EQUIPMENT AND METHOD FOR HARDENING IN THE RARE GAS

ATMOSPHERE

DESCRIPTION OF THE INVENTION Technical Field The scope of the invention is construction of the equipment and method enabling simple hardening of tools and constructional parts in the rare gas atmosphere as there are argon or helium, without harmful appearances of oxidation, decarburizing or absorption of nitrogen on the surface of hardened steel parts. According to the international classification of patents, the invention belongs to the fields C21D9/00, C21D1/74 and F27B11/00. Technical Problem

Hardening of steel parts consists of heating up to the hardening temperature and quick transporting to the quenching bath in which there is performed transformation of microstructure of austenitized steel to the hard martensite. As hardening temperatures are high, a hardening process should not be performed in the air atmosphere that would harmfully affect the changes of chemical composition and features in the superficial layer of parts which are the most important for their application regarding resistance to wear. For the purpose of preventing harmful appearances on the surface of hardened parts, hardening is carried out in various equipment that have to prevent or limit undesired reactions with the surface of steel parts. There are applied various types of protective gas atmospheres, including vacuum as a very diluted atmosphere with as low oxygen contents, that it can not react with the steel surface. At application of protective gas atmospheres, quenching is carried out in separate baths, most often in oil baths. In the industrial application, for heating up at hardening temperature, more types of protective gas atmospheres are applied which by origin may be assigned to the following groups:

- purified generator-atmospheres obtained by incomplete burn-out of hydrocarbon, containing nitrogen N ₂ , carbon monoxide CO, carbon dioxide CO ₂ , hydrogen H ₂ , water vapor H ₂ O, - dissociated methanol CH ₃ OH>CO + 2 H ₂ ,

- dissociated ammonium 2NH ₃ >N ₂ + 3 H ₂ ,

- synthetic nitrogen atmospheres with dissociated methanol: N ₂ , CO, H ₂ .

AU these gas atmospheres contain reductive gas components carbon monoxide CO or hydrogen H ₂ , neutralizing the oxygen from the air, thus preventing the most disadvantageous appearance - oxidation of steel surface. However, all these gas components are reactive because in a certain way they can react with the surface of a steel part, changing its chemical composition and consequently physical properties; hardness, toughness, resistance to tribological wear. Depending on the level of temperature and composition of a gas atmosphere, as well as chemical composition of hardened steel, in "protective" gas atmospheres of such a kind, the following, for hardening, undesired processes, take place: - decarburizing: CO ₂ + /C/>2 CO and H ₂ O + ICI > CO + H ₂ , the consequence of which is the lower hardness and resistance of a surface to wear in exploitation of tools;

- carburizing: previous reactions in reverse direction, the consequence of which is less toughness and endurance limit of parts in exploitation;

- absorption of nitrogen: N ₂ >2/N/, the consequence of which is stabilization of austenite and consequently a larger share of remained austenite in hardened martensite microstructure.

These secondary changes of chemical composition in the superficial layer of steel are often not visible from the surface, because the oxidation is prevented, however the changes of physical properties are present below the surface and affect the endurance limit of parts in exploitation. This should be prevented, particularly at hardened tools of complex form of working surface that can not be machine grinded after hardening which is the case in engravings that are made by electro erosion or high-speed CNC-machining. Background Art Existing equipments described in patent documents US5265851, AU2003280366, ES 8200405 and DE3525635 are multipurpose equipments intended for performing various thermal processes: hardening, carburizing, carbonitriding, nitriding in protective and active gas atmospheres where there is prevented oxidation of a surface, however not other mentioned harmful changes of chemical composition. Differences among them are in the way of preventing additional oxidation at transporting heated charge to emerging into a quenching bath. The first three mentioned patent documents contain solutions with translatory transporting the charge with the retort and dropping the charge in the quenching bath with intensified flushing with active gas atmosphere neutralizing a surrounding air, as

