RU1767886C - Steel pieces hardening method - Google Patents

Abstract

FIELD: surface thermal treatment. SUBSTANCE: before numerical heatings of surface layer by pulses of high power current electronic beam piece is subjected to preliminary heating by continuous electronic beam with power density of q = (2V/d)ρ·H(СТ_melt+L), with V electronic beam motion speed; d the beam diameter; H given thickness of melted surface layer; ρ, C, T_melt and L density, thermal capacity, melting temperature and specific temperature of treated steel melting correspondingly. EFFECT: steel pieces hardening method is effective in surface thermal treatment. 1 tbl

Description

 The invention relates to heat treatment of the surface of structural materials and products from them with highly concentrated energy flows and is an improvement of the known method of hardening steel products.

The known method consists in the fact that the surface of the products is repeatedly heated to the hardening temperature by pulses of a high-current electron beam with the parameters:
pulse duration (0.5-3) ^. 10 ^-6 s;
electron energy 10-50 keV;
energy density 2-5 J / cm ² .

, (1) где q плотность энергии пучка, Дж/см²;
ρ, c, a соответственно плотность в кг/см³, удельная теплоемкость, Дж/кг^.К, и коэффициент температуропроводности стали, см²/с;
Т_о исходная температура изделия, ^оС;
Т₁ температура отпуска стали, ^оС.The number of pulses N 150-300, and the interval between them is selected at least
τ _o

, (1) where q is the beam energy density, J / cm ² ;
ρ, c, a, respectively, density in kg / cm ³ , specific heat, J / kg ^. K, and thermal diffusivity of steel, cm ² / s;
T _{about the} initial temperature of the product, ^about C;
T ₁ temperature tempering steel, ^about C.

The disadvantage of this method is that the maximum values of microhardness, depending on the grade of steel, do not exceed 1400-1700 kg / mm ² , which limits its capabilities.

 The aim of the invention is to improve the quality of products by increasing the microhardness of the hardened layer.

ρH(CT_пл+L), (2) где V скорость движения электронного пучка;
d его диаметр;
H заданная толщина оплавленного поверхностного слоя;
Т_пл и L температура плавления и удельная теплота плавления обрабатываемой стали соответственно.To do this, in a known manner according to ed. N 1682403, previously before multiple heating of the layer surface by pulses of a high-current electron beam, it is heated by a continuous electron beam with a power density equal to
q

ρH (CT _PL + L), (2) where V is the velocity of the electron beam;
d is its diameter;
H is the predetermined thickness of the fused surface layer;
T _PL and L melting temperature and specific heat of fusion of the processed steel, respectively.

 The main factor determining the increase in microhardness is the stress wave, which propagates from the irradiated surface into the interior of the product. The source of stress wave excitation is the effect of thermoelasticity caused by the rapid expansion of the surface layer heated to high temperatures when the material is irradiated with a high-current electron beam. The interaction of the stress wave with the crystal lattice of the target material leads to an increase in the density of defects in the region of its propagation. Multiple loading of surface layers by a stress wave with an increase in the number of irradiation pulses provides the effect of accumulation of defects and, accordingly, an increase in the degree of hardening of the zone (increase in microhardness). With the number of pulses 150-300 and the parameters of the high-current electron beam specified in the main invention, the maximum possible defect density for a given steel grade and, accordingly, high microhardness are achieved.

Pretreatment of the surface layer of the steel product with a continuous electron beam leads to the melting of the surface layer to a predetermined depth and its subsequent sharp cooling (quenching from a liquid state). As a result, a very dispersed quenching structure is formed in the surface layer, which provides, for example, for the surface of U7A steel, microhardness N 1080 kg / mm ² (table), which is ≈ 400 kg / mm ² higher than the microhardness of steel quenched in the traditional way, and in several times greater than the microhardness of steel in the delivery state (≈ 200 kg / mm ² ).

 The power density q necessary for melting the surface layer of a given depth by a continuous electron beam is determined as follows.

When moving along the surface of an infinite metal plate of an electron beam, the relationship between its parameters and the geometric dimensions of the molten zone can be found from the relation [2]
η _t Qd = (πB ^2/4) H˙ρ (CT _pl + L), (3) where QI˙U˙η _U / V heat input;
I beam current;
U accelerating voltage;
V is the speed of the beam;
η _u effective efficiency of the processing process, η _U ε ₁ / ε ₂ ;
ε _{1 is the} energy introduced into the surface layers of the target;
ε ₂ energy supplied to the target surface;
d is the diameter of the electron beam;
The width of the fused surface layer;
H specified thickness of the fused surface layer;
T _pl the melting temperature of the processed material;
L specific heat of fusion of the processed material;
With the heat capacity of the processed material;
η _t thermal efficiency of the processing process, η _t ε ₃ / ε ₁ ;
ε ₃ part of the energy introduced into the material spent on its melting.

