Quantity "M" a measure of cooling capacity of quenching media.

Mudura, Pavel ; Munteanu, Alexandru ; Vesselenyi, Tiberiu 等

Abstract: In this paper an original physical entity is determined through the cooling speed curve which can be regarded as a measure of cooling capacity of a quenching media used in hardening of steel machine parts. There are also presented some proprieties of this entity, which will result in a better correlation of cooling conditions with yielded structures. Keywords: characterization of quenching liquids, cooling curves, cooling capacity.

1. INTRODUCTION

As it can be reasoned form CCT curves, of a certain steel, structures obtained from austenite by continuous cooling are depending in a great measure on the aspect of the temperature-time curve (cooling law), respectively on this curve's position compared to the CCT curves. Further more, the cooling law of a certain point from the interior of that steel probe, depends on the interaction between the heated probe and the cooling media.

The most complete information regarding the cooling ability of a certain cooling media, is given by the cooling curves (temperature-time curves and respectively cooling speed--temperature curves), which can be obtained at the cooling of certain probes made from metal (Ni, Cu, Ag), and which usually are not presenting phase transformations at cooling with those cooling media (***, 1998). This explains why the cooling curve method is most frequently used, for finding the cooling capacity of a certain media, also existing a few national standards in this field (Bodin & Segerberg, 1993).

This paper presents an original quantity, found with the help of cooling curves, which measures directly the intensity of cooling, in a temperature range, of a certain point from the interior of a steel probe, formerly heated in austenite domain and then cooled with the help of a certain cooling media.

2. DEFINITION OF "M" QUANTITY.

The cooling of a probe (regardless of the metal which is manufactured of), can be made by producing a contact between the probe, heated at over critic temperature, and a certain cooling media, which is at a considerably lower temperature. As a function of the probes surface temperature every media has a certain ability to extract the heat stored in the probe, so a point within the probe will be more or less intensely cooled (Mudura, 2000a). It is easy to see that the cooling of the probe is a process that depends, through the thermo physical proprieties of the probe and cooling media, on the couple probe-cooling media.

It follows, (admitting that the cooling of a point from the interior of a certain probe is best represented by the cooling curves obtained for that point) (Mudura et al, 2000b), (Mudura et al, 2001), that the characteristic quantities, which derives from this curves will depend on the selection of that particular couple. In these conditions cooling capacity characterization of a quenching media, using the cooling curves, will depend on the steel used to manufacture the probe. In order to highlight this characterization in a direct link with austenite transformations at cooling, in different temperature ranges, it is necessary to find more representative quantities, which also can be correlated with the results of quenching, represented by the obtained and measured hardness. These quantities, frequently used, given in table 1 (***, 1998), are: cooling time on different temperature intervals and cooling speed at different probe temperatures.

For the measuring of cooling capacity of a certain cooling media, by cooling a certain point from the interior of a metallic probe, we define a quantity noted by us "M", expressed by:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where: [v.sub.r] (T) is the cooling speed, in [degrees]C/s, of a representative point; [T.sub.1], [T.sub.2] ([T.sub.1]>[T.sub.2])--temperatures which are defining the interval for which the cooling intensity is found, in [degrees]C.

As geometric interpretation, the "M" quantity, (fig.1.a)), represents the area between the lines T=[T.sub.1], T=[T.sub.2], axis OT and the curve [v.sub.r] = f(T). From the point of view of cooling, "M" represents, by the area of this surface, the intensity of cooling on the interval ([T.sub.1], [T.sub.2]).

For the quantity "M", on the cooling curve T = f(t), (fig.1 b)), corresponds another quantity namely the time needed to cool the respective point from temperature [T.sub.1] to temperature [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. It can be observed that any of these two quantities represents the intensity of cooling on a certain temperature interval. In order to analyze which of these two quantities represents better the cooling intensity on a certain temperature interval, there will be considered a set of cooling laws (1,2,..,7) formed from two segments each (a) and (b). These two segments correspond to the cooling domains used at steel hardening: cooling until the entering in the domain of martensite forming (MS-MF) then cooling in the domain MSMF. We admit that each of these cooling laws represents the cooling of different points of a probe, for example from the temperature of 100[degrees]C to the temperature of 0[degrees]C, on the same time interval (60 seconds) (figure 2). The values of quantities [M.sup.100.sub.0] and, [DELTA][t.sup.100.sub.0] for each cooling law are given in table 1

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

After the analyses of the values of the two quantities, the following observations can be made:

--the quantity [DELTA][t.sup.100.sub.0] has the same value (60 sec.) for all the cooling laws in spite of the fact that these laws are different, characterized by different slopes (a and b, fig.2);

--the quantity [M.sup.100.sub.0]has different values admitting a minimal value corresponding to the cooling law which has a constant cooling speed (curve 4);

--every cooling law 1 ... 3 admits another cooling law symmetrical to the point of coordinates ([DELTA][t.sup.100.sub.0]/2, [DELTA][t.sup.100.sub.0]), so that the two symmetrical cooling laws has the same value of the quantity., (table 1).

