(A Practical Low Cost Method)

by Jack Oakey
L&N Metallurgical Products Company
Ellwood City, PA

Introduction
In today's world, businesses are being driven by their customers, owners, and competitors to increase production and decrease operating costs while producing products of ever-increasing quality. The producer of ductile iron castings is no exception to these pressures. In the midst of these demands from all directions, the ductile iron foundry operator must be particularly concerned that the proper degree of nodularity is achieved for each and every part that is produced and sent out to a customer without sacrificing production rate and increasing the costs of the operation.

All of you are familiar with the potential dire and catastrophic consequences that the production of improperly processed products can have on any particular foundry.

First off, there is the question of are you even aware of having produced bad material. If you are aware of it, there is always the uncertainty that all of the defective parts have been properly identified and segregated. Of course, if any of the bad material reaches the marketplace, there are always the potential problems of costly recalls, or far worse, liability lawsuits that could possibly put the company out of business.

Commonly Used Methods for Nodularity Testing
Currently, the most common methods used by ductile iron foundries for the determination of nodularity are microexamination and emission spectrometry.  Both of these methods leave something to be desired in everyday foundry use.


Microexamination
The problems of using microexamination to determine the degree of nodularity for cast parts are many. First off, the technique is very slow and only provides information after the fact, well after the metal has been cast.  Because of the delay in obtaining results, bad castings can get into the downstream system and this triggers a "scramble" to segregate parts. This occurrence results in loss of production and potential increased scrap rates. Further, the microexamination technique can be greatly influenced by the subjectivity of the individual performing the examination. It is only reliable when the personnel are well trained and consistently utilize very good technique. The learning curve is very high and can be quite costly to perfect.

Spectrometry
The use of an emission spectrometer provides compositional information faster than the microexamination method. However, spectrometry still requires sample preparation time and the results also become available after the fact or production delays are taken while awaiting the results. In addition, the equipment required is quite expensive and must be properly maintained.

Low Cost Thermal Analysis Technique
The remainder of the presentation will focus on a patented low cost thermal analysis technique that is being marketed by L&N Metallurgical Products.  This system can tell the operator if the magnesium content of the treated
iron is above an established threshold level. The system consists of specially prepared disposable thermal analysis cups, referred to as MgCUPs, and an instrument, MgLAB, that provides the pass-fail information through analysis of the cooling curves.

How System Works
It is known that the use of tellurium in a thermal analysis cup provides a carbidic solidification structure and an analysis of the resulting cooling curve can be used to determine the carbon and silicon composition of the metal. An example of the cooling curve for this situation is shown in Figure 1. However, in the presence of magnesium, tellurium can not prevent the iron from solidifying with a graphitic eutectic structure. The cooling curve for this situation is shown in Figure 2.  Further, if sufficient sulfur is present in the cup to neutralize completely any magnesium that may be present in the iron, the tellurium again becomes effective in achieving carbidic eutectic solidification. With the magnesium neutralized, the metal again acts as if no magnesium were present and the resulting cooling curve is as shown in Figure1.

The MgCUP/MgLAB system utilizes the above-described changes in cooling curve behavior to provide PASS/FAIL information in real time to the operator on the foundry floor.

The process starts with a specially prepared thermal analysis cup that contains not only tellurium, but a specific amount of sulfur that is consistent with the lowest acceptable, or threshold, level of magnesium for the particular foundry under consideration. Following the addition of magnesium to the iron, a MgCUP is poured and the MgLAB instrument analyzes the resulting cooling curve to determine if sufficient magnesium is present. It is recommended that the temperature of the iron that is poured into the MgCUP be greater than 2390^oF so that good cooling conditions are obtained in the cup before any measurements are made.

The MgLAB analyzes the cooling progress of the poured sample and looks at the time it takes for the sample to cool from one predetermined temperature level to another temperature level. When the temperature falls to 2138^oF the timer is activated. When the temperature reaches 2075^oF the timer is halted and the elapsed time is evaluated. A relatively long cooling time, greater than a minute, indicates graphitic eutectic solidification. A relatively short cooling time, less than a minute, indicates either carbidic eutectic solidification or a mixture of carbidic and graphitic eutectic solidification that produces a "mottled" structure.

Figure 3 illustrates the various curves that can result with the use of the MgCUP. The first, 3a, shows a graphitic eutectic and indicates that there is sufficient magnesium present, over and above that required to react with
the sulfur addition. As can be seen, the time elapsed as the sample cools through the two temperatures of interest is greater than one minute. The result indicated by the MgLAB instrument is PASS. Figure 3b illustrates the
case of a "mottled" structure and figure 3c is an example of a carbidic structure. The elapsed time between the two temperatures of interest for both of these cases is less than one minute. For both of these situations there is insufficient magnesium in the iron to meet the threshold level and the MgLAB would indicate FAIL.

The MgLAB instrument can also be connected to remote devices to show results to the operating floor and to computers for data storage and archiving purposes. A printer can be hooked up to the MgLab for a hard copy record of test results.

Through experience, different foundries have found that different levels of magnesium are required as the target threshold levels required to achieve satisfactory nodularity. This thermal analysis system can be readily adapted to accommodate various threshold levels of magnesium through adjustment of the amount of sulfur that is incorporated into each thermal analysis MgCUP. Hence, the system can be tailored for each individual foundry.

Advantages of Thermal Analysis System
By using the MgCUP/MgLAB system on a routine everyday basis, the foundry operator can bring about a number of advantages to the operation.

The test can be performed very quickly and requires only two minutes from start of test to the PASS/FAIL indication from the MgLAB instrument. As a result, no bad iron castings should get into the system. If the results from the MgCUP indicate FAIL, the iron can be pigged immediately or, time permitting, further magnesium additions can be made. Either way, improperly treated material will not be cast. There will be no stoppages of production and defective parts will not have to be retrieved from the downstream system.

Test subjectivity and the human element is essentially eliminated from the test results. Further, no operator training is needed to obtain the results. Pouring a MgCUP is no different than pouring a standard thermal analysis cup. The MgLAB instrument then takes over, interprets the information and provides the PASS/FAIL results without human interpretation.

Summary
To summarize, the L&N Metallurgical Products MgCUP/MgLAB system offers a low cost, rapid method to determine if sufficient magnesium has been added to the base metal.

The system requires no training on the part of the operator and human intervention or interpretation does not enter into the process.

The utilization of the system can lead to a number of advantages for the foundry operator that relate to quality of product, yield, and production rate.

However, the system may not be applicable or cost effective for all foundries. Current practices and experiences, as well as past history, may indicate that this thermal analysis approach is not of interest or cost beneficial for your foundry.

 
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