WO2010029564A1

WO2010029564A1 - Nodulizer for the production of spheroidal graphite iron

Info

Publication number: WO2010029564A1
Application number: PCT/IN2009/000396
Authority: WO
Inventors: Suhas Keshav Paknikar
Original assignee: Suhas Keshav Paknikar
Priority date: 2008-07-15
Filing date: 2009-07-10
Publication date: 2010-03-18

Abstract

A nodulizer alloy for the production of high strength, high ductility spheroidal graphite iron is disclosed. The nodulizer alloy of the present invention is economical, efficient and produces pearlite grade spheroidal graphite iron with uniform and controlled microstructure.

Description

NODULIZER FOR THE PRODUCTION OF SPHEROIDAL GRAPHITE IRON

FIELD OF THE INVENTION

The present invention relates to a nodulizer alloy.

Particularly, the present invention relates to a nodulizer alloy for making pearlitic grade spheroidal graphite iron.

DEFINITIONS OF TERMS USED IN THE SPECIFICATION

In this specification, the following terms have the following definitions as given alongside.

Spheroidal graphite iron: Spheroidal graphite iron also known as ductile iron or spheroidal graphite cast iron or nodular cast iron means iron in which graphite is present in spheroidal form instead of flakes and has higher mechanical strength, ductility and increased shock resistance.

Nodulizer: A nodulizer is an alloy which is added to molten cast iron to change the morphology of graphite present in cast iron from flakes to spheroids.

Pearlite: Pearlite is a two-phased, lamellar structure composed of alternating layers of alpha-ferrite (88%) and cementite (12%).

Ferrite: Ferrite is a solid solution of carbon in iron which is soft and ductile, it is found in some types of cast iron and spheroidal graphite iron. Cementite: Cementite or iron carbide is hard, brittle and crystalline compound of iron and carbon that is found in some types of cast iron and spheroidal graphite iron.

BACKGROUND OF THE EVVENTION & PRIOR ART

Cast iron is an iron alloy consisting about 95% by weight iron, 2.1% to 4% by weight carbon and 1% to 3% by weight silicon. Cast iron is characterized by its high carbon content and brittleness. When molten cast iron solidifies, the carbon precipitates as graphite, forming tiny and irregular flakes within the crystal structure of the metal. While graphite enhances the metallurgical properties of cast iron, the flakes disrupt the crystal structure and provide a nucleation point for cracks, producing the characteristic brittleness in cast iron.

Ductile iron or spheroidal graphite iron or nodular cast iron is a type of cast iron that is obtained when molten cast iron is treated with an alloy to form nodular graphite upon solidification. This is done by addition of nodulizer alloys or nodulizers to produce the graphite as spherical nodules rather than flakes, maintaining the matrix structure of the metal, thus, inhibiting the creation of cracks and providing enhanced strength and ductility/ The nodulizer alloy can be added to the molten cast iron in the ladle or it could be added immediately before entering the mold or could be added into a portion of the gating system within the mold. Commonly used nodularizing elements' include magnesium, calcium and cerium.

Magnesium cannot be introduced into the molten iron for nodularization until the sulfur content in the molten iron has been reduced to 0.005% to 0.01% because when the sulfur content is high the graphite will solidify into flakes instead of nodules. Thus, a desulphurization step is essential before the nodularization step for the production of ductile iron and is usually followed by an inoculation step. Magnesium is also a difficult element to introduce into molten iron for nodularization since in its pure state it has a boiling point (1107 ⁰C) well below the temperature of molten iron, a low solubility in iron and a much lower density than iron and a high tendency to be lost as magnesium oxide or magnesium vapor. Thus, generally magnesium is alloyed with another material with higher density and melting point.

Magnesium can be alloyed with denser elements like nickel and lately ferro- silicon. Though ferro-silicon is most commonly used along with magnesium for the nodularizing purpose, silicon if allowed to reach relatively high values can pose problems in the later stages of manufacture of the ductile iron. The presence of silicon can also result in the formation of siliceous slags, which need to be removed. Also, the reaction between magnesium in the magnesium-ferro-silicon alloy and molten iron can be violent even with a magnesium content of 5% to 10%. In the past, magnesium was alloyed with nickel or nickel was added in small quantities while making ductile iron, as nickel is also beneficial at imparting strength to the cast iron, however, nickel is expensive and not easily available. In addition to nodulizer alloys, pearlite stabilizers are also added to ductile iron to further enhance its yield and tensile ^• strength. The pearlite stabilizers help in increasing the pearlite content of the ductile iron which helps in improving its metallurgical properties. Pearlite is a eutectoid structure that comprises alternate layers of ferrite and cementite, influencing the hardness; fatigue properties, wear characteristics and the machinabilty of ductile iron. Commonly used pearlite stabilizers are copper, tin and manganese; these are expensive and add impurities in the final product. Thus, there is a need for a nodularizing alloy and pearlite stabilizer that is economical, gives high residual magnesium in the final product and also produces ductile iron with high toughness and higher ductility for use in the automobile industry, railway applications, connecting shafts and milling machinery.

