TECHNOLOGY OF INDUCTION ELECTROTHERMAL PROCESSING

The induction electrothermal treatment technology was first used for induction heating and heat treatment of automotive parts. In 1937-1938 together with the laboratory of prof. V. P. Vologdina mastered the surface hardening of the crankshaft of the crankshaft of the ZIS-5 engine in the production line of the machining workshop on a high-frequency semi-automatic machine tool. The proportion of heat treatment using induction heating for surface hardening of parts on the ZIL with respect to other methods of surface hardening is more than 61% (cars ZIL-164A and ZIL-157К).

Surface hardening during surface heating

This is the most common type of heat treatment using induction heating.

The depth of the hardened layer is determined by the depth of the layer heated to the quenching temperature. Parts made of steel 45, 40X, 40HNMA, etc. are subjected to local surface hardening. In most cases, local surface hardening is used to increase the wear resistance of parts whose safety margins are large enough (distribution and crankshafts, axles, switching rods, expandable cams, etc.) .

In some cases, local hardening is also used to harden the most loaded sections of parts in order to increase their overall strength (for example, hardening the semi-axle pipe of a ZIL-164A vehicle). Previously, the pipe was made of normalized steel 40X or improved (for buses), and due to the strength of the material of the pipe, the structural strength of the most loaded section was ensured.

After analyzing the distribution of the working stresses on the pipe, surface hardening was strengthened by the location of application of the largest alternating bending moment. As a result, the strength of the carbon steel pipe 45, hardened to a depth of 2-4 mm at HRC 50-62, exceeded the strength of the pipe from normalized or improved 40X steel. The introduction of the process of strengthening the semiaxial tube made of steel 45 in 1959 yielded 230,000 rubles. savings (in the new scale of prices).

The use of local hardening to increase the strength of parts is one of the main directions in the development of induction heat treatment at the plant.

Surface hardening with deep heating

In case of surface induction heating for quenching, considerable generating powers are required (0.8–2 kW per 1 cm2 of the heated surface). In addition to the need for significant heating capacity, in some cases, there are great difficulties in the implementation of surface heating for quenching on the circumference of parts of a complex profile (for example, gear teeth).

The use of dual-frequency heating for gears, in which surface heating can be obtained along the tooth profile, requires the use of very high power currents of sound and radio frequencies. This limits the scope of induction hardening of surface heating products.

In our factory, hardenable steel has been developed and used for hardening gears (critical diameter 6–15 mm); the quenching depth is due to the limited hardenability of steel [2]. Heating for quenching is carried out at low specific powers (0.2-0.05 kW / cm2).

The use of relatively low power, no need to concentrate this power strictly on a given complex surface of the part makes the use of this steel with the specified depth heating useful for heat treatment of gears and other parts with a thin section of the strengthened element.

Reduced hardenability 55PP steel during quenching after deep induction heating is used for the manufacture and heat treatment of the driven cylindrical gear of the rear axle of the ZIL-164 car instead of ZOHGT steel after cementation and hardening [3]. This method is also applied at the Gorky Automobile Plant [4].

In fig. 1 shows the macrostructure of hardened pinion of 55PP steel after deep induction heating. Heating at a frequency of 2500 Hz, power 140-150 kW, heating time 76 seconds, cooling time 6 seconds, the rate of issue of the part 120 seconds.

The macrostructure of the driven gears of the rear axle ZIL-164 of 55PP steel after induction hardening

Image1. Macrostructure of the driven gears of the rear axle ZIL-164 from steel 55PP after induction quenching.

The introduction of this process in 1961 in the flow of machining yielded savings of 172,000 rubles. The use of steel with limited and adjustable hardenability opens up new fields of application of induction heating for heat treatment of not only gears, but also other mass parts.

Surface hardening of malleable ferritic cast iron

The surface hardening of ductile ferritic cast iron provides a very advantageous combination of properties in a cast iron casting – high wear resistance of the hardened surface under conditions of dry friction while maintaining high strength characteristics.

For the first time, our factory mastered the hardening of high-frequency parts from ductile ferritic cast iron, and this process was introduced into the flow machining line for brake shoes and spacer sleeves [5].

Under normal surface heating for the hardening of steel parts in ductile ferritic cast iron, the diffusion dissolution of carbon throughout the entire volume of the layer does not have time to complete. To increase the rate of diffusion saturation, a high heating temperature of 1000-1050 ° C is used. The mode of heating for surface hardening is divided into three stages:
1) heating at low power density up to 600-700 ° С;
2) forcing the heating by increasing the specific power of heating to 1000-1050 ° С;
3) holding at this temperature.

The total heating time for quenching is 80-130 seconds. Experience has shown that water can be used as a cooling medium during quenching.

Hardening of malleable ferritic cast iron expands the scope of induction heating for heat treatment.

Due to the possibility of surface hardening of ductile ferritic cast iron, the steel forged bracket of the rear additional spring is replaced by a cast iron cast bracket.

In fig. 2 shows the original and tempered structure of ductile ferritic cast iron.

