RU2326175C2 - Method of periodic hardening of saws - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of heat treatment of metals and may be used in operation of band and disk cutting saws. To increase the saw resistance, service life, and efficiency, new hardened metal areas are formed on the saw teeth points periodically, on teeth with initial hardened layer size decreased as a result of periodic teeth sharpening.

EFFECT: increase in saw resistance, service life, and efficiency.

2 cl, 2 dwg, 1 tbl

Description

The invention relates to the heat treatment of metals and can be used in the operation of tape and circular saws for cutting materials from wood, plastic, composite, metal and other materials. The method includes the periodic formation of sections of hardened metal on the tops of the saw teeth, the size of the initial hardened layer of which decreased as a result of their regular sharpening and operation.

Band and circular saws work in difficult conditions. In the process, due to friction, the temperature of these saws increases significantly, they experience enormous dynamic loads (tensile, impact, and periodic bending, after which the saws must return to normal) and continuous abrasive wear. Due to the difficult operating conditions, these saws are dull quite quickly. They have to be sharpened and thinned periodically, while each time reducing the width (diameter) of the saw (for example, the usual width of the band saw is from 35 to 55 mm; the width of the saw, at which its strength is still ensured, is 25 mm, up to this size the width of the saw decreases in 20-30 hours of its correct operation), i.e. saws are consumables that have to be changed often enough, and therefore, they should be cheap enough.

Taking into account the specifics of operating band saws, special, largely contradictory, requirements are presented to them.

1. Elasticity to ensure that the saw returns to normal after bending.

2. Hardness, the high value of which increases the time intervals between the need for sharpening the saw.

3. Sufficient softness, which allows the saw material to bend and stretch without tearing the blade.

4. Cheapness.

Most manufacturers of band saws follow the traditional path - the creation of materials that to one degree or another satisfy these requirements, and the production of saws from these materials. Therefore, a large number of saws made of various steels are sold on the market, each of which has its own advantages (i.e., it satisfies more of one of the above requirements).

It should be noted that the softness requirement is imposed on the saw body working on bending and stretching, and the hardness requirement is imposed on the actually working part of the saw, on its tooth. Therefore, some manufacturers supply band saws with a hardened tooth to the market (it is impossible to harden the entire saw, because the material becomes much more brittle, which increases the likelihood of the saw body breaking during bending and tensile work).

Various manufacturers harden the saw teeth in various ways, and hardening modes are a strictly guarded secret. For example, Hakansson, which was founded in 1944, is the progenitor of band hardening technology for band saws and the oldest manufacturer of band saws with hardened teeth in the world. In 1946, they were the first in the world to develop a technology for hardening teeth of a band saw by induction current, known as high-frequency hardening. For over 40 years, this technological discovery has been a well-kept secret and until now a patent for this technology gives the company products an advantage over other manufacturers (detailed information on these saws can be found on the website of Avangard LLC [avanguard.ru]).

Another method of hardening the teeth of the saws involves hardening the teeth using a twin oxygen-acetylene burner, the nozzles of which are located on both sides of the saw, and the torches are directed at a certain angle to the saw blade. Thus, the well-known American company SIMONDS hardens the teeth of its saws (detailed information on these saws can be found on the GLOBAL EDGE ™ website [lentochnye_pily.ru]).

The disadvantage of this method is that the hardness obtained by quenching is unstable. The hardness of the cutting edges of the saw teeth varies from 55 to 64 HRC. In addition, an additional vacation operation is required. The disadvantages of this method are associated with the difficulty of accurately dosing the energy deposited in the tooth and the long time the tooth is heated (heating is carried out due to heat conduction from the heated tooth surface, in contrast to the high-frequency hardening, in which the entire volume of the hardened part of the tooth is heated).

Currently, various companies are conducting intensive research on alternative methods of hardening teeth, which would provide an increase in the operational qualities of saws. This search led, in particular, to the development of a new method of hardening saws [RF No. 2033437, C21D 9/24, publ. 1995.04.20]. In this method, the processing is carried out by continuous emission of a CO ₂ laser, the laser beam being directed from above onto the back surface of the teeth and the saw blade is moved.

