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New technology of steel ball thin layer carburizing

The main failure mode of steel ball is contact fatigue spalling, but the current steel ball quality standard only checks the crushing load and hardness, but does not require the contact fatigue performance of steel ball. Taking the bicycle steel ball as an example, the crushing load is 15500n in the national standard, 16700n in the Ministry and the hardness is HRC60-65. In order to meet the high standard requirements for crushing load, the manufacturers all produce according to the high-temperature carburizing and cooling quenching process (hereinafter referred to as the original process), and the thickness of steel ball carburizing layer reaches 1.2-1.4mm. Although such a thick infiltration layer makes the crushing load meet the requirements, the impact on the contact fatigue performance of steel balls is unknown. In addition, because the process adopts high-temperature carburizing and cooling quenching, it not only has long production cycle and high production cost, but also precipitates uneven network carbide in the carburizing layer and ferrite in the center, which is not conducive to the improvement of contact fatigue performance.

20210908014446 98207 - New technology of steel ball thin layer carburizing

This paper will study the new process without reducing the crushing load of steel ball, so as to improve its fatigue performance, reduce the cost and increase the economic benefit. Secondly, lower carburizing temperature will be used as far as possible to improve the service life of furnace tank (because the carburizing tank used in production is welded from steel plate, and the change of carburizing temperature has a significant impact on its service life).

Research items and methods

Research projects

  • (1) Steel balls were carburized and directly quenched at different temperatures (9000c, 9050c, 9100c, 9200c), and their crack tendency was investigated.
  • (2) Determine the relationship between carburizing layer thickness, carburizing temperature and carburizing time.
  • (3) Effect of carburizing temperature and carburizing depth on crushing load.
  • (4) Effect of kerosene drop on crushing load.
  • (5) At the optimum temperature, the effect of carburized layer depth on contact fatigue properties.
  • (6) Through the above research, a new process conducive to improving the quality of steel balls and reducing the cost is put forward.

Test materials and samples

The test material is No. 15 steel, and its chemical composition is 0.15% C, 0.15% Si, 0.40% Mn, 0.03% s and 0.02% p. The sample is processed and formed by the material Ф 6 steel ball.
2.3 research methods and equipment

  • (1) Rts-45-12 drum type gas carburizing furnace is adopted for carburizing.
  • (2) Three steel balls are crushed by vwpl universal testing machine, and the average crushing value shall prevail.
  • (3) Kg type fatigue testing machine is used for contact fatigue test, with loading of 200kg, rotating speed of 2200r / min, 9 grains each time, rolling wear, needle pitting as the failure standard, and 1H of industrial inspection as qualified.
  • (4) The fracture was analyzed by jxa840 scanning electron microscope.
  • (5) The carburized layer concentration is measured by Y-2 X-ray analyzer.

Analysis of test results

Crack tendency of steel balls directly quenched after carburizing

The steel balls directly quenched after carburizing at different temperatures and the steel balls not quenched in the original process shall be pickled respectively to check the cracks. The results are shown in Table 1.

Table.1 Effect of different carburizing temperatures on crack tendency

Treatment conditions(0 )

900

905

910

915

920

930

Original process not quenched

Number of 100 cracks(Unit)

7

7

8

6

6

5

7

It can be seen that the number of cracks is between 5% – 8%. The crack shape, width and depth are basically the same. This shows that the crack is produced in the process of rolling ball, not quenching, so the direct quenching method should be feasible.

Effect of carburizing temperature and carburizing time on carburizing layer thickness

For samples treated at different temperatures (9000c, 9050c, 9100c, 9150c, 9200c, 9300c) and different carburizing times (2.5h, 3h, 3.5H, 4.5H), the carburizing depth is measured. Some results are shown in Table 2.

Table.2 Relationship between carburizing temperature, carburizing time and carburized layer thickness

2.5h 3.5h 4.5h
900 0.55mm 0.75mm 0.9mm
930 0.73mm 0.93mm 1.1mm

It is concluded that at the same temperature curve, the initial carburizing speed (V) is large, the carburizing speed (V) decreases with the increase of time, and the carburizing layer thickens with the increase of time. This is now analyzed.

