distinctions, it will be much easier to differentiate between them.
3) The identification of the steel type must be carried out for parts with unknown steel grade.
If the discriminator understands the basic use of steel and is familiar with the common sense of the materials that should be used in the manufacture of the various parts, it can be of great help in identifying sparks.
One factor to consider is whether dendritic explosion occurs during spark grinding. If there is dendritic bursting, this can be further inferred from the following situations:
① If the dendritic explosion occurs normally and there is no spark in other special shapes, it is mainly carbon steel (dead and semi-dead steel).
At this point, if the pattern is a split rupture and the rupture pot is relatively sparse, it indicates that the carbon content is low and belongs to the low carbon range of carbon steel.
If the pattern is secondary, tertiary or a small amount of multi-split dendritic explosion, the explosion amount is medium and the distance between explosions is clear, indicating that the carbon content of the sample is about 0.4% C , and belongs to carbon steel in the medium carbon range.
If the explosion is a multi-forked tree-shaped explosion, the explosion amount is large and the distance between explosions is small, indicating that the carbon content is high and belongs to high-carbon steel. When the explosion is crowded, it confirms that the carbon content is high.
② If the blast exhibits a dendritic pattern and a feathery appearance, this indicates that the steel contains very low silicon content. The carbon content can be roughly estimated based on the amount of explosion, which can help in inferring the type of steel.
③ To roughly identify the type of steel:
- Obvious dendritic bursts are present at the end of the flame beam with large branches, disordered lines and strong burst force. Most of them belong to the manganese steel group of structural steel alloys.
- Pure and regular fireworks, bright colors, straight and thick aerodynamic explosions, mainly belong to chromium-containing structural steel.
- Obvious straight-tailed flowers from the tip of the weapon appear at the end of the flame beam, with dendritic explosions contained to some extent. Most of them belong to chrome-molybdenum steel, chrome-manganese steel and other structural steel groups.
- Particularly bright knots before explosion or explosions with silicon bracts mainly belong to the silicon-manganese steel and chrome-silicon steel groups of structural steel or spring steel.
- If spear-tail flowers and silicon-containing flower buds appear, and the dendritic burst shrinks significantly, it mainly belongs to silicon-manganese-aluminum and silicon-manganese-aluminum-vanadium structural steel.
- Regular dendritic bursts occur and nickel-containing bracts appear before the burst. The decarburized layer on the surface of the steel is apparent, and most of them belong to chromium-nickel alloy structural steel.
- Foxtail flowers are present, and the tree-shaped explosion is mainly attached around the foxtail. The streamline of the root is bright, not dark red, indicating that it belongs to Kesimiduo's tungsten alloy structural steel.
