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04 Mar 2014
Ductile Iron Treatment Optimization

            Ductile iron production has come a long way since its discovery during the 1940s. Even though magnesium and rare earths have both been shown to produce spheroidal graphite, magnesium is the element of choice commercially. There are many methods used to convert base iron to ductile iron.

            The result of ductile iron conversion is measured or evaluated by the following factors

            -  Nodularity - maximize

            -  Nodule count - maximize

            -  Chill carbides - minimize

            -  Inverse chill - minimize

            -  Shrinkage porosity- minimize

            -  Inclusions - minimize

           

        Unique properties of magnesium

            Magnesium has a low boiling point, high reactivity with oxygen and low solubility in iron making reliable additions to iron difficult. To minimize Mg reactivity calcium is added to the master alloy. Magnesium content of the alloy is also restricted to lower the reactivity. To increase solubility silicon  is raised in the vicinity of the alloy by using ferrosilicon as cover material. To satisfy different needs many alloys were developed with combinations of Mg and rare earths.

 

       Different treatment process

            Many different treatment processes have evolved over the years to achieve better quality ductile iron at the lowest cost. Many variables affected the selection of a particular treatment process. Some of the factors are:

            -  Base melt quality including sulfur content

            -  Temperature of treatment, holding and pouring

            -  Delays in metal handling

            -  Casting section modulus

            -  Ease of late inoculation

            These factors affect the treatment technique due to the unique properties of magnesium and resulted in the following processes, which are still used in commercial production:            - Tundish            -  Flow thru processes           -  Pure magnesium processes-Converters, plunging,pressure vessels           -  Wire feeders           - In-mold process        

          Nodularity and nodule count

          It has been shown by many that the nodule shape is best when the magnesium residual is just enough as too much will deteriorate the nodule shape from fully spheroidal. Nodule count can be maximized by sound base iron melting practice and good inoculation practice. Cooling rate affects both the nodule count and the nodule shape. Fast solidifying iron results in better nodule shape than slowly cooled iron for the same magnesium residuals. Larger sections require increased magnesium residual and late inoculation reduces the magnesium requirement. When rare earths are added to the iron the amount of magnesium required is also reduced. As some of the magnesium measured is in the form of magnesium sulfide, final iron sulfur level affects the magnesium needed to result in nodular graphite.

                What is good MgFeSi ?

                 We should not consider and concentrate on the quality of MgFeSi only the materials compositon in the certificate, bacuase sometime the value in the certificate is good but the performance of material is not good as the certificate

No. Description  Comparison for MgFeSi Advantage /Disadvantage 
Good  Poor 
The Must       
1 Raw materials of production  Produce from the quartz and iron ore  Produce from Ferro remelting silicon and Steel scrap The primary produced product have more purity, less oxidation and other oxide 
2 Manufacturing process  Melting by Arc furnace  Melting by small induction furnace  More constancy product quality and less oxidation 
3 Melting reaction atmosphere (in manufacturing) Reduction  Oxidation  Less Oxidation and oxide 
4 Casting method  Rapid solidification by water cooled process with thin thickness (<1/2") By conventional thick batch cast slads (>1.5 "), /(Pan Cast ) 1) Faster solidification              2) Homogeneous materials        3) Less oxidation and oxide during solidification                 4) Uniform chemical and metallurgical distribution 
5 Thickness of product after casting  Thin (15-20 mm.) Thick                       (over 30 mm.) Controlled materials size 
6 Content of MgO  Low                   (below 0.5%) High  1) High Active Mg                     2) High Mg recovery and less consumption (adding at <1.0%)                              
7 Sizing of materials  Sizing uniformity controlled by thin cast process  Not uniformity sizing  1) Many loss (too small size)     2) Too strong reaction (Too big size) 
Recommended       
1 Content of total Rare earth (Re) (depend on the quality of base metal, if has low Sulphure, it no need high Re) Optimum (1.4-2.2%)  Too low or too high  1) Good for nodularity and nodule count                                                                        2) Too expensive (too high Re) 
2 Content of Cerium (Ce) in RE  Optimum (approx. 70% of total Re) Too low or too high  1) Good for nodularity              2) Chunky graphite (if too high) 
3 Content of Lathanum (La) in total Re  Optimum (approx. 30% of total Re) Too low or too high  1) Good for nodule count           2) Less micro shrinkage 
4 Content of Calcium (Ca)  Optimum (1.7-2.2%) Too low or too high  1) Smooth reaction                   2) Too strong reaction (too low Ca)                                       3) Too short reaction time  (too low Ca)                                   4) Cause to dross and inclusion (too high Ca)

 

 
 
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