How to Identify Carbon Steel and Stainless Steel According to the Spark During Grinding?

1. Why do sparks appear in steel spark identification?

Answer: When the steel is ground on the wheel with a certain pressure, the steel is ground into fine grains due to the grinding of the wheel.

The particles are heated by mechanical grinding and are ejected by the centrifugal action of the rotating wheel.

The heated particles run in the air and are oxidized by contacting with the air, so that the heat generated is enough to make the steel particles close to the melting state and emit light.

The trajectory of the bright spot is the streamline.

When particles are oxidized, iron oxide film (2Fe+O2=2FeO) is first formed on the surface layer of iron.

In the grain, because the steel contains carbon and exists in the form of carbide, the carbide decomposes at high temperature to release carbon (such as Fe3C – Fe+C in the presence of carbon Fe3C).

The released carbon reacts with iron oxide on the surface of glume to form gaseous carbon monoxide.

The surface iron oxide reduced by carbon atoms can take oxygen from the air and then be oxidized.

Then, it reacts with the carbon inside the particle, so that more and more carbon monoxide gas can accumulate inside the particle.

When the expansion capacity of the internal gas is greater than the surface tension of the outer layer of particles, explosion occurs, which is the generation of the form of fireworks.

After one burst of particles, if there is carbon not participating in the reaction inside the finer particles, the oxidation reaction as described above can be carried out again, followed by the second burst, and even the third and fourth burst, which forms a dendritic pattern.

It can be seen that the explosion is caused by the combustion of carbon.

Therefore, the amount of explosion in carbon steel is directly related to the amount of carbon in the steel.

The higher the carbon content in carbon steel is, the more the explosion occurs, and vice versa.

When the dendrite explosion occurs, with the increase of carbon content in the steel, the split level and pollen increase.

How to Identify Carbon Steel and Stainless Steel According to the Spark During Grinding? 1

2. What is the name of each part of the spark?

Spark patterns, various parts and forms generally include:

1) Firebeam

When the test sample is ground on the grinding wheel, all spark forms are called fire beam.

The fire beam is divided into three main parts:

The fire beam near the grinding wheel is called the root fire beam;

The middle part is called the middle fire beam: the end part of the fire beam far away from the grinding wheel is called the tail fire beam. See Fig. 12-1.

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2) Streamline

When grinding steel, the grinding particles fly out at high speed, and the bright lines are called streamline.

According to the shape characteristics of streamline, there are three common forms: straight streamline, wavy streamline and intermittent streamline. As shown in Fig. 12-2.

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3) Firecracker

A burst pattern occurs midway along the streamline.

There are three common types of fireworks: dendritic fireworks, feathery fireworks and bracts.

The branch-shaped fireworks are similar to the branches, with more or less branches, including two forks, three forks and many forks;

There are levels of split, including primary split, secondary split and multiple split.

Feather-shaped explosion is a special form of rimmed steel explosion, which looks like feathers. The bracts are special expansion forms that occur midway along the streamline, and include bursting before and after the expansion part.

See Fig. 12-3. If the bract flower appears at the tail of streamline, it is also called bract flower tail flower.

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4) Node

The point where the streamline bursts halfway is called a node.

Some fireworks have bright and plump nodes, while some fireworks have no obvious nodes.

5) Awn line

When the spark bursts, the streamer is called the awn line.

Dendritic fireworks can be regarded as the collection form of most awn lines.

6) Pollen

Sparks in the form of dots between the exploding awn lines or near the streamline.

7) Tail flower

Cauliflower is an abnormal form of streamline tail.

There are three kinds of common tail flowers: fox tail flower, spear tip tail flower and magnolia tail flower. As shown in Fig. 12-4.

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How to Identify Carbon Steel and Stainless Steel According to the Spark During Grinding? 6

Fig. 12-4 Tail flower form

8) Color

The color and lightness of the whole flame beam or part of the spark.

3. What is the equipment and operation for spark identification?

The main tool for spark identification is the grinder.

The grinding machine can be desktop or portable.

Bench grinder is suitable for inspection of steel samples and small shaped parts.

Portable grinder can be used to identify batches of steel in workshops and warehouses.

The power of the motor used for the bench grinder is 0.5KW, and the rotating speed is about 3000 rpm.

The motor for portable grinder has a power of 0.2KW and a speed of 2800 rpm.

Excessive power and speed will make sparks scattered, which is not conducive to identification.

