HEAT TREATMENT OF DIES MADE OF CHROMIUM STEEL

I. V. SLOSMAN

Introduction

One of the essential tasks in the struggle for technical progress is the task of introducing into production practice more advanced methods of metal processing. From this point of view, along with such positive developments in the work of our industry as the use of cast tools made of high-speed steel, abrasive tools, high-speed methods of machining, and other measures contributing to increased labor productivity, one should note the tendency toward an ever broader spread of cold stamping as a method of metal processing.

A most important circumstance, almost always determining the labor productivity of stamping shops and the cost of their products, is the service life of dies in operation. This service life depends on many causes, but, as an investigation [1] has shown, the principal factors affecting the durability of dies are the correct choice of steel for the dies and the conditions of their heat treatment. The present work was devoted to solving this problem as applied to a typical case of operation of cold-blanking dies.

Fig. 1. Reduced. 1—die, 2—punch, 3—steel strip before blanking, 4—steel strip after blanking, 5—blanked parts.

Dies for blanking magnets in the cold state (Fig. 1 shows a set of such a die and the magnets blanked by it) were made of U10 steel. The service life of these dies in operation was short. Their failure was due to the fact that, after a certain duration of operation, the length

the working part of the punches and the thickness of the die became, as a result of frequent regrinding caused by deformation of the working edges of the die and punches, so small that further operation of the die became practically impossible. The low service life of the dies in operation appreciably increased the cost of the products and, moreover, often impeded the work of the stamping shop. The use of various heat-treatment variants for dies made of U10 steel, as well as the use of high-quality carbon steel of grade U10A, did not give noticeably better results. In order to achieve an effective increase in the service life of blanking dies, it was decided to proceed to the manufacture of these dies from alloy steel.

Purpose of the Work

The purpose of the present work was to select, for the dies indicated above, the most suitable steel grade, and also to establish such a heat-treatment regime for the dies as would ensure their greatest service life when operating under severe conditions—in blanking flat magnets in the cold state.

Selection of Steel

In deciding the question of selecting the composition of an alloy steel for dies, it was necessary to bear in mind that the alloying element introduced into the steel had, above all, to promote the attainment of higher mechanical properties in the steel. Chromium was chosen as such an element.

It is known that the hardness of carbon steel increases markedly under the influence of chromium. This is explained by the fact that when chromium is introduced into steel, chromium carbides and complex chromium-iron carbides appear in it along with iron carbides; these possess a higher hardness than iron carbides [2]. While increasing the hardness of steel, chromium does not make it brittle. G. Z. Nesselsstrauss, who studied the question of the critical points and structure of chromium steel, quenched specimens of high-carbon chromium steel (0.85–1.40% C; 1.4–6.6% Cr) from a fairly high temperature—1100°—in ice water. It was thereby established that not a single specimen produced cracks [3]. From other works it is known that the presence of chromium (up to 15%) in high-carbon steel promotes an increase in its impact toughness. The influence of chromium on steel that is very important for practice also consists in the fact that chromium, according to numerous investigations, reduces the deformation of steel products during their hardening. All these qualities of chromium have strengthened its use in the manufacture of tool steel: at present the most widespread type of alloy tool steel is steel alloyed with chromium.

Characteristics of the Selected Steel

In the work, two grades of chromium steel were investigated: a highly alloyed steel—grade Kh12M—and a low-alloyed steel—grade Kh. The standard chemical composition of these steel grades is given in Table 1.

Table 1

Chemical composition of steels of grades Kh and Kh12M (OST—14958)

Studies of steel grade Kh12M showed that it possesses properties very valuable for cold blanking dies.

The most important positive properties of Kh12M steel are its high hardness in the hardened state, good hardenability, slight warping and absence of cracks during hardening, and, finally, its relatively low cost, owing to its simple composition, which does not require additions of nickel, tungsten, or low-carbon ferrochrome. The addition of small amounts of molybdenum and vanadium, the optional possibility of which is provided for by the OST standard, contributes to increasing the impact toughness of this steel.

Kh12M steel also has certain shortcomings. It is a steel of the ledeburite class; therefore rolling ingots of such steel is very difficult. A second shortcoming of this steel is its difficult machinability on machine tools, since it has a comparatively high hardness even in the annealed condition (up to 255 Brinell). Another shortcoming of Kh12M steel is its sensitivity to the formation of grinding cracks. Therefore, articles made from it must be ground with soft wheels, removing a small chip layer in one pass.

The study of steel grade Kh was undertaken in order to establish the advisability of manufacturing blanking dies from a steel less alloyed than Kh12M steel and cheaper.

Experimental Part

Specimens for Investigation

The investigation was carried out on steel specimens whose chemical composition is given in Table 2.

Table 2

Chemical composition of the steel investigated

Fig. 2. Magn. 400. Kh12M steel after annealing.