This method allows to obtain a hardened layer in the form of relatively thin strips (0.5-0.7 mm) located on the rear surfaces of the saw teeth. The main advantage of this hardening method is that the main part of the tooth material remains plastic (it is not hardened), as a result of which the likelihood of brittle tooth decay is significantly reduced, especially when a tooth gets on solid inclusions in wood (for example, knots).

The disadvantages of this method are:

- a change in the angle of inclination of the front surface of the teeth due to the difference in the strength characteristics of the material of this surface, which requires more frequent sharpening of the saw;

- insufficient depth of the obtained hardened layer on the cutting edges of the saw (0.6-0.7 mm), which limits the use of this method when hardening saws that are sharpened during operation;

- the high cost of equipment necessary for such hardening;

- High qualification of the personnel working in this operation.

The disadvantages noted above lead to the fact that the method described above is practically not used when hardening the saw teeth.

- 15 час. Обычный незакаленный зуб служит

4 часа. Однако при суммарном возможном уменьшении ширины пилы в 20 мм отличия в сроке службы оказываются не так значительны: для пилы с первоначально закаленным зубом

часов, для пилы с незакаленным зубом

часов, т.о. общее время работы увеличивается примерно на 40%. В случае если при достижении зубьев пилы границы области закалки зубья закалить заново, срок службы пилы можно существенно увеличить (более чем в 2 раза).In all of the above methods, hardening provides a significant increase in tooth strength, and therefore, a decrease in its wear. Such a saw at the beginning of its operation, until the periodic sharpening of the tooth is lower than the hardening zone, dulls much more slowly, which reduces the cost of its periodic sharpening and increases the service life of the saw (during operation, the width of the saw with an unhardened tooth is ~ 0.8 mm / hour , with a hardened tooth ~ 0.2 mm / hour). With the usual size of a hardened tooth of 3 mm, the time of its service, until it reaches the limit of hardening,

- 15 hours An ordinary non-hardened tooth serves

4 hours. However, with the total possible reduction in the saw width of 20 mm, the differences in the service life are not so significant: for a saw with an initially hardened tooth

hours, for a saw with an unhardened tooth

hours, i.e. total operating time is increased by approximately 40%. If, when the teeth of the saw reach the boundaries of the hardening region, the teeth are hardened again, the service life of the saw can be significantly increased (more than 2 times).

In order to increase the life of the band saws, the experts of the well-known German company “Banholzer GmbH” (saws “BANSO FLEX-BACK”) proposed a method for deep hardening of band saws. By this method, not only the tooth itself is tempered, but also the part of the saw body located next to it [information of Center for Cutting Tool LLC]. Such hardening allows you to increase the number of regrind of the teeth of the saw. At the same time, the material of the cavities between the teeth also turns out to be hardened, which significantly increases the likelihood of brittle fracture of the saw in these places and does not significantly increase the strength of the hardened layer (hardening is performed only up to HRC 61-62) and imposes rather strict restrictions on equipment in which such saws can be used (in particular, on the minimum diameter of a band saw pulley).

A known method of hardening the teeth of the saws, including hardening the tops of the teeth of the saw (8I 94812A, S2Sh9 / 24, 01/01/1953). This method, according to the applicant, can be considered as a prototype of the claimed method.

The objective of the invention is to increase the durability, service life and efficiency of the saws while ensuring the durability and wear resistance of the product as a whole.

The solution to this problem is provided by using the proposed set of essential features.

The set of essential features

The method of periodic hardening of saws, including hardening the tops of the teeth of the saws, characterized in that the repeated and all subsequent hardening of the teeth is carried out in the case when the residual height of the hardened zone (b) at the top of the teeth reaches a value determined from the relation:

b = K × A,

where A is the height of the tooth, K is the coefficient = 0.2-0.5;

and the height of the new hardening section on the tops of the teeth (L) is chosen from the condition:

L = R × A × K,

where R is the coefficient = 1.2-3,

where A is the height of the tooth, K is the coefficient = 0.2-0.5,

at the same time, hardening is carried out by hardening by the action of energy released by the RF current or laser radiation in a pulsed mode;

The invention is illustrated by illustrations.

