Comparison of Modification by Cooling and Modification by Small Additions

N. D. Tyuteva

(Presented by Prof. A. N. Dobrovidov, Doctor of Technical Sciences)

Modification by Cooling

By “modification by cooling” we mean the following: modification by cooling is a change in the structure and physico-mechanical properties of an alloy under the influence of accelerated cooling during casting. In the present case we are speaking of steels that harden over a wide range of cooling rates, for example, high-speed steels. The change in the properties of an alloy under the influence of accelerated cooling may be analogous to the change in the properties of an alloy when modified by additions of a modifier.

The greater the cooling rate of an alloy, the more nonequilibrium is the state of the system. In nonequilibrium systems, a different distribution of alloying elements among the phases is observed, as well as different quantitative ratios between the phases than in equilibrium systems.

In high-speed steel, for example, when the cooling rate during casting is changed, it is possible for the amount of the carbide phase to change and for the chemical composition of the carbides to change.

To investigate the influence of the cooling rate of a casting (modification by cooling) on the amount of the carbide phase and its chemical composition in high-speed steel, we cast 11 heats of steel of the composition indicated in Table 1.

Table 1

Conditions for Casting the Melts

The change in the amount and chemical composition of the carbide phase of high-speed steel as a function of the cooling rate during casting is given in Table 2.

Table 2

Analyzing the data of Table 2, the following conclusions may be drawn:

1. The amount of carbide phase in high-speed steel increases with an increase in the cooling rate during casting.

2. The largest amount of carbides is obtained at cooling rates on the order of 148° and 32°/sec (casting in a steel chill mold).

3. With an increase in the cooling rate, we obtain carbides poorer in carbon and vanadium. Consequently, under accelerated cooling the solid solution is enriched in carbon and vanadium, i.e., it is more highly alloyed.

As is known, in the heat treatment of forged high-speed steel one seeks to obtain a highly alloyed solid solution in order to improve the cutting properties of the steel.

1. When the cooling rate is increased from 7°/sec (casting in sand molds) to 32°/sec (casting in steel chill molds), the amount of carbon in the solid solution increases by 32.1%, vanadium by 29.5%, and tungsten by 10.0%.

2. The cooling rate has almost no effect on the chromium content in the carbide phase.

Modification by Small Additions

We have attempted to substantiate experimentally the theoretical proposition: boron accelerates the chemical reaction of carbide formation in high-alloy steels.

The basis for this proposition was provided by the results of preliminary experiments on the influence of boron on the amount of carbide phase in high-speed steels. The preliminary experiments showed that very small amounts of boron added to steel sharply increase the amount of carbide phase. For a more complete experimental verification, heats with a constant boron content were cooled at varying rates. The composition and weight amount of carbides isolated by the method of electrolytic dissolution were determined. During slow cooling, the composition and amount of carbides changed only under the influence of boron. The influence of modification by cooling in this case was excluded.

During accelerated cooling (chill casting of small specimens, with a cooling rate of tens of degrees per second), the amount of carbides increases owing to modification by cooling.

At a high cooling rate, the influence of the modifier is suppressed by the influence of modification by cooling. During slow cooling—cooling in sand molds (7°/sec), in heated ceramic molds (0.8°/sec)—the change in the amount of carbides, under equal cooling conditions, can be explained only by the influence of boron. The influence of boron was revealed by comparing heats cooled in the same way but with different boron contents. We observed that the slower the cooling of the casting, the more sharply expressed is the influence of boron on the amount of carbides. The influence of boron was studied on cast steels of the composition given in Table 3.

Table 3

Conditions for recording the cooling rate

A specially made ceramic crucible was placed in a shaft resistance furnace. A thermocouple of molybdenum with a tungsten alloy was introduced through the wall of the crucible. The thermocouple junction was not protected and was located on the central axis of the crucible.

At low cooling rates, the current generated in the thermocouple was directed to Kurnakov’s apparatus.

The heats were obtained under the following cooling conditions:

Where the inertia of the mirror galvanometer had an effect, a loop oscillograph was used.

At high cooling rates the current was directed to the vibrator of the oscillograph. High cooling rates were obtained when pouring into metal (steel or copper) molds.

To obtain very low cooling rates, the steel in the crucible was cooled together with the crucible (curve, Fig. 1). The cooling rate in

Fig. 1.

heated to 500° is characterized by the cooling curve taken with a Kurnakov apparatus (Fig. 2). The cooling rate in the steel mold is characterized (the ordinate is enlarged 10 times) by an oscillogram (Fig. 3).

The change in the amount of carbides with changing boron content and cooling rate of the heats is given in Tables 4 and 5. The change in the chemical composition of the carbide phase of cast high-speed steel as a function of boron content and cooling rate during casting is given in Tables 6 and 7.

In Table 2 the weight amount of carbides in heats cooled at different rates is compared. All heats were divided into 2 groups.

