Nodular graphite is obtained by spheroidizing and inoculation treatment, which reduces the cutting effect of graphite on matrix, effectively improves the mechanical properties of cast iron, and obtains higher plasticity, toughness and strength.  Nodular cast iron is a kind of high strength cast iron material developed in the 1950s. Its comprehensive performance is close to that of steel. Based on its excellent performance, nodular cast iron has been successfully applied to some parts with complex force, high strength, toughness and wear resistance requirements.  Nodular cast iron has rapidly developed into a very [TJC STEEL]widely used cast iron material, second only to gray cast iron.  People demand more and more high performance of ductile iron.  Many kinds of spheroidizing methods can be applied in the production of nodular cast iron. These methods have their own advantages and disadvantages, so it is necessary for enterprise engineers to choose and apply them reasonably according to the actual production conditions.  

 

Pressure Plus Magnesium Method  

Because the boiling point of magnesium (1107℃) is low and difficult to dissolve in liquid iron, and the temperature of liquid iron can reach 1500℃ during spheroidization, magnesium is easy to react violently in liquid iron, resulting [TJC STEEL]in a low absorption rate.  When the pressure of medium around magnesium increases, the boiling temperature of magnesium increases correspondingly, the burning loss of magnesium decreases, and the absorption rate of magnesium increases.  Based on this principle, pressure plus magnesium method was developed.  According to the different way of building pressure, can be divided into external pressure type and self-built pressure type two pressure and magnesium method.  The early use of external pressure was to place a treatment package full of liquid iron in an airtight pressure tank and compress air or nitrogen to establish the required pressure.  Another is the use of magnesium vapor self-built pressure inside the liquid iron package, which is add pure magnesium to seal the liquid iron package, magnesium in liquid iron package quickly generate a large amount of magnesium vapor, part of the steam passes through the liquid iron is absorbed by molten iron, the other part of escape and space within the bag quickly established accordingly with liquid iron temperature of saturated vapor pressure, then no longer boiling evaporation and loss of magnesium.  The advantage of the pressure and magnesium method is that the spheroidization treatment is carried out with pure magnesium, and the absorption rate of magnesium is high, up to 70% ~ 80%, and there is no smoke and dust in the process, and the working environment is good.  Disadvantage is the processing equipment requirements and higher cost;  Complex and strict operation;  The treatment time is long and[TJC STEEL] the temperature of liquid iron is more.  In the process of spheroidization, the pressure is great and the accident is easy to happen.  

 

Flowing Method 

Punching method is the most widely used spheroidizing treatment method at home and abroad.  The treatment package used is usually dike type spheroidizing treatment package.  In order to reduce the intensity of the reaction between molten iron and magnesium and the rate of evaporation of magnesium vapor, the thrusting method usually uses alloy spheroidizer with low magnesium content.  When spheroidizing treatment, the spheroidizing agent is first loaded into one side of the dam, covered with ferrosilicon alloy, slightly tightened, and then covered with rust-free iron filings, steel plates or other covering agents.  When spheroidizing, liquid iron should be flushed into the other side of the liquid iron bag as much as possible.  The absorption rate of magnesium is generally 30% ~ 50%.  In order to improve the spheroidization effect, the ratio of height to diameter of the treatment [TJC STEEL]package can be increased.  Using low magnesium alloy spheroidizing agent;  Reasonable temperature and covering dose of liquid iron.  The advantages of the punching method are simple treatment methods and equipment, easy operation, greater flexibility in production, the required technical content is also low, but the deficiency is the spheroidizing process of magnesia, dust pollution is more serious;  The absorption rate of magnesium is low.  

 

Turning Bag Method  

Subcontracting is a spherification process developed and patented by George Fischer Company.  The method uses pure magnesium as spheroidizing agent, which is suitable for the treatment of liquid iron with high sulfur content. It can make the impurities such as magnesium sulfide and magnesium silicate separate well from liquid iron. The reaction between magnesium and liquid iron is not very violent, and the liquid iron is less cooled, so it is safe to use and the absorption rate of magnesium can reach 60% ~ 80%.  The specific technological process is that before spheroidizing treatment, the subpackage is first laid horizontally, quantitative liquid iron is injected, then the spheroidizing agent is added into the reaction chamber, the device is locked tightly, and the cover is closed.  Then rotate the liquid iron bag to put it vertically, then the liquid iron enters the reaction chamber through [TJC STEEL]the hole in the reaction chamber, and the flow rate is related to the area of the hole and the static pressure in the liquid iron bag.  When the pressure exceeds the static pressure of molten iron in the bag, the molten iron stops entering, and the latent heat of vaporization of magnesium decreases the temperature in the reaction chamber.  The vapor pressure also decreases, and liquid iron enters the reaction chamber again. This automatic adjustment can make magnesium react with liquid iron more smoothly.  In addition, the small holes in the reaction chamber in the subcontract are easily blocked by molten iron or molten slag, so it is difficult to clean and maintain the size of the small holes. Therefore, it is difficult to continuously treat molten iron by the subcontract method.  