Figure 1 - saw tooth with a residual height (b) of the hardened zone.

Figure 2 - saw tooth after hardening (height of the hardened zone of the tooth - L).

During operation, the saw’s teeth become stupid and sharpened periodically. The main advantage of drinking with a hardened tooth is a significantly longer time between sharpening. However, with periodic sharpening, grinding of the hardening zone of the tooth occurs, which leads to a decrease in the time interval between the necessary sharpening of the teeth. The limit of the permissible level of this parameter in our experiments was considered to be a 20% reduction in time between periodic sharpening of teeth ((t _po -t _p ) / t _po * 100% = 20%, where t _po and t _p are the time between periodic sharpening immediately after quenching and with the current development of the quenching zone, respectively). Experiments indicate that a decrease in the boundary of the hardened layer below this level leads to a sharp decrease in the time interval between sharpening. Coefficient K shows the border of the zone of residual hardening of the tooth, experimentally determined on the basis of the acceptable level of reduction in time. On the saws of different manufacturers, the criterion of admissible time between periodic sharpening is fulfilled at different residual height of the hardening zone (b). This explains a fairly wide range in which the coefficient K.

The coefficient R determines the maximum height of the hardening zone, which does not lead to problematic situations when sawing and operating them (i.e., it does not increase the likelihood of cracks in the root of the tooth during setting and chipping of the teeth during operation of the saw). The maximum hardening zone, which was determined experimentally, is also significantly different for saws of different brands and different manufacturers.

During the experiments, a correlation was observed between the residual hardening height of the teeth and the height of the hardening area at the tops of the teeth. Therefore, the coefficient K was also included in the formula for determining the height of a new quenching site.

The review shows that the development of technology for hardening the teeth of band saws is in the direction of increasing the intensity of energy exposure to the processed object.

The development of electronics, micro- and nanotechnology has led to the fact that modern technologists have the opportunity to use electromagnetic (EM) radiation of great intensity for their own purposes. Such radiation allows you to get in the processed product specific volumetric energy input at the level of 10 ⁹ W / m ³ . The possibility of using high-intensity EM radiation has fundamentally changed a number of areas of electrical technology.

One of the areas in which intense EM radiation has found its application is pulsed hardening of the teeth of band saws by high-frequency currents (HF).

Using an intense HF induction field, in a fraction of a second, only the surface and relatively shallow near-surface layers can be heated (the depth of these layers is _hsl ~ 0.3-0.5 Δ, where Δ is the penetration depth of the electromagnetic field into the metal, which, in turn, is uniquely determined by the frequency supply voltage f) of the steel product to the phase transition temperature, and then instantly stop the heating. With high-intensity induction heating, the temperature profile practically repeats the distribution profile of the heating sources, which in induction heating are usually characterized by the depth of penetration of the electromagnetic wave into the heated material. In this case, the high cooling rate of the heated layers, necessary for hardening, can be ensured by the removal of heat into the unheated product and the radiation from its heated surface. Such a quenching method allows for significantly higher cooling rates and, therefore, better quenching modes. Moreover, the obtained metal structures in most cases do not require an additional operation — tempering (mandatory in traditional technology).

An important factor is the possibility in this case not to use liquid cooling, which allows the process to be carried out in “clean” conditions. As a result of the development of high-intensity RF quenching, a technological operation that required sophisticated equipment and highly qualified personnel can be fully automated and carried out in non-factory conditions.