In group I, heats cooled at rates of 300, 148, 32, and 7 degrees per second were compared.

In group II, heats cooled at rates of 0.78, 0.58, and 0.18 degrees per second were compared.

Cooling at rates on the order of fractions of a degree per second did not noticeably increase the amount of carbides in comparison with heats cooled at rates \(>300\) and 148 degrees per second. This can be explained by:

1) a change in the chemical composition of the carbides. During slow cooling, qualitatively different carbides precipitated, for example, those less alloyed with tungsten; 2) the fact that cooling at rates on the order of fractions of a degree per second in highly alloyed high-speed steel does not cause noticeable decomposition of the austenite–martensite mixture; 3) the circumstance that the amount of carbides in high-speed steel is a function of the austenitizing temperature. In our case it is approximately the same, since the alloy was quenched from the temperature of the solidus line; therefore there is no sharp difference in the amount of carbides.

Fig. 2

The results of a study of the change in the amount of carbides with the austenitizing temperature of high-speed steel are given in the work of Kaiser and Cohen [1].

Thus, we consider it rational to divide the heats into two groups according to cooling rates and to compare heats with one another within one group.

Carbides have a very strong effect on the properties of high-speed steel and on the results of its heat treatment. The technique for isolating carbides is anodic dissolution. The electrolyte composition is a saturated solution of potassium chloride with an addition of 0.5% citric acid. The current density is 0.02 A/cm². The duration of anodic dissolution is from one to three hours. Dissolution is carried out with continuous cooling of the electrolyte by running water. The cathode is a copper coil used for cooling the electrolyte. The temperature of the water passing through the cooling coil is \(+2^\circ\mathrm{C}\), and the temperature of the electrolyte in the bag near the specimen is from \(+10\) to \(+12^\circ\mathrm{C}\).

Fig. 3

To determine the ratio of the weight of carbides to the weight of the entire dissolved portion of the specimen, the latter was weighed before and after anodic dissolution. The precipitate from the bag and from the specimen was transferred to a Schott filter, dried to constant weight, and washed with hot water. The filter with the precipitate was then dried in a drying oven at a temperature of \(105^\circ\), likewise to constant weight. The difference in weight between the ignited filter and the filter with precipitate gave the weight quantity of the carbide precipitate. The difference in the weight of the specimen gave the quantity of dissolved metal.

Before removal of the precipitate, the specimen was carefully placed in a beaker with hot water; vigorous evolution of gases adsorbed by the surface of the specimen was observed. With a large number of specimens, the gas evolution was so intense that it gave the impression of boiling water. Such liberation of the specimen from oxygen, in our opinion, should reduce the tendency of the carbide powder to oxidize.

Table 4

Table 5

Table 6

Table 7

The percentage content of carbides in the alloy was determined as the ratio of the weight of the separated carbides to the weight of the dissolved part of the specimen. The anodic dissolution conditions and the composition of the electrolyte were taken by us from the work of N. A. Saverina [2].

A number of works [1, 3, 4] are concerned with the study of carbide phases in high-speed steel.

We investigated the amount of carbide precipitate in steel hardened from the maximum temperature, namely from the temperature of the solidus line (melting temperature).

Conclusions

Comparing modification with boron with modification by cooling, the following conclusions may be drawn.

1. Modification by cooling in melts without boron, at a cooling rate of 300°/sec (cooling in a copper mold) and at a rate of 32°/sec (cooling in a steel mold), gives an amount of carbides equal to that which we have with modification by 0.015% boron under conditions of slow cooling (7°/sec—cooling in sand molds). Boron in an amount of 0.015% replaces modification by cooling.

2. The amount of carbides increases sharply in melts with boron when cooling at a rate of 0.18°/sec (melt K 6) and 0.78°/sec (melt No. 3).

3. The lower the cooling rate during casting, the more sharply expressed is the influence of boron on the amount of carbides.

4. The amount of carbides increases with increasing boron content at a given cooling rate. The greatest difference in the amount of carbides was obtained at a cooling rate of 0.18°/sec and a boron content in the melts of 0.5%, i.e., with the slowest cooling in our experiments.

5. At all cooling rates, from high rates of 150°/sec down to 0.18°/sec, under the influence of boron there is observed a depletion of the carbides in tungsten, carbon, and vanadium, and a weakly expressed enrichment in chromium.

Literature

1. F. Kaiser and M. Kohen, Metal Progress, June, 1952.
2. Saverina N. A. Method for Separating Carbides and Nitrides from Stainless Steel. Factory Laboratory, No. 8, 1952.
3. Prosvirin B. I., Kreshanovskii N. S., and Ginsburg E. S., “Foundry Production,” No. 5, 16–19, 1954.
4. Blickwede and M. Kohen, Trans. Amer. Inst. min. metallurg. Engr, 1949.