DE2322132C3 - Process for the production of spheroidal graphite cast iron - Google Patents

Description

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The invention relates to a method of manufacture of high tensile strength nodular cast iron by heat treatment in the form of quenching from the Austenitizing temperature and subsequent tempering or in the form of an isothermal transformation.

As a rule, mainly special steels are used as materials with high tensile strength. These special steels however, do not contain any precipitated graphite and can therefore neither stick nor oil they have an intrinsic sliding property, so that their usability at high speeds and under high load is limited. Another disadvantage of such special steels is that when they are used as machine components because of their low damping ability of the question of vibration absorption Attention needs to be paid.

Cast iron is usually superior to such special steels in terms of wear resistance and damping capacity. Even spheroidal graphite cast iron, which is superior to conventional types of cast iron in terms of strength and toughness, only has a tensile strength of 70 to 90 kg / mm ² . Tensile strength values of more than 100 kg / mm ² can only very rarely be achieved even after the starting material has been melted, after a graphitization treatment, after quenching and tempering.

Thus, when using cast iron instead of steel, it is inevitable that the wall thickness (of the respective Part), which leads to an increase in the weight and size of the machine component concerned leads. It is particularly serious that no light profile castings can be made from cast iron permit.

From "The Foundry", May 1949, pp. 112/115 and 218/230 is the hot formability of cast iron known with spheroidal graphite. However, this reference says nothing about tensile strength values at various Rolling degrees.

From the VDI-Zeitschrift, Vol. 93, No. 3, Jan. 21, 1951, p. 54 it is also known that spheroidal graphite cast iron can be subjected to all heat treatment processes customary for steels and gray cast iron. However, it is also pointed out that a type of cast iron can be given plastic properties by annealing for a short time, which by annealing at 720 ^° C. with a reduction in strength

can still be increased.

From US-PS 20 87 765 it is known to subject cast iron, which is brittle before the treatment, to a hot annealing treatment. For this purpose, the cast iron is subjected to a heat treatment (tempering treatment) in order to precipitate coal. It takes a long time and takes place on white cast iron, which is in a hypereutectoid state and has an extremely hypoeutectic mass. The carbon is deposited _{in it in the form of Fe 3 C.} The treatment is a special heat treatment at a temperature of around 820 to 840 [deg. ^] C. for a longer period of time, followed by gradual cooling to 650 [deg.] C. and finally, after the temperature of 650 [deg.] C. is reached, rapid cooling.

From the magazine "Gießerei-Praxis", No. 9/1971 (May 10), p. 157/169, it is known that niobium in cast iron can be alloyed. The addition of niobium improves in spheroidal graphite cast iron or conventional cast iron for example the wear resistance if the cast iron in question does not undergo any thermal deformation treatment is subjected.

Effects achieved by quenching nodular cast iron from the austenitizing temperature are finally described in Iron & Steel, Dec. 1959, pp. 559/560.

In view of this state of the art, the invention lay the underlying task of a spheroidal graphite cast iron with the tensile strength of special steels comparable and the tensile strength of conventional rolled cast iron or heat-treated special cast iron produce superior tensile strength, wherein in the resulting spheroidal graphite cast iron the inherent properties of cast iron as such, such as high wear resistance and damping ability, remain.

The subject of the invention is thus a process for the production of high tensile strength spheroidal graphite cast iron by heat treatment in the form of quenching from the austenitizing temperature and the following Tempering or in the form of an isothermal transformation, which is characterized in that one the spheroidal graphite cast iron before the heat treatment of a hot forming with a deformation coefficient Subjects from 30 to 80%.

The reason for the limitation of the degree of deformation in the hot forming of spheroidal graphite cast iron to 30 to 80% is to be found in the fact that the mutually dependent influence of hot forming and subsequent heat treatment does not fully come into play at a degree of deformation of less than 30% and that the machinability becomes worse at a degree of deformation of over 80%. As can be seen from the examples below, a cast iron with a tensile strength of 170 to 190 kg / mm ² can be obtained if the material is quenched from a temperature above the A! Holding point after hot forming and then tempered. If a certain toughness is required, a cast iron having a tensile strength of 160 to 170 kg / mm ² and suitable toughness can be produced by subjecting the material to an isothermal heat treatment after hot working.