 

Cored Wire Feeding Method  

Core-wrapped wire feeding method was first applied in steelmaking industry, and then popularized in casting industry.  At present, wire feeding technology is widely used in the production of nodular cast iron in industrial developed countries, but the application of this technology in the production of nodular cast iron started late in China, and has not been widely used in the production of nodular cast iron, is in the promotion stage.  The application of wire feeding method to produce nodular cast iron is simply to insert core-coated wire with magnesium and other alloy elements directly into the liquid iron for spheroidizing treatment so as to produce nodular cast iron. The whole spheroidizing process can be completely automated.  The diameter of commonly used core-clad [TJC STEEL]wire is generally 9mm and 13mm, and the magnesium content of the built-in powder alloy is generally 25% ~ 30%.  A certain amount of RE, Ca, Ba is added to improve the performance of castings when special needs are met.  Thread feeding machine can set the wire feed speed, feed wire length, thread feeding mode parameters, such as processing, thread feeding machine through the drive mechanism will pack core according to set good parameters configuration continuously inserted into the sealed package processing package of iron liquid, due to the high pressure generated by the function, core cover from the effective air flow and package according to certain speed continuous insertion of iron liquid,  In this way, the instant mass explosion of magnesium vapor can be avoided to ensure the safe addition of magnesium alloy, and the mass escape and burning loss of magnesium can be avoided to improve the absorption rate of magnesium in liquid iron.  Generally speaking, the performance and quality of alloy [TJC STEEL]core-clad wires, as well as the feeding speed and feeding quantity are the key factors to ensure the success of feeding spheroidization.  The shape of the treatment package, temperature of molten iron, sulfur content of raw liquid water and sealing property of the package cover are also important factors affecting the effect of spheroidizing treatment.  The advantages of core-fed wire spheroidization are: good desulfurization and deoxidation effect, less cooling, relaxed requirements on the original liquid iron;  The absorption rate of magnesium is high and relatively stable, and the fluctuation range of residual magnesium content is small.  Less dust and magnesia in spheroidizing process.  It can realize precise and automatic control of alloy addition quantity.  

 

Cover Package Method  

The covering method was invented by the British iron Research Association and is widely used in the production of ductile iron abroad.  In the spheroiding process of the cover method, the addition of alloy is the same as that of the punching method. Then, the cover is placed on the treatment package and its periphery is sealed, and the liquid iron is injected into the cover, and the liquid iron will flow into the cover through the liquid iron injection hole on the side of the cover (the liquid iron shall not be[TJC STEEL] directly aligned with the alloy stacking place).  In this way, the outside gas and the package can be completely isolated, reduce the oxidation of magnesium, burning loss, improve the absorption rate of magnesium (generally 60% ~ 65% or above), improve the working environment.  After the spheroidization, the cover was removed.  The spheroidization effect of cap coating method is closely related to the correct selection of injection hole diameter of cap coating liquid iron.  Correct diameter of liquid iron injection hole can ensure that a certain height of liquid iron is kept in the cover.  The time of molten iron flowing into the cover is the same as the time of spheroidization.  The capping method not only retains the advantages of simple equipment and easy operation, but also overcomes the disadvantages of magnesium oxidation burning loss, low absorption rate, large consumption of spheroidizing agent and poor working environment in the punching method.  Over the years, casting workers have been making use of the advantages of covering method to produce nodular cast iron, and at the same time, they are constantly trying to overcome the shortcomings of this method in use: it is difficult to lift the covering and operate;  When cupola is used for continuous iron extraction, the weight of liquid iron is difficult to be accurately quantified.  After continuous improvement, the [TJC STEEL]spheroidizing process has been widely promoted and applied.  

 

Reaction Chamber Method

The spheroidizing agent is placed in a specially designed reaction chamber in the pouring system. During the pouring process, molten iron reacts with the spheroidizing agent after flowing through the reaction chamber.  In order to ensure the stability of spheroidizing treatment and reduce burning loss, the size of reaction chamber and pouring system should be calculated strictly.  In general, the reaction chamber is arranged in the runner under the sprue.  

Type ii internal magnesium absorption rate is high, up to 70% ~ 80%, no magnesia light, no smoke, no spheroidization recession, suitable for mechanized molding assembly line production.  The shortcoming is that it has strict [TJC STEEL]requirements on the temperature of liquid iron, sulfur content, composition of spheroidizing agent, lumpiness of spheroidizing agent, size of reaction chamber and design of pouring system. The slight change of these factors may cause the change of spheroidizing effect.  In addition, this method is easy to produce slag.  

 

In general, each of the above six spheroidizing treatment methods has its own advantages and disadvantages. It is hoped that each nodular cast iron production enterprise can comprehensively evaluate each spheroidizing treatment method [TJC STEEL]according to its own production conditions and conditions, and choose its own spheroidizing treatment method to ensure the quality of nodular cast iron.