Too small power and speed will make it difficult to grind alloy tool steel and high-speed steel containing tungsten, or even produce no flame beam.

The grinding wheel shall have a grain size of 46 # or 60 # (preferably 60 #) and medium hardness (ZY1~ZY3) Corundum (GZ or GB) vitrified bond grinding wheel, the size of the bench grinder is 150~200mm, and the thickness is 20~25mm;

The grinding wheel of portable grinding machine can be used with a diameter of 90~120mm and a thickness of 16~20mm.

1) Be familiar with the performance of tools

Do not change the tools (grinding machine, grinding wheel granularity) frequently.

Familiarity with the performance of tools is an important part of spark identification.

Because the wheel speed and particle size of different grinders change, the spark form will be different.

2) Maintain the sharpness and roundness of the grinding wheel friction surface

The sharpness and roundness of the grinding wheel friction surface shall be maintained regularly to maintain a certain projection force.

(The grinding wheel is not sharp, so the streamline is reduced; the roundness of the grinding wheel is not round, and the steel will jump when rubbing with the grinding wheel, so the roundness of the grinding wheel should not be too small.).

3) Use standard blocks to correct the impact of the environment

Before work, try to identify the standard sample first to correct the influence of the objective environment.

The brightness of the working environment has a great influence on the spark observation.

4) Choose a good workplace

The identification site should not be too bright, but it does not need to be completely dark.

It is better to keep the brightness consistent to ensure the observation power of identification.

Generally, outdoor operation is not suitable.

If it is operated in the open air, a movable tarpaulin shall be made and covered with black cloth to avoid the influence of strong light from rabbits.

5) Standard steel sample of self-made steel grade

A set of standard steel samples with known steel grades shall be provided. The more complete the steel samples, the better.

For comparison in learning and identification.

The standard steel sample must undergo chemical analysis to determine the correct content of each element.

4. What is the significance of spark identification?

Answer: As a technical worker in the machinery manufacturing industry, you will encounter the problems of metal material selection and heat treatment.

When selecting and manufacturing mechanical parts, if the materials are not properly selected, or other steel grades are mixed during heat treatment during part processing, the performance of the parts can not meet the requirements of processing and use, but also bring economic losses or serious accidents.

To ensure the correct use of steel, it is necessary to know the variety and performance of the steel used.

Different varieties of steel have different numbers and marks.

Steel grade identification is to analyze and identify the steel used when the steel grade is unknown, so as to use the steel correctly and reasonably.

Therefore, the identification of steel grade is an indispensable and important link in the manufacturing process of machinery

The methods of steel identification can be divided into chemical and physical methods: chemical analysis is correct and reliable, but it is only applicable to steel sampling inspection.

The procedures are complicated, which requires more time and cost, so the on-site work is not applicable:

Although the physical identification method is not as accurate and reliable as the chemical analysis, it can reach a certain degree of accuracy through the experience accumulated by people’s long-term practice and understanding, so it is suitable for the preliminary analysis on site, with the characteristics of fast and convenient.

There are many kinds of physical identification methods, among which “spark identification method” and “Taoist analysis method” are the most simple and practical.

The steel identified by “spark identification method” is not only simple and rapid, but also does not damage the steel, so it is widely used.

Steel is one of the important raw materials for machinery manufacturing.

When a large number of steel products enter the factory, they may be mixed due to repeated turnover of transportation, storage, etc.

In the production process, the quality shall be the first pass before steel feeding.

The steel grade shall be rechecked and checked before the heat treatment of steel parts.

Different steel grades have different heat treatment conditions and process requirements. Don’t mistake the heat treatment process.

When parts need to be scrapped in time after scrapping, it is necessary to know the steel grades of the materials used for the parts, which must be identified.

5. What are the effects of alloy elements on spark changes?

Carbon is the main element of steel grade, and its activation form changes with the increase of carbon content.

6. What is the spark pattern of commonly used steel?

Spark patterns of common steels are as follows:

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Fig. 12-6 30 Steel

The whole flame beam is yellow, with thick streamline in the middle, slightly thin streamline at the root, slightly large fireworks at the tail, and long awn streamline hanging slightly.

The secondary explosion has many branches, and the explosion nodes are bright.

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Fig. 12-7 40 Steel

The length of the fire beam is slightly increased. All the fireworks are secondary burst, and the awn line is long and thick.

