Patent Application
20190003001
The present invention relates to the technical field of oil-immersion cooling of workpieces in heat treatment of metals, in particular to an oil-immersion quenching cooling precursor that is separated into a plurality of sections to expel gas bubbles and an oil-immersion quenching cooling method.
In the production process, most axle-type workpieces are heated and quenched in a vertically hung state in order to reduce flexural deformation of the workpieces in the heating and quenching cooling process. The quenching mentioned here includes a quenching process for a purpose of obtaining a martensitic structure in certain depth as well as a cooling process for obtaining a fine pearlite structure in large-diameter workpieces by oil-immersion cooling. Until now, the consensus in the industry is that there are two major factors that have influence on the quenching cooling rate of workpieces: one factor is the cooling characteristics of the cooling medium, and the other factor is the effective thickness of the quenched workpiece. To study the influence of the cooling characteristics of a cooling medium on the cooling rate, a cooling characteristic curve of the cooling medium must be first detected and plotted. The curve reflects the cooling characteristics of the cooling medium in vapor film stage, boiling cooling stage, and convection cooling stage, and reveals a one-to-one relationship between the surface temperature and the surface heat flux density of the workpiece in each of the stages. In addition, it is common knowledge in the art that the workpiece cools very slowly in the vapor film stage because the thermal conductivity of a vapor film is very poor. In other words, the gas in a vapor film doesn't flow. In fact, there is no report on the flow of gas in a vapor film up to now.
Regarding the influence of the effective thickness of workpiece on the cooling rate, it is widely believed in the industry that the parts of the same workpiece that have the same effective thickness usually obtain the same quenching cooling effect. Then, in the quenching process of workpieces in the same furnace, the cooling effect of the parts of the same axle-type workpiece should be essentially the same without any obvious difference, because the parts have the same effective thickness. This is the widely accepted belief in the industry. There is no knowledge or report contrary to that belief in any relevant literature. In view of that, in productions and applications, for axle-type workpieces that are immersed and quenched in a vertical state, the as-quenched hardness is usually checked only at specified spots, and the uniformity of the as-quenched hardness of the entire workpiece is seldom checked in the axial direction. However, in actual detection the as-quenched hardness in the axial direction is not uniform on axle-type workpieces obtained by cooling with the existing quenching method; the hardness values at specified local spots can't truly reflect the quenching cooling effect of the entire workpiece. After such workpieces with non-uniform as-quenched hardness are put into use, some problems may occur, such as compromised mechanical properties and shortened service life, etc., and accidents may even occur in the service life of the parts.
Presently, to ensure that the workpieces meet the specified quenching quality, some measures can be taken to improve the cooling uniformity and as-quenched hardness of the workpieces, such as stirring. In general, the uniformity of as-quenched hardness of a workpiece can be improved by improving the uniformity of cooling of the workpiece. However, due to the difference in shape complexity between workpieces and hardenability between materials, uniform cooling does not ensure uniform as-quenched hardness. It is believed that the uniformity of temperature of the quenching oil can be improved if the quenching oil is stirred well, and thus the uniformity of cooling and uniformity of as-quenched hardness of the workpiece can be improved. Moreover, stirring can strengthen the heat exchange between the workpiece and the quenching oil and thereby improve the as-quenched hardness of the workpiece. However, it is impossible to obtain the same cooling effect for different workpieces quenched in the same furnace or different parts of the same workpiece in quenching oil at a uniform temperature, due to the characteristics of medium stirring and complexity of stirring problems. In addition, a uniform quenching oil temperature doesn't mean uniform as-quenched hardness of the workpiece, due to the influences of workpiece shape and position. Consequently, even workpieces quenched in the same furnace often have the problem that some of the workpieces have quenching deformation or unacceptable hardness.
In a first aspect, the present invention provides an oil-immersion quenching cooling precursor that is separated into a plurality of sections to expel gas bubbles, to improve the as-quenched hardness and uniformity of hardness of an axle-type workpiece. The workpiece is an axle-type workpiece or workpiece that has section(s) in an axle form; the oil-immersion quenching cooling precursor is an axle-type workpiece or workpiece that has section(s) in an axle form on which several separation rings are arranged in the axial direction to separate the workpiece into a plurality of sections in the axial direction before oil-immersion quenching cooling, and the workpiece is separated into a plurality of sections to expel gas bubbles.
The separation rings are distributed on the surface of the workpiece in the axial direction.
The separation rings are machined integrally with the axle-type workpiece.
At least one separation ring is arranged on the workpiece.