For efficient operation, the saw teeth are “bred” (deviated from the vertical axis of the saw). Depending on the manufacturer and the sawing conditions, the teeth of the band saws are bred to values of up to 0.4 mm, either each tooth in turn in different directions, or through one tooth. As a result, the main load falls on a part of the lateral surface of the tooth (region 1 in figure 2). These parts should have the greatest hardness. Moreover, the fact that the rest of the tooth remains unhardened significantly reduces the likelihood of brittle tooth decay when it enters solid inclusions during sawing. Considering that only part of the tooth surface should have the highest hardness, only this part can be hardened, and the hardened layer depth (g) should be selected from the relation: g = Kl × D, where Kl is the coefficient = 0.01-1.0, D is the thickness of the saw band. The depth of the hardened layer will be from 0.01 mm (purely surface hardening of the most loaded part of the tooth) to 1 mm (through hardening of the tooth and strip).

The experimental results obtained by the authors indicate that, using induction heating, changing the frequency of the current, the pulse duration, and the specific input power, it is possible to obtain a hardened layer depth from 1 mm (the tooth is hardened to the entire depth) to 0.2 mm in the band saw tooth.

The method is as follows.

In the normal maintenance cycle of the saw, which includes sharpening and wiring, in the case when the minimum residual height of the hardened zone at the top of the teeth is less (Fig. 1) b = K × A, where A is the height of the tooth, K is the coefficient = 0, 2-0.5, one more operation is added - hardening. Moreover, the height of the newly hardened zone at the top of the tooth is selected from the ratio:

L = R × A × K, where R is the coefficient = 1.2-3.

The proposed method differs from all existing at the moment also in that the hardening of the teeth of the saw is carried out many times as the development of previously hardened layers.

For each saw tooth using high-intensity heating (for such heating high-frequency currents or laser radiation can be used) for a short period of time (0.1-1.0 s), a region is created in the metal whose temperature exceeds the quenching temperature. A short heating time is necessary to ensure modes in which cooling could be achieved by removing heat to unheated areas of the saw.

The most effective way of heating metal for quenching is heating using volumetric heat sources, which can provide a high-frequency induction heating method. The effectiveness of this method is due to the fact that with its help it is possible to achieve a minimum heating time, minimal energy loss, accurate dosing of the input energy. High-frequency electromagnetic exposure allows, due to the volume contribution of energy, to heat the entire tooth (or most of it), and not just its surface, as happens when using other methods. The volume character of the RF magnetic field by the absorption metal is usually described using the penetration depth (Δ). The penetration depth is the distance from the surface of a semi-infinite body made of the material to be processed, at which the complex amplitude of the intensity of the alternating magnetic field decreases 2.72 times. Typically, the penetration depth is determined by the formula:

where ρ is the resistivity of the processed material, μ is its relative magnetic permeability, f is the frequency of the supply voltage.

If we want to achieve uniformity (in tooth thickness) of the input energy, it is necessary that the penetration depth be comparable with the thickness of the metal being processed 0.5 · h≤Δ≤2 · h. This condition can be rewritten as:

In inequality (1), μ is the relative magnetic permeability of steel under processing conditions (i.e., at operating values of temperature and magnetic field strength).

The quenching temperature of the steel is above the Curie temperature. When heated above this value, the relative magnetic permeability of the steel abruptly decreases to unity. In this case, the distribution of energy sources along the depth of the tooth also changes abruptly.

To fulfill the condition of uniform energy input, the frequency of the supply voltage in the "cold" mode (up to the Curie temperature) must be selected based on the ratio: 6 · 10 ⁴ · ρ / (h ² · μ) ≤f ₁ ≤10 ⁶ · ρ / (h ² · μ), moreover, 7≤μ <16, and in the "hot" mode - from the relation:

6 · 10 ⁴ · ρ / h ² ≤f ₂ ≤10 ⁶ · ρ / h ² . The optimal frequencies for the "hot" mode are much higher than for the "cold" mode. Those. when heating at one frequency, it is impossible to simultaneously satisfy the uniformity of the energy input in both “cold” and “hot” modes. Therefore, quenching systems are usually designed based on the high frequencies that are characteristic of “hot” modes, and ensure uniform heating in its last stage, where this uniformity is most important.