According to the invention, taking into account the properties of conventional cast iron, surprisingly one can be achieved Manufacture high tensile strength cast iron. The advantages of carried out according to the invention Measures are in particular that from

such a cast iron relatively lightweight grain manganese

compact machine parts with high mechanical strength phosphorus

can be produced. sulfur

It is recognized that light profiles can only be very remainder of iron

difficult to pour. Even if a lightweight profile has been produced by casting, it is more expensive Measures (which sometimes make up more than half of the prime cost) to surface to edit. If, for example, a 1 to 3 mm thick sheet is desired, this can be done according to the invention Hot-work cast iron to near the desired thickness, reducing the machining allowance to the Outermost (0.05 to 0.20 mm) is reduced. In this way the prime costs can be compared sharply lower than normal cast iron. If niobium and / or tantalum in the cast iron melt is (are) contained, the wear resistance increases with decreasing durability, and consequently the machinability is improved.

With a preferably selected content of 0.05 to 2.5% niobium and / or tantalum, regardless of grain boundaries, very small and hard spherical carbides and nitrides of these elements (spherical diameter 1 to 5 μ, ίτι) in a concentration of distributed over 100 / mm ² in excreted form. If the material is used as a sliding part, the contact surface pressure is consequently evenly distributed, with a suitable bearing effect being established. If the material is cooled beforehand, the carbides mentioned do not dissolve sufficiently in the austenite, with the result that the hardenability decreases and even a thin sheet does not show any overcooling efiect and can be machined without difficulty. If, for example, a niobium-containing spheroidal graphite cast iron is rolled, then heated to a temperature of 930 "C and held at this temperature for 30 minutes and finally cooled, there is no martensite precipitation in the base material, so that it can be worked with light if the niobium is missing, a part with a wall thickness of less than 3.2 mm shows a hardening effect and when it cools down naturally becomes very hard, so that it has to be tempered or annealed for machining.

Niobium and / or tantalum contents of less than 0.05% are not sufficient to form spherical niobium and / or tantalum carbides and nitrides ^{above 100 / mm 2.} Niobium and / or tantalum contents above 2.5% are no longer acceptable from an economic point of view, since these elements are expensive. Their content should therefore be kept as low as possible with the best possible wear resistance of the later material. The spherical carbides and nitrides of niobium and / or tantalum are, as such, fairly uniform and distributed uniformly in the matrix and in a very small form. Consequently, the influence of the spherical carbide and nitride precipitates on the wear resistance is not noticeably different if only one or if both elements are added.

The following examples are intended to illustrate the process according to the invention in more detail.

Example 1

A cast iron containing spheroidal graphite of the composition:

0.29% 0.10% 0.012%

was converted in a conventional manner into a plate having a thickness of 20 mm, a width of 100 mm and a length of 600 mm, whereupon the obtained plate was surface-machined to a thickness of 15 mm, a width of 60 mm and a length of 500 mm became. After one hour of pre-heating to a temperature of 1050 C ^ü of the sample was immediately hot rolled, its thickness to 10.5 mm (degree of deformation 30%), 7.5 mm (degree of deformation 50%), 4.5 mm (degree of deformation 70%) or 3.0 mm (degree of deformation 80%) has been reduced. After hot rolling, specimens were mm a thickness of 2.0 mm a width of 20 and a length of 180 mm (in each deformation) produced by machining Bearbettung heated in a salt bath to a temperature of 890 ⁰ C and 850 ⁰ C. held for one hour at the respective temperature, then quenched with oil, and finally 30 min annealed at a temperature of 300 ⁰ C long. The tensile strength values of the individual test specimens were then measured. The results obtained are summarized in the following table I:

Table I. Austenitizing temperature 850C Deformation 890 C 152.4 kg / mm ² Degree (%) 175.2 kg / mm ² 153.2 kg / mm ² 35 80 161.0 kg / mm ² 137.5 kg / mm ² 70 143.1 kg / mm ² 120.7 kg / mm ² 60 125.6 kg / mm ² 116.0 kg / mm ² 50 123.5 kg / mm ² 109.0 kg / mm ² 30th 121.2 kg / mm ² 40 0

Comment:

The values given in the table represent mean values for the tensile strength from two measurements in one direction to the rolling direction parallel direction.

