RU2581691C1 - Method for surface hardening and stabilisation of low-hardness of articles - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: invention relates to machine building and can be used for surface hardening and stabilisation of torsion shafts during processing by sources with high concentration of energy. Method of surface hardening of torsion shafts involves change of level of laser thermal effect on treated surface by setting required temperature level heating surface and providing required speed υ, m/s, moving treated surface, which is determined by formula:

and pitch l, m, movement of surface in transverse direction is set equal to: l ≤ 0.8·D_pp, wherein D_pr and D_pp are dimensions of heating zone of treated surface at single laser flash, correspondingly along its movement and in transverse directions, m; λ is time between two successive laser flashes, s; k = 0.2-0.5 is coefficient of overlapping of heating zone in two successive laser flashes, determined depending on level of laser thermal action and required temperature level heating of treated surface.

EFFECT: technical result consists in improvement of quality of processed surface by preventing formation of high hardening stresses.

1 cl, 2 dwg

Description

The invention relates to mechanical engineering and can be used for surface hardening and stabilization of non-rigid products when processed by sources with a high concentration of energy.

Known methods of surface hardening of metals by a laser beam [1, 2, 3, 4], in which the processing mode is controlled by changing the energy of the pulse, its duration and diameter of the spot, assuming that the energy distribution over the spot is close to uniform.

The disadvantages of the known methods should be considered the inability to simultaneously implement the various stages of thermal exposure (annealing, hardening and tempering) under conditions of a single surface irradiation, the complexity of the interrelation of the mechanical characteristics of the resulting structures with processing modes.

There is also known a method of surface hardening of metals by a laser beam by changing the level of thermal effect on the treated surface with variable reflectivity [5].

The disadvantages of this method are the complexity of processing the configured surfaces and the need to provide a technological combination of the characteristics of the dye with the hardening modes.

A known method of surface hardening and stabilization of non-rigid products by changing the level of laser thermal exposure to the treated surface [6]. The heat treatment is carried out by polarizing the laser beam into a strip with a variable radiation intensity and scanning along this strip, while the degree of polarization is set according to the accepted heat exposure conditions taking into account the fixed scanning speed and the isothermal exposure stages of the treated area are successively implemented at different temperature levels, and the temperature is changed with optimal speeds for hardened metal. In this case, the scanning speed is assigned experimentally by the value of the specified microhardness in the hardened metal layers corresponding to the quenching state.

The disadvantage of this method is the low quality, since most of the scanned laser beam provides heating of the surface to the level of hardening, and a smaller part to the level of tempering, due to which the tempering of the surface is incomplete and the product loses its shape. In addition, the polarization of the laser beam into a narrow strip reduces the area of its impact on the treated surface and, as a result, reduces the processing performance.

The closest analogue of the claimed invention is a method of surface hardening of torsion shafts by changing the level of laser heat exposure on the treated surface, including setting the required temperature level for heating the hardened surface of the torsion shafts by providing the necessary speed of movement of the treated surface, v, m / s (JP 62109924 A, C21D 1/09, 05/21/1987).

The disadvantage of this method is the low quality of processing.

The objective of the invention is to improve the quality of processing and increase processing productivity.

EFFECT: absence of residual stresses on the treated surface, stabilization of the machined parts and increase of processing productivity.

The problem is solved in that in the method of surface hardening and stabilization of torsion shafts by changing the level of laser thermal exposure to the treated surface, the required temperature level for heating the surface is set by providing the necessary speed of movement of the treated surface, which is determined by the formula:

, $$\upsilon =k\cdot \frac{D_{pr}}{\lambda }$$

,

and the step of moving the surface in the transverse direction is set equal to:

l≤0.8 · D _pp ,

where D _pr and D _pp are the dimensions of the heating zone of the treated surface during a single laser flash, respectively, along the surface displacement and in the transverse directions, m;

λ is the time between two consecutive laser flashes, s;

k is the coefficient of overlap of the heating zone is the reciprocal of the number of laser flashes during the movement of an arbitrary portion of the treated surface through the laser exposure zone, determined depending on the intensity of the laser radiation and on the required heat exposure mode: k = 0.2-0.5.

By setting such a speed of movement of the treated surface at which the step of moving the surface between adjacent laser flashes is less than the size of the heating zone, so that each of its sections is repeatedly exposed to laser radiation, the surface temperature in the laser exposure zone rises to the hardening temperature, even at low laser radiation intensity . In addition, due to the thermal conductivity of the metal and the small step of the displacement of the surface, each of its sections experiences thermal effects from laser flashes even after it leaves the zone of laser exposure. This ensures the creation of a surface with an extended length of the tempering zone of the surface after quenching, which helps to remove residual stresses from this surface. This ensures a higher surface quality. And due to the fact that hardening is provided even at a reduced intensity of laser radiation, then at the same power, the size of the laser exposure zone can be increased, which helps to increase the processing productivity. Since the degree of heating of the treated surface in the peripheral sections of the young laser exposure is less than in the central sections, the step of transverse movement of the treated surface after each longitudinal passage is set smaller than the size of the heating zone in accordance with the proposed dependence. This increases the uniformity of the heat treatment and, thus, helps to improve the quality of processing.

