RU2690386C1 - Method of tempering parts from low-carbon boron-containing steel - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: to obtain self-sharpening effect on cutting edges of the tool, a protective shield is first installed on the cutting edge, the tool with the screen is placed into an inductor immersed in a cooling medium containing the following in wt. %: ethylene glycol 15–19, urotropin 0.5–1.5, water is the rest, wherein the medium temperature is maintained from 4 to 8 °C, and the part is heated by cycles 3–5 times, in the temperature range from 1150–1270 °C to 650–730 °C.EFFECT: invention can be used, in particular, at hardening of cutting tool from low-carbon boron-containing steels.1 cl, 3 tbl, 4 dwg

Description

The invention relates to the field of metallurgy and heat treatment of metals and can be used in the hardening of parts from low carbon boron steels: working bodies of agricultural machines (disks, lancet paws, plowshares), tools (knives, hammers), suspension parts (racks, stabilizers, torsions) and others, which are subject to increased requirements for durability.

To improve the structure of steel and increase wear resistance, various types of heat treatment of metals are widely used, based on the use of cyclic thermal effects, which received the general group name - thermal cycling treatment (TCO) [Fedyukin, V.K. Thermocyclic treatment of steel and cast iron. - L .: Mechanical Engineering, 1977. - 384 p.]. Unlike other types of heat treatment, structural and phase transformations at TCW are performed multiple times at varying heating / cooling temperatures. The need for multiple repetitions of treatment at given temperatures, as a rule, is due to the desire to accumulate changes that radically improve the quality of products and give them properties unattainable with a one-time heat treatment.

The nature of the phase interaction of the components in steel largely determines the effectiveness of the effect of the TCP on the change in its structure and properties. So, in the case of complete miscibility of the components in the solid state, TCT is not accompanied by a change in the number of phases in the system, and structural changes in such steels can be associated only with the effects of microplastic deformation and subsequent recrystallization, which increases their plasticity [Fedyukin, V.K. , Smagorinsky, M.E. Thermocyclic treatment of metals and machine parts. - L .: Mashinostroenie, 1989. - 255 p.]. When the components are solubility in each other in the eutectic and peritectic systems, the character of TC processes changes - there is the possibility of diffusive mass transfer through solid solutions, diffusion division of extended particles in the eutectic and excess phases, as well as the possibility of spheroidization and coagulation of phase inclusions. At TCTs of such steels, significant interfacial stresses and temperature gradients occur during repeated (multiple) diffusion transformations, which lead to an increase in the number of transformation centers and, as a result, to grain refinement and improvement of the complex material properties: increase in toughness, strength, wear resistance, ductility.

In iron-carbon alloys doped with Cr, Ti, V, Mo and other carbide-forming elements, a phase transformation also occurs during TCT, which plays a crucial role in dividing the carbide mesh — from a continuous carbide mesh of a lamellar structure, as a result of SCT, insulated globular dispersed carbides form, which are located along the boundaries of austenitic and other grains, and within the grain. The efficiency of the TTO effect on the structure and properties of steel is largely determined by the technological parameters: mode (upper and lower temperatures in the cycle, number of cycles, and also heating / cooling rates).

Until now, there are no data on the use of thermal cycling for tempering parts under high-frequency heating conditions while simultaneously cooling them in liquid media in order to ensure the desired properties (hardness, wear resistance) and self-sharpening effect for the cutting tool edges, agricultural machines and pr [Bareyan, A.G. Self-sharpening knives of cutting mechanisms from laminated materials // New promising materials and technologies for their production: Sat. scientific tr. - In 2 tons. - T. 1. - 2004.].

Thus, a method of heat treatment of carbon steel is known, including its multiple heating at a rate of 50 ÷ 150 ° C / min to a temperature Ac ₁ + (30 ÷ 50 ° C), cooling in a cycle in air to a temperature of 590 ÷ 610 ° C and cooling after the last cycle in water or oil [A. No. 1379322 USSR, cl. C21D 1/78. The method of thermocyclic processing of carbon steel].

