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10«ШУШМИМ-ЛШИПШУ» #2И©Ш, 202 / TECHNICAL science
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TECHNICAL SCIENCE
УДК 538.9:536
Moskalenko A.A., Razumtseva O. V., Protsenko L.N.
Institute of Engineering Thermophysics of NAS of Ukraine, Kyiv.
DOI: 10.24412/2520-6990-2021-21108-37-41 EXPRESS METHOD FOR RESTORING THE COOLING PROPERTIES OF MINERAL OILS IN INDUSTRIAL HEAT TREATMENT TECHNOLOGIES
Abstract.
The paper considers the problem of the stability of the cooling properties of quenching media based on mineral oils under the conditions of their operation in continuous industrial technologies for thermal hardening, namely quenching of metal products. Optimal in terms of efficiency, time consumption, economy and ecology methods of restoring the characteristics of quenching mineral oils (QMO) deteriorated during long-term operation have been proposed and experimentally confirmed.
Keywords: quenching of metal products, cooling ability (CAb) of liquid medias, quenching mineral oils (QMO), testing of new and post-exploitation quenching oils, industrial mineral oil restoring methods.
Introduction.
The cooling properties of natural mineral oils enhance the strength of steel products during heat treatment. These cooling media, despite the high cost, certain fire hazard and risk of negative environmental impact during disposal after operation, traditionally continue to occupy leading positions in industrial technologies. Strict conditions of hardening technologies, numerous cyclic contacts with metal products superheated to 850-950°C cause rapid aging of oils and, as a result, significant losses of this valuable non-renewable natural resource. During long-term operation, aging, depletion of mineral oils and deterioration of cooling quenching properties are observed.
To maintain the stability of the strength characteristics of parts in the technologies of metal products' hardening, periodic testing and replacement of quenching media are required. During operation, mineral oils accumulate products of destruction, oxidation and various mechanical impurities, which can significantly reduce the initial chemical and physical properties of oils and, accordingly, their cooling ability. Oils with such changes in composition are not able to fully meet the requirements for metal product strength and must be replaced with a new quenching media. An alternative to this costly solution is the regeneration of used oils [1].
To restore the properties of used oils, various types of technological operations are used based on physical, physicochemical and chemical processes. Usually, the following sequence is observed:
- mechanical - is filtration in order to remove solid impurities from the oil;
- thermophysical - is evaporation of free water and vacuum distillation;
- physicochemical - is coagulation, adsorption.
If necessarily, chemical methods for the regeneration of oils are used, which are more expensive and require the use of rather complex equipment.
With the right approach to organize the oil processing procedure, it gives the enterprise a number of positive results: first of all, ensuring stable high
strength properties of products, as well as the ability to manage the economical and rational use of technological resources, reduce the volume of waste and negative environmental impact.
With all the advantages of the listed results of the regeneration of used oils, for their implementation it is necessary to interrupt the technological process of heat treatment of parts: to drain the oil for processing it with various reagents, to pass the oil through filtering materials, apparatus, etc. All this is unacceptable and unprofitable for a real continuous technological process of thermal hardening.
In this work, the effective test express method for restoring the quenching properties of oils with minimal cost and timeframes of the technological process shutdown is proposed and experimentally tested.
The essence of the proposed method is as follows:
- in a reduced mode, mechanical filtration of the oil is carried out to remove scale and other types of waste and contaminants in the sediment of the quenching bath;
- a comparison of the temperature and cooling rate graphs is made for the used quenching mineral oil and the samples of the pure original mineral oil before start of the operation;
- in case of a significant decrease of the used oil's cooling capacity, which is expressed in a slowdown of the thermal probe's cooling process, the appearance on the dT curve (cooling time) in the temperature range of 770-640°C of specific horizontal "step" typical for film boiling modes of oil on the surface of the thermal probe, a decrease in the values of dTmax and its shift to the zone of the thermal probe's lower temperatures, a decision is made on the need for regeneration;
- laboratory modeling of the required volume of oil regeneration in the quenching bath is carried out by stepwise, metered replacement of a part of the used oil with the original, new oil, with a new probe testing procedure repetition after the next replacement and
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comparison of its' characteristics with the original quenching media ones;
- with a sufficient convergence of the results of the added and original oil samples, the renewal operation in the same proportion in the quenching bath is performed, which allows to almost completely reestablish the initial cooling ability of the oil.
The determination of mineral oils' cooling properties was carried out on a laboratory equipment created at the Institute of Technical Thermophysics of the National Academy of Sciences of Ukraine (Fig. 1) for multi-factor diagnostics (temperature control with video recording) of metal probes, cooling and testing of quenching media taking into account the principles of
ISO 9950 methodology [2].
