RU2124068C1 - Method of thermochemical treatment of parts - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: method includes stepped nitrogenation of parts at 520 ± 10 and 540 ± 10 C and cooling them in dissociated ammonia medium to 400 C, then in 100% air medium with cooling rate no higher than 25 C/h to 180 C, after which parts are cooled in oil. Method enables corrosion resistance of parts to be increased by ca.1.2 times and also using commercial equipment available, improving technological effectiveness by combining oxynitrogenation, oxidation, nitrogenation processes into single continuous process, improved environmental purity, and reduced explosion hazard. EFFECT: enhanced process efficiency. 1 tbl

Description

 The invention relates to metallurgy and is intended to increase the resistance of steel nitrided parts against corrosion and can be used in mechanical engineering, for example, in excavation, for hydraulic drive parts (rod), instead of a chromium plating.

A known method of gas nitriding of steel products by AS SU 852965A, 08/08/81, C 23 C 8/26, including treatment first in ammonia at 530 - 540 ^o C, then in a mixture of ammonia and propane-butane with an equal ratio of 570 - 580 ^o C, followed by cooling in oil .

The disadvantages of this method are:
the presence of pores in the nitride layer, which reduce the corrosion resistance of nitrided parts;
higher internal stresses of the nitrided layer after cooling with T = 570 - 580 ^o C in oil, deformation and warping reduce the applicability of the method in the manufacture of high-precision (precision) parts.

 The purpose of the invention is to increase the corrosion resistance of nitrided parts by oxidation.

This goal is achieved by the fact that in the method of chemothermal treatment of parts, including step nitriding and cooling in oil, nitriding is carried out at temperatures of 520 ± 10 ^o C and 540 ± 10 ^o C, and cooling is carried out in a stream of dissociated ammonia to 400 ^o C, then medium containing 100% air with a cooling rate of not more than 25 ^o C per hour to 180 ^o C, after which they are cooled in oil.

The selected limiting cooling temperature of 400 ^o C in a medium of dissociated ammonia is explained by the fact that at this temperature ammonia dissociation practically stops (degree of dissociation 5-6%), a small amount of hydrogen is contained in the medium and the formation of explosive oxygen-hydrogen mixture (explosive gas).

At the same time, at this temperature in the air diffusion processes of deazotization and oxidation occur with the formation of an oxycarbonitride phase and a dense layer of oxide Fe ₃ O ₄ on the surface of the nitride layer.

 The pores of the nitride layer are healed with oxide.

 Based on the foregoing, cooling conditions with temperatures above 400 "have not been investigated.

The cooling rate of 25 ^o C per hour provides a gradual increase in the thickness of the dense oxide film and increase its mechanical strength.

 An increase in the cooling rate reduces the time of formation of the oxide film, its thickness, mechanical strength, and reduces the corrosion resistance.

The cooling temperature in the industrial oil of 180 ^° C. is determined by the boiling point of the industrial oil. When parts with such a temperature are immersed in oil, oil vapors are formed which, under pressure, penetrate into the smallest open pores of the oxide film and condense there, closing the pores.

 The resulting oil vapor in the presence of atmospheric oxygen leads to further oxidation of the nitrided surface, the appearance of a saturated black color characteristic of the oxidized surface.

Example. Hydraulic actuator rod simulators (samples in the amount of 3 pieces of each variant) ⌀ 36 mm, 100 mm long with a polished and polished surface made of 40X steel with a hardness of 215 - 245 HB were subjected to chemical-thermal treatment in the following modes:
1. The proposed method.

Nitriding in a medium of dissociated ammonia with a degree of dissociation of 20-40% with exposure at 520 ± 10 ^o C for 9 h and 540 ± 10 ^o C for 5 h, cooling in an ammonia stream to 400 ^o C, blowing the retort with compressed air for 5 min, cooling in air with a cooling rate of 25 ^o C per hour to 180 ^o C, cooling of samples in oil.

 2. Experienced method.

According to the proposed change in cooling rate, which was taken 35 ^o C per hour.

 3. The known method I.

Nitriding in a medium of dissociated ammonia with a degree of dissociation of 20-40% at 570 ± 10 ^o C for 14 hours, then purging with oxygen and nitrogen to create an atmosphere containing 35% oxygen and 65% nitrogen (1: 3 ratio), exposure to this medium 6 s, cooling in water by pouring water into a retort with samples, tempering in oil at 150 ^o C for 40 minutes

 4. The known method II.

Nitriding in a medium of dissociated ammonia with a degree of dissociation of 20-30% at a temperature of 520 ^o C ± 10 ^o C for 14 h with cooling to ≈ 40 ^o C in a stream of ammonia, transfer to an electric furnace and subsequent thermal oxidation in an air environment with step heating and holdings at 150 - 200 ^o C, 350 - 400 ^o C, 450 - 500 ^o C for 30 min in each temperature range and at 520 ^o C for 1.34 hours

 In order to assess the corrosion resistance of nitrided samples in comparison with those chrome plated, an additional 3 samples were subjected to galvanic chrome plating (X40.tv).

All samples were subjected to corrosion tests in a moisture chamber at a temperature of (55 ± 2) ^o C and a relative humidity of 98 - 100% for 10 days with moisture condensation on the samples.

 The electrode potential was evaluated as an indirect indicator of corrosion resistance.

 A 3% aqueous solution of NaCl was used as an aggressive electrolytic medium. The potential was determined by the potentiometer R-307.

 The test results are shown in the table.

 An analysis of the results of corrosion tests indicates that the boundary values of the declared parameters are selected correctly. An increase in the cooling rate of more than 25 degrees. per hour degrades corrosion resistance.

 Continuous nitriding and oxidation processes in one furnace, ease of process control by temperature and gas parameters indicate the high technological effectiveness of the proposed process in comparison with the known ones.

Claims (1)

A method of chemical-thermal treatment of parts, including stepwise nitriding and cooling in oil, characterized in that nitriding is carried out at temperatures of 520 ± 10 ^o and 540 ± 10 ^o C, and cooling is carried out in a stream of dissociated ammonia to 400 ^o C, then in a medium, containing 100% air with a cooling rate of not more than 25 ^o C per hour to 180 ^o C, after which they are cooled in oil.

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