The disadvantages of the analogue are: duration (4-8 h); the use of complex and metal-intensive thermal equipment for the implementation of thermal cycling (zone furnaces with moving parts (hearth, pushers, etc.), a set of two or more furnaces with fixed temperature transfer centers, etc.), which, nevertheless, does not provide adequate technological parameters process, due to the high inertia of heating / cooling parts in furnaces; high labor intensity; decarburization of the surface of the part.

Partially, these disadvantages are eliminated when using the method of thermocyclic treatment of steel types 20, 20Л and 20 ФЛ, including their threefold treatment with accelerated heating above the austenization temperature to 900 ° C, homogenization of austenite at this temperature and cooling to a temperature below Ac ₁ at a speed of 5 ° C / s [A. №1315487 USSR, MKI C21D 1/78. The method of thermocyclic treatment of medium carbon low-alloy steels].

The closest in technical essence is a method of hardening cutting tools of low carbon boron steel, including heating the tool with high frequency currents by means of an inductor immersed in a cooling medium and cooling (BY 15422, C21D 1/10, 02.28.2012).

The disadvantage of the prototype and analogues is the inability to obtain different hardness over the cross section of the sample (part), on its surface and in the core, or different hardness on one and the other side of the blade, which does not provide the self-sharpening effect required on the cutting edges of the tool and working bodies of agricultural machines.

The use of accelerated heating, high-speed cooling and an increase in the upper temperature in the cycle up to 900 ° C in the above technical solutions allows to reduce the heat treatment time, reduce the laboriousness of the process and reduce the intensity of the surface decarburization of the part surface, but still requires complex and metal-intensive thermal equipment when it is heated.

The objective of the present invention is to simplify the hardware design in the implementation of heating parts and getting her self-sharpening effect on the cutting edges.

This problem is solved by the fact that in the method of hardening a cutting tool of low carbon boron steel, which includes heating the instrument with high frequency currents by means of an inductor immersed in a cooling medium and cooling, a protective shield is installed on the cutting edge of the instrument, and the instrument is heated and cooled in a thermocyclic mode the number of cycles 3-5 in the temperature range from 1150-1270 ° C to 650-730 ° C, while using a cooling medium with a temperature of from 4 to 8 ° C and composition, in wt. %:

ethylene glycol 15-19

urotropin 0.5-1.5

water the rest

The screen is made of copper with a thickness of 3 mm in the form of a cutting edge.

When implementing the proposed method of quenching, instrumentation is simplified at the heating stage - due to the use of a high-frequency electromagnetic field, and also provides a self-sharpening effect - due to different intensity and HDTV heating mechanisms from different sides of the part.

The implementation of the proposed method is illustrated by the following examples.

Example 1. The choice of hardened parts.

As an example of the details for the implementation of the proposed method and the establishment of optimal limits for the parameters of the thermal cycling process, the lancet paw of the Kuzbass tillage complex manufactured by ANITIM JSC (Barnaul) from 30MnB5 steel, a characteristic and commercially available representative of the whole group of low carbon boron containing steels, was chosen presented in standard EN 10083-3.

Example 2. Determination of the characteristic wear pattern.

The characteristic wear pattern and its parameters (shape, dimensions) at the articulated paw were determined experimentally, for which purpose a complex of spring field work (stubble incorporation, accumulation and preservation of moisture, hulling, plowing, direct sowing of cereals, cultivation) was carried out on various soil types of the Altai Territory : experimental fields SEC "Tambovsky" Romanovsky district; the fields of the agricultural farm tribal plant (collective farm) "Steppe" of the German National District.

Units for tillage and sowing were completed on the basis of K-700 (701) “Kirovets” tractors with a sowing complex. Kuzbass PK-8.5. Tillage was carried out at a speed of 7-8 km / h, periodically monitoring the state of the surface of the working bodies of the complex, the aggregates in the farms were treated with 2800 ÷ 3000 ha of arable land, which amounted to an average time of 100 ÷ 107 ha / paw.

The research program in 2017 included an assessment of the wear rate of the working bodies and determination of parameters of the wear pattern of the jibbed feet depending on the abrasive component in the soil and the development time. During the experiments, the following parameters were measured and determined: weight wear, width (wear) of the paw blade in the middle part of the wing and at the edge, as well as nose wear and then the wear pattern was determined according to the test results (see Fig. 1).

