CAST BLANKING DIES

I. O. Khazanov

(Presented by Prof. Dr. A. N. Dobrovidov)

Technology for Producing Cast Blanks for Blanking Dies

Obtaining high-quality blanks for blanking dies is associated with certain requirements of a technological nature. The steel used for blanks must forge well and have high thermal conductivity; carbides must be uniformly distributed throughout the entire volume of the blank, etc.

Steels Kh12, Kh12F, Kh12FT, Kh12M, and others, used for manufacturing the working parts of dies, while possessing high wear resistance, often do not satisfy the specified requirements. To obtain the proper microstructure of the steel, forging is necessary in order to break up the carbide network, followed by annealing for spheroidization of the carbides and reduction of hardness. Heating the steel for forging is carried out in the range 880–1180°C; heating to 1200–1250°C already leads to melting of the eutectic and to the formation of cracks during forging. However, even by forging it is not always possible to obtain a homogeneous fine-grained structure. After forging, carbide banding is observed in the blanks, and this leads to chipping of the edges in dies.

The aim of the present work was to obtain a finer primary structure in castings of small volume by means of their rapid cooling. The castings obtained can be used as blanks for the matrices and punches of blanking dies without preliminary forging. In addition, obtaining blanks by casting makes it possible to use the large amount of metal that becomes scrap in die manufacture, as well as worn dies. As a result, up to 90% of the metal that usually goes to scrap is recovered [1].

For casting the blanks, waste steel of the Kh12 type, worn punches, and pieces of matrices were taken. The scrap was remelted in a high-frequency unit of type AZ-43 in an acid crucible with a capacity of 2.5 kg. The duration of melting for a charge weighing 2.3 kg was 25–30 min. To protect the bath from oxidation, broken glass was used. It is necessary that the entire surface of the metal be covered with slag, but the glass layer should not be thicker than 5–6 mm.

After the charge has completely melted, the temperature of the steel is measured with an optical pyrometer, deoxidation is carried out, and modifiers are introduced immediately before pouring. The steel pouring temperature is 1440–1460°C. The steel is usually deoxidized 1–2 min before casting with aluminum, at the rate of 0.1% aluminum by weight of the heat. To refine the structure after deoxidation, a modifier was introduced into the steel. Ferrotitanium was used as the modifier, at the rate of 0.2% titanium by weight of the heat. Because of the small weight of the heat, the steel was poured manually, directly from the crucible. During casting the stream must be continuous, and the pouring time as short as possible. In our case, the actual time for pouring liquid steel into the permanent mold was 30–40 sec.

Design of the crucible for melting steel

The inductor is made of copper tubing 12 mm in diameter, slightly flattened, and consists of three turns. Sheet asbestos-cement 6 mm thick was used as the material for the crucible frame. The posts and nuts for fastening were made of textolite. For lining the crucible, pure quartz sand was used with an addition, for bonding, of ten percent refractory clay. The upper part of the crucible is coated with a mixture of sand and asbestos (15–20% by weight of the sand), with liquid glass added as a binder. After thorough drying, the crucible usually withstands 15–20 heats. To prevent cracking of the crucible walls after the heats are completed, dry sand is poured into it to provide more uniform cooling.

Under laboratory conditions we carried out a series of experimental heats for casting blanks for dies and punches for blanking dies. Casting was carried out into a rotating permanent mold mounted on a centrifugal machine with a vertical axis and an adjustable number of revolutions (Fig. 1).

Fig. 1.

The permanent molds for casting pieces for dies and punches were made of steel 3. The design of the permanent mold is shown in Fig. 1. More than 100 heats were poured into the permanent mold, and the mold remained in good condition.

When designing permanent molds, the mold walls should be made as thick as possible. As a material for manufacturing permanent molds, low-carbon grades of steel may be recommended in cases where

the cast parts are of low weight (for example, punches). In the case of casting large blanks, it is advisable to use heat-resistant ferritic grades of steel with a minimum carbon content as the material for making the permanent mold. Experience in the use of such steels in permanent-mold casting of cutting tools gives good results.

Fig. 2.