GB Patent No. 1,058,402 discloses a magnesium master alloy for treatment of iron and steel melts, particularly for producing spheroidal graphite cast iron. The master alloy disclosed in GB 1,058,402 comprises 20% to 48% magnesium (Mg), 5% to 24% rare earth metals, atleast 40% silicon (Si) and/or nickel (Ni) and/or copper (Cu), the remainder being iron (Fe) and/or manganese (Mn). In the above cited alloy, large quantity of rare earth metals are required to raise the magnesium yield of the alloy, these rare earth metals are expensive, thus, making said master alloy expensive.

US Patent No. 4,173,466 discloses a treatment agent for use in nodularizing cast iron comprising iron (Fe), magnesium (Mg), calcium (Ca) and optionally rare earth metals, alkaline earth metals and tin (Sn). The treatment agent proposed in US 4,173,466 consists of magnesium (Mg) to calcium (Ca) ratio of 1 :1 to 8:1 and is used in a proportion of 0.5% to 3.0% by weight of the molten metal. A recent study on the effect of magnesium and calcium as spheroidizers/nodulizers on the graphite morphology in ductile cast iron proved that when magnesium and calcium were used in a ratio 1:1 a flaky graphite microstructure was obtained while a higher ratio of magnesium to calcium yielded a chunky, stubby graphite microstructure. The study also proved that the use of calcium as a nodularizing agent produces grey flake cast iron. Also, the above cited method requires the use of large quantity of the treatment agent for nodularizing cast iron, which adds to the cost of producing ductile iron.

CA Patent No. 1213148 discloses a process for producing an iron alloy containing magnesium, used to produce ductile and compacted graphite irons. The alloy disclosed in CA 1213148 consists of 0.5% to 4.0% Mg, optionally 0.1% to 10% Si, 0.5% to 6.5% carbon (C), 0.05% to 2.0% cerium (Ce), and/or rare elements, 0.1% to 10% Ni and remainder being Fe. The above cited process aims at retaining the Mg in the alloy and this is achieved by rapidly solidifying the alloy melt in a chill mold. The process uses a separate apparatus for producing the alloy which is provided with a proper temperature control to rapidly solidify the alloy melt. This makes the above process expensive as it involves additional cost of the apparatus and complex as the process demands an accurate temperature control.

US Patent No. 4,363,661 discloses a method for improving the mechanical properties of ductile iron; by using an alloy that produces a microstructure consisting of a pearlite matrix with uniformly distributed graphite nodules. In US 4,363,661, the iron melt is treated with a pearlite stabilizer comprising tin (Sn) and/or antimony (Sb), cerium (Ce), lanthanum (La) and manganese (Mn) to obtain a pearlite matrix and a nodularizing agent comprising of at least one of Mg, Ca or lithium (Li) and rare earth metals like Ce and La. The above cited method uses two different additives for producing the high strength ductile iron and large quantity of rare earth metals are used; thus, the costs involved in the above process are high. CN Patent No. 1,147,022 discloses the use of a nodulizer alloy comprising iron (Fe), silicon (Si), magnesium (Mg) and rare earth metals (RE) and separately adding copper (Cu) element during the nodularization process for making ductile cast iron. In CN 1,147,022, the nodularization is carried out during the melting process of iron wherein 1.4% of the FeSiMgRE nodulizer and Cu element are separately added to the molten metal, the % of Cu used in the mixture being greater than 35%. In CN 1,147,022, the invention describes a process exclusively used for making cold rolls for manufacturing steel pipes. The nodulizer alloy used in the process contains rare earth metals, thus, it is expensive.

In the prior art, it is seen that almost all of the nodulizer alloys used for producing spheroidal graphite iron use rare earth metals/misch metal mainly to obtain high residual magnesium in the alloy. The rare earth metals are in high demand for their use in making hybrid car motors, superconductors, high-flux magnets and electronic polishers, and there is a growing concern over the depletion of these materials. This makes the rare earth metals expensive and scarcely available. Thus, there is a need for a nodulizer alloy that does not require the addition of rare earth metals, is economical and produces iron with better metallurgical properties.