The microstructure of the investigated steel in the annealed state is shown in Figs. 2 and 3.

In the microstructure of steel Kh12M (Fig. 2), chromium–iron carbides of various shapes and sizes are clearly visible, distributed comparatively uniformly in a mainly sorbite-like matrix. The hardness of the steel after annealing was 217 Brinell.

The microstructure of steel Kh after annealing (Fig. 3) consists of a basic matrix of sorbite-like pearlite with very fine and uniformly distributed carbides. The hardness of this steel after annealing was 207 Brinell.

Fig. 3. Magnification 400. Steel Kh after annealing.

From each grade of steel, the following specimens were prepared for investigation:

1. Specimens of square cross section with a notch for impact-toughness testing; these same specimens also served for determining hardness after quenching and tempering.
2. Special specimens of round cross section for testing the ability to retain impact toughness after repeated impact-bending loads.
3. Experimental punches and dies.

Determination of the quenching temperature

Experiments to determine the quenching temperature were carried out on impact specimens 55 mm long, with a cross section of \(10 \times 10\) mm, with a notch in the middle of the specimen 2 mm deep. The specimens were heated for quenching in an electric furnace, the temperature of which was measured by a platinum–platinum-rhodium thermocouple, the hot junction of the thermocouple being placed on the specimens being heated.

The heating temperature for quenching was varied for steel Kh12M in the range \(860—1040^\circ\) at intervals of \(20^\circ\), and for steel Kh in the range \(770—910^\circ\), likewise at intervals of \(20^\circ\). At the heating temperature, the specimens were held in the furnace for 15 minutes, after which they were cooled in oil. On specimens treated in this way, hardness, impact toughness, and microstructure were studied. The results obtained for specimens of steel Kh12M are given in Table 3 and in Fig. 4.

The results obtained show that, with an increase in the quenching temperature, the hardness of the steel at first increases, reaching a maximum value—65 units—at a temperature of \(980^\circ\), and with a further increase in the quenching temperature begins to decrease somewhat. The specific impact work, measured on a pendulum impact machine with a capacity of 15 kgm, for all quenched specimens is within the limits of up to \(1\ \text{kgm}/\text{cm}^2\) and, as the quenching temperature rises, gradually decreases.

Table 3

Hardness and impact toughness of hardened specimens of Kh12M steel

Fig. 4. Dependence of hardness and specific impact work on the quenching temperature.

The microstructure of the hardened specimens of Kh12M steel is shown in Figs. 5–7. Fig. 5 presents the microstructure of a specimen quenched from a temperature of 860°. It consists of a troostite-martensite matrix, in which chromium-iron carbides of various sizes and outlines are distributed—chiefly coarse carbides having, in section, the form of irregular polygons.

Fig. 5. Magnification 400. Kh12M steel, quenching 860°.

Figs. 6 and 7 show the microstructure of specimens quenched, respectively, from temperatures of 980° and 1040°. From comparison of these figures it is evident that, with an increase in the quenching temperature, the microstructure of the steel changes in the fol-

directions: in the main field the relative amount of the martensitic constituent increases, the chromium–iron carbides become finer and more rounded in shape, and their distribution becomes more uniform. When quenching from a temperature of \(1040^\circ\), grains of retained austenite become noticeable in the microstructure, which well explains the reduced hardness of specimens quenched from this temperature.

Fig. 6. Mag. 400. Steel Kh12M, quenching \(980^\circ\).

On the basis of the results obtained, for steel Kh12M a quenching temperature of \(980^\circ\) was adopted, as the temperature at which, after quenching, the steel has the best microstructure and hardness. This temperature is also the most suitable because the hardness of the steel, with small deviations from this temperature—unavoidable under production conditions—changes quite insignificantly.

Fig. 7. Mag. 400. Steel Kh12M, quenching \(1040^\circ\).

Data on the dependence of the hardness and impact toughness of steel Kh on the quenching temperature are given in Table 4 and in Fig. 8.

These data show that the hardness of steel Kh increases rapidly with an increase in the quenching temperature up to \(810^\circ\), reaching 64 units at this temperature. With a further increase in temperature—to \(890^\circ\)—the hardness does not change appreciably, and with an even greater increase in the quenching temperature it begins to fall. This decrease in hardness is explained, as in the case of steel Kh12M, by an increase in the amount of retained austenite. The obtained

the results show that the hardness of quenched steel Kh does not change appreciably when the heating temperature for quenching is varied over a fairly wide temperature interval—from 810 to 890°.

Table 4

Hardness and impact toughness of quenched specimens of steel Kh

Fig. 8.

The microstructure of steel Kh quenched from different temperatures is described below.