Example 3. Optimization of the parameters of the TCO process on samples of steel 30MnB5.

From the flat parts of the surface of the selected part (wings of the paw), prototypes were cut out, 35 ÷ 50 × 30 × 6 mm in size, in the amount of 27 pieces, which, to ensure repeatability and reproducibility, were divided into 9 batches of 3 pieces each.

The prepared samples were placed in an enclosing five-turn water-cooled solenoid inductor made of a copper tube ∅ 10 mm and connected to the ELSIT 70-100 / 40 high-frequency inverter, after which high-speed high-frequency heating of the samples was carried out in a coolant containing the following ingredients, masses. %: ethylene glycol 10 ÷ 20; urotropin 0,5 ÷ 2,0; water - the rest.

The investigated compositions of the cooling medium are shown in table 1.

Samples (3 pcs.) Were heated in the inductor under the same conditions (frequency, heating time, number of heating cycles) while on one side (reverse) the sample surface was preliminarily protected from excessive exposure to the high-frequency electromagnetic field by a screen made of M1 brand copper ( GOST 495-92), 1 ÷ 3 mm thick (see Fig. 2).

To determine the optimal quenching temperature, the entire batch of samples was quenched at temperatures from 1050 to 1300 ° C.

In the temperature ranges: 1050 ÷ 1100, 1100 ÷ 1150, 1150 ÷ 1200, 1200 ÷ 1250 and 1250 ÷ 1300 ° С, three samples were quenched to determine the optimal number of thermal cycling cycles (2 ÷ 6 times).

The optimum temperature of the cooling medium was determined by the selected sample heating temperature and the need for high-speed cooling.

For example, at a sample heating temperature of 1050 ÷ 1100 ° C - the coolant temperature was 8 ° C, if the sample was heated to a temperature of 1150 ÷ 1200 ° C, the coolant had a temperature of 4 ° C, and at a temperature up to 1300 ° C it was 2 ° C, which provides a predetermined gradient of hardness over their cross section with the same structure of the hardened material.

To measure the temperature of the samples, a tungsten-rhenium thermocouple with a diameter of 0.2 mm was used, which was welded to a plate made of steel 30MnB5 using capacitor welding, as well as a ProfPiP V7-46M universal digital voltmeter in the dc range 0 ÷ 50 mV. The total error of temperature measurement with this instrumentation using the ADC device does not exceed 4 ÷ 5%.

* Note: the investigated number of cycles of TTsO in a certain mode is shaded; the sign “+” - the effect of self-sharpening appears, the sign “-” - the effect of self-sharpening does not appear.

The choice of heating parameters of the samples for quenching was carried out first without a protective environment, and then, according to the established parameters, quenching was carried out already with a coolant. The final heat treatment (tempering) of the samples was carried out in a muffle furnace at a temperature of 450 ° C for 2 ÷ 2.5 hours.

The manifestation of the self-sharpening effect was investigated in parts heat-treated (hardened) in modes that meet the optimal process parameters at the appropriate temperature of the cooling medium, as well as the thickness, shape and dimensions of the protective screen.

The investigated quenching modes, process parameters, the results of measuring the hardness of the sides of the sample surface and the manifestation of the self-sharpening effect are shown in Table 2.

Example 4. Determination of the design and parameters of the protective screen for the hardened part.

The design and parameters of the protective screen were determined by the characteristic wear pattern of the hardened part (pointed paw) installed earlier (see note 1).

Thus, the shape of the figure of the surface of the screen (see Fig. 3) was performed coinciding with the shape of the figure of the upper (convex) side of the pointed paw, bending it from a flat sheet of copper billet along the line from directly onto the surface of the new part, with the length of the line setting no less than the experimentally established maximum width of the characteristic figure of wear of the arched paw, defined in its nose part.

The length of the wings of the screen - and performed no less than the length of the wings of the pointed blade, determined along its cutting edge, and the distance between the wings of the screen - b was performed not less than the distance between the wings of the new pointed blade, minus 2c.

With this value of the dimensional parameters of the screen a, b, c, complete overlap of the part wear pattern with them is achieved.

