Customer Services Manager, Elkem AS, Foundry Products Division
The objective of this paper is to provide an overview of some of the most common metallurgical defects found in the production of ductile cast iron today. The examples shown have all been determined during the examination of samples in Elkem’s Research facility in Norway. Whilst many foundries recognise the defects, an appreciation of the possible causes, and therefore cures, is not always apparent. The causes and cures for the different problems are examined in the paper. Emphasis is made on shrinkage problems, probably the most common problem seen by Elkem’s team of service engineers around the world.
Metallurgical defects in ductile iron can be very costly to the foundry, not only because the part has to be remade or rectified, but due to the unfortunate fact that many defects are not revealed until after the expensive machining stage. Care in the selection of raw materials, good process control in the melting stage and proper metal handling procedures will go a long way to the prevention of defects. Further, a routine for logging and recording of defect occurrences will reveal which are the major problem areas, allowing for a systematic elimination of the defects. This paper will examine the most common defects, starting with shrinkage. Deterioration of affordable steel scrap qualities, use of incorrect inoculants and nodularisers plus the pressures to get castings out of the door as fast as possible has led to an increase in the incidences of shrink/porosity related cases seen by Elkem’s team of technical service engineers. Indeed, the ductile iron foundry, which truthfully claims not to have shrinkage concerns is the exception to the rule.
Other common defects may be divided into two basic categories:
- Those related to nodule shape and size, such as compacted graphite structures, exploded and chunky graphite,
graphite floatation, spiky graphite and nodule alignment.
- Those related to inclusions/abnormalities within the matrix, such as flake graphite surfaces, slag inclusions,
carbides and gas. These problem areas are described to aid recognition of the defect and causes are discussed
together with possible cures.
There are many causes of shrinkage in ductile iron, experience globally has shown that about 50% of shrinkage defects are related to sand systems, feeding and gating. The other 50% may be attributed to metallurgical factors such as carbon equivalent, temperature, inoculation or high magnesium residuals.
Figure 1: Typical sub-surface shrinkage defect with dendrite arms partly covered with graphite sticking out.
When a shrink or porosity is detected in a casting, there are several immediate and simple steps that can be taken to identify the cause of the problem.
Firstly, the geometry of the casting should be examined to determine whether the location of the defect is close to a sharp radius or a potential hot spot. At the same time, the sand in the region of the shrink should be examined to look for any soft spots. Sand integrity accounts for a high proportion of shrinkage defects and a worn seal on the moulding machine, for example, resulting in a lower sand compaction can often be the cause of an unexplained sudden outbreak of shrinkage.
The second avenue of investigation should be the gating / runner designs and the feeding of the casting. Whilst many foundries have computer aided design systems, patterns are often altered slightly over the years at shop floor level and can be significantly different from the original design. Also, changes to the feeder specification can lead to different burn characteristics and metal solidification patterns. This can affect the amounts of feed metal available to different parts of the casting.
Metallurgically, there are many factors that can affect the shrinkage tendency.
Figure 2: Effect of magnesium ******* on shrinkage
Magnesium, apart from being one of the most powerful carbide stabilizers, has a marked effect on the shrinkage tendency of ductile irons. Foundries operating at the higher end of the magnesium range, 0.05% or above, will find that the iron is more prone to shrink than foundries operating at lower, but very acceptable, levels, say 0.035-0.04%. Both under-inoculation and over-inoculation can cause shrinkage. In the case of under- inoculation, not enough dissolved carbon is precipitated as graphite. Graphite nodules have a far lower density than the matrix and to precipitate the low density, high volume graphite has an overall expansion effect, which helps to counter the natural tendency of the iron to shrink. With over-inoculation, too many nucleation points are active early in the solidification, resulting in an early expansion and sometimes large mould wall movements. Later in the solidification, when feeders become inactive and contraction takes place, there is no graphite coming out from solution to counteract the contraction and the result is shrinkage between the eutectic cells. In many foundries, the microstructure shows even sized nodules (accounting for the fact that the section cuts through nodules in 2-dimensions). Many foundry men still consider this to be a good structure, even though the iron is prone to shrinkage. Nodularizers and specialist inoculants are available these days, which help to counter shrinkage by giving a skewed nodule distribution.
Figure 3: The same base iron treated with two different nodularisers resulting in a) Skewed nodule distribution b) Unskewed nodule distribution
A skewed nodule distribution indicates that some nodules are being created late in the solidification process and the drawing of graphite from solution at this stage is a very effective way to counter shrink. Most inoculants act almost instantaneously and this gives the even nodule size effect. Once the potency of the inoculant has gone, then there is no driver to create nodules late in the solidification and shrinkage can be the result. More recently, nodularisers have been developed by Elkem that have the same effect of producing the skewed and shrink reducing nodule distribution curve.
A low carbon equivalent, or metal that has been held for some time at temperature, due to a mechanical breakdown, for example, is also prone to shrinkage. In these cases, the inherent nuclei within the melt will be low and some preconditioning may be necessary to achieve a good level of nucleation.
within the structure. There are several causes of this, the most common being that the nodularisation process has partly failed. Incorrect weighing of the nodulariser or the use of the wrong nodulariser are possible reasons for the failure, although a long holding time in the ladle or excessive temperatures can be contributory factors.
Figure 4: Sample with compacted graphite present in the matrix due to partly failed
Another cause of CG particles in the matrix is an incorrect sulphur level in the base iron. Many foundries melt both grey and ductile charges and segregation of returns is essential. During the nodularisation process, the first reactions that take place are a desulphurisation and deoxidation, these elements combining preferentially with the magnesium. The base sulphur level must be accounted for in the calculation of MgFeSi charge weight. A note of caution here with regard to the addition of the MgFeSi to the ladle or treatment vessel. To add the MgFeSi early to a hot ladle and then hold the ladle for several minutes until the moulding line calls for metal is bad practise as the alloy will be burning or oxidising in the bottom of the ladle during this time. Higher and more consistent recoveries can easily be achieved by adding the alloy just before tap from the furnace.
As the compacted graphite mentioned above may commonly be attributed to the nodulariser, then low nodule counts tend to be a function of the inoculant. Figure 5 shows a low count compared to the foundry’s normal practise. Avoiding long holding times in the furnace and prolonged pouring time post-inoculation will help to achieve consistent nodule counts, as will improving the responsiveness of the iron via preconditioning. The use of a specialist powerful inoculant will give the most consistent results.