2. 3. 4.
Vo N. Nikitin, A. I. Liverov, and N. V. Shavrikova, '~echanical properties of low-alloy steels with 4% Mn," Stal', No. 2, 168 (1975). V . A . Tsukanov, Alloying of Structural Steel with Manganese [in Russian], Mashgiz, Moscow (1959). I . E . Brainin et al., "Recovery before the initial stage of strain aging in steel 08kp," Izv. Akad. Nauk SSSR, Met., No. 3, 152 (1974).
EFFECT OF OXIDIZING PROCESSES ON CRAZING OF DIE-CASTING MOLDS V. A. Kovrigin, S. A. Yurasov
B. S. Starokozhev,
and
UDC 620.186.4:621.746.073
Crazing is the main reason that die-casting molds used in casting copper and aluminum alloys are put out of commission. It is considered [i] that crazing is the result of thermal fatigue. The cracks appear after 300-400 casting cycles. A correlation has been found between the crazing resistance and mechanical properties. The crazing resistance is lower for steel with low ductility. Thermal embrittlement has an especially negative effect. Considerably less attention has been given to the interaction of the die-casting mold with the liquid alloy and oxygen in the process of casting. This work concerns the reasons for the failure of die-casting molds used in casting copper- and aluminum-base alloys, taking into account the physicochemical processes that occur at the boundary between the liquid and solid phases. We investigated plungers made of steels 5KhNM and 3Kh2V8. They were given the standard heat treatment at the Moscow Automobile Factory to hardness HRC 36-41 and HRC 40-46, respectively. The pressure was maintained at a level of 15 kgf/mm 2. The production rate was 20 castings per hour. Samples were cut from plungers that had been in operation for different times. Microsections were prepared by the standard method, and also sloping sections. Electron microscopic, x-ray spectral, and metallographic analyses were made. It was found that cracks form in the first 50 cycles of casting brass LS-59-I. The number of cracks is small and they are located mainly along the axis of the plunger. The depth of the cracks does not exceed 0oi mm. The cracks are narrow and the walls of the cracks are covered with a thin layer of lead and zinc oxides. After 5000 cycles a network of cracks covers the entire surface of the plunger. The deepest cracks are 3 mm deep. The position of the cracks indicates that they develop
Fig. i. General view of cracks in a die-casting mold used for casting brass. 400 ×.
Moscow Automobile Factory. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 51-53, September, 1980.
688
0026-0673/80/0910-0688507.50
© 1981 Plenum Publishing Corporation
Fig.
2
Fig.
Fig. 2. Distribution of elements the axis of the crack. 200 x.
in the steel and brass along
Fig. 3. Distribution of zinc and lead in the scale crack in a die-casting mold used for casting brass. from cracks that occur in the very beginning 0.2-0.3 mm are located between them. Almost
3
formed in a 1500 ×.
of the process. Fine cracks with a depth of all cracks are filled with scale.
The scale has a layered structure through the depth of the crack (Fig. i). The scale is solid at the walls of the crack, but the central section of the scale is defective, with pores and voids. Brass penetrates into the crack along defects (Fig. 2). Studies of the chemical composition of the scale showed that it consists mainly of iron oxides. However, there are also inclusions of zinc and lead oxides (Fig. 3). In the scale on steel 5KhNM there are sometimes nickel oxide inclusions. A substantial increase in the concentration of nickel is found at the metal--scale interface. The distribution of molybdenum in the scale is similar in steel 5KhNM, and the distribution of tungsten in steel 3Kh2V8. A film of chromium spinel was found at the same interface. This distribution of alloying elements is characteristic of the oxidation process in steels containing Ni, W, Mo, and Cr at 700-900°C in air. There is a difference only in the concentrations of nickel and tungsten oxides in the layer of iron oxides. This is evidently explained by the presence in the scale of low-melting zinc and lead oxides (Tm= 718 and 587 ° respectively), inducing rapid oxidation of nickel and tungsten [2]. The penetration of zinc and lead into the scale and fine cracks from the liquid metal is probably due to the low heat of evaporation of these elements. No diffusion observed. Only shallow After especially extracting
of the components
of brass
slightly
cracks
oxidized
into the steel in sections
between
are observed when aluminum-base
cracks
alloys
is
are cast.