The longitudinal cross-section of the separation ring can be a rectangular shape, sloped shape, stepped shape, triangular shape, or other irregular shape.
The top of the separation ring can be a flat top, domed top or spire top.
The length (L) of a part of the separation ring that is coupled with the outer surface of the workpiece in the axial direction of the workpiece (i.e., base thickness) is 1-20 mm.
The length (h) of an outer edge of the separation ring in relation to the outer surface of the workpiece in the radial direction of the workpiece (i.e., height) is 1-10 mm.
The spacing (b) between adjacent separation rings is 10 mm-200 mm.
The spacing between the separation rings may be even or uneven.
In a second aspect, the present invention provides an oil-immersion quenching cooling method for a workpiece, comprising: heating up the oil-immersion quenching cooling precursor described above and immersing the oil-immersion quenching cooling precursor into oil to accomplish quenching cooling.
In a third aspect, the present invention provides a workpiece processing method, comprising the oil-immersion quenching cooling method described above and a procedure of removing the separation rings to obtain a workpiece with required dimensions after quenching cooling.
The separation rings are removed by cutting, and the cut parts are cooled.
The workpiece processing method further comprises a tempering procedure of the workpiece, after which the procedure of removing the separation rings is executed.
In a fourth aspect, the present invention provides a workpiece obtained with the processing method described above, wherein, the workpiece is an axle-type workpiece or workpiece that has section(s) in an axle form.
Compared with the prior art, the present invention attains the following beneficial effects:
1. Improving inherent quality of the workpiece: utilizing the oil-immersion quenching cooling precursor provided in the present invention in the quenching process can effectively improve as-quenched hardness and uniformity of as-quenched hardness of a workpiece (especially an axle-type workpiece), reduce quenching deformation of the workpiece, and improve fatigue life of the workpiece.
2. Saving alloy element resources: since utilizing the oil-immersion quenching cooling precursor provided in the present invention can improve as-quenched hardness of a workpiece, steel with lower hardenability can be used instead of steel with higher hardenability for some types of workpieces, and therefore alloy element resources are saved.
3. Reducing production cost: for some type of workpieces, when the oil-immersion quenching cooling precursor provided in the present invention is used as a transitional workpiece in the quenching cooling process, ordinary quenching oil can be used in the quenching process to obtain the same quenching cooling effect that can be obtained with fast quenching oil.
4. Improving production efficiency: the oil-immersion quenching cooling precursor and oil-immersion quenching cooling method provided in the present invention can greatly shorten the quenching cooling time of a workpiece, and that effect is even more obvious when the oil-immersion quenching cooling precursor and oil-immersion quenching cooling method provided in the present invention are used for large-size axle-type workpieces.
5. The oil-immersion quenching cooling precursor provided in the present invention has advantages including: simple principle, excellent and steady effect, and extremely high uniformity, etc. Therefore, if the oil-immersion quenching cooling precursor and oil-immersion quenching cooling method provided in the present invention is used in combination with the existing process and method, i.e., separation rings are worked out at appropriate parts of a workpiece before quench heating, it is possible to greatly reduce or even eliminate defective products in the quenching process.
In recent years, the inventor of the present invention has studied liquid-immersion quenching cooling processes of a variety of test samples through extensive tests, and has found that the parts of the same workpiece that have the same effective thickness usually can't obtain the same quenching cooling effect in the quenching cooling process. For example, though the upper part and lower part of an axle-type workpiece have the same effective thickness, their cooling effects are quite different from each other. Up to now, there is no such report in relevant literatures or the industry. Moreover, except for the inventor of the present invention, no one has reported that the gas in a vapor film on the surface of a workpiece can flow and the vapor film can release gas bubbles in a liquid-immersion quenching process. Furthermore, no one has reported that the gas flow in the vapor film and the released gas bubbles have influence on the cooling rate and cooling uniformity of the workpiece.
Over a decade, the inventor has carried out extensive video recording and observation on the quenching cooling process of samples in oil and water and has conducted analytical study on numerous observed phenomena. It was found that the gas in the vapor film can flow and the flow pattern has influence on the cooling rate and cooling uniformity of the workpiece. In addition, it was found that there is no definite one-to-one correspondence relationship between the transition from vapor film cooling mode to boiling cooling mode on the surface of a workpiece and the surface temperature of the workpiece. Rather, there is a special law. The inventor has concluded from the research findings that two new factors have influence on the cooling rate and cooling uniformity of a workpiece in the quenching process: first, the gas flow pattern in the vapor film, and second, the sequence of transition from vapor film cooling mode to boiling cooling mode. These new factors revealed a theory on why an axle-type workpiece quenched in a vertical state can't obtain high and uniform quenching cooling hardness. In addition, utilizing the new theory, the invention has put forward a quenching cooling method in the present invention to solve such problems.