it contains reductive components CO and H ₂ . The patent document DE3525635 describes the solution without translatory transporting a charge that has vertical lifting and dropping a charge with three height positions and vertical lifting and lowering the furnace with retort and the extension that is open on the bottom. By lowering the furnace the extension emerges in the oil bath, by which there is achieved sealing of the retort with the extension towards the surrounding air. In all four embodiments, introduction of a gas atmosphere is on the top of the retort and outlet of pressed out atmosphere on the bottom of the retort, because reactive gas atmospheres have identical or smaller density than pressed out air. Therefore, the mentioned equipments are declared and intended for performing thermal processes: hardening, carburizing, carbonitriding, annealing and tempering in protective atmospheres preventing oxidation or in reactive gas atmospheres delivering to the steel carbon and nitrogen. Gas atmospheres containing more than 5% of flammable components; carbon monoxide CO and hydrogen H ₂ ignite on that outlet by means of special permanent torch with continuous ionized flame supervision and reserve connection of nitrogen for quick flushing in case of appearance of under-pressure in the retort, as well as safety blockades for application at temperatures below 750 ⁰ C. Because of possible change of composition of carbon and nitrogen in superficial layer of steel, gas atmospheres of such a kind and equipments are not adequate for hardening of tool steels in which any change of chemical composition would be very harmful. Therefore mentioned constructions cannot use gas atmospheres of higher density that the air density as it is argon, used as the protective gas in welding technologies. Only rare gases as helium He, neon Ne, argon Ar, krypton Kr, xenon Xe and radon Rn are fully inert and do not react with the steel surface and do not change the chemical composition of the surface of heat treated steel. Essence of the Invention Developed construction of the equipment on the quenching basin has fastened vestibule with the seal, to which by lowering a retort the gastight sealing is achieved and with special introduction and retaining the rare gas, argon or helium there is enabled full protection of a surface of hardened parts with minimum amount of rare gas. Argon is significantly denser than the air and therefore it gravitationally presses out the air from below upwards, whereas in case of application of extremely thinner helium, it presses out the air downward. For application of one or the other rare gas, valves on the upper and lower part of the closed system let in the necessary gas amount and maintain the necessary over-pressure in the

retort and vestibule. The lift lowers vertically the holder with the charge from the retort through the open double door and vestibule to the quenching bath. In this way hardening without harmful appearances of oxidation, decarburizing, carburizing or nitrogen absorption is achieved which is not the case in application of the solution from the mentioned patent documentation. Moreover, rare gases are non-toxic, non-inflammable and non-explosive and thus devices are simpler and cheaper. Argon and helium are rather expensive gas, however consumption in an developed equipment is very small because the air exchange takes place by the principle of gravitation pressing because of big difference in density to the air, which is not the case in application of gas atmospheres with nitrogen having identical density as the air, when the exchange takes place by the principle of dilution that requires ten times larger gas amount.

The basic significant advantage of a developed equipment in relation to the equipment described in the document DE3525635 is, that it enables application of a rare gas argon that is fully inert towards steel, whereas the device described in the document DE3525635 requires introduction of protective or reactive gas from the top of retort to the bottom of the vestibule, which excludes the possibility of application of heavier gas argon. Hardening without any changes of composition of steel on the surface is particularly important for hardening of tool steel, because at heating in gas atmospheres applicable in the document DE3525635, except basic protection against oxidation, it is neither possible to prevent the change of carbon content in the steel surface (decarburizing, carburizing), nor nitrogen absorption.

Other advantages of the equipment according to the invention in relation to the equipment described in the document DE3525635, are the following:

- construction of closing a double door of the retort enables effective thermal insulation within the lower retort part and not only below the retort, which is insufficient because of non-uniform temperature in the retort which requires more significant lifting the charge to the upper position,

- construction of lift with the guide within the vestibule fastened on both sides, enables a transport of small charge holder, which at quenching may lower the charge to the bottom of the quenching bath contrary to the solution in DE3525635 in which a batch holder, because of the construction of the extension, should be long and lowers the charge only a bit under the level of quenching medium and therefore it is necessary to

introduce the system of rapid circulation of quenching medium, in order to ensure necessary intensity of cooling and to prevent overheating and evaporation of quenching oil,

- gravitationally pressing out the air with a heavier gas enables a small consumption of gas, contrary to the equipment according to DE3525635 which uses gas atmospheres which, because of identical density, exchange the air by the principle of dilution, whereby the consumption of gas is ten times bigger than the volume of retort and vestibule. Moreover, the vestibule fastened above the quenching bath enables retention of a larger part of used gas, because argon stays in the vestibule till the next batch, whereas in the mentioned patent, by lifting the furnace, the complete gas content gets lost.

Description of Drawings

Fig. 1 shows the equipment for hardening in rare gas atmosphere, variant 1, heating up position. Fig. 2 shows the equipment for hardening in rare gas atmosphere, variant 1, quenching position.

Fig. 3 shows the equipment for hardening in rare gas atmosphere, variant 1, charging and de-charging position.

Fig. 4 shows the equipment for hardening in rare gas atmosphere, variant 2, heating up position.

Fig. 5 shows the equipment for hardening in rare gas atmosphere, variant 2, quenching position.

Fig. 6 shows the equipment for hardening in rare gas atmosphere, variant 2, charging and de-charging position. Description of invention with examples of performing

The developed equipment for hardening consists of three basic vertically placed parts; quenching basin 12, vestibule 7 and furnace 1 through which the lift 8 lifts resp. lowers the charge holder 4 upon opening thermally insulated double door 5. There are developed two variants of construction embodiments. In the first variant with two-part vestibule 7, opening the vestibule 7 is done by lifting the furnace 1 with the retort 2 leant against the seal 6 of the top of vestibule 7 and the lift 8 has two positions by height.