Induction Electrothermal Technology

Image2. Macrostructure of ductile ferritic cast iron:
a – initial; × 100; b – hardened layer; × 250.

Induction hardened tempering

The need to carry out tempering after quenching with induction heating means a break in the technological cycle in the production processing line due to the long duration of the normal tempering process (1-1.5 h). Reducing the duration of tempering to a time commensurate with the machining and surface hardening operations (1-2 minutes) is one of the conditions ensuring the introduction of the process of induction heat treatment into the machining flow. The duration of the process is reduced, mainly due to an increase in tempering temperature.

Self-release

When surface hardening with induction heating, only 50-80% of the heat produced by the part during heating is used directly to heat the hardened layer. The rest of the heat is accumulated in the core of the part and with a time limit for the forced cooling of the hardened surface can be used for self-draining. Studies have shown that with self-tempering, the duration of which does not exceed 10-60 seconds, and the surface temperature is 60-80 ° C higher than the designated temperature of a conventional furnace leave, the effect of normal tempering and self-tempering is the same. The great advantage of self-tempering is the possibility of holding it immediately after quenching, which allows you to effectively prevent quenching cracks. Currently, when processing more than 80% of parts undergoing induction hardening, self-tempering is used. Self-tempering replaces low-temperature furnace tempering.

However, self-tempering can also be used to replace high-temperature tempering (improved piston pin for preparing the structure for surface hardening).

The piston pin is made of normalized steel 45. To ensure the strength of the finger and mainly to obtain a finely dispersed structure that makes it possible to reduce the time required to transfer the layer heated to quenching to austenite for about 0.7 sec, the piston pin is pre-improved. Improving the finger is reduced to volumetric induction heating it and then to short-term cooling of the surface with a water shower so that, if the cooling time is limited, the core structure is sorbitol or troosto-sorbitol, with a hardness of HRC 28-35, corresponding to the structure of a conventional improvement. A detailed description of this process is given in [6].

Electric release

Under the conditions of induction heating and cooling after quenching, it is not always possible to ensure self-tempering, especially on thin sections of parts and parts of complex shape. In such cases, it is advisable to replace the usual tempering by electric escape during induction heating with currents of increased or industrial frequency. At the same time, the reduction in the tempering time is achieved by increasing the tempering temperature, which can be easily achieved by uniformly heating the product over the cross section of the hardened layer.

High-temperature electrical tempering for HRC 23-35 hardness after surface hardening is carried out at the threaded end of a semi-axial tube made of steel 45. Improving the thread is necessary to increase the crushing and shearing strength.

The power output is carried out in the same inductor as for heating for quenching. In many cases, for electrical output, it is advisable to use a power frequency current.

At present, an inductor for electric axle of the rocker arms of the ZIL-130 valves supplied from a 50 Hz network through a transformer is manufactured and tested at ZIL. Such a heater is mounted together with an induction hardening device and constitutes a single unit for induction heat treatment.

Frequency selection

The frequency of the current determines the choice of parameters of induction heating and equipment.

The rational choice of the current frequency is related to the economic efficiency of the process. In order to choose the right frequency in this case, a table can be used, made up on the basis of production experience.

Frequency performance table

Frequency in Hz Rational hardening depth in mm The minimum diameter when heated for quenching in mm
possible desirable
1000 3-17 22 44
2500 2-11 14 28
8000 1-6 8 16
70000 0,5-3 2,5 5

Prospects for the introduction of induction heating technology

Specialized heat treatment plants have mechanized loading and parts dispensing.

It is intended to introduce heat treatment processes for heavily loaded parts (semi-axes, steering knuckles, pivots of steering knuckles, etc.).

In connection with the development and development of steels of limited hardenability, it becomes possible to use inductive heating for hot rolling of gears and subsequent surface hardening in a production line of mechanical processing.

The gas carburizing process with induction heating, developed and implemented at the plant, allowed to solve the issue of manufacturing gears in a single production line of the machining workshop, due to the transition to the manufacture of a new car, was replaced by grouting and carbonitriding.

With the further development of technological processes, the development of the carbonitriding technology with induction heating will be continued.

The works on the use of induction electric heating for the thermomechanical treatment of springs are carried out at the plant.

The introduction of new technological processes and the expansion of induction electrothermal processing will reduce the complexity and cost of manufacturing automotive parts, improve the reliability of operating vehicles and improve working conditions for workers.

Bibliography:

M.O. Rabin, A.G. ORLOVSKY
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, № 6. 1963

1. Shepelyakovsky K.3., Ryskin S.E. Technique of induction heating. Mashgiz, 1949.
2. Shepelyakovsky K.3. “Mitom”, 1960, № 12.
3. Shepelyakovsky K.3. Automotive Industry, 1962, No. 10.
4. Natanzon E. I. “Automotive Industry”, 1962, No. 8.
5. Rabin M.O. “Foundry”, 1954, № 9.
6. Shklyarov I.N., Ognevsky V.A. “Messenger of Mechanical Engineering”, 1959, No. 9.
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