The present invention relates to a method for the production of ductile cast iron, comprising controlling and correcting the composition and physical properties of the cast iron melt. More particularly, the method comprises the steps of determining the inherent structural characteristics of the melt by thermal melt analysis and making corrections that are necessary.
Thermal analysis has long been applied for determining the characteristics of metal melts. The thermal analysis method allows a sample of the melt to be solidified in a suitable vessel provided with one or more thermocouples by means of which the temperatures which prevail as the sample solidifies are recorded. The precipitation of different phases and phase transformations can be seen from the temperature-time curves.
In the production of ductile iron, it is essential that the amount of structure modifying agent present is sufficient to provide nodular graphite precipitation. There must, of course, also be a sufficient amount of nucleating agent present.
U.S. Pat. No. 4,667,725 teaches a method of determining the structure characteristics of a melt which will permit determination of the metallographic structure which develops during the solidification of a metal melt. The method comprises extracting a sample from the melt and permitting the melt to solidify in a sample vessel provided with two thermosensors, one of which is placed in the centre of the sample volume and the other of which is placed in the vicinity of the inner wall of the sample vessel. During the process of solidification, the various temperatures in time are recorded through both thermosensors. From these recordings, values of miniumum and maximum points, duration of level temperatures, shoulders, derivatives, etc. are used to predict the structure characteristics that will be obtained. The method according to U.S. Pat. No. 4,667,725 is mainly directed to the production of compacted cast iron, which has been a challenging task for the person skilled in this art. However, this technique can also be used to predict the structure characteristics of a melt intended for casting ductile cast iron. A typical melt for casting ductile cast iron has a C.E. value of 4.4-4.6, i.e. a hypereutectic cast iron. The method will only disclose that nodular graphite solidification will be obtained. No information relating to excessive amounts of structure modifying agent can be obtained. This means that even if the amount of modifying agent is sufficient to obtain a good cast product at the moment of extracting the sample, the modifying agent content may have decreased, by fading or burning-off, during the process of casting and thus the moulds that are filled last may possibly receive a cast iron melt which contains insufficient amounts of modifying agent.
Fading is contingent on the actual process applied and the particular production equipment used. For example, in one particular foundry, fading was established to be 0.003% Mg with each 5 minute period. The casting period was about 15 minutes in this case. In other words, the modifying agent content (in this case the magnesium content) was found to decrease by a total of 0.009% during the whole casting period.
The above-mentioned problem has forced the foundryworker to use a relatively high excess of modifying agent, so as to guarantee a completely nodular graphite cast iron throughout each production run. The use of an excess quantity of modifying agent, such as magnesium, has the following drawbacks:
1. It makes the process more expensive; PA1 2. It increases the tendency to produce residual carbide formations; and PA1 3. It may form unwanted oxide inclusions in the castings.
There is an apparent need for a method which will allow the addition of modifying agent, such as magnesium and corresponding metals to be decreased.