Preparing the Permanent Mold for Pouring

The temperature of the molten metal poured into the permanent mold is 1440–1460°C, but the temperature of the wall of the permanent mold in contact with the liquid metal fluctuates around 900°, and therefore it is especially important to have a high-quality coating on the walls of the riser of the permanent mold and of the feeders of the gating system. To eliminate possible burn-on, the walls of the permanent mold that come into contact with the molten metal must be thoroughly coated with protective paint and dried at a temperature of 250–300°C. As the paint we used chromium oxide, thoroughly ground in natural drying oil. The degree of consistency is the same as for painting metal products. The paint is applied to the surface of the permanent mold heated to 250°. It is desirable to paint and dry the gating system twice. With good painting of the permanent mold, up to 5–7 heats can be poured into it. Immediately after casting, as soon as the metal has solidified, the permanent mold is disassembled and, if necessary, touched up. The paint dries very quickly, and the permanent mold is again ready for pouring. In the case of casting small parts in a massive permanent mold (punches), it should be painted and dried the first time (the gating system), while the working surfaces of the permanent mold should only be sooted. The surface of castings obtained with sooted walls of the permanent mold is considerably cleaner than when casting in a permanent mold with painted surfaces. After casting and disassembly of the permanent mold, the resulting castings are sent for annealing according to the schedule corresponding to the softening anneal for the given grade of steel [2].

Influence of the Pouring Process and Mold Design on Blank Quality

The design of the permanent mold has a direct effect on the quality of the castings obtained; therefore, when designing new permanent molds it is necessary to take into account the specific features of the casting method.

A sufficiently massive riser should be provided in order to reduce the probability of a shrinkage cavity appearing in the casting. The gates should be made as large as possible, since otherwise the metal may solidify on its way to filling the mold. In addition, it is desirable to locate the gates in the upper parts of the castings, so that the shrinkage cavity partly passes into the gate and the riser.

If the rotational speed of the permanent mold is insufficient, laps and films may form on the surface of the castings, and the metal may prove insufficiently dense. The mold cavities most distant from the riser may be poorly filled with metal. The impact of the metal jet should be taken by a graphite or ceramic insert at the bottom of the permanent mold, and not by the walls of the mold; this will protect the bottom of the permanent mold from melting and will not lead to premature solidification of the metal in the riser (Fig. 4).

We cast punches and blanks for die blocks in a permanent mold with a radius of 90 mm, at rotational speeds from 150 to 600 rpm. After casting, fractures were examined at the places where the castings were separated from the riser.

Analysis of the fractures showed that the surface and quality of the castings were good at 300–350 rpm. At higher rotational speeds the metal seeps through discontinuities in the joints of the permanent mold, which leads to shrinkage cracks.

To determine the percentage of burn-off of the principal elements in the steel, experimental heats were carried out without charge correction. The chemical composition of the remelted steel scrap is given in the table. In its chemical composition the initial steel corresponds to grade Kh12F1 steel, but has a somewhat elevated content of carbon and silicon (sample 1).

The degree of oxidation of the elements depends entirely on the duration of the melting and on the continuity of the slag covering over the entire surface of the bath. Judging from the results of remelting, in the case of rapid melting of the charge and a short melting time, the burn-off of the elements is insignificant.

Chemical analyses of a large number of heats showed that all the blanks obtained corresponded to an average composition between Kh12F and Kh12F1. The vanadium content in all heats varied from 0.4 to 0.65%. Consequently, the first remelting of the scrap can be carried out without charge correction.

To refine the structure, 0.2% titanium was introduced into the steel of all heats. According to spectral and chemical analysis data, the titanium content in the heats ranged from 0.12 to 0.16%.

From the cast blanks, four experimental dies were manufactured at the plant, three of which have already undergone testing.

The average life between regrindings of the first die was 12–15 thousand blankings; for the second die, the life between regrindings was 18–19 thousand blankings; and for the third, about 29 thousand blankings. The life between regrindings of factory dies fluctuates very widely between 10 and 18 thousand blankings, although it is rarely greater.

The life of cast dies between regrindings proved to be (especially for the last two dies) higher than that of forged factory dies. The total life of the dies was as follows: for the first die, 175 thousand blankings; for the second die, 200 thousand; and for the third die, 300 thousand blankings.

It should be noted that the first two dies failed for reasons other than wear of the working parts.

The life of factory dies is not stable, but a die that has withstood 400–450 thousand blankings is considered good. However, even among forged factory dies, dies with a number of blankings of 200–250 thousand are encountered extremely often.

Conclusions

The manufacture of blanks for blanking dies by casting makes it possible to use up to 90% of the scrap of scarce steel (including remelting of worn-out die parts), eliminate forging, improve the structure of the steel owing to rapid cooling of the blanks during casting in a chill mold, and increase die life by selecting the optimum chemical composition of the steel and using modifiers.

The life of cast dies is equal to the average life of dies manufactured according to factory technology.

References

1. V. A. Chernikov. Cast blanking dies. Abstract of Candidate’s dissertation, Tomsk Polytechnic Institute Press, Tomsk, 1954.

2. Yu. A. Geller. Tool Steels. Metallurgizdat, 1955.