Fig. 1. Experimental setup scheme [3]: 1 - thermal probe; 2 - control thermocouple; 3 - cooling liquid; 4 -transparent glass container; 5 - heating furnace; 6 - temperature control unit; 7 - analog-to-digital converter (ADC); 8 - web-camera; 9 - web-camera lighting; 10 - video adapter; 11 - data processing software (IQ Lab);
12 - computer
To register the data obtained during the experiment, to process and calculate the thermophysical parameters of the cooling process, a special IQlab software was used [4, 5]. The experimental sample, i.e., the thermal probe (TP), was a cylinder made of heat-resistant chromium-nickel alloy X18H9T with parameters of D=10 mm, H=50 mm, at the geometric center of which a chromel-alumel thermocouple was installed. Before cooling, the thermal probe was heated up to a temperature of 810°C. During the cooling of the thermal probe in the tested environment, the thermocouple data was recorded at a rate of 10 measurements per second and transmitted through the ADC to the computer. The processing of the experimental data array by IQlab software made it possible to obtain graphical dependences of the temperature of the thermal probe and the rate of its cooling on time, as well as, by solving the inverse thermal conductivity problem, to calculate such important characteristics of the heat transfer process on the surface of the thermal probe as the heat flux density
and the heat transfer coefficient depending on temperature and time. The volume of mineral oil samples in the experiments was 0.25 dm3, the temperature was 50 +/- 3°C. It is a solution based on mineral oil of Industrial - 20A (I-20A) type and special additives. These additives optimize the cooling rate, resist aging and thermal degradation of the quench medium during operation, minimize the formation of sludge and facilitate its removal from the surface of the part.
The experiments were carried out on: a) pure (new) quenching medium; b) on a used sample after 6 months of intensive operation in the thermal shop of a machine-building plant; c) the same used samples with the addition of pure (new) quenching medium in different percentage ratio (10%, 20%, 30% and 50%).
The results of the experiments, temperature curves were obtained, the cooling rate of the thermal probe and statistical characteristics of the thermal probe's cooling process was calculated. The results of the experiments and calculations are presented below.
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Fig. 2. TP Temperature during cooling process in QMO: Tn - new original quenching oil; Tus - used quenching oil; T„.10.20.30.50 - used quenching oil with the addition of
10-20-30-50% of new original oil
Fig. 3. TP Cooling rate (dT) in quenching mineral oil versus time: dTn - new original quenching oil; dTus - used quenching oil; dT^w-w-^o - used quenching oil with the addition of10-20-30-50% of new original oil
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Cooling rate, C/s
0 20 40 80 SO 100 120 140
-------------------------------------
Time, s
Fig. 4. TP Cooling rate (T*) in quenching mineral oil versus TTP and time: Tn - new original QMO; TUS - used QMO; T10-20-30-50 - used QMO with the addition of10-20-30-50% of new original QMO
Table 1.
Statistical characteristics of the thermal probe's cooling process
Characteristics (unit) Value
n us n10 n20 n30 n50
TP Cooling time from 810 to 600°C, s 3.5 5.5 5.2 4.7 4.4 3.9
TP Cooling time from 810 to 400°C, s 6.8 8.0 7.7 7.4 7.1 7.0
Maximum cooling rate, °C/s 128.6 112.2 111.5 137.5 136.1 131.7
TP Temperature at max. cooling rate, °C 641 561 557 602 582 586
TP Cooling rate at temperature of 300°C, °C/s 8.7 9.7 10.1 9.3 8.5 8.7
2 s. 2,5 s. 2,9 s. 3,15 s. 3,9 s. Fig. 5. Video shots of boiling modes evolving during two-phase cooling of a thermal probe in the mix of used
mineral oil with 30% of new oil addition
Results Discussion.
Long-term use of quenching liquid in the process of metal products quenching can lead to almost complete loss of its cooling properties. This work is devoted to solving this problem. The experiments carried out confirmed the possibility of a shortened procedure of
the coolant regeneration by the used QMO partial replacement with a new original QMO, which eliminates the need of a complete replacement of the law-quality cooling medium. The efficiency of regeneration was assessed by restoring the cooling properties of QMO by means of comparing ISO 9950 standard tests indicators
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of quenching mineral oils, the temperature and time of the thermal probe during the cooling process, and the presence of a prolonged film boiling regime, which is undesirable when quenching metal products. The elimination of the film boiling process, especially the absence of local film boiling areas, ensures a stable and uniform cooling process and decreases the potential risk of parts' deformation.