20,000 cycles of casting brass, similar processes occur. Only the depth and the width of the cracks increase substantially, which leads to difficulty in the castings.
Let us consider the development of cracks from the viewpoint of continuum mechanics. Solution of the elastic--plastic problem [3] indicatesthat ifthe surfaceof themold isheated suddenly to a temperature near the temperature of the molten metal then plastic compression occurs on the surface. With equalization of the temperature through the section of the diecasting mold the plastic region will extend into the depth and tensile stresses will occur in the surface zone. The highest stresses will occur at the surface at the first moment and can be determined by the formula
amax ~ 1 ~ - -
~
Ga&TJ,
where o T is the yield strength; G, shear modulus; ~, thermal expansion coefficient; temperature difference between the molten alloy and the die-casting mold.
AT,
Substituting numerical values into the equation, we find that the tensile stresses reach i00 kgf/mm 2 for casting aluminum-base alloys in a cold die-casting mold, and 150 kgf/ 2 mm for casting brass.
689
Preliminary heating of the mold to 300-400 ° and application of an anticrazing lubricant reduces tile residual stresses ~50%. However, they substantially exceed the yield strength of the steel at the temperature of the molten brass. Since liquid zinc greatly embrittles steel [4], cracks should occur in the mold at the very beginning of operation. Crack growth is limited in ductile steel by the plastic flow in the crack tip. In brittle steel the crack propagates to approximately half the depth of the surface zone, the depth being ~0.5].0 mm. The cycle is repeated in manufacturing subsequent castings, although the is lower, since the surface on which the highest tensile stresses are reached against the layer of molten metal, the thickness being equal to the length of At a certain stage amax even becomes lower than the ultimate tensile strength growth ceases as the result of thermal cycling.
stress level is protected the crack. and crack
Further development of cracks occurs due to oxidation of the walls. The specific volume of the oxide is much larger than the specific volume of the steel. The oxides spread the crack and create tensile stresses in the tip. In turn, the tensile stresses intensify oxidation. In the process of thermal cycling the edge of the crack is displaced, brittle oxides break off, the alloy penetrates into the crack and prevents extraction of the casting, and the die-casting mold is put out of commission. In this case the crack width is 0.5-1.0 mm. It should be noted that stresses similar in character to those mentioned above occur in the sections of the mold between the cracks formed at the beginning of the process. However, umax is much smaller than the stress determined by the formula. With a fairly small width of the section amax does not exceed the yield strength. Consequently, the fine cracks located between deep cracks are thermal fatigue cracks, development of which is also promoted by oxidation of the steel. In casting of aluminum-base alloys the stresses are relatively small, the strength of the steel is fairly high, oxidation processes develop only slightly, and the Rebinder effect is absent. CONCLUSIONS i. Die-casting molds for copper-base alloys are put out of commission by cracks formed at the beginning of the process and developing mainly due to oxidation of the steel. 2. Failure of die-casting molds for casting aluminum-base alloys is due to low-cycle fatigue of the steel under the influence of thermal stresses. Oxidation is not a major factor in the development of cracks in this case. 3. Alloying to increase the service life of die-casting molds for copper-base alloys should be directed toward increasing the scale resistance, and toward increasing the heat resistance for casting aluminum-base alloys. LITERATURE CITED 1.
2. 3. 4.
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V. M. Vladimirov, Preparation of Dies, Die-Casting Molds, and Equipment [in Russian], Vysshaya Shkola, Moscow (1974), p. 371. Jacques Benard (editor), Oxidation of Metals [Russian translation], Vol. 2, Metallurgiya, Moscow (1969), p. 261. H. Parkus, Irregular Thermal Stresses [in Russian], Izd. FML, Moscow (1968), p. 186. W. Rostoker et al., Embrittlement Under the Influence of Liquid Metals [Russian translation], IL, Moscow (1962), p. 46.