To accelerate the heating process and obtain a uniform temperature distribution over the cross-section, it must be carried out either with sequential switching from f ₁ to f ₂ as soon as the temperature of the entire sample being processed reaches the Curie point, or immediately at two frequencies f ₁ and f ₂ .

In order to confirm the possibility of obtaining a hardened area with the parameters proposed in the application, and the effectiveness of the proposed method, a series of experimental studies was carried out.

To conduct research on this technological process, we have developed and manufactured an experimental sample of a quenching device. This sample consisted of:

1) a transistor generator operating at a frequency of 440 kHz, with an average power of 2.5 kW;

2) a block matching the parameters of the generator and the load;

3) an inductor of a special form;

4) devices for positioning teeth of band saws.

The developed sample of the hardening device made it possible to set the pulse duration t _imp in the range of 0.05-1 sec with an accuracy of ± 5% and to regulate the power invested in the saw tooth (P _on ) from 0.1 to 2 kW per pulse.

With a long pulse-periodic mode (IPR) of work for many teeth of the developed sample, the deviation in pulse duration did not exceed ± 3%, in power - ± 2%. Further studies suggest that these deviations will not affect the repeatability of the results.

The ability to adjust the pulse time and the input power allowed to carry out a range of studies on the hardening of band saws of various brands and manufacturers. The studies were conducted for saws CR-300 and CR-400 ("Carl Rontgen"), C-75 ("Lenz"); HSWM ("Womaco") and expensive saws of American manufacturers: "Wood-Mizer", "Simonds", "Lenox".

After each series of experiments on the equipment, studies of the results were carried out. In the process of research, the following parameters were controlled.

1. The hardness of the hardened teeth at three points on their lateral surface (1 - the tip of the tooth, 2 - 2 mm from the tip of the tooth, 3 - 4 mm from the tip of the tooth).

2. Sufficient impact "fragility" of the hardened tooth.

The data obtained experimentally were compared with similar parameters of band saws hardened in the factory by the manufacturer.

As an example, in table. The results of comparing the hardness of the tooth of a band saw CR-300 (Carl Rontgen) at three points of its lateral surface on samples with “proprietary” hardening and samples obtained in optimal operating conditions of the developed experimental equipment (two different inductors) are presented.

Table Average Hardness at three points on the side surface hardened tooth and its maximum deflection (measured hardness of the teeth ^of one meter CR-300 bandsaw length ( «Carl Rontgen»)) "Firm" hardening Quenching using an experimental setup at different pulse durations τ = 0.4 s τ = 0.5 s one

2

3

The table shows the experimental results obtained at the same average power. It should be noted that the results obtained did not depend on how this average power was organized in time. In the process, the following modes of energy contribution to saw teeth were investigated:

1) with constant power,

2) using pulse-width modulation (with different ratios of pulse duration and pause),

3) with a change in the energy input according to a certain law (rising and falling exponentials, isosceles triangular momentum, momentum in the form of a right triangle).

From the above table. the data shows that the hardness of the tooth and the reproducibility of the results obtained using the developed equipment are at least not lower than that of saws with a “company” hardening.

In order to conduct a series of experiments on the continuous operation of band saws hardened according to the developed technology in real woodworking production and the possibility of their multiple hardening, the experimental equipment created was launched in the laboratory of the Irkutsk State Technical University.

The experimental results showed that, subject to the technological parameters of sawing, a periodically hardened saw required sharpening much less often, which significantly increased its service life.

Claims (2)

1. The method of periodic hardening of saws, including hardening the tops of the teeth of the saw, characterized in that the re-hardening of the tops of the teeth of the saw is carried out in the case when the residual height of the hardened section (b) on the tops of the teeth reaches a value determined from the ratio: b = K · A, where K is the coefficient = 0.2-0.5; A - tooth height, mm, and the height of the site of repeated hardening at the top of the tooth is chosen from the ratio: L = R · A · K, where R is the coefficient = 1,2,3. 2. The method according to claim 1, characterized in that the hardening is carried out by exposure to the tops of the teeth of the saw with high-frequency currents or laser radiation in a pulsed mode.

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