Addition of 10% of new, pure oil to the used oil leads to a decrease of film boiling process duration by only 20%. Addition of 30% pure oil to the used one reduces film boiling time by 60%. Finally, after adding 50% pure original oil to the spent QMO, the experimental film boiling time is reduced by 80% and is as close as possible to the characteristics of the new original oil.The presence and duration of film boiling is a predominant negative indicator when evaluating the cooling properties of mineral oil samples used as quenching medium, since low cooling rates at the initial stage of the process increase the potential risk of "soft" areas on the surface, i.e., the local hardness of the part in the process of hardening reduces.
The foregoing allows us to conclude that the addition of 50% of the original QMO to the waste oil almost completely restores its cooling capacity. At the same time, 50% of the cost of the new coolant is saved, in contrast to the option of its complete replacement, and, accordingly, the negative environmental impact and the inevitable costs of disposing of used mineral oils are significantly reduced. The obvious advantage for continuous mass production is that such regeneration of the cooling medium allows its cooling properties to be restored without prolonged shutdown of the technological process, which notably reduces financial and time losses. Importantly, as the life of the quench oil increases, the stability and continuity of industrial production increases.
Conclusions.
1. Experiments have shown that the statistical indicators of the cooling process of a standard thermal probe in QMO after 6 months of operation in the industrial heat treatment technologies for steel products' quenching significantly differ from the original quenching medium.
2. In case of used mineral oil, the cooling intensity decreases in the temperature range of 800-6800C, a prolonged film boiling regime arises and reduces the cooling rate and microstructural transformations of the metal, so it is necessary to obtain a high concentration of martensite microstructure to ensure the hardness and operational strength of the product.
3. A cost-effective express method of QMO regeneration by partial replacement of the used mineral oil is proposed, restoring the cooling capacity of QMO. Adding 50% pure original to used QMO almost completely restores its cooling capacity. This allows us to speak about saving the cooling medium by 50%, in contrast to the option of completely replacing it with a new one, and allows proportionally reducing the
financial costs for the regeneration of the cooling medium.
4. The advantages of the method are simplicity, cost-effectiveness in comparison with the procedures of purification, filtration, centrifugation, logistics costs, etc., the factor of the short duration of the operation, which does not require a long shutdown of the continuous production cycle.
5. The results of the work give grounds to recommend the industrial heat treatment technologies with QMO regeneration as those that ensure the stability of products' quality under condition of establishment and observation of certain control operations' adjustment as follows:
a) to keep detailed record of the mass of the metal undergoing heat treatment in each quenching bath;
b) to carry out control testing of the quenching medium according to ISO 9950 method and evaluate the stability of its characteristics in comparison with the initial ones regularly, at least once a quarter;
c) to determine the need for and the optimal amount of the next preventive adjustment of the quenching medium composition based on laboratory tests' results.
Acknowledgment. The authors are grateful to Boris Shchegolev and Alexandra Moskalenko for highly qualified assistance during the experimental setup and text of the article preparation.
References
1. Totten G.E., Dossett J. I., Kobasko N. I., "Quenching of Steel," ASM Handbook; Steel Heat Treating Fundamentals and Processes, Vol. 4A, 2013, pp. 91 -157.
2. ISO 9950:1995: Industrial Quenching Oils Determination of Cooling Characteristics-Nickel-Alloy Probe Test Method, International Standard, International Organization for Standardization, Geneva, Switzerland. - 1995.
3 Moskalenko A.A., Simachenko A.V., Zotov E.N., Dobrivecher V.V., Deineko L.N., Kimstach T.V., Protsenko L.N. Development of a hardware and software complex for determining the cooling properties of quenching media. Construction, materials science, mechanical engineering. Collection of scientific papers. Series: 2009 Starodubov Readings. Dnipro city, 2009, 99-105 pp.
4. Dobrivecher V.V., Zotov E.N., Kobasko, N.I., Morgunyuk V.S., Sergeev Yu.V. IQLab software complex, commercially distributed by Intensive Technologies LTD (iqlab@itl/kiev.ua).
5. Zotov E.N., Moskalenko A.A., Dobrivecher V.V., Kobasko, N.I., Deineko L.N. IQLab software use to select the optimal modes for the heat treatment of steel products. Collection of reports of the 6th International Conference "Equipment and technologies for heat treatment of metals and alloys", (OTTOM-6), part II, Kharkiv, NSC KIPT, IPC "Contrast", 2005.-106-115 pp.
