CS271765B1 - Method of modular cast iron's two-stage treatment - Google Patents

Abstract

The solution concerns a method of the two-step treatment of ductile cast iron for thick-walled castings. The basic ductile cast iron flow is composed of 1.5 to 2.3 %weight of silicon, 2.7 to 3.6 % by weight of carbon, 0.01 to 0.06 % by weight of phosphorus, 0.03 to 0.08 % by weight of magnesium, and eventually 0.2 to 0.60 % by weight of copper, in the first step a magnesium foundry alloy is added and during the casting in the second step magnesium alloy containing 42 to 52 % by weight of silicon, 0.5 to 3 % by weight of magnesium, 0.5 to 1.3 % by weight of aluminium and 0.3 to 1.2 % by weight of calcium is added in the amount of 0.1 to 0.5 % by weight of the casting weight.

Description

BACKGROUND OF THE INVENTION The invention relates to a two-stage nodular cast iron treatment for the production of castings having a wall thickness above 100 mm by adding magnesium master alloy to the nodular cast iron having a composition of 1.5 to 2.3% by weight. % silicon, 2.7 to 3.6 wt. % of carbon, 0.01 to 0.06 wt. % phosphorus, 0.03 to 0.08 wt. % magnesium, optionally 0.2 to 0.60 wt. copper, in the first stage, and master alloys in the second stage, into a stream of basic ductile iron.

Ductile cast iron is known with good mechanical properties even in thick cross-sections and with regularly eliminated grained, spherical, graphite shape and with a free carbide-free matrix made of high-purity pig iron. In some cases, it is recommended that lead or copper contents be controlled to ensure a regular grained graphite shape. However, all methods known to date are highly sensitive to batch assembly and results are not always reliable. British Patent Specification No. 1,478,218 discloses a process for producing ductile cast iron by first preparing, using special magnesium-free refining agents, an imperfect ductile cast iron, which is further processed to the desired ductile cast iron quality by additives containing less than 3% of magnesium or pure magnesium, whose additive is less than 0,04%. However, the properties of this ductile iron in thick-walled castings are not known.

Further, processes for the manufacture of ductile cast iron with the addition of additives which provide the desired grained graphite shape in the ductile cast iron, are merely added to the mold cavity or into the metal stream before it enters the mold cavity. However, none of these processes reliably guarantees the formation of a grainy graphite shape in the thick walls of the casting, i.e. above 100 mm.

The above mentioned drawbacks are eliminated by a two-stage treatment of ductile cast iron for the production of castings with a wall thickness above 100 mm by adding magnesium master alloy to the ductile iron having a composition of 1.5 to 2.3% by weight. % silicon, 2.7 to 3.6 wt. % of carbon, 0.01 to 0.06 wt. % phosphorus, 0.03 to 0.08 wt. % magnesium and optionally 0.2 to 0.60 wt. copper in the first stage and master alloys in the second stage into the malleable cast iron stream according to the invention. It is based on the addition of a magnesium alloy having a composition of 42 to 52 wt. 0.5 to 3 wt. % magnesium, 0.5 to 1.3 wt. % aluminum, 0.3 to 1.2 wt. % calcium, in an amount of 0.1 to 0.5% by weight. casting weight.

The basic production consists of obtaining ductile cast iron with a well-formed grained, spherical graphite shape and a matrix without free carbides, even in thick-walled castings, which achieves high ductility and strength even in the heat center of very mass castings.

In this way, a ductile cast iron is obtained without extraordinary demands on the purity of the starting feedstock, which is also characterized in thick sections, i.e. over 100 mm and in very thick sections, i.e. from 250 to 500 mm, high strength and in particular ductility above 6%.

In the two-stage malleable cast iron process according to the invention, an alloy containing both silicon and magnesium is added to the finished malleable cast iron just prior to cooling in the mold cavity, thereby creating conditions for the formation of a large number of stable graphitizing nuclei. A large number of graphitizing nuclei condition the formation of a large number of graphite grains, which grow only to a smaller size. Smaller graphite grains are for a long time when cast iron is stable in the liquid state and at eutectic dwell time and the austenitic envelope surrounding it in the liquid state is not broken. Small grains of graphite are not affected by such a high buoyancy and therefore do not float to the surface even at higher carbon and silicon contents, so-called flotation of graphite does not occur. Due to the fact that a large number of graphite nuclei was formed by adding a master alloy FeSiMg, respectively. FeSiMgCe, just before the metal enters the casting cavity, even in thick-walled castings of ductile cast iron, a regularly grained graphite shape and a basic structure without free carbides are achieved.

CS 271 765 Bl

The process of the two-stage malleable cast iron treatment according to the invention is described in more detail below on a specific embodiment.

Example

Of ductile cast iron of a chemical composition: 3,45% mass C, 0.20 wt. % Mn, 2.03 wt.

Si, 0.050 wt. S, 0.03 wt. Cr, 0.02 wt. Cu, 0.01 wt. % Ni, 0.0007 wt. Sn and 0.049 wt. Mg, were cast, to verify mechanical values and structure, test castings with a wall thickness of 250 mm and a cube cast with an edge of 500 mm. Fe SiMg master alloy having the following composition: 47 wt. Si,

2.5 wt. Mg, 0.9 wt. AX, 0.60 wt. Ca, in an amount of 0.3 wt. weight of the casting produced. Results from the performed tensile tests taken from the casting centers: the following average values from the three cast tests were found:

casting with a wall thickness of 250 mm: yield strength 0.2 336 MPa 594 MPa 9.6% fortress ductility thrust 500 mm cube casting: yield strength 0.2 256 MPa fortress thrust 361 MPa ductility 9.2%

The structure consisted of ferrite and perlite and graphite in a regular spherical to partially irregularly spherical shape in a 500 mm cube casting.

The comparative melting results, without using the invention, were as follows:

2. Steps of adding alloy with a wall thickness of 250 mm cube casting with 500 mm edge:

yield strength 0.2 tensile strength elongation

358 MPa 403 MPa

4.2%

244 MPa 356 MPa

2.5%

The process according to the invention can be used in the production of castings of ductile cast iron with a large wall thickness, i.e. above 100 and above 250 mm, with the mechanical values being extremely high due to the grainy shape of the graphite. A particular application is for castings of high wall thickness, which have so far been made only of steel or castings, where, due to the complexity of the shapes, cooling is extremely slow and the solidification rate corresponds to thick-walled castings. Specifically, for castings forming parts of large compressors, basic parts of large grinding equipment and the like.

Claims (1)

Process for two-stage ductile cast iron for the production of castings having a wall thickness above 100 mm by adding magnesium master alloy to the ductile iron having a composition of 1.5 to 2.3% by weight. % silicon, 2.7 to 3.6 wt. % of carbon, 0.01 to 0.06 wt. % phosphorus, 0.03 to 0.08 wt. % magnesium, optionally 0.2 to 0.60 wt. copper in the first stage and adding the master alloy in the second stage to the ductile iron stream, 2. A process according to claim 1, characterized in that a magnesium alloy of 42 to 52% by weight of silicon, 0.5 to 3% by weight of magnesium, 0.5 to 1.3% by weight is added in the second step. % aluminum, 0.3 to 1.2 wt. 0.1 to 0.5% by weight of the casting.