The invention is illustrated by drawings, where in FIG. 1 shows a laser processing circuit; FIG. 2 - the temperature dependence of the elementary section of the treated surface at various depths on its position relative to the laser exposure zone.

Part 1 moves at a speed of 9 relative to the stationary zone of laser irradiation 2. The laser irradiation zone has the shape of an ellipse with the dimensions of the axes D _pr and D _pp . A Cartesian coordinate system xoy was drawn from the center of the laser exposure zone. Axis oX is directed along the displacement vector of part 1, axis oy is in the transverse direction. In the laser irradiation zone 2, periodically over a period of time λ, laser flashes of the same intensity are carried out, which cause heating of the workpiece surface 1. The speed of the part is determined by the formula

$$\upsilon =k\cdot \frac{D_{pr}}{\lambda }\left(\mathrm{one}\right)$$

The value of k is chosen in the range of k = 0.2-0.5.

In FIG. 1, a small rectangle shows an elementary section 3 of the workpiece surface 1 at its various positions 1 ... 7 relative to the laser exposure zone at the time of each laser flash. The distance between two adjacent positions of the elementary section Δx = λ · υ. In the first position, the elementary region has not yet entered the laser irradiation zone 2, but due to the thermal conductivity of the metal at the time of the laser flash, its temperature increases compared to the ambient temperature. At positions 2, 3, and 4, the elementary region is in the zone of laser irradiation and experiences a dual nature of temperature change: at the time of the laser flash, it rises sharply, and in the interval between laser flashes due to heat removal deep into the metal, it decreases. But since the time λ is small, during this time the surface temperature that it receives at the time of the laser flash does not have time to cool to ambient temperature. Therefore, with the next laser flash, the resulting temperature is summed with the residual temperature from previous laser flashes. Thus, when the elementary section is moved from position 2 to position 3, the surface temperature gradually increases and reaches the quenching temperature. Since heat is removed exponentially, after leaving the laser exposure zone up to point 4, the surface temperature drops sharply, which ensures the hardening mode. At point 4, the temperature reaches the tempering temperature and can then be stored at points 5 and 6 due to the following factors. Firstly, at points 4, 5 and 6, with each next laser flash, taking into account the small value Δх = λ · υ, the temperature can increase so as to maintain the tempering temperature previously accumulated. Secondly, in accordance with the exponential dependence of temperature on time, the cooling rate decreases with decreasing temperature. Therefore, at a tempering temperature, surface cooling is slower than at a quenching temperature. All this contributes to the fact that the tempering time of the surface is stretched, which helps to remove stresses that occurred during the hardening process and stabilize the geometric parameters of the product.

After the complete passage of the part along the axis ox, it is moved in the transverse direction (along the axis oy) by the amount:

$$l\le 0.8\cdot D_{pp}\left(2\right)$$

Due to the overlap of the laser exposure zone during the first and second passes, the difference in temperature effect of the heating zone on the treated surface in its various sections and the length of the surface displacement in the heating zone are leveled. This helps to improve the quality of processing.

Example. The surface of a metal part having the following mechanical properties of the material is subjected to laser treatment: density of the material of the product ρ = 7800 kg / m ³ ; thermal diffusivity α = 5 · 10 ^-6 m ² / s, specific heat s = 800 J / kgK. The laser creates on the surface to be treated a zone of laser irradiation of circular shape with a diameter of D = D _pr = D _pp = 2 · 10 ^-3 m. The specific amount of heat generated by the laser radiation source q = 4.8 · 10 ⁶ J / m ² , the period of laser flashes λ = 0.05 s.

We take k = 0.33 and using the formula (1) we determine the speed of movement of the part:

$$\upsilon =0.33\cdot \frac{2\cdot {10}^{-3}}{0.05}=0.013m/\mathrm{from}.$$

For these conditions, a computer experiment was conducted, the results of which are shown in FIG. 2. The axis ox is located horizontally, along which the part moves, the values of the surface temperature of the part are shown vertically. The origin is located in the center of the laser exposure zone. The initial position of the considered point of the workpiece surface corresponds to the initial position of the elementary surface section in FIG. 1. This starting point is located outside the laser exposure zone at a distance x = -1.65 mm from its center. At this moment, another laser flash occurs in the laser exposure zone, as a result of which the temperature slowly rises to the temperature θ = 55 ° C at the initial point.

