Something Has Been Changed

Art Spengler Lecture No. 1

There are always reasons why things go wrong. Often they are apparent; sometimes the causes are obscured by other things. This is particularly true when dealing with the causes of casting scrap. To find the cause, an investigative procedure must be followed, which is designed to meet the needs of the foundry. 

The production of castings can be a relatively simple procedure, or extremely complicated, depending on the types of castings being produced and the metals from which they are poured. Here are a few simple steps which should always be followed to determine the source of casting defects and their cause.

1. Examine the evidence. Be absolutely sure that the castings are, in fact, defective, and not the product of overzealous quality control. Changed standards/increased inspection.
2. If the castings have been rejected on the basis of metal composition or mechanical properties, be sure that all test are verified. If necessary, have the verification made by a reputable commercial laboratory. This is particularly important if there is any doubt about the validity of test results or there is a dispute with a casting purchaser.
3. If the reason for rejection is dimensional, be sure that a relatively large number of castings have been examined by more than one inspector.
4. When variation in machining allowances are involved, determine if new machining fixtures are being used or a different method layout could have been used. (locating points)
5. The next step is to assess the initial or apparent area of accountability. For example; is the defect the result of:

   a. Molding problems; such as a crush, shift, cope raise, ram-off; swells; sand in the mold and so on.

   b. Does the defect appear to be related to core-making; such as a blow, erosion, broken cores.

   c. Is it a sand control problem; inclusions, washes, porosity, due to wet sand, hot sand, veining, buckles.

   d. Was the defect caused by pouring; was it a mis-run; was the casting poured short, or is the defect a cold-shut due to low pouring temperature.

   e. Another consideration is gating or feeding.

   f. Finally, was the casting defect caused by melting practice or a metallurgical deficiency i.e., Slag, soft or hard casting.

    

6. When the source of the defect is determined, the cause must be found and eliminated. This involves objectively reviewing every detail of the practices and procedures being used in the area of operation, suspected to be responsible for the casting defect. It is always well to remember that a casting defect, which is occurring in all castings produced, is usually associated with molding materials or melting practices. When the defect occurs only in a single casting, the cause can usually be traced to a change in the practice or technique used to product that one casting.

Put the occurrence of casting defects in the proper perspective.

For example, a typical gray iron production foundry will normally generate from 2.00% to 6.00% casting scrap. A jobbing foundry will often operate with scrap rates of up to 15% or more. This of course must be included in casting price.

At the present time, the majority of casting defects in production foundries are associated with molding and pouring. This is estimated to be about 85% of the total castings scrapped. The types of defects involved are:

1. Run-outs.
2. Poured-short.
3. Miss-run or poured-short.
4. Broken mold.
5. Crushes.
6. Sand in the mold; dirt.

These types of scrap are usually associated with people-problems, and to some extent, equipment maintenance. They can be controlled by aggressive supervision that understand casting production. Approximately 15% of casting scrap can normally be associated with:

1. Sand Control
2. Core Making
3. Melting Practices

These areas of foundry operations are often the least understood. When they get out of control, the result can be a major disaster, with the possibility of losing a days production or worse. This is why personnel working in these three phases of foundry operation require close and continuous supervision by knowledgeable supervisors.

A recent survey shows that there are 211 different categories of ferrous casting scrap. Of these, 42 are attributable to melting practices and melting materials. There are 126 types associated with core making and sand control. The balance of 83 categories of casting defects, are caused by molding, pouring, and cleaning. These are a few examples of what can happen when management does not give proper consideration to changes which occur in the overall scheme of operations.

Look at the following case histories to see what not to do.

• An example of improper definition of scrap was in a large production foundry. This foundry was owned by a large corporation, which utilized all of the latest and most modern management techniques such as computerized cost control, profit centers. And delegation of authority. In addition, the parent company promoted very hard and competitive interdepartmental management policies. This foundry had an on-going casting scrap level ranging from 8% to 22%. The alleged cause of scrap was attributed largely to the metal. This was done in spite of the fact that the iron produced always met both tensile properties and chemical composition requirements. The foundry had an excellent laboratory, which was used very effectively to control the melting operations.
  