The inventor has further found that the gas in the vapor film on the surface of a workpiece can flow in the liquid-immersion quenching process and the hot gas will be released in a form of gas bubbles from the top of the vapor film. Based on this the inventor has drawn the conclusion that the gas flow in the vapor film has influence on the cooling rate and cooling uniformity of the workpiece. In addition, in the quenching cooling process, the transition from vapor film cooling mode to boiling cooling mode is realized by occurrence of a hyper-spreading spot and demarcation line borrowing long before the thickness of the vapor film reaches zero. Finally, by summarizing the newly discovered phenomena and research findings, the inventor has concluded two new factors that have influence on the cooling rate and cooling uniformity of a workpiece in the liquid-immersion quenching process: one factor is the gas flow in the vapor film on the surface of the workpiece; the other factor is the sequence of transition from vapor film cooling mode to nucleate boiling (hereafter simply referred to as boiling) mode.
In the first factor, the inventor has found that the gas flow pattern in the vapor film is as follows: the gas in the vapor film is evaporated from the liquid surface at the outer side of the vapor film. In the gas in the vapor film, the gas in the inner layer closest to the outer surface of the workpiece has the highest temperature, while the gas in the outer layer closest to the liquid surface has the lowest temperature. Namely, the gas temperature distribution in the vapor film is very uneven. For axle-type workpieces, in the vapor film along the vertical surface, the temperature of the gas in the outer layer close to the liquid surface in the vapor film is essentially the same, regardless of the vertical position (usually, the temperature of the gas in the outer layer is slightly higher than the boiling temperature of the cooling medium). Since there is a high temperature difference between the gas in the inner layer and the gas in the outer layer, the gas in the vapor film will flow.
Possible gas flow patterns include laminar flow and circulative convection flow.
Wherein, the gas in the inner layer flows upward and becomes the laminar flow layer that flows upward vertically along the surface of the workpiece. The gas transferred upward in the laminar flow layer is always the part of gas that has the highest temperature in the gas at the same elevation. The gas transferred in the laminar flow layer is released in a form of gas bubbles into the quenching liquid from the top of the vapor film above the workpiece. The gas evaporated from the liquid surface below the workpiece is released from the vapor film above the top of the workpiece continuously. In the upward flowing process, since the laminar flow layer is the closest to the surface of the high-temperature workpiece and is further heated by the high-temperature surface, the temperature of the gas in the laminar flow layer is increased continuously, and causes decreased heat dissipation rate at the surface above the workpiece. In other words, the cooling effect above the workpiece is weakened. Such an action may be regarded as a heating action of the gas in the laminar flow layer to the workpiece surface above the gas. Consequently, in the outer surface of the workpiece that has the same effective thickness, the surface of the upper part is cooled more slowly than the surface of the lower part; moreover, the longer the axle-type workpiece is, the longer the path along which the laminar flow layer is heated is, and the higher the difference between the cooling rate of the upper part of the workpiece and that of the lower part of the workpiece is. For any axle-type workpiece (or any workpiece that has section(s) in an axle form, i.e., sections in an axle form on workpieces in other shapes), such a problem inevitably exists when the axle-type workpiece is quenched in a vertical state.
The circulative convection mentioned here only happens in the gas within a range from the intermediate portion in the vapor film to the liquid surface, as shown in
In the second factor, the inventor has found that the law of sequence that has influence on the transition of the vapor film is: on the surface of the same workpiece having the same effective thickness, such a transition can happen starting from a vapor film area that is so small that it can be referred to as a “spot”, by dint of thickness fluctuation of the vapor film, only after the surface temperature is decreased to be lower than a characteristic temperature value (T0—minimum surface temperature of the workpiece, at which the transition from vapor film cooling mode to boiling cooling mode absolutely can't happen); the small vapor film area where the transition starts is referred to as a hyper-spreading spot (the term “hyper” is used here because the thickness of the vapor film at that spot is still quite large and not reduced to zero yet when the transition happens). The boundary between the boiling cooling area and the vapor film area after the transition happens on the surface of the workpiece is referred to as a demarcation line. Then, as the demarcation line spreads towards the vapor film area, the vapor film portion where the demarcation line has spread over transitions gradually. Due to the transition pattern on the surface of a workpiece having the same effective thickness is sequential. Such a transition of the vapor film on the surface by dint of the arrival of the demarcation line is referred to as demarcation line borrowing.