At the point x = -1 mm, another laser flash is carried out and the surface temperature rises to θ = 720 ° C at a depth of 0.1 mm from the surface and to θ = 300 ° C at a depth of 0.3 mm from the surface. In the next two laser flashes, the temperature rises and reaches a quenching temperature even at a depth of z = 0.3 mm. The next laser flash occurs at the moment when the surface point in question is outside the laser exposure zone at a distance of x = 1.65 mm from the center. In the interval between these laser flares, the surface temperature drops sharply and reaches θ = 325 ° C at almost all considered depths z. Such a sharp cooling of the surface leads to its hardening. But since the temperature did not drop to 20 ° C, but to 325 ° C, this prevents the formation of very high quenching stresses.

When the point in question is moved to the position x = 2.3 mm, another laser flash is carried out. From FIG. Figure 2 shows that the temperature at this point increases for a short time to approximately θ = 330 ° C, and with further movement of the point up to the position x = 3.4 mm, the temperature slowly drops, but remains within the tempering temperature θ> 160 ° C, shown by the horizontal dash-dotted line. This ensures the mode of hardening and tempering of the surface after hardening, which contributes to the achievement of high surface quality.

After the first pass, the surface returns to its original position and then shifts in the transverse direction by an amount determined by the formula (2):

l≤0.8 · 2 · 10 ^-3 = 1.6 · 10 ^-3 m.

We accept l = 1.6 · 10 ^-3 m.

We calculate the processing performance of the proposed method. Consider, for example, the processing time of a low-rigid shaft with a diameter d = 10 mm and a length L = 100 mm. Under the above laser processing conditions, the processing time will be equal to:

$$T_m=\frac{\pi \cdot D\cdot L}{60\cdot l\cdot \upsilon }=\frac{\pi \cdot 10\cdot \mathrm{one\; hundred}}{60\cdot \mathrm{1,6}\cdot 13}=\mathrm{2,5}m\mathrm{and}n.$$

If polarization of the laser beam is used, then the size of the laser hardening zone l decreases sharply several times, up to a thin line. If we substitute this value of l in the above formula, we can determine that the processing time will increase several times and, therefore, the processing performance will decrease. Thus, the problem of increasing processing productivity of the proposed method.

The proposed method is very effective to use for surface hardening and stabilization of non-rigid products such as torsion shafts. In the manufacture of torsion shafts, their surface is subjected to bead-hardening with subsequent low-temperature tempering. But during bead-blasting, high stresses are formed on the surface of the shaft, which during tempering lead to severe deformation of the shaft. The deformation is removed by dressing and subsequent grinding, after which shot blasting and tempering are again repeated. Such processing can be carried out several times. It turns out a very time-consuming processing. The proposed method of hardening and tempering of the surface allows you to perform hardening and tempering in one operation, thereby increasing the processing productivity and to ensure high quality of the treated surface.

The technical and economic efficiency of the proposed processing method is as follows:

1. Improving the quality of products, since laser hardening is carried out in several cycles, which improves the structure of the material, and the tempering of the surface after hardening is extended in time and therefore provides active removal of stresses after hardening and stabilization of the shape of the products.

2. Improving processing performance, since the size of the laser exposure zone can be increased at the same laser power.

3. The expansion of technological capabilities of the method, because it can be used instead of other currently used inefficient technologies for manufacturing low-rigidity parts such as torsion shafts.

Information sources

1. Safonov A.I., Tarasenko V.M., Skoromnik V.I. Laser hardening of cutting tools: Overview inform. - M.: VNIPIEIlesprom, 1989.S. 52.

2. Patent RU No. 2004603, 15.12.1993.

3. Patent RU No. 2127768, 03.20.1999.

4. Patent RU No. 2047661, 10.11.1995.

5. Patent RU No. 2197541, 01/27/2003.

6. Zotov G.A., Pamfilov E.A. Improving the durability of wood cutting tools. M .: Ecology, 1991.S. 300.

Claims (1)

The method of surface hardening of torsion shafts by changing the level of laser thermal influence on the treated surface, including setting the required temperature level for heating the hardened surface of the torsion shafts by providing the necessary speed of movement of the treated surface, characterized in that the speed of movement of the hardened surface of the torsion shafts, υ , m / s determined by the formula:
$$\upsilon =k\cdot \frac{D_{pr}}{\lambda }$$

,
and the step l, m of the movement of the hardened surface in the transverse direction is set equal to:
l≤0.8 · D _pp ,
where D _pr and D _pp are the dimensions of the heating zone of the treated surface in a single laser flash along the movement of the hardened surface and in the transverse directions, respectively, m; λ is the time between two consecutive laser flashes, s; k = 0.2-0.5 - the coefficient of overlap of the heating zone with two consecutive laser flashes, determined depending on the level of laser thermal exposure and on the required temperature level of heating of the treated surface.

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