The screen thickness - d was determined experimentally, on samples of steel 30MnB5 similarly approx. 3, with the difference that the samples were hardened at the optimum values of the process parameter (heating temperature, cooling temperature, number of TC cycles), a copper screen M1 1-5 mm thick was used and the hardness of the hardened samples was determined after their tempering in a muffle furnace at a temperature 450 ° C for 2 ÷ 2.5 hours. The results of experiments to determine the optimal thickness of the screen are shown in Table 3.

Example 5. Hardening parts of the proposed method.

First, in real conditions, the wear figure of the lamella part is determined (see Fig. 1), then technological equipment (protective copper screen) is made, the shape of which overlaps the wear figure, the dimensions of which are not less than parameters a , b, c of the wear figure, and thickness, m 3 mm, then the screen is installed on the front side of the lancet paw on the cutting edge side and the entire assembly is placed in an inductor and heated with high frequency currents in the high-frequency inverter ELSIT 70-100 / 40 with the following optimal process parameters: current - 80 ÷ 90%; heating time - 1.5 ÷ 2 min; heating mode - TC; number of cycles 3 ÷ 5; the upper temperature of the cycle is 1150 ÷ 1270 ° С; the lower temperature of the cycle is 650 ÷ 730 ° C, placing the equipment in a protective (cooling) environment with a temperature of from -4 to -10 ° C, the following composition of the mass,%: ethylene glycol - 15 ÷ 19, water - 80 ÷ 84, inhibitor (hexamine) - the rest.

The invention is further illustrated by the following figures (drawings).

FIG. 1, a hardened part (projection) is shown with an experimentally characteristic wear figure, where: 1 is a wear figure (shaded), 2 is a part surface, 3 is a cutting edge.

FIG. 2 shows a sample hardened by the proposed method and protected on one side with a copper screen from excessive exposure to a high-frequency electromagnetic field, where: 1 - inductor branches, 2 - water cooling inductor, 3 - sample, 4 - copper screen, 5 - ceramic spacer, 6 - protective (cooling) medium.

FIG. 3, the construction (shape) of the protective copper screen is shown, and its main parameters a , b, c thickness - d.

FIG. 4 shows an example of the organization of the assembly (tooling) of a protective screen with a hardened part according to the proposed method, which is then placed in a cooling medium, where: 1 is a protective screen, 2 is a hardened part (lance), 3 is a wear pattern, 4 is cutting edge.

Specified in the application parameters of the proposed method of hardening parts from low carbon boron-containing steels are determined experimentally and are optimal as they achieve the stated objective of the invention and obtain the desired technical result - simplification of instrumentation at the stage of heating the part and getting its self-sharpening effect on the cutting edges.

Simplification of the hardware design is achieved by using high frequency currents for heating the hardened part, special inductors optimized in the form of the part, and performing HD TV heating in TCT mode, which allows not only surface heating of the workpiece (due to induction of eddy currents in the high-frequency electromagnetic field its skin layer), but also volumetric heating of the deeper layers of the workpiece and its core - with heat transfer, which ultimately allows replacing the volumetric heating under quenching parts in furnaces for high-performance, high-tech high-frequency heating.

The hardening effect of the hardened part made of steel 30MnB5 is achieved by implementing the proposed method in the optimal intervals of the values of TC process parameters (the number of cycles of TC, the upper temperature of the cycle, the lower temperature of the cycle, the temperature of the cooling medium, the composition of the cooling medium), as well as the shape and sizes of the protective copper screen.

Thus, the number of cycles of TTsO, is selected from the interval of 3 ÷ 5, which allows to ensure uniform heating of the billet over the cross section due to thermal conductivity and heat transfer to its core from the surface skin layers located on the front and back sides of the billet. The number of cycles less than 3 times, for example, 2, is insufficient for uniform heating of the workpiece to the desired thickness and does not provide it with a self-sharpening effect (see Table 2). The number of cycles of TTsO above 5 times, for example 6, is unnecessary, since a uniform heating of the workpiece occurs already at 5 cycles.