OBJECTS OF THE INVENTION

An object of this invention is to provide a nodulizer alloy for producing spheroidal graphite iron.

Another object of this invention is to provide a nodulizer alloy for producing spheroidal graphite iron having high tensile and yield strengths.

Still another object of this invention is to provide a nodulizer alloy for producing spheroidal graphite iron having high ductility.

Yet another object of this invention is to provide a nodulizer alloy for producing spheroidal graphite iron that is economical.

One more object of this invention is to provide a nodulizer alloy for producing spheroidal graphite iron with uniform and controlled microstructure.

Still one more object of this invention is to provide a nodulizer alloy with minimal tramp elements for producing spheroidal graphite iron.

Yet one more object of this invention is to provide a nodulizer alloy for producing spheroidal graphite iron, the process for which is efficient, inexpensive and requires less number of stages.

An additional object of this invention is to provide a nodulizer alloy that can be used for any method of producing spheroidal graphite iron such as direct pour method, sandwich method and tundish method. SUMMARY OF THE INVENTION

In accordance with the preferred embodiment of the present invention, a nodulizer alloy for the production of high strength, high ductility spheroidal graphite iron is provided, said alloy comprising:

• a nodularizing element which includes 5% to 30% magnesium; and

• a carrier element comprising of 70% to 95% copper.

Typically, in accordance with this invention, said nodulizer alloy contains at least 60% of said carrier element.

Typically, in accordance with this invention, other nodularizing element or elements including calcium, lithium, yttrium and silicon are used in small quantities along with magnesium in said nodularizing alloy.

Typically, in accordance with this invention, the total amount of said nodularizing elements in said nodulizer alloy not exceeding more than 30%.

Typically, in accordance with this invention, 5% to 10% of nodularizing element including silicon is added in said nodulizer alloy.

Typically, in accordance with this invention, a method for the production of high strength, high ductility spheroidal graphite iron by adding said nodulizer alloy, wherein the extent of said alloy required is 0.3% to 0.5% (by mass) of the molten metal. Typically, in accordance with this invention, a method for making a nodulizer alloy for the production of high strength, high ductility spheroidal graphite iron, said method comprising of the following steps:

• melting pure copper in a furnace at a temperature of 1100 ⁰C to 1200 ⁰C;

• reducing the temperature in the furnace to 800 ⁰C to 850 ⁰C;

• adding magnesium to the copper solid solution in the furnace at 800 ⁰C to 850 ⁰C;

• stirring the mixture to obtain a homogenous copper-magnesium solid solution; and

• cooling the copper-magnesium solid solution in metallic dies to produce the nodulizer alloy.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

The invention will now be described with the help of the accompanying drawings, in which,

FIGURE 1 is a photograph showing the unetched niicrostructure of the spheroidal graphite iron produced using the nodulizer alloy at xlOO magnification, in accordance with the present invention;

FIGURE 2 is a photograph showing the etched microstructure of the spheroidal graphite iron produced using the nodulizer alloy at xlOO magnification, in accordance with the present invention; and FIGURE 3 illustrates the graphical representation of the mechanical tests performed on the spheroidal graphite iron produced using the nodulizer alloy, in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described with reference to specific embodiments which do not limit the scope and ambit of the invention. The description relates purely to the exemplary preferred embodiments of the invention and their suggested application.

The present invention envisages a nodulizer alloy for the production of high strength, high ductility spheroidal graphite iron, said nodularizing alloy comprising a nodularizing element which includes 5% to 30% magnesium; and a carrier element comprising 70% to 95% copper. At least 60% of said carrier element is used in said nodulizer alloy for the purpose of obtaining a pearlite matrix in the spheroidal graphite iron produced. The use of copper as the carrier element helps in increasing the pearlite content of the final product thus imparting high strength. The molten cast iron treated using said nodulizer alloy does not require any additional manganese to increase the pearlite content in the structure.

Copper in said nodulizer alloy also helps in avoiding the formation of any flare-ups due to the reaction of magnesium with molten cast iron and the reaction is non-violent. Thus, said nodulizer alloy can be used for any method of producing spheroidal graphite iron such as direct pour method, sandwich method and tundish method. Also, since magnesium is retained in the final product, the spheroidal graphite iron thus produced has high nodularity, high nodule count and uniform and controlled microstructure.

Other nodularizing element or elements including calcium, lithium, yttrium and silicon can be used in small quantities along with magnesium in said nodularizing alloy. However, the total amount of said nodularizing elements in said nodulizer alloy should) not exceed 30%, wherein, 5% to 10% of nodularizing element including silicon is also added in said nodulizer alloy. The nodulizer alloy of the present invention is to be used in the extent of 0.3% to 0.5% (by mass) of the molten cast iron.