There are more fireworks in the whole fire beam, and pollen is beginning to appear. The tail of the fire beam is big, and the color is bright yellow·

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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, the number of streamline and fireworks increases, the streamline is thick and the awn line is long, there is a proper amount of pollen between the streamline, the streamline emits forcefully, the degree of bursting is larger, the nodes are bright, the number of fireworks at the tail is significantly more than that of 40 steel, and the color is yellow and bright.

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Fig. 12-9 50 Steel

The length of the fire beam is equivalent to that of 45 steel.

The explosion pattern is large, the number of streamline and explosion increases, the streamline is thick and the awn line is long, there is pollen between the streamline, the streamline is powerful, the degree of explosion is large, the nodes are bright, the number of explosion at the tail is obviously more than that of 45 steel, and the color is yellow and bright.

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Fig. 12-10 20Cr Steel

The whole flame beam is yellow, the streamline is slightly thick and long, the shape line is straight, and the middle part to the pouring tail is slightly sagging.

A single multi branch single explosion and carbon structural steel with the same carbon content are slightly more regular than the explosion pattern, the explosion degree is large, and the nodes are brighter.

At this time, the presence of chromium shows the role of lengthening and cracking.

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Fig. 12-11 40Cr Steel

The spark beam is yellow and bright, with many streamline lines: the secondary burst compound flower is large in shape, neat and regular, and the number of fireworks is large. The awn line is long and thick, and the flower angle is clear and separable.

There is a proper amount of pollen, with a large burst degree, thick streamline, and slightly drooping from the middle to the tail. There is a big branch chrysanthemum, and the burst degree is more intense.

At this time, the medium carbon low inscription still has the function of promoting explosion,

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Fig. 12-12 20CrMo Steel

The flame beam is shorter than that of 20Cr steel, the streamline is slightly thin, and there are multiple bifurcations and single explosion at one time.

Compared with 20CrMo, the explosion pattern has shrunk, the degree of explosion is weakened, the nodes are not very bright, the color is yellow, and the tail of streamline has gun tip tail flowers.

At this time, molybdenum has the property of inhibition.

Although chromium is an explosive element, it coexists with molybdenum, and its properties become subordinate.

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Fig. 12-13 40CrMo Steel

The color of 42CrMo steel flame beam is slightly darker than that of 40Cr steel, the streamline is similar to that of 40Cr steel, the secondary explosion compound flowers form, there is an appropriate amount of pollen, and the nodes are bright, but the explosion patterns are irregular and messy, the degree of the explosion is slightly weakened, there is a gun tip tail flower at the tail, and there is no obvious gun tip tail flower of 20CrMo steel.

From this, it can be seen that the carbon content has a certain impact on molybdenum.

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Fig. 12-14 60Si2Mn Steel

The length of the fire beam is moderate, the streamline is reduced, and slightly thick, and most of them burst twice.

Part of them burst for three times, with small flower type, obvious node of silicon bud, few and short awn lines, slightly weak burst degree, and no pollen.

Spark color and explosion node are not bright.

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Fig. 12-15 GCr15 Steel

The length of the fire beam is moderate, with many streamline and triple burst patterns.

The streamline is slightly thin, and the streamline is densely covered with branch shaped fireworks.

The amount is large, the pattern is small, and the awn line is thin and irregular.

There is a large amount of pollen between awn lines, the nodes are not very obvious, and the color is orange.

The internal organization is troostite pearlite in hot rolling state, with long fire beam, thick streamline, and three times of burst.

The burst intensity is intense, the awn line is long, there is a large amount of pollen between the awn lines, the burst node is bright, and the tail pattern is long in the middle.

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Fig. 12-16 Cr12MoV Steel

The fire beam is thin and extremely short, and the streamline is wavy and intermittent, with many and thin.

The fireworks are very strong. The sparks burst into three flowers, which have many branches and are big stars, with many broken flowers and pollen. The flowers are full of fire.

The streamline end has obvious gun shaped tail flower due to molybdenum content, and the streamline at the tail is slightly thick, so the material feels hard when rubbing.

Color: yellow to orange. Spark form is not different from Cr12.

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Fig. 12-17  5CrMnMo Steel

The fire beam is the longest, the streamline is medium in thickness, and the burst is second strong.

All of them burst three times, sometimes with a few flowers, and there are molybdenum gun tip tail flowers.

The flower shape is a multi branched three or four segment star shaped flower+lancet tail flower. The awn line is dense, the flower amount distribution area accounts for 55~60% of the whole fire bundle, the flower shape is large, and the flower angle is large.