Through research, the inventor has found that T0 is higher than the boiling temperature of the cooling medium by about 100° C. at the most (although sometimes actually higher by about 20° C.) , instead of higher than the boiling temperature of the cooling medium by hundreds of degrees Celsius as widely believed in the industry. In fact, the temperature at which the transition happens on the surface is much lower than T0. That is to say, in the quenching cooling process, the temperature range of the surface that can be covered by the vapor film at any part of the workpiece starts from a temperature near the quenching heating temperature of the workpiece (e.g., about 850° C.) and extends as far as to a temperature higher than the boiling point of the quenching liquid by several dozens of degrees Celsius. The findings indicate that: if different cooling patterns are sorted by their contribution to the workpiece quenching cooling effect, the cooling pattern that makes the greatest contribution is the cooling pattern before the transition, i.e., the cooling pattern exists when the surface of the workpiece is covered by the vapor film. For that reason, in practical applications, the cooling rates at different parts of the surface of a workpiece can be determined roughly according to the arrival time of the demarcation line: a part where the demarcation line arrives earlier is cooled faster, while a part where the demarcation line arrives later is cooled more slowly.
As shown in
The results of tests and observations also demonstrate that the cooling time in the boiling cooling mode on the surface of a sample is usually very short, while the cooling time in the vapor film cooling mode is much longer relatively in the quenching cooling process.
The degree of cooling non-uniformity in the quenching cooling process of an axle-type workpiece in vertical state can be demonstrated in a simple test. A Φ20×135 mm cylindrical sample is heated to 850° C. and then quenched in base oil in a vertical state.
Hereunder an analysis is made with reference to
Based on the above finding, the inventor puts forward a solution to the problem: separating the laminar flow layer of the vapor film that should extend continuously from the bottom end of an axle-type workpiece to the top end into a plurality of sections, and enabling each of the sections to release gas bubbles from the top. In addition, the separation rings configured to separate the laminar flow layer can transition soon after the workpiece is immersed into the liquid, so that demarcation lines required for transition can be provided to the vapor film in the sections near the separation rings, as shown in
The present invention will be further detailed in embodiments, but those embodiments should not be understood as constituting any limitation to the present invention.
The oil-immersion quenching cooling precursor that is separated into a plurality of sections to expel gas bubbles in the present invention will be detailed in examples of axle-type workpieces. In the machining work before quenching, several separation rings 1 are worked out on the surface of an axle-type workpiece, as shown in
In the oil-immersion quenching cooling precursor and the oil-immersion quenching cooling method provided in the present invention, the action principle of the separation rings is as follows: the base thickness of the separation ring is much smaller than the diameter of the workpiece substrate; therefore, within a very short time early in the liquid-immersion quenching, the majority of surfaces of the separation rings can be cooled to lower than the boiling temperature of the cooling medium, as shown in
To prove the effect of the oil-immersion quenching cooling method provided in the present invention and verify the difference between the method provided in the present invention and the existing method in terms of cooling effect, the following experimental examples are used.
Two samples are taken, wherein, one sample is a sample 1a without separation ring, the other sample is a sample 1b with separation rings, both of the two samples have dimensions of Φ30 cm×135 cm, the axial cross section of each separation ring is in a trapezoid shape, the top part is a horizontal surface, the bottom part is a beveled surface, the base thickness is 2 mm, the top thickness is 1 mm (the top thickness refers to the length of the separation ring calculated from the farthest point from the workpiece substrate in the axial direction), the height is 3 mm, the spacing between the separation rings is 25 mm, both the sample with separation rings and the sample without separation rings are heated up to 850° C. under the same conditions, and then are quenched in the same oil in a vertical state.
A sample 2b with separation rings and a sample 2a without separation rings are worked out from the same 45 lb steel bar. Both test substrates have dimensions of Φ20×135 cm, except that the sample 2b with separation rings has four separation rings. The shapes, dimensions, and spacing of the separation rings are the same as those in the experimental example 1. Both samples are heated up to 850° C. under the same conditions, and then are cooled in the same fast quenching oil in a vertical state. The diagram on the left in
From the diagrams: the bottom end of the sample 2a without separation rings is cooled faster, and a hyper-spreading spot occurs there at 5.5 s; the top end is cooled more slowly, and a hyper-spreading spot occurs there later; furthermore, the demarcation line at the bottom end spreads upward more quickly, and the demarcation line at the top end and the demarcation line at the bottom end meet each other at 40 mm from the top at about 23.1 s. The demarcation line spreading takes 17.6 s, from 5.5 s to 23.1 s.