The microstructure of steel quenched from a temperature of 770° is characterized by a troostitic matrix with carbides of various sizes distributed unevenly in it. The microstructure of the specimen quenched from a temperature of 790° differs from the preceding one in having finer and more uniformly distributed carbides. The microstructure of the specimen quenched from a temperature of 830° consists of acicular martensite with very fine, uniformly distributed carbides.

With a further increase in the quenching temperature, the martensite becomes acicular, and the needles become the coarser the higher the heating temperature.

Thus, steel Kh has the most favorable microstructure when quenched from a temperature of 830°. This temperature lies within the above-mentioned temperature range, quenching from which gives practically no change in the hardness of the steel. Therefore, a quenching temperature of 830° was adopted for steel Kh.

Establishing the tempering temperature

Experiments to establish the tempering temperature were carried out on impact specimens, die specimens, and special specimens for testing the ability to retain impact toughness after repeated impact-bending loads.

Tempering of Kh12M steel specimens, quenched from 980°, was carried out in the temperature range 100–500° at intervals of 30°; tempering of Kh steel specimens, quenched from 830°, was carried out in the temperature range 100–320°, also at intervals of 30°. All specimens were held at the tempering temperature for 1 hour, after which they were cooled in air. On the specimens treated in this way, hardness, impact toughness, the ability to retain impact toughness after repeated impact-bending loads, and microstructure were studied. The results obtained are described below.

The hardness of Kh12M steel specimens, their impact toughness, and their ability to retain impact toughness after repeated impact-bending loads as a function of tempering temperature are given in Table 5 and in Fig. 9.

Table 5

Dependence of the hardness of Kh12M steel specimens, their impact toughness, and their ability to retain impact toughness after repeated impact-bending loads on tempering temperature.

In the graph: Kh12M steel after quenching from 980°; \(a_k\), kgm/cm²; HRC; dependence of properties on tempering temperature; \(t^\circ C\).

Fig. 9.

From the data presented it is evident that, as the tempering temperature is increased, the hardness of the steel first decreases to 55 units at a temperature of 350°. Then, in the temperature range 350–410°, the hardness of the steel becomes higher, reaching 59 units at a temperature of 410°, and with a further increase in tempering temperature it decreases sharply. The “secondary” hardness observed during tempering of Kh12M steel in the temperature range 350–410° is due to the fact that in this steel, after quenching, a certain amount is retained

the amount of retained austenite, whose transformation into martensite takes place at about 400°. However, this temperature range could not be recommended for tempering dies made of Kh12M steel, since the specific impact work is then very low.

The specific impact work increases with increasing tempering temperature and at temperatures of 290–350° reaches a maximum value—0.86 kgm/cm². As for the phenomenon of temper brittleness observed at temperatures close to 400°, its cause is apparently the process of transformation into martensite of the retained austenite of the hardened steel.

As mentioned above, special specimens were also tempered for testing the ability to retain impact toughness after repeated impact-bending loads. We shall briefly discuss the idea and method of this test.

Every test is, of course, the more valuable the closer its conditions approximate the conditions of actual service of the products. From this point of view, in establishing a heat-treatment regime for cold-punching dies, we faced the task of finding, on steel specimens, the regime under which, along with sufficient hardness, the steel has the highest ability to resist impact, since when punching articles from hard magnetic steel the working parts of the die are subjected to the action of a strong impact load. This task was solved by studying hardness and specific impact work on ordinary impact specimens subjected to various heat-treatment regimes.

Fig. 10. Specimen for testing the ability to retain impact toughness after repeated impact-bending loads.

For the working conditions of a cold-punching die, it is also very characteristic that blows of the die on the steel being stamped are repeated many times at equal intervals of time and with the same force. Wear of the dies is a consequence of these blows. Obviously, the best heat-treatment regime for dies will be the regime under which the die retains the properties imparted to it by heat treatment for the greatest possible number of blows, or at least these properties will not begin to fall sharply until after the greatest possible number of blows. And since one of the most important properties for die steel is impact toughness, the test we conducted, recommended by Academician N. T. Gudtsov, had the purpose of determining at what tempering temperature the steel has the best ability to retain impact toughness after repeatedly repeated impact loads.

To carry out this test, round specimens were made, 150 mm long and 15 mm in diameter, with a notch in the middle of the specimen 5 mm wide, 1 mm deep, and with a fillet radius of 3.5 mm. A photograph of such a specimen is shown in Fig. 10.

Such a specimen was mounted in a special testing machine in a horizontal position and subjected to blows from a steel ram weighing 5 kg falling from a height of 3 cm; after each blow the specimen was automatically turned through 180°. The testing rate was 100 blows per minute. After 5,000 and 30,000 blows the specimens were broken on a pendulum impact tester of 30 kgm capacity.