As we all know, in the carburizing process, the carburizing speed is affected by the decomposition of kerosene, the absorption of activated carbon atoms and the diffusion of carbon atoms.

  • (1) The decomposition temperature of kerosene is about 8750c. When the temperature is higher than 9000c, it decomposes completely. It may be considered that it is not affected by temperature and time.
  • (2) The adsorption of activated carbon atoms is mainly related to the composition infiltrated into the steel and the precipitation rate of activated carbon atoms. Therefore, the infiltration rate mainly depends on the diffusion process.
  • (3) According to Fick’s first law, increasing the concentration gradient on the surface of the infiltration layer is an important way to speed up the infiltration rate. In the initial stage of carburizing, a large number of activated carbon atoms are chemically adsorbed and strongly cracked by the carbon poor surface. Therefore, the carburized layer of steel is mainly formed by the high concentration gradient of the outermost layer of the carburized layer, resulting in a very high carbon concentration gradient. Therefore, at the beginning of carburizing, the carburizing rate is relatively large. With the infiltration of carbon atoms, the carbon concentration gradient gradually decreases. In this way, the carburizing rate also slows down.
  • (4) At the same carburizing time, the carburizing layer thickens with the increase of temperature. This is because the activity of activated carbon atoms increases with the increase of temperature, so the diffusion rate also increases. Therefore, the carburizing depth increases with the increase of carburizing time and carburizing temperature, but the carburizing rate decreases with the extension of time.

Effect of carburizing temperature and carburizing layer thickness on compressive load

The samples with different carburizing temperature and different carburizing layer depth are crushed, and the compressive crushing value is measured. The following conclusions are obtained: when the carburizing temperature decreases, the carbon layer becomes thinner, but the compressive crushing value does not decrease.
The reason for this is that the compressive crushing load of carburized and quenched steel balls depends on the strength of the carburized layer (thickness and concentration gradient) and the core. On the one hand, fine martensite structure is obtained on the surface layer, which improves the surface structure and the strength and toughness of the carburized layer; On the other hand, all high-strength low-carbon martensite (about HRC45) can be obtained in the heart, which is also conducive to the increase of crushing load. The original process carburized for a long time at a higher temperature (9300c) and then quenched at 820 ℃ with the furnace, which not only obviously coarsened the martensite structure of the carburized layer, but also precipitated network carbides on the surface during the furnace cooling process, which worsened the surface structure, reduced the hardness and increased the brittleness; More ferrite precipitates or troostite is formed in the heart. The core structure is coarse lath martensite and troostite, and the hardness is about HRC36. To sum up, the use of low temperature thin-layer carburization and direct quenching is conducive to the improvement of compressive crushing load of steel balls.
In addition, the spectrum analysis shows that the carburized layer carbon concentration of the sample carburized at low temperature (9150c) for 5h is flat. The carbon concentration of the carburized layer in the original process is too high, and the concentration gradient is not smooth enough after 1.25h diffusion. This can also be proved by test fracture analysis.
The crushing fracture of the new process is a typical ductile quasi cleavage, with tearing edge and ductile separation. The crack develops along the transition layer, and its concentration gradient transition is relatively gentle, so that the carburized layer is closely combined with the core matrix. When subjected to external force, it is not easy to peel off, which reduces the crack propagation ability and improves the compressive strength. The crack is generated and propagated along the grain boundary, about 0.5mm from the surface. On the contrary, the fracture of the original process is brittle quasi cleavage. The fracture crack extends from the surface layer to the transition layer, and then along the transition layer. It undergoes intergranular development quasi cleavage, reducing the compressive strength. Part of the crack is intergranular fracture, on the other hand, it is brittle fracture, and the fracture distance from the surface is about 0.6mm.

Effect of dropping amount on crushing value

Carry out variable drop test at the preferred temperature, measure its compressive crushing value, and obtain Table 3 and Figure 2. The test condition is 9150c and carburizing for 5h. It can be seen that the compressive crushing value of kerosene drops at 6ml / min is higher.