3. PROPRIETIES OF "M" QUANTITY.

Starting from the above mentioned observations, the following proprieties of quantities [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are formulated:

([P.sub.1]): In a certain temperature interval [T.sub.1]-[T.sub.2], theoretically an infinite number of distinct cooling laws can exists which has the same value for the quantity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

([P.sub.2]): The values of quantity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] which characterize different continuous cooling on the same period of time from temperature [T.sub.1] to temperature [T.sub.2], admit a minima which corresponds to the constant cooling speed.

([P.sub.3]): Cooling laws which represents a continuous cooling from temperature [T.sub.1] to temperature [T.sub.2], in a time period [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], symmetrical to each other compared to the middle of the constant cooling speed segment are characterized by the same value of the quantity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Having in sight the fact that at continuous cooling of austenite, the cooling speed is one of the main parameters, which influence the structural transformations and the internal stress state, it is considered that the quantity "M" defined by relation (1), is more suggestive for the specialists in the field of heat treatment. As a result we propose that the intensity of cooling of a point within a probe to be expressed by the "M" quantity. In case of cooling from austenite temperature, the temperature intervals which corresponds to formation of: ferrite, pearlite, bainite and martensite will be correlated with quantities: MF, MP, MB, MM, which are specific for a certain type of steel-cooling media couple.

4. CONCLUSION

The quantity "M", defined with the help of cooling speed curves ([v.sub.r]= f(t)), presents the following advantages:

--it offers a more refined characterization of the intensity of cooling of a point within a steel probe in a certain temperature interval;

--allows a more precise characterization of cooling media, offering new possibilities for realization of a link between thermal cooling capacity and metallurgic hardening capacity;

--offers new possibilities in the field of controlled cooling.

5. REFERENCES

Bodin, I., Segerberg, S., (1993) Measurement and Evaluation of the Power of Quenching Media for Hardening, Heat Treatment of Metals, 1993. 1, p. 15-23.

Mudura, P., Vermesan, G., Munteanu, A., Vermesan, H.,(2000) Heat Treatment--Quenching Liquids (in Romanian), Editura Universitatii din Oradea, ISBN 973-8083-64-8, Oradea.

Mudura, P.,(2000) Contributions regarding quenching liquid characterization,(doctoral thesis), (in Romanian) Universitatea Tehnica din Cluj-Napoca,

Mudura, P., Ungur, P., Pop, M., But, A. (2001) Quenching liquid cooling capacity test used at steel hardening, vol. 3 al Conferintei de cercetare si dezvoltare nr. 25, Academia Maghiara de Stiinta, sectia Stiinte Agrare, comisia Tehnica agrara, tiparit de Universitatea St. Istvan, Godollo--Hungary, 2001.

*** Les Fluides de Trempe, (1998) Edition PYC Livres, Paris, Les dossiers techniques de l'ATTTT--Comision Fluides de Trempe.

Table 1. Values of quantities [M.sup.100.sub.0] and
[DELTA][t.sup.100.sub.0] for cooling laws given in figure 2

Cooling [DELTA]T [DELTA]t [v.sub.r]
law [degrees]C s [degrees]C/s

1 a 80 5 16
 b 20 55 0,36
2 a 60 10 6
 b 40 50 0,8
3 a 40 10 4
 b 60 50 1,2
4 -- 100 60 1,66
5 a 60 40 1,5
 b 40 20 2
6 a 50 50 1
 b 50 10 5
7 a 20 55 0,36

 M = [v.sub.r] x [DELTA]T
 [degrees]C/s x [degrees]C
Cooling
law [M.sub.a]; [M.sub.b] [M.sup.100.sub.0]

1 a 1280 1287,2
 b 7,2
2 a 360 392
 b 32
3 a 160 232
 b 72
4 -- 166 166
5 a 90 170
 b 80
6 a 50 300
 b 250
7 a 7,2 1287,2