The upper temperature of the cycle is 1150 ÷ 1270 ° С, which is guaranteed (by 120 ÷ 130 С °) exceeds the temperature point Ac ₁ (pearlite eutectoid transformation) for steel 30MnB5. The lower temperature of the cycle is 650 ÷ 730 ° C, which is guaranteed (by 5 ÷ 30 C °) below the temperature of the Ac ₃ point (complete dissolution of ferrite during heating and the beginning of austenite formation) of this steel. Heating parts made of low carbon boron steel (30MnB5) above the optimum temperature of 1270 ° C, for example, to 1300-1350 ° C, at the upper boundary of the cycle, the cost center leads to a sharp increase in the size of austenitic grain, and cooling the workpiece below the optimum temperature of 650 ° C, for example, to 550 ÷ 600 ° C, at the lower boundary of the cycle, is impractical, since the transformation of austenite - ferrite + cementite by the time this temperature is reached is already complete.

The temperature of the cooling medium should be in the range from 2 to 8 ° C, which ensures a guaranteed martensitic structure of the hardened part. Exceeding the temperature of the cooling medium above 8 ° C, for example 9 ÷ 10 ° C, does not provide the necessary cooling rate and obtain the required martensite structure with a hardness of up to 52 HRSe over the entire cross section of the sample. Lowering the temperature below 2 ° C, for example (1 ÷ 0) ° C, does not change the structure of the hardened material, but requires additional technical efforts to achieve lower temperatures and requires changing the composition of the coolant.

The optimal composition of the ingredients of the coolant provides the necessary critical rate of cooling samples for martensite and corresponds to # 1, 2, 3 (Table 1). So the composition of the coolant No. 4 no longer provides a stable surface hardness if it contains 10% ethylene glycol instead of 15%, due to its crystallization. The composition of the coolant No. 5, when the content of ethylene glycol is above 19%, for example 20 ÷ 23%, is also undesirable, due to the unjustified overrun of the expensive component.

The optimal content of ethylene glycol in the coolant in the range of 15 ÷ 19% is associated with the state diagram of this system, which ensures the automatic achievement and maintenance of temperatures in it from 2 to 8 ° C, without an increase in viscosity.

Urotropin is selected as an affordable and cheap corrosion inhibitor, its content in the coolant from 0.5 to 1.5% provides the necessary anti-corrosion properties. The excess of the content of the inhibitor above 1.5%, for example up to 2.0%, is impractical because of the overrun of this ingredient, and the resulting solution foams strongly. The decrease in the concentration of urotropin in the solution below 0.5%, for example 0.3%, no longer provides its necessary anti-corrosion properties, and it is also difficult to provide it technically.

The thickness of the protective copper screen - up to 3 mm is also optimal (see Table 3) and is associated with the depth of penetration of currents into the metal, which at frequency 40 ÷ 70 kHz for copper is δ = 2.4-2.8 mm. With such a thickness of the protective screen, the entire skin layer is located in its volume, and the hardened part under it is heated only by heat transfer. On the other side (not protected by the screen), the part is heated by the skin layer, which is located under the inductor branch. Thus, the different heating intensity on different sides of the part provides its different hardness (10 ÷ 12 HRC units) on the surface protected and unprotected by the screen, which, in the end, allows to get the effect of its self-sharpening. As can be seen from the table. 2, the optimum hardness on the hardened surfaces of the part is achieved on samples No. 1-12, where the self-sharpening effect is ensured.

When the optimal parameters of the proposed method were established, the lancet paws of the Kuzbass PK-8.5 sowing complex (28 pieces), made of steel 30MnB5, were strengthened, the wear resistance of which increased 1.5–2 times while maintaining the cutting properties of the working member due to the effect self-sharpening at up to 100 ÷ 107 ha / paw.

Claims (5)

1. The method of hardening cutting tool of low carbon boron steel, including heating the tool by high-frequency currents by means of an inductor immersed in a cooling medium and cooling, characterized in that a protective screen is installed on the cutting edge of the tool, heat and cooling of the tool is carried out in thermocyclic mode with cycles 3-5 with heating to the upper temperature of 1150-1270 ° С and cooling to the lower temperature of 650-730 ° С, while using a cooling medium with a temperature from 4 to 8 ° C and composition, in wt.%: ethylene glycol 15-19 urotropin 0.5-1.5 water the rest 2. The method according to p. 1, characterized in that the screen is made of copper with a thickness of 3 mm in the shape of the cutting edge.

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