Table.3 Relationship between drop amount and compressive crushing value

ml/min 4 6 8
N 17800 18000 1750

Kerosene is an organic liquid containing carbon, in which alkanes CnH2n + 2 account for 60% – 65%, cycloalkanes CnH2n account for 20% – 30%, and other hydrocarbons cnh2n-6 account for 7% – 10%. It can only be cracked at high temperature (lower limit 8750c). After cracking, there is more excess carbon, which is easy to form carbon black and coking. With the increase of drop amount and carbon potential, the infiltration rate is accelerated, the thickness of carburized layer is increased, and its compressive crushing value is increased. However, this does not mean that the larger the drop amount, the better. When the drop amount is too large, the carbon potential in the furnace will increase and carbon black will be generated, which will surround the carburized steel ball and reduce the infiltration rate; In addition, due to the increase of carbon potential, excessive concentration gradient is generated in the transition zone between carburizing and surface layer, and cracks are easy to produce and expand in the transition zone. Since the thin-layer carburizing time is short and carried out under the condition of unbalanced carbon potential, the eutectoid concentration is required on the part surface, so the drop amount (7ml / min) can be appropriately increased during just carburizing. When the drop amount is too small, the carbon potential concentration is not enough, resulting in uneven carburization and unstable compressive crushing load.

To sum up, 6ml / min is suitable for carburizing at 9150c and 5h.