  In spite of this, metallurgists and melting superintendents were hired and fired about once a year. Scrap meetings were a farce, gating problems, gas porosity caused by sand and cores, molding problems, venting, and so on were always attributed to metal by the collective management. Unfortunately, no matter how many metallurgists, and melting superintendents were hired, the scrap was not reduced. One day, someone in corporate management hired a foundry manager who was a foundryman and was able to put the scrap problem in its proper place. Now scrap levels are down to 3%. Another similar situation where no action was taken, that foundry is now closed. It is interesting to note that a recent survey shows that there are 43 types of ferrous casting scrap caused by melting practice and melting materials and approximately 152 kinds of casting scrap can be attributed to other causes.
  
• Blind emphasis on melting material costs alone, without consideration for quality, can result in disaster. There is a large gray iron foundry located in the east. This foundry produced castings for a very cost-conscious industry where the mechanical properties of the gray iron produced are not considered important. There was no metallurgist. They did have a melting superintendent who operated a series of large electric furnaces. Laboratory facilities were adequate for controlling the melting operation and determining the mechanical of iron produced.
  
  This foundry suddenly developed an ongoing scrap level of 20+% and was in the process of going broke. The causes of casting scrap were pinhole porosity, miss-runs, and low tensile properties (below the minimum of 25,000 psi). The problem was transient.

1. Some days there was no scrap.
2. Some days there was 60% scrap due to pinhole porosity.
3. Some days tensile properties were below 25,000 psi with pinholes.
4. Some days tensile properties were low and there were no pinholes.
5. There were also occasions when there were many castings scrapped because of mis-runs and cold-shuts.

The local management and a number of consultants attributed scrap to these causes:

1. The pinholes were attributed to wet sand, nitrogen from furan cores and improper venting. Efforts were made to improve sand control and Ferro-titanium was added to control the nitrogen problem. These efforts were unsuccessful.
2. The cause of low tensile strengths below 25,000 psi was attributed to a need for an alloy addition. An addition of copper resulted in no improvement. Addition of 0.3% chromium also did not improve the mechanical properties.
3. Unfortunately none of the conventional solutions worked. The problem was caused by the melting materials used. This foundry uses a furnace charge consisting of foundry returns and 50% cast iron scrap. An examination of the cast scrap used in the furnace charge revealed the presence of large quantities of cast iron fittings with leaded joints. There were also large quantities of aluminum-zinc die casting scrap mixed with the cast iron. In fact, streams of metal were seen running out of the pre-heaters. Layers from 6 in. to 12 in. of mixed aluminum, lead, and zinc, were found in the pit under the pre-heaters. Even though a large quantity of the non-ferrous metals melted off of the cast scrap in the pre-heater, a considerable amount remained in the scrap and was mixed into the cast iron. The result was that low tensile properties were caused by the presence of leas in the iron, which resulted in the formation of mesh graphite. Further investigation revealed that when castings contained 0.03% to 0.07% aluminum, pinholes occurred. As the aluminum content of the iron increased from 0.10% to 1.0%, the pinholes disappeared, but incidence of mis-runs and cold-shuts in the castings drastically increased. Shortly after the real casting scrap was revealed, a dependable and more expensive source of cast scrap was immediately found. Today scrap in this foundry is below 3%.

• The next example demonstrates how a commercial testing laboratory can cause a small steel foundry to scrap 20% of stainless steel produced. The foundry produces very precise shell-molded stainless steel castings used in extremely critical applications. The specifications are such that the mechanical properties of each heat required certification. About 25% of the tensile bars from so-called defective heats revealed that they were bent and apparently had been pulled in an unaligned position. This foundry is now using another testing laboratory and there has only been one heat of stainless steel scrapped since December 1976. This error, made by an inexperienced technician, cost the small foundry many thousands of dollars.
  