Expression (3) obtained for any concentrated flows regardless of the thermal conditions in the treatment zone can be written in a simpler form, based on the conditions of surface hardening of steels from the melt. So, for example, cooling rates V _o (10 ⁴ -10 ⁵ ) K / s, necessary for the formation of high-strength layers during quenching from the liquid phase, are provided if the thickness of the molten layer does not exceed 500 μm. Our experiments showed that in this case, the value of B does not practically differ from the value of the diameter of the electron beam d. It is also known [2] that the maximum theoretical value of η _{T is} 0.484, and its experimentally measured values for electron beams are in the range of η _T (0.43-0.52). The value η ₄ for electron beams is considered equal to unity.

H·ρ(CT_пл+L).Thus, taking B d, η _T 0.5, η _U 1 and substituting their values in (3), we obtain
q

H ρ (CT _pl + L).

Further heating of the surface to the temperature of quenching by pulses of a high-current beam (heating conditions selected in accordance with (1) leads to a further increase in surface quality: microhardness values increase for U7A steel from 1080 to 1850 kg / mm ² , and for steel ШХ15 from 1220 to 1920 kg / mm ² (tab.).

 The increase in microhardness is due to the fact that repeated loading by a voltage wave of a surface layer quenched from the melt, which has a very dispersed structure, leads to a significant accumulation of defects, possibly a change in the chemical composition of the boundary zones and, as a result, an increase in microhardness (table).

EXAMPLE 1. Samples of U7A steel products after thermal hardening and tempering were placed in a chamber of an electron-beam installation, where their surface was irradiated with a continuous electron beam. The predetermined thickness of the fused layer was 180 μm. To obtain such a thickness of the melted layer in accordance with expression (2), the following parameters of the electron beam were chosen: accelerating voltage U 35 kV, beam current I 80 mA, beam diameter on the sample surface d 3 mm, power density q 3.8 ^. 10 ⁴ W / cm ² . The beam velocity was V0.42 m / s. The fused layer had a characteristic dendritic structure and a clear boundary with the initial structure. The thickness of this layer, determined metallographically, agrees well with that specified in formula (2).

After irradiation with a continuous electron beam, the samples were transferred to the chamber of another electron-beam installation, where their surface was additionally treated with a pulsed high-current electron beam with electron energy eU30 keV, pulse duration τ _U 1 μs, and energy density q 3 J / cm ² in the beam. The interval between pulses in accordance with expression (1) was 10 s, the number of pulses N was changed from 20 to 300.

 After processing, the samples were removed from the chamber, cut in the direction perpendicular to the irradiation surface. After grinding, microhardness was measured on the surface and at various distances from the surface using the PMT-3 device at a load of 50 g.

The thickness of the hardened layer was taken as the distance from the irradiated surface, where the microhardness is maximum, to the place where the microhardness is equal to the microhardness before irradiation. When processing samples with a continuous electron beam alone, the thickness of the hardened layer is almost equal to the thickness of the melted layer due to the sharp boundary of the melt zone with the initial structure. Subsequent additional cyclic treatment with a pulsed high-current electron beam leads to a significant increase in microhardness in the melted layer and an increase in the thickness of the hardened layer. The results of such processing are shown in the table: maximum microhardness on the surface H _μ 1850 kg / mm ² , the thickness of the hardened layer 230 μm.

PRI me R 2. Samples of products made of steel ШХ15 after thermal hardening and tempering were placed in the chamber of an electron-beam installation, where their surface was irradiated with a continuous electron beam. The predetermined thickness of the molten layer was 170 μm. To obtain such a thickness of the melted layer in accordance with expression (2), the following parameters of the electron beam were chosen: U 43 kV, I 160 mA, d 3 mm. With these parameters, the power density q is 9.8 ^. 10 ⁴ W / cm ² . The velocity of the beam V 1.25 m / s.

After irradiation with a continuous electron beam, the surface of the samples was additionally treated with a pulsed electron beam with parameters eU 30 keV, τ _U 1 μs, q 3 J / cm ² . The interval between pulses in accordance with expression (1) was 12 s. The number of pulses N was varied from 20 to 200.

The irradiated samples were studied in the same manner as in Example 1. The following results were obtained: maximum microhardness on the surface H _μ 1920 kg / mm ² , the thickness of the hardened layer 320 μm.

 Thus, the preliminary processing proposed as an improvement made it possible to obtain hardened layers with microhardness on the surface by ≈ 30% higher than in the main invention, while the processing time was reduced by a factor of 2–3 due to a reduction in the number of pulses.

Claims (1)

METHOD FOR STRENGTHENING STEEL PRODUCTS by ed. St. N 1682403, characterized in that, in order to improve the quality of products by increasing the microhardness of the hardened layer, prior to repeated heating of the surface layer by pulses of a high-current electron beam, it is heated by a continuous electron beam with a power density q equal to

where v is the velocity of the electron beam;
d beam diameter;
H is the predetermined thickness of the fused surface layer;
ρ, C, T _pl and L density, specific heat, melting temperature and specific heat of fusion of the processed steel, respectively.

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