Relationship between contact fatigue strength and penetration depth

This needs to be analyzed.
The fatigue test of steel balls with different penetration depth is carried out at 9150c. It is found that the fatigue life increases with the increase of penetration layer. After reaching a certain value, it decreases. The fatigue strength of 0.8mm carburizing layer depth (9150c carburizing 5h direct quenching) is 10 times higher than that of the original process. After analysis, the reasons for the improvement of steel ball fatigue strength are summarized as follows.
(1) The surface residual compressive stress is increased.
High surface residual compressive stress was obtained after shallow carburizing and direct quenching. The surface residual stress is formed due to the different transformation order of austenite in the core and carburized layer into various structures and the different specific volume of high carbon martensite on the surface.
Specifically, when the steel ball is subjected to a certain pressure, the crack will crack perpendicular to the stress direction. If there is a high residual stress on the surface of carburized layer, it can resist the cracking force of steel ball caused by external load, so as to improve the compressive crushing value. When the infiltration layer is too deep, there will be excessive residual austenite free carbides, which are harmful to the residual compressive stress. If the carbon content of the surface layer is too high and there are a large number of residual austenite and other abnormal structures, the residual tensile stress will appear on the surface layer of the carburized layer. The higher the carbon content in martensite, the greater the specific volume and the greater the lamination stress.
The test shows that the surface carbon concentration of carburized layer of low carbon steel tends to saturation after 2.5-3h. When the carburized layer depth is less than 0.65mm, the carburized layer carbon concentration does not reach saturation. At this time, the specific volume of martensite increases with the increase of carburized layer carbon content, and the residual compressive stress increases with the increase of specific volume of martensite. When the infiltration depth is greater than 0.65mm, with the increase of carbon content, the surface residual austenite increases, the surface martensite transformation variable decreases relatively, and the residual compressive stress decreases. At this time, the infiltration depth, the core low-carbon martensite decreases relatively, the surface high-carbon martensite increases greatly, and the surface residual compressive stress also decreases. The contact fatigue performance of the steel balls treated by the original process is not good, which belongs to the latter case.
Formation mechanism: there is a great difference in chemical composition between the infiltrated layer and the core, resulting in the change of martensite MS in the infiltrated layer. In the process of quenching and cooling, the core is often transformed into martensite first, while the surface layer has not reached the MS point and is still in the plastic supercooled austenite state. The stress caused by volume expansion caused by the core transformation is very easy to be absorbed by the plastic deformation of the surface layer. When the temperature drops to the surface MS point, the volume expansion caused by martensitic transformation in the infiltrated layer is restricted by the strengthened core, resulting in the state of surface compression and core tension.
(2) Martensite and retained austenite in carburized layer were refined.
High carbon martensite is formed by coherent shear. When it grows to meet other martensite sheets or grain boundaries, it will produce impact and form a stress field. Because high carbon martensite is very brittle, it can not eliminate stress through deformation or slip, resulting in microcracks. The crack sensitivity increases with the increase of martensite needle length. According to the principle of heat treatment, the thickness of acicular martensite on the surface will directly affect the contact fatigue life of the infiltrated layer surface. Microcracks in coarse acicular martensite are natural crack sources causing contact fatigue failure.
For the steel balls produced by the original process, not only the infiltrated layer martensite is coarse (grade 6-7), but also the residual austenite is coarse and unevenly distributed. At the same time, due to cooling and precipitation of non-uniform network K, the martensite transformation variable on the surface is relatively reduced, and the carbon concentration on the surface is about 1.0%, which will further reduce the strength and toughness of the infiltrated layer. For the steel balls produced by the new process, the surface acicular martensite is fine (Grade 5), the surface concentration is about 0.8%, there is almost no K, and a small amount of residual austenite is evenly distributed in the fine acicular martensite matrix, so as to reduce the brittleness of the surface and improve the fatigue performance.
Formation mechanism: the contact fatigue life of carburized surface is related to M. high carbon martensite is needle flake, hard and brittle. The thicker the needle, the more brittle, and often accompanied by microcracks. Under the action of external load, its crack expands rapidly. Due to the existence of retained austenite, the loaded surface has a certain plastic deformation and the width of the contact surface increases, which correspondingly reduces the compressive stress of the contact surface and improves the service life. On the other hand, due to the effect of plastic deformation, austenite is induced to transform into martensite, resulting in work hardening, which also improves the service life. In addition, in the fracture process, the crack mainly propagates along the martensite region, which is difficult to pass through the residual austenite. Therefore, under the action of certain stress, once the crack developing along the martensite reaches the martensite and residual austenite surface, the crack will stop developing. Only when the applied load is increased, the crack will produce bifurcation and bypass the retained austenite to continue to develop. Because the crack generates bifurcation and absorbs energy, it is conducive to the improvement of toughness. On the contrary, if there is tensile stress on the surface, the shear stress caused by mutual sliding is promoted, and the shear stress caused by mutual sliding is promoted. Therefore, residual austenite distributed around martensite can improve the ability of materials to resist crack propagation.
After fatigue wear, the retained austenite of the sample treated by the new process is significantly less than that of the original sample, and the martensite is smaller. Therefore, martensite is fine and has good toughness, which can strengthen the surface and form a compressive stress state, which is conducive to improving its fatigue strength. It is found that some retained austenite still exists when fatigue failure occurs, which is still beneficial to toughness.
The residual austenite on the surface of the sample treated by the original process is excessive and distributed in blocks at the coarse m-needle and edge. When fatigue failure occurs, the change of residual austenite volume can not be seen basically, the phase transformation strengthening effect is greatly reduced and the service life is affected.

