Nalazite se na CroRIS probnoj okolini. Ovdje evidentirani podaci neće biti pohranjeni u Informacijskom sustavu znanosti RH. Ako je ovo greška, CroRIS produkcijskoj okolini moguće je pristupi putem poveznice www.croris.hr
izvor podataka: crosbi !

Possibilities of heat transfer control during quenching (CROSBI ID 540128)

Prilog sa skupa u časopisu | sažetak izlaganja sa skupa

Liščić, Božidar Possibilities of heat transfer control during quenching // Materiali in tehnologije / Leskovšek, Vojteh ; Smoljan, Božo ; Jager, Heimo et al. (ur.). 2008. str. 11-12

Podaci o odgovornosti

Liščić, Božidar

engleski

Possibilities of heat transfer control during quenching

Quenching is a nonstationary thermodynamic process the aim of which is to attain the required level of superficial hardness, as well as adequate hardness distribution on the cross-section of a hardened workpiece, with minimum deformation and size change. These two requirements are opposite to each other. A higher quenching intensity to achieve greater depth of hardening increases deformation and size change.Therefore it is necessary to control the dynamics of heat extraction from the workpiece i.e. to optimize the quenching parameters and control the quenching intensity during the whole quenching process. From the first moment when a workpiece is immersed in a quenchant, three different proce- sses start and develop simultaneously:the thermodynamical one, the metallurgical one and the mechanical one. The transformation of the microstructure does not start on the whole cross-section simultaneously, but gradually from the surface to the core only then when a particular point attains the temperature A1.This fact makes it possible ( at least for bigger cross-sections) to intentionally change the heat extraction dynamics i.e. to control the quenching process. For numerical simulation of every quenching process it is necessary to know the temperature dependent heat transfer coefficient (HTC), which depends on many influential factors: geometrical, fluid-dynamical, material's dependent and surface condition's dependent.There is a fundamental difference between the HTC for quenching in liquid quenchants and for gas quenching.The usual way to calculate the HTC is to measure the temperature at a point below surface of a specimen or probe and by applying the inverse heat conduction method to calculate the heat flux on the surface and the surface temperature.For estimation of the HTC one needs to know: the cooling curve at the location where the temperature is measured, thermophysical properties of the workpiece's material and the temperature of the quenchant.It is important to distinguish between the HTC calculated for the small laboratory test specimen and for real workpieces.The HTC calculated for small laboratory test specimen cannot be used for quenching simulation of real workpieces in practice because of the following reasons: real workpieces have bigger mass ( in case of cylindrical parts have bigger diameters) ; their cooling times are much longer, and some parameters which occur in practice (bath temperature, agitation rate and direction, position and loading arrangement in a batch) cannot be taken into account at laboratory tests.Therefore for calculation of HTC in case of real cylindrical workpieces the Liscic/ Nanmac quench probe was used.It is a cylinder of 50 mm diameter and 200 mm length instrumented with three thermocouples (TC) the outher of which of special design ( U.S.Patent No.2, 829.185) measures the temperature at the very surface.By applying the Temperature Gradient Method the heat flux density and the HTC can easily be calculated.To calculate realistic values of HTC for workpieces in practice the quenching probe should be of the same shape as the workpiece (cylinder ; plate ; ring) and of similar dimensions (volume to surface ratio). The TC should have the smalles possible diameter and its position should be as close to the surface as possible, in order to minimize the damping effect, the time lag and the TC response time. Changing intentionally the heat extraction dynamics during quenching in liquid quenchants, especially when workpieces of thin cross-sections are involved, is practically impossible or limited to special quenching techniques. Two of such examples are: Intensive Quenching and Delayed Quenching.At Intensive Quenching usually plain water is used but its flow velocity is extremely high, so that(at the IQ-3 variant) from the beginning of quenching eliminates the film as well as the boiling phases.As a consequence suddenly a hardened crust around the workpiece is formed in which high compressive stresses exist.The most important feature of this technology is the interruption of cooling at the moment when the compressive stresses attain its maximum.As the results from practice prove, the distortion of parts is less and the fatigue-strength is increased so that the working life of workpieces is substantially increased, even if lower alloyed steels are used.At Delayed Quenching the first part is slow cooling (usually in air) followed by immersing the workpiece in a liquid quenchant.The cooling rate within the surface region is low, but after immersion in a liquid quenchant it becomes high during the period when the structure transformation in the core takes place.The characteristic feature of this technology is the discontinuous change of cooling rate. From the earlier works of Shimizu and Tamura it is known that the pearlite transformation in case of discontinuous change of cooling rate differs from what it should be according to the CCT diagram, and de- pends on the consumed incubation before the discontinuous change of cooling rate occured. They have also given explanation why at delayed quenching an inverse hardness distribution on the workpiece's cross-section (i.e.higher hardness in the core than at the surface) can occur. Recent investigation by Liscic and Totten has shown that Polyalkilen-Glykol (PAG) solution of high concentration can be used for a preprogrammed and reproducible delayed quenching. The results of this investigation enable two important conclusions.The first is: the dynamic of heat extraction at quenching is responsible for the hardness distribution achieved. The second one is: a controlled delay quenching has a much greater potential to achieve through-hardening, compared to conventional quenching process. Gas Quenching. High Pressure Gas Quenching (HPGQ) in vacuum furnaces is a modern and promising technology with many advantages compared to liquid quenchants. Cooling takes longer time, hence some quenching parameters can be intentionally changed during quen- ching.Because cooling follows the Newton's law there is no uncontrollable changes in heat transfer.The main shortcoming of HPGQ, especially at workpieces of big cross-section made of steel having low hardenability, is the inability to attain the required hardness in the core, due to insufficient quenching intensity.According to the Newton's law there are two possibili- ties to remedy this situation.The first one is to increase the heat transfer by increasing the gas pressure and its velocity, which is usually applied but is limited because of economical reasons (furnace design and blower's power required).The other one is to increase the temperature difference between the workpieces' surface and the circulating gas by transient spraying with liquid nitrogen which suddenly increases the heat flux, because of the very low temperature of evaporated nitrogen.By using both possibilities simultaneously i.e.a combination of main gas stream (of adequate pressure and velocity) and transient spraying of liquid nitrogen, new Controllable Heat Extraction Technology (CHE) can be developed. It would enable a broad spectrum of quenching intensities and eliminate the above described shortcoming of the HPGQ.While the change of gas pressure or of its velocity requires a certain period of time, spraying of liquid nitrogen can start and be interrupted momentarily, which enables a very flexible control of the quenching intensity. Moreover providing that the furnace is equipped with necessary control system and software programme, fully automatic control of the heat extraction during quenching is possible.

quenching ; heat extraction dynamics ; delayed quenching ; gas quenching

nije evidentirano

nije evidentirano

nije evidentirano

nije evidentirano

nije evidentirano

nije evidentirano

Podaci o prilogu

11-12.

2008.

nije evidentirano

objavljeno

Podaci o matičnoj publikaciji

Materiali in tehnologije

Leskovšek, Vojteh ; Smoljan, Božo ; Jager, Heimo ; Jenko, Monika

Ljubljana: Inštitut za kovinske materiale in tehnologije

978-961-91448-9-3

1580-2949

1580-3414

Podaci o skupu

2nd International Conference on the Heat Treatment and Surface Engineering of Tools and Dies

predavanje

25.05.2008-28.05.2008

Bled, Slovenija

Povezanost rada

Strojarstvo

Poveznice
Indeksiranost