A typical method of making a nodulizer alloy for the production of high strength, high ductile spheroidal graphite iron in accordance with this invention comprises the following steps:

Pure copper is melted in a furnace at a temperature of 1100 ⁰C to 1200 ⁰C. When the copper is in the molten state the temperature of the furnace is reduced to 800 ⁰C to 850 ⁰C, the copper solution forms a copper solid solution at a lower temperature. The temperature of the furnace is reduced to accommodate magnesium, since magnesium has a melting point of 650 ⁰C and boiling point at 1107 ⁰C. The reduction in temperature will prohibit the loss of magnesium as vapor. Magnesium is added to the copper solid solution in the furnace at 800 ⁰C to 850 ⁰C. The copper-magnesium solid solution is mixed with a stirrer to obtain a homogenous mixture containing the copper rich solid solution and a eutectic which comprises the copper solid solution and copper-magnesium phase. The copper-magnesium solid solution is poured in metal molds and allowed to cool to produce the nodulizer alloy. THE INVENTION WILL NOW BE DESCRIBED WITH THE HELP OF FOLLOWING EXAMPLES

EXAMPLE 1

The nodulizer alloy was manufactured in a foundry where pure copper wire was melted at approximately 1200 ⁰C. The temperature in the furnace was reduced to approximately 800 ⁰C, pure magnesium was added to the copper solid solution, the solution was stirred to obtain a homogenous mixture. In said alloy, 87% copper and 13% magnesium was used. Magnesium was completely dissolved in the copper solid solution. The microstructure of said nodulizer alloy showed as a cast structure consisting of copper rich solid solution and a eutectic. The eutectic was composed of a copper rich solid solution and a copper-magnesium phase. Ingots of the alloy were obtained by immediately transferring said solid solution of said alloy in the metal molds and allowed to cool.

EXAMPLE 2

Step 1 : The nodulizer alloy from EXAMPLE 1 was placed at the bottom of an open ladle and was covered with cold rolled close annealed (CRCA) steel punchings. This ladle was placed in an induction furnace where the melting was done at approximately 1550 ⁰C.

Step 2: Cast iron charge comprising scrap steel with low Mn content and pig iron containing low sulfur (S) and phosphorus (P), was melted at 1550 ⁰C in another furnace. The composition of the cast iron charge was 3.5% to 3.6%

C, 2.5% to 2.6% Si, < 0.2% Mn with traces of S and P, and the microstructure showed flake type large graphite particles. Step 3: The nodulizer alloy from Step 1 was placed in the centre pocket of an open ladle. Small quantity of inoculant was added to the nodulizer alloy, said inoculant consisting of barium, silicon and iron.

Step 4: Cast iron melt from Step 2 was poured in the open ladle containing the nodulizer alloy from Step 1. The quantity of said alloy used was 0.5%

(by mass) of the molten cast iron. The reaction was completed within 2-3 minutes at a temperature of approximately 1550 ⁰C.

Step 5: The molten spheroidal graphite iron produced in Step 4 was poured in , metal molds. During pouring, post inoculation was done using barium, silicon and iron inoculant.

TESTS PERFORMED ON THE SPHEROIDAL GRAPHITE IRON PRODUCED USING THE NODULIZER ALLOY OF THE PRESENT INVENTION

TEST 1 : MICROSTRUCTURE ANALYSIS OF MOLTEN SPHEROIDAL GRAPHITE IRON

Four samples were collected during the beginning and end of the pouring process of the molten spheroidal graphite iron produced in EXAMPLE 2 from the open ladle to the metal molds; to check the decreasing percentage of magnesium in the spheroidal graphite iron with time, also called as the fading effect of magnesium. First sample was collected from the ladle immediately after the treatment, second sample was collected intermediately during pouring the molten metal from the ladle into the metal castings, third sample was collected from the first metal casting and fourth sample was collected from the last metal casting. The above samples are labeled as 1, 2, 3 & 4, respectively. The results are tabulated below^

It was observed from the above results that the spheroidal graphite iron produced using the nodulizer alloy of the present invention showed reduced magnesium fading effect.