(3) The improvement of effective hardening layer is conducive to the improvement of fatigue performance.
When the alternating contact stress of steel ball occurs, the maximum stress is often on the surface or sub surface. The fatigue crack source generally occurs in the surface layer of 0.1mm-0.3mm, which is proved by experiments. Therefore, in order to improve the fatigue performance, we should focus on improving the hardness and strength of the dangerous layer, rather than thickening the infiltrated layer. The test shows that when the thickness of the infiltrated layer is less than 0.75mm, the hardness is increased by 4-5HRC compared with the original process, which improves the strength of the dangerous layer and the fatigue performance. This is also confirmed by the surface morphology analysis of fatigue failure.
Contact fatigue damage is actually the process of crack initiation and propagation. The whole fracture process can be understood through fracture analysis. According to the characteristics of crack initiation and spalling, contact fatigue can be divided into pitting and spalling. Where the crack originates on the surface, the fibrous spalling is pitting corrosion, and the surface pitting of the new process steel ball after 13.5h load operation belongs to this kind; If the crack originates from the surface and flakes off, it is spalling. Large deep spalling occurs on the surface of the steel ball treated by the original process due to insufficient strength after 1.5h load operation.
The reason for this is the low surface hardness of the original process steel ball, which is caused by the irregular distribution of a large amount of residual austenite in the carburized layer. Concentration analysis shows that the surface carbon concentration of the steel ball is about 1.0%, and the carburizing temperature is high, so a large amount of residual austenite is produced. Under the action of contact stress, although there is mutagenic martensite, the quantity is small and the transformation strengthening effect is not obvious. Under the action of stress, there is relative sliding between soft residual austenite and hard martensite, which makes the crack initiate and expand to intergranular fracture, and finally the quasi cleavage fracture appears. The original process steel ball experienced slip intergranular fracture quasi cleavage and fatigue spalling. The residual austenite on the surface of the new process steel ball induces martensitic transformation, and the surface has good strength and toughness. When fatigue failure occurs, only shallow quasi cleavage fracture occurs.
The above tests show that the strength and toughness of retained austenite depends on the mechanical stability of retained austenite, that is, there must be a certain amount of retained austenite on the one hand, and induce martensitic transformation under contact stress on the other hand.
In a word, increasing the hardness of the surface or sub surface under high stress is an effective way to improve the fatigue life, not by increasing the depth of the infiltration layer.
The main form of steel ball failure for bicycle is fatigue failure, but because the fatigue failure test takes a long time, most enterprises use crushing load to detect. But there is no corresponding relationship between them. From the test results, the fatigue strength value of the new process is the largest when the thickness of the carburizing layer is equal to 0.75mm, while the crushing resistance value increases with the thickening of the carburizing layer, that is, it increases with the extension of carburizing time, but the fatigue strength decreases at the same time. After the wheel load is 500kg, it can be seen that the load of steel ball does not exceed 2250n, which is why the long-time carburizing method with high crushing load is not selected, and the heat treatment method with high fatigue strength and low crushing load is adopted.

New process of steel ball heat treatment

Comprehensively investigate the compressive crushing load and contact fatigue strength. According to the above test analysis, the heat treatment process is 9150c carburizing for 5h and direct quenching based on the principle of economy and improving product performance, quality and service life.

Economic benefit analysis

  • (1) The production of steel ball in the original process takes 7.25h, while that in the new process is 5.67h, which is shortened by 1.58h, and the production efficiency is increased by 1.58 / 7.25 = 21%.
  • (2) The carburizing temperature is reduced from 9300c to 9150c, and the time is shortened, which prolongs the service life of the carburizing furnace.
  • (3) The time is shortened, the dropping amount is reduced, and the amount of kerosene and methanol is reduced.
  • (4) The service life of wear test is increased from 1.5h to 13.5h, which has obvious economic and social benefits.

Conclusion

  • (1) The new process of steel ball thin layer carburizing has many advantages over the original process and is beneficial to production.
  • (2) The new process improves the structure of the steel ball, obtains all the low-carbon martensite with high strength in the center, obtains fine acicular martensite with good strength and toughness in the infiltrated layer, reduces the brittleness caused by high-carbon martensite and the content of residual austenite in the surface layer, increases the residual compressive stress, improves the ability to resist surface cracks, and due to the induced strengthening of martensite, The surface strength, toughness and fatigue strength of the infiltration layer are improved, so that a higher compressive crushing load is obtained with a thinner infiltration layer.
  • (3) The new process makes the steel ball obtain good metallographic structure and reasonable carburized layer, and the concentration gradient is gentle. Direct quenching is adopted to increase the flow rate of quenching medium water, improve the core strength and increase the supporting effect on the hardened layer. The good properties of infiltration layer and core ensure that the fatigue strength is greatly improved and the crushing load is reduced when the infiltration layer is thin.
  • (4) The hardness of the effective hardened layer is improved, the strength of the dangerous area prone to cracks is strengthened, and the fatigue strength is effectively improved.
  • (5) The new process shortens the production cycle, improves the production efficiency, reduces the material consumption, improves the product quality, and has good social and economic benefits.

Source: Network Arrangement – China Steel Balls Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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