Comparative example

Example 1 was repeated with the exception that the test specimens were merely hot-rolled, however not quenched and left on. Tensile strength measurements in carried out a direction parallel to the rolling direction. The results obtained are in compiled in Table II below:

Table II

Degree of deformation (%)
0 30 50

60

70

80

76.0 80.1 84.5 92.3 101.0 100.0

carbon
Silicon

3.70%
2.64%

Tensile strength (kg / mm ² )

Comment:

The results contained in Table II are the mean values from two measurements.

As can be clearly seen from Tables I and II, a high tensile strength cannot be achieved if only a hot forming was carried out, Table III

or if it is simply quenched and tempered. The high tensile strength can in fact only be achieved can be achieved if both measures are combined.

Example 2
A cast iron containing spheroidal graphite d = r deformation-austenitizing temperature

degree (%) 930 C

850X

55 169.5 kg / mm ²

172.3 kg / mm ²

composition: 3.38% carbon 3.47% Silicon 0.41% manganese 0.08% phosphorus 0.011 ° / sulfur 0.14% chrome 0.32% niobium Remainder iron

was converted in a conventional manner into a plate with a thickness of 10 mm, a width of 100 mm and a length of 600 mm. The plate obtained was then converted into a 6.0 mm thick, 85 mm wide and 500 mm long plate by surface treatment. After one hour of pre-heating to 1000 ⁰ C, the sample was immediately hot rolled, whereby the thickness to 4.55 mm (reduction ratio Vo 24), 3.5 mm (degree of deformation 42%) or 2.7 mm (degree of deformation 55%) was reduced. By machining samples were mm a thickness of 2 $, mm a width of 20 and a length of 180 prepared tnm, whereupon held at austenitising temperatures of 930 ° C or 850 ° C, this 30 minutes, then in a salt bath of 280 ⁰ C quenched and left for one hour and finally cooled in air (so-called isothermal heat treatment). The tensile strength of each test piece was then measured twice in a direction parallel to the rolling direction. The mean values of these measurements are given in Table III below

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35 As is evident from Table II, has a treated cast iron according to the invention a high tensile strength, comparable to the tensile strength of special steels, on.

Nodular cast iron produced according to the invention therefore has better wear resistance than steels and common types of cast iron, and the wear resistance shows neither parallel nor perpendicular to Rolling direction an anisotropy.

According to the invention processed spheroidal graphite cast iron with high tensile strength can with regard to their Tensile strength can therefore be greatly improved if the types of cast iron containing niobium and / or tantalum usual composition before the heat treatment of a hot forming with a degree of deformation subject to from 30 to 80%.

This increased tensile strength is far greater than merely by hot forming or heat treatment. Such a further in terms of its tensile strength improved cast iron can have properties comparable to those of special steels are. In addition, all the excellent properties of cast iron, such as oil retention capacity, Wear resistance and high damping ability have been retained.

Spheroidal graphite cast iron produced according to the invention can thus be on previously inaccessible for cast iron Areas of application are used. For example, industrial plants, components and machine parts made of cast iron made in accordance with the present invention with lower cost can be manufactured with a lower weight.

Claims (2)

Patent claims: 1. Process for the production of high tensile strength Nodular cast iron by heat treatment in the form of cutting: 3ns from the austenitizing temperature and subsequent tempering or in the form of an isothermal conversion, thereby characterized in that the spheroidal graphite cast iron before the heat treatment of a Subjected to hot forming with a degree of deformation of 30 to 80% 2. The method according to claim 1, characterized in that a spheroidal graphite cast iron is used, the carbides and nitrides of niobium and / or tantalum corresponding to a proportion of niobium and / or tantalum from 0.05 to 2.5 percent by weight, based on the total starting material. IO