TEST 2: MICROSTRUCTURE ANALYSIS OF SOLIDIFIED SPHEROIDAL GRAPHITE IRON

The spheroidal graphite iron produced in EXAMPLE 2 was allowed to cool and solidify in the metal molds. Four sample test bars of the spheroidal graphite iron were obtained and a detailed analysis was performed on the same to determine its microstructure. These samples are labeled as 5, 6, 7 & 8, respectively. The results are tabulated below:

FIGURE 1 illustrates the unetched microstructure of the spheroidal graphite iron obtained in TEST 2. The microstructure is represented by the numeral 100, it shows a ferrite base 112 surrounding the graphite nodules 110 and compacted graphite 114. A high nodule count, very high nodularity and even sized nodules were observed in 100. A nodularity of more than 95% can be obtained using said nodulizer alloy.

TEST 3: MICROSTRUCTURE ANALYSIS OF THE ^"MATRIX STRUCTURE OF SPHEROIDAL GRAPHITE IRON The spheroidal graphite iron obtained in TEST 2 was chemically etched to study the matrix structure. Chemical etching of the spheroidal graphite iron surface was done using concentrated nitric acid and ethyl alcohol. FIGURE 2 illustrates the etched microstructure of the spheroidal graphite iron. The microstructure which is represented by the numeral 200 shows the graphite nodules 110 and compacted graphite 114 surrounded by ferrite base 112 and the rest being nearly dense pearlite 210. It is observed from 200, that the spheroidal graphite iron produced using the nodulizer alloy of the present invention has a high pearlite content. TEST 4: MECHANICAL TESTS OF THE SPHEROIDAL GRAPHITE

IRON

Sample test bars of the spheroidal graphite iron obtained in TEST 2 were analyzed to determine its strength and ductility.

The spheroidal graphite iron thus produced is of grade 500/7 and has a tensile strength of 613 N/mm² to 688 N/mm², a yield stress of 427 N/mm² to 482 N/mm², a breaking load of 77 KN to 87 KN and a ductility of 8% to 13%.

A ductility of 3% is a standard requirement for applications using ductile iron. The spheroidal graphite iron produced using said process has high tensile and yield strength and a very high ductility making it suitable for applications in critical components requiring high grade ductile iron such as connecting shafts, hubs, mining machinery, railway applications and the like. Unlike most of the nodulizer alloys particularly ferrro-silicon- magnesium alloy, wherein copper is usually added to obtain a pearlite matrix, said process does not require any additional copper.

FIGURE 3 shows the stress-strain diagram of the mechanical tests performed on the spheroidal graphite iron. The graph is represented by the numeral 300 displays the displacement in millimeters (mm) on the X-axis and the load in Kilo Newton (KN) on the Y- axis. From graph 300, it is observed that the spheroidal graphite iron produced using the nodulizer alloy of the present invention has a breaking load 87 KN, represented by point 310. TECHNICAL ADVANCEMENT

A nodulizer alloy is provided for the production of high strength, high ductility spheroidal graphite iron, as described in this invention has several technical advantages including but not limited to the realization of:

• a nodulizer alloy for producing spheroidal graphite iron having high tensile and yield strengths;

• a nodulizer alloy for producing spheroidal graphite iron having high ductility;

• a nodulizer alloy for producing spheroidal graphite iron that is economical;

• a nodulizer alloy for producing spheroidal graphite iron with uniform and controlled microstructure;

• a nodulizer alloy with minimal tramp elements for producing spheroidal graphite iron;

• a nodulizer alloy for producing spheroidal graphite iron, the process for which is efficient, inexpensive and requires less number of stages;

• a nodulizer alloy that can be used for any method of producing spheroidal graphite iron such as direct pour method, sandwich method and tundish method.

In view ot the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

Claims

Claims:

1. A nodulizer alloy for the production of high strength, high ductility spheroidal graphite iron, said alloy comprising:

• a nodularizing element which includes 5% to 30% magnesium; and

• a carrier element comprising of 70% to 95% copper.

2. A nodulizer alloy as claimed in claim (1), wherein said alloy contains at least 60% of said carrier element.

3. A nodulizer alloy as claimed in claim (1), wherein other nodularizing element or elements including calcium, lithium, yttrium and silicon are used in small quantities along with magnesium.

4. A nodulizer alloy as claimed in claim (1 & 3), wherein total amount of said nodularizing elements in said alloy not exceeding more than 30%.

5. A nodulizer alloy as claimed in claim (1), wherein 5% to 10% of nodularizing element including silicon is added in said alloy.

6. A method for the production of high strength, high ductility spheroidal graphite iron by adding said nodulizer alloy as claimed in claim (1 to 5), wherein the extent of said alloy required is 0.3% to 0.5% (by mass) of the molten metal.

7. A method for making a nodulizer alloy for the production of high strength, high ductility spheroidal graphite iron, said method comprising of the following steps:

• reducing the temperature in the furnace to 800 ⁰C to 850 ⁰C;