Color: the fire beam is bright yellow, and the nodes are yellow to white. The resistance is less strong when grinding.

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Fig. 12-18 3Cr2W8V Steel

The fire beam is relatively long, the streamline is very thin, wavy and intermittent, the burst is weak, the amount of flowers is small, and the flower shape is bald fox tail, and small.

Body color: fuchsia.

Bald and solitary, light cherry red.

It feels very resistant when grinding.

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Fig. 12-19 W6Mo5Cr4V2 Steel

The fire beam is short, orange yellow, bright in the middle and tail, and dark red in the root.

There are discontinuous streamlines and a few wave streamlines.

The streamline is not very thick, and the length is medium;

The tail streamline is thick, showing willow leaf like tail flowers, and the tip is slightly bald.

The fireworks are large in shape and few in quantity.

There are few awn lines, which are bald.

Streamline tail droops.

7. What are the precautions for spark identification?

Answer: It is difficult to judge the steel grade of the tested sample according to the observed spark pattern.

Because the tiny differences of spark patterns are often difficult to describe and express accurately, the discriminator must study and experience in his repeated practice.

1) Identify and check the required steel grades of known parts.

At this time, it is only necessary to use spark identification to confirm whether it belongs to the steel grade used.

If it is to identify a batch of parts, the first one shall be carefully observed and analyzed.

If it is confirmed that steel No. 1 is used correctly, the part shall be lightly ground to observe the basic characteristics of sparks when it is least worn.

Keep this feature in mind, and the rest of the parts can be ground with light pressure.

In this way, it is not only conducive to identification, but also can make the parts less worn, so as to avoid affecting the appearance and use.

2) Two steel grades are known to be mixed for identification.

At this time, we must pay attention to the basic differences between the two steel grades in spark patterns.

When their respective characteristics and main differences are mastered, it is very convenient to distinguish between them.

3) Steel grade identification shall be conducted for parts with unknown steel grade.

In this case, if the discriminator understands the basic use of steel and is familiar with the common sense of what materials should be used to make various parts, it will bring great convenience to spark identification.

Whether there is dendritic explosion when grinding sparks.

If there is dendritic explosion, it can be further inferred from the following situations:

① Dendritic explosion occurs normally, and there is no spark in other special forms.

At this time, it is mostly carbon steel (killed and semi killed steel).

If its pattern is a split burst, and the burst pot is relatively sparse, it indicates that the carbon content is low and belongs to the low carbon range of carbon steel;

If its pattern is a secondary, tertiary or a small amount of multi split dendritic explosion, the amount of explosion is medium, and the distance between the explosion and the explosion is clear, it indicates that the carbon content of the sample is about 0.4% C, belonging to the carbon steel in the medium carbon range;

If the explosion is a multi forked tree shaped explosion, the amount of explosion is large, and the distance between the explosions is small.

When the explosion is crowded, it indicates that the carbon content is high and it belongs to high carbon steel.

② If the explosion has dendritic explosion and the basic form is feathery explosion, it belongs to rimmed steel with very low silicon content, and its carbon content is roughly estimated according to the amount of explosion, so as to infer the steel grade.

③ Roughly judge the steel type:

At the end of the flame beam, there is obvious dendrite explosion from the big branches.

The explosion pattern is large, the awn line is disordered, and the explosion strength is large.

Most of them belong to the alloy structural steel of the manganese steel group;

The fireworks are neat and regular, bright in color, straight and thick in streamline, and mostly belong to chromium containing structural steel;

At the end of the flame beam, there are obvious straight tail flowers of the gun tip, and the dendritic explosion is restrained to a certain extent.

Most of them belong to the structural steel of chromium molybdenum steel, chromium manganese steel and other steel groups;

The nodes before the explosion are particularly bright or the explosion with silicon bracts occurs, which mostly belongs to the structural steel or spring steel of silicon manganese and chromium silicon steel groups;

Both spear tip tail flowers and silicon containing bud flowers appear, and the dendrite explosion shrinks significantly, which mostly belongs to silicon manganese aluminum and silicon manganese aluminum vanadium structural steel;

The explosion is a regular dendritic explosion, and nickel containing bracts occur before the explosion.

The decarburized layer on the steel surface, the bright part of the so-called “rough cut” bracts is obvious, and most of them belong to chromium nickel alloy structural steel;

Fox tail flowers appear.

The tree like explosion is basically attached around the fox tail.

The root streamline is not dark red but bright. It belongs to tungsten alloy structural steel in Kesimiduo.

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