• There are many gray iron foundries that have intermittent and unexplainable chill control problems. Here is an example of what can happen: In this particular circumstance, there was a foundry producing thin section castings. The melting department was responsible for melting up to the tip of the furnace spout. The molding department assumed responsibility for inoculation of iron and pouring. No one paid any attention to pouring and inoculation. Casting scrap due chilled edges was over 20%. The iron pourer threw the inoculant on the foundry floor and not in the pouring ladles. There was at least a foot of inoculant on the foundry floor in from of the melting units. The foundry was sold, and the new management has corrected the problem. It is amazing how often things like this happen.
  
• When the demand for metal in a ductile iron foundry over-runs the treating capacity, here is an example of what happens. The castings contained unreacted treatment alloy (5% magnesium ferrosilicon alloy). The problem was solved by installing another treating station and using a treatment ladle.
  
• This is what can happen when an overzealous engineer takes control: One man in a three-man holding crew on a large cope and drag unit was eliminated. Castings scrap due to crushes increased from 1% to 25% in one day. Later, the installation of a mold-closing unit reduced manpower requirements to one man.
  
• Here is an example of a change which caused a serious scrap problem: In steel foundries, nails are often used in cope and side-walls of large molds. The ails act as chills and reinforce the mold. A cost cutting representative from consulting firm said they were not necessary. The result was the loss of three days casting production. The moral is to try one and see what happens before changing everything.

Now For Some Good News

•  There was a relatively small gray and ductile iron foundry which has been very successful and produced high quality castings. In fact, premium prices are paid for their castings because of the exceptionally high quality. This is particularly true with regard to dimensional tolerance, consistency of mechanical properties and chemical composition. The casting scrap in this jobbing foundry ranges from 2% to 6% for all causes. Castings scrap related to metallurgy and metallurgy practice is less that 1% of total scrap.

How is this done?

1. Competent experienced personnel are employed.
2. The causes of casting scrap are understood, recognized and controlled.
3. Well-defined melting practices have been established for each type of metal produced.
4. A small, but adequate, laboratory is used to control the melting practice.
5. First quality melting materials are always used.  After all, metal cost are only 15% of the casting costs.  So why take a risk for a very small saving?
6. Furnace charges are calculated.
7. Records of furnace charges, resulting chemical compositions and mechanical properties are recorded and analyzed on a daily basis for trends and variations.

This is what putting casting scrap caused by melting practice in proper perspective means.  Metallurgical control of melting practices, which results in low scrap levels, can be achieved.  Simply use good judgment and understanding in both large and small foundries.  It also applies to all of the other phases of casting production.

Here are some of the more important, but often overlooked, causes of gray and ductile iron casting defects.

Gray iron is an iron-carbon-silicon, which contains an aggregate of graphite flakes.  In order for the flakes to form, it is necessary for some types of nuclei to be present in the molten iron.  These nuclei can come from many sources.  Nuclei become smaller with time and temperature until they are not large enough to initiate a crystal growth, they are only an embryo.  Nuclei can come from many sources:

1. Small particles of graphite from pig iron or cast scrap if the section size is large enough and cools slow enough to precipitate and grow a crystal of hexagonal structure large enough to pull carbon from the liquid iron and continue to grow.
2. Crushed graphite electrodes to nucleate graphitic carbon from the liquie iron.
3. The addition of inoculants.

If the molten iron does not have hexagonal nuclei present, graphite will form too late and the result will be carbidic iron and porosity. 

Another factor, which must be considered simultaneously, is the fact that when gray iron solidifies, it expands in volume because graphite has 3.54 times the volume of iron. If fact, recent studies have shown an expansion of about 3% and a high cell count. What does this mean to a foundry man? It means that the furnace charge materials and inoculating practices drastically influence casting yield. This is true, both in electric furnace and cupola melting.