Magnesium is not an easy element to add to a cast iron melt, as it has lower density than iron, limited solubility in cast iron, a boiling point below the freezing point of cast iron and a vapor pressure above 1 atmosphere (as seen in Fig. 2) for temperatures relevant for production of ductile cast iron. To overcome these disadvantages of Mg, various methods for the addition of Mg have been developed and tested over the years.
It all started as a simple ladle treatment because that was state of the art at the time. Cast iron production at that time was grey cast iron with iron coming from acid slag cupolas with S-levels of around 0.1 to 0.14wt%. To obtain ductile iron it was early recognized that the S-level should ide- ally be lower than 0.04wt% prior to treatment and below 0.02wt% after treatment1
to consistently achieve good duc-
tile iron. In order to achieve this, high amounts of magne- sium had to be added to the melt or a pretreatment/desul- furizing step to take the S-level down had to be applied prior to treatment. For the ladle treatments master alloys like NiMg and MgFeSi were used with Mg-levels in the range of 10 to 40 wt%.3,4,5
Due to the high sulphur level in the base iron, the devel- opment of the treatment process focused on ways to in- troduce high amounts of Mg to combine the two steps of pretreatment/desulfurizing and treatment in one step. Re- actions were violent with low Mg recovery and heavy slag generation. Slag defects were also dominating as a primary reason for scrap. Of the processes aiming to introduce high amounts of Mg to desulfurize and nodulize in one step, the pure Mg-converters and cored wire have survived and can still be seen today.
Development of the basic slag cupola made it possible to melt a low S base iron. The melt rate was choked down due to the heavy slag load in the furnace and the extra coke re- quired to melt it. The need at many locations to return to higher production rates eventually led to the development of the continuous desulfurization process. Nitrogen gas stir- ring via porous plugs was developed. Calcium carbide, and eventually lime-fluorspar desulfurizing reagents were used.
Simultaneously, induction melting and holding furnaces were becoming more cost effective and provided a flexible melting arrangement very suitable for ductile iron produc- tion. Access to charge materials with low sulphur levels also made it possible to make base iron for ductile iron produc- tion with S-level below 0.02wt%. Today 0.01wt% base S is common with foundries being cautious to avoid S levels well below this level due to nucleation issues and a propen- sity for iron carbide formation.
As the production and demand of ductile iron increased, the need to find consistent and cost efficient production process- es continued. Two directions have evolved to accommodate this need. One focuses on making a cost effective treatment process by introducing low amounts of pure Mg or high Mg containing master alloys. The other direction looked at de- veloping a cost efficient way through introducing dilute Mg alloys to get a calm reaction and reduce the negative impact of high residual Mg content on shrinkage tendency and car- bide formation. Treatment methods focused on minimizing the addition of Mg such as ladle and inmould treatments.
Since the start of ductile iron production, the simple ladle treatment has undergone various developments. All of them aim to improve the yield and consistency of the treatment and thus favor adding as little Mg as possible.6,7
This started
with the master alloy added to the bottom of the ladle and iron poured over. Recovery improvements were made with the addition of alloy pockets to contain the MgFeSi alloy and with cover materials to delay the start of the Mg reaction un-
a) Grey Cast Iron
b) Ductile Cast Iron
Figure 1. Principle overview of the production steps for grey versus ductile cast iron production.
8
Figure 2. Relationship between temperature and vapor pressure of pure magnesium.4
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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