The results of the tests carried out are presented in Table 5 and in Fig. 9, where \(a_{k1}\) denotes the specific impact work of the specimens after 5,000 blows, and \(a_{k2}\)—after 30,000 blows.

On the basis of the data obtained, for Kh12M steel a tempering temperature of 300° was adopted as the temperature at which the best combined results are obtained for all types of tests: at this tempering temperature, high impact toughness and the ability of the steel, after repeated impact-bending loads, to retain this toughness are combined with hardness sufficient for die operation.

The microstructure of steel quenched from a temperature of 980° and tempered at a temperature of 290° is shown in Fig. 11. It consists of a martensitic matrix with a large amount of uniformly distributed carbides of rounded form.

Fig. 11. Magnification 400. Kh12M steel, quenching 980°, tempering 290°.

To verify the adopted tempering temperature on specimens close in size and shape to punches, and also in order to determine the holding time of dies during tempering, 6 punch specimens were made. All these punches were quenched at a temperature of 980° and tempered at a temperature of 300°. During tempering, 2 punches were held in the furnace for one hour, 2 for three hours, and 2 for six hours. The hardness-measurement results given in Table 6 showed that, within the investigated time interval, the hardness as a function of holding time at a tempering temperature of 300° practically does not change.

Table 6

Dependence of the hardness of Kh12M punches on holding time at the tempering temperature
(quenching 980°, tempering 300°)

In the microstructure of all the punches, likewise, no difference was noticeable in comparison with the microstructure shown in Fig. 11. On this basis, for dies made of steel Kh12M, a holding time at the tempering temperature of 1 hour was established.

Taking into account that the thermal conductivity of such a high-alloy steel as Kh12M is considerably lower than the thermal conductivity of carbon steel, the adopted heat-treatment schedule for dies made of Kh12M steel provided for preheating at a temperature of 650° during heating for quenching.

The graph of the adopted heat-treatment schedule for dies made of Kh12M steel is shown schematically in Fig. 12.

Fig. 12.

To establish the tempering temperature of steel Kh, the same tests were carried out as for steel Kh12M. The results obtained are given in Table 7 and in Fig. 13, where \(a_{k1}\) denotes the impact toughness after 5000 impact-bending loads.

Fig. 13.

From the data presented it is evident that, with increasing tempering temperature of steel Kh, its hardness continuously decreases. The impact toughness of this steel increases with increasing tempering temperature up to 230°. In the temperature range 230–290° the phenomenon of temper brittleness occurs, and with a further increase in the tempering temperature the impact toughness begins to rise rapidly. However, at these high tempering temperatures (above 300°)

hardness of the steel becomes already insufficient for the successful operation of cold blanking dies.

Table 7

Dependence of the hardness of specimens of steel Kh, their impact toughness, and their ability to retain impact toughness after repeated impact-bending loads on the tempering temperature

On the basis of the foregoing, for dies made of steel Kh a tempering temperature of 230° was adopted, as the temperature at which the steel possesses high impact toughness, a better ability than at other temperatures to retain impact toughness after repeated impact-bending loads, and a hardness sufficient for die operation.

Fig. 14. Magnification 400. Steel Kh, quenched at 830°, tempered at 230°.

The microstructure of steel Kh, quenched from a temperature of 830° and tempered at a temperature of 230°, is shown in Fig. 14. It consists of a martensitic matrix in which a large quantity of very fine carbides is uniformly distributed.

The adopted heat-treatment schedule for dies made of steel Kh is shown schematically in Fig. 15.

Testing the Dies in Operation

The question of the correctness of the choice of steel grade and heat-treatment schedule for any tool can be finally resolved only by means of production tests. To carry out such tests, it was

six sets of experimental dies were made—3 of each grade of steel. On one of the dies made of Kh12M steel, many fine grinding cracks formed during grinding (see the note above), and therefore it was not tested. The remaining 5 dies were tested in operation under ordinary production conditions.

Fig. 15.

These tests gave the following results:

The durability of the dies made of steel Kh, both between individual sharpenings and overall, exceeded the durability of dies made of carbon steel by approximately 1.5 times.

The durability of dies made of Kh12M steel is considerably higher: it exceeds the durability of dies made of carbon steel by 2.5–3 times.

Conclusions

Dies for blanking products from hard sheet steel in the cold state can successfully be made from high-chromium steel of grade Kh12M.

The durability of such dies is 2.5–3 times higher than the durability of dies made of carbon steel.

As the heat-treatment regime for dies made of Kh12M steel, the regime shown in graph 12 may be recommended.

References

1. Krasilshchikov M. S. Methods for Increasing the Durability of Blanking and Drawing Dies, 1948.
2. Gudtsov N. T. Course in Special Steel, 1938.
3. Hessehlshtraus G. E. Critical Points and Structure of Chromium Steel. Journal of the Russian Metallurgical Society, 1911, 5.