In the other variant with the door 6a on the vestibule 7, the retort is firmly connected with the vestibule 7 on which lateral side there is the sealed door 6a through which charging and de-charging on the charge holder 4 in the middle position is done, whereas the upper position is for heating up and the lower one for quenching. Descriptions of both variants of the equipment follow.

Description of variant 1 with sealing between the bottom of the retort 2 and the top of vestibule 7

In the bell-type furnace 1 there is built in the retort 2 where the charge 3 is heated up, which is at side charging positioned on the charge holder 4 with the length of retort opening 2 and which is during heating up between two wings of thermally insulated door of the retort 2. Below the opening of the furnace 1 there is vestibule 7 with a gas-tight sealing 6 against which the bottom of the furnace 1, leans. In the middle of the device there is the lift 8 which upon opening the door 5 of the retort 2 lowers the charge holder 4 from the upper position to the lower position in the quenching basin 12 with quenching bath 13 in which there is the heater 9, cooling device 10 and mixing device 11 of the bath 13. In the lower part of the vestibule 7 there is the valve 14 for introduction of necessary amount of argon and on the outlet of the retort 2 there is the valve 15 for pressing out the atmosphere and maintenance of necessary overpressure of argon in the retort 2 and vestibule 7. The hardening process takes place as follows: side charging to the charge holder 4 in the upper position of the lift 8 when the furnace 1 is in the upper position, lowering the furnace 1 and sealing the connection of the retort 2 and vestibule 7, closing the retort door 5, flushing vestibule 7 and the retort 2 with the necessary amount of argon, heating up the charge 3 to the hardening temperature, opening the retort door 5, lowering the charge holder 4 by the lift 8 to the lower position in the quenching bath 13, lifting the furnace 1 with retort 2 to the upper position, lifting hardened charge 3 to the upper position of the lift 8, de-charging to the side outside the hardening equipment.

Upon lifting the furnace 1, in the vestibule 7 there remains argon that is thicker than the air and at the beginning of a new hardening cycle it is necessary to replace only the air in the retort 2, and thus the costs for argon are very low. Description of the variant 2 with the door 6a on the vestibule 7

In the bell-type furnace 1 there is built in the retort 2 in which heating up of the charge takes place, which is at charging on the side put on the charge holder 4 with the length of

the retort opening 2, which is during heating up between two wings of thermally insulated door of the retort 5. Below the opening of the furnace 1 there is the firmly connected vestibule 7 with gas-tight sealed door 6a on the lateral side of vestibule 7. In the middle of the equipment there is the lift 8, which upon opening the retort door 5 lowers the charge holder 4 from the upper position to the lower position in the quenching basin 12 with the quenching bath 13. In the lower part of the vestibule 7 there is the valve 14 for introducing argon and on the outlet of the retort 2 there is the valve 15 for pressing out the atmosphere and maintenance of necessary over-pressure of argon in the retort 2 and vestibule 7. Hardening process is as follows: charging on the side to the charge holder 4 in the middle position, closing the sealed door of the vestibule 6a, flushing of the vestibule 7 and retort 2 with the necessary amount of argon, lifting the charge in the upper position of the lift 8 in the retort 2, closing the retort door 5, heating up the charge 3 to the hardening temperature, opening the retort door 5, lowering the charge holder 4 with the lift to the lower position in the quenching bath 13, closing the retort door 5, lifting hardened charge 3 to the middle position of the lift 8, opening the vestibule 6a door, de-charging to the side out of the hardening equipment.

Argon density is 1,784 kg/m ³ and the air density approximately 1,3 kg/m ³ . Instead of pure argon in the equipment there may be used non-flammable mixture with maximum 4 % of hydrogen for neutralizing small amount of oxygen and water vapor in a gas with lower purity level.

In application of thinner helium of density 0,1785 kg/m ³ , gas introducing in the closed system is reverse than in the case of thicker argon: helium is introduced by the valve 15 on the top of the retort 2 and atmosphere gets out at the bottom of vestibule 7 through the valve 14 maintaining the over-pressure in the closed system. The choice of a rare gas depends on a local price and availability of gases. Industrial application

Developed equipment and method are applied in the following examples of industrial application: 1. Hardening of cold forged spiral tubes for arms made of alloyed construction steels. 2. Hardening of tools made of high-alloyed steels.

Reference numbers

1. Furnace

2. Retort

3. Charge 4. Charge holder

5. Retort door

6. Vestibule sealing 6a. Vestibule door

7. Vestibule 8. Lift

9. Heater

10. Cooling device

11. Mixing device

12. Quenching basin 13. Quenching bath

14. Valve

15. Valve