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Metal Casting Technologies : September 2006
88 BacktoBASICS I Defect Diagnosis in Iron Castings INTRODUCTION t is difficult to imagine a manufacturing process as inherently prone to variation as metal casting. Not only are castings produced from variable materials and production methods, but also the way metals behave on pouring and solidification can vary according to how they are melted and treated. Coupled with this, appropriate corrective actions are sometimes contradictory according to the alloy under consideration, an example of this is shrinkage defects in steel castings compared to graphitic cast irons. Often, the approach taken to defect diagnosis is haphazard and based on a "hit or miss" strategy that more often than not, fails to address the underlying cause. Accurate diagnosis of casting defects and the implementation of appropriate corrective actions can be demanding, but can be minimised if the problem is approached in a systematic manner. Frequently, foundrymen will have a preconceived idea of a defect's cause and set about to prove their opinion. This approach is fraught with danger and often results in wasted time and money while the real cause of the problem goes undetected. It is important to approach defect diagnosis with an open mind and in a systematic manner. A SYSTEMATIC APPROACH A systematic approach to defect diagnosis may include the following steps:  ■ Visual examination ■ Microscopic examination ■ Chemical analysis ■ Occurrence survey Visual examination Visual examination refers to observations made with the naked eye or perhaps with the assistance of a magnifying glass or low powered portable microscope. During the visual examination process, characteristics such as size, shape, location and internal characteristics of the defect are observed. Is the defect round or irregular in shape? Is it located on all surfaces or only on the cope? Is the defect uniformly distributed or confined to core or chill surfaces? Is it located near ingates or remote from them? What are the internal characteristics of the defect? Is the surface smooth or dendritic? Answers to these questions can often distinguish between shrinkage, inclusions and gas defects. Microscopic Examination Microscopic examination will often yield additional information about the defect. It is particularly important to properly prepare the sample for microscopic examination, this includes mounting specimens and following correct polishing techniques. In the case of iron castings, care should be taken not to tear out graphite, as graphite shape and location can be an indicator to a defects cause. Microscopic examination will often reveal exogenous or indigenous inclusions in castings that are not apparent to the naked eye and can be a valuable indicator to a defects origin, such as manganese sulphide inclusions in iron castings. The scanning electron microscope (SEM) and electron microprobe can be valuable in determining the composition of inclusions and characteristics of the metal matrix in the vicinity of a defect. Chemical analysis Chemical analysis will often be of assistance in drawing conclusions on a wide range of defects such as pinholes, blowholes and shrinkage. Chemical analysis will often be used to support microscopic observations. Occurrence Surveys Occurrence surveys are particularly valuable for diagnosing ongoing defects and will often yield valuable information and help identify the condition that favour defect formation. Occurrence surveys can highlight changes in raw materials or practices, whether defects are occurring early or late in the day, from the first or last metal from a ladle. Occurrence surveys can be as narrow as the period of pouring one ladle to as broad as the time of the year i.e. are the defects more prevalent in times of high humidity. For occurrence surveys to be effective it is important to maintain detailed defect records and the use of such tools as cast-on date and time clocks on castings, particularly mass-produced castings is essential. Figs. 1 & 2. The preceding proposed procedure can be used to help diagnose most common defects in iron castings including: J.F. Meredith -- Casting Solutions Pty Ltd Figure 1. Cast-on time clock on high volume iron casting. www.metals.rala.com.au