From the diagram of demarcation line spreading on the sample 2b with separation rings: the demarcation lines in the three middle sections separated by the separation rings start to spread at 6.2 s, and the moment when the last piece of vapor film disappears is at 8.5 s; the demarcation line spreading takes 2.3 s only. The sample with separation rings is cooled faster. Since the cooling processes of the three sections are the same, it indicates that the cooling effect is uniform and steady.
After the quenching cooling, the separation rings on the sample 2b are ground off. Then, at the middle parts of the sections separated by the separation rings, the surface hardness is measured in the axial direction respectively. On the sample 2a without separation rings, the surface hardness distribution in the axial direction is measured directly. Next, the surface hardness distribution curves of the two samples in quenched state are plotted, as shown in
The comparison between the two curves shows: From bottom to top, the sample 2a without separation rings obtains 50 HRc hardness only within a range smaller than 30 mm from the bottom end, and then the hardness begins to decrease; the hardness drops to be lower than 20 HRc rapidly within the range of 50 mm to 80 mm from the bottom end, and drops to a minimum value of about 18 HRc at about 90 mm from the top end; then, the hardness gradually returns to 25 HRc at the top end; the maximum hardness and minimum hardness on the surface are 50 HRc and 18 HRc respectively, with 32 HRc difference between them. This result matches the part where the demarcation line spreading is accomplished. In contrast, the surface hardness curve of the sample 2b with separation rings in the axial direction is very steady, and is always at about 50 HRc.
In this experiment example, three conclusions can be drawn: 1. The sample with separation rings obtains higher and more uniform as-quenched hardness. 2. If no separation ring is arranged on the sample, a steel material that has better hardenability has to be used (e.g., 42CrMo) for the sample to obtain as-quenched surface hardness not lower than 50 HRC along the full length of the sample. Thus alloy element resources can be saved by applying the method provided in the present invention. 3. After separation rings are applied, the surface hardness distribution curve is independent of the sample length. That is to say, no matter how the sample length is increased, the same as-quenched hardness value can be obtained along the full length.
The two samples used in this experiment are taken from the same 42CrMo bar, have dimensions of Φ20×135 mm, wherein, one sample is a sample 3a without separation rings, the other sample is a sample 3b with separation rings, and the shapes, dimensions, and spacing of the separation rings are the same as those in the experimental example 1. The two samples are heated up to 850° C. under the same conditions; then the sample 3b with separation rings is cooled in 60 SN base oil in a vertical state, while the sample 3a without separation rings is cooled in fast quenching oil in a vertical state. In this experimental example, 60 SN base oil replaces the original fast quenching oil to quench the sample 3b with separation rings, and the result is compared with the result of the sample 3a without separation rings quenched in fast quenching oil. Table 1 shows the comparison of cooling characteristics between the 60 SN base oil and the fast quenching oil (at 50° C. oil temperature, without stirring).
The Table 1 indicates that there is a great difference in cooling performance between the fast quenching oil and the base oil, and the cooling rate attained with the fast quenching oil is much higher than that attained with the base oil.
After the quenching is finished, the as-quenched surface hardness is measured in the axial direction on the two samples. Table 2 shows the comparison of surface hardness between the two samples.
It is seen from the test results in Table 2: when the sample with separation rings is quenched in 60 SN base oil, it obtains as-quenched hardness almost equivalent to that obtained by the sample without separation ring quenched in fast quenching oil. This indicates that the base oil can replace the fast quenching oil to reduce cooling medium cost. The reason why the surface hardness of the workpiece without separation ring is uniform as indicated in the Table 2 is as follows: because a 42CrMo bar is used for the samples in this experimental example, it is difficult to meet the requirement for as-quenched hardness if 60 SN base oil is directly used for the quenching; the requirement for as-quenched hardness can only be met if the fast quenching oil is used. However, fast quenching oil is not only expensive and increases application cost, but also involves higher carried quantity. It is a challenge to add a quenching tank to receive the fast quenching oil on a production site that is already crowded. The method provided in the present invention can meet the requirement for as-quenched hardness with just 60 SN base oil.
The oil-immersion quenching cooling precursor that is separated into a plurality of sections to expel gas bubbles and the oil-immersion quenching cooling method provided in the present invention are capable of improving the inherent quality of a workpiece, saving alloy element resources, improving production efficiency and reducing production cost, and are suitable for industrial application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201510516350.8 | Aug 2015 | CN | national |
| 201520633659.0 | Aug 2015 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2016/086405 | 6/20/2016 | WO | 00 |
