Simple Solutions That Work! Issue 9

DESIGN PRICIPLES FOR CAST IRON The fundamental difference between iron and other alloys is the expansion that occurs as graphite precipitates during solidification. In most situations, the casting can become “self-feeding” after the onset of expansion and no further feeding is required. The object of designing a feeding system for iron castings is to provide feed metal for the contraction of the liquid alloy as well as the contraction of the solidifying iron prior to the start of expansion; once the expansion begins, a well-designed feeding system should control the expansion pressure to ensure that the casting is self-feeding during the remainder of solidification. This is in contrast to other alloys such as steel, where feed metal must be supplied to the casting during most or all of solidification and there is no expansion involved. Another major difference between cast irons and other alloys has to do with the mechanism involved in “piping”, or the onset of feeding behavior in the feeder. In practice, only one feeder should be used on each “feeding zone” in an iron casting; if multiple feeders are placed on the same zone of a casting, then typically one feeder will begin piping while the other feeders will not. Often, porosity will be seen at the contact point of non-piping feeders. The requirement for a single feeder within a single zone of the casting is probably the design rule which is violated most often in iron foundries. We often see designs where two or more feeders are feeding the same zone within a casting, and the resulting casting exhibits porosity, often at the contact point of one of the feeders. The tendency of many foundry engineers is to add more feeders to try and resolve the porosity issue; in fact, this is exactly the wrong approach and will worsen the situation. To correctly design a feeder system, we must answer the question: Is this casting composed of a single feed zone, or are there multiple zones and, if so, what is the location and size of each zone? To make this determination, we introduce the concept of the Transfer Modulus. Feed zones within the casting are defined by knowing where within the casting it is possible for liquid metal to flow from one point to another in response to expansion pressures. If there is no possibility of metal flowing from one area of the casting to another as expansion begins, then each of these areas forms a separate feed zone and each may require its own correctly-designed feeder (but no more than one). The analysis of a casting begins with consideration of the Casting Modulus. This is defined as the volume:surface area ratio of various areas of the casting, and has been used for many years to estimate the order of solidification of different parts of the casting. The Casting Modulus (Mc) allows us to estimate which part of the casting will solidify first and which will solidify last. In steel castings, this information is immediately useful to indicate where feeders should be placed and what size they should be (the Modulus of the feeder should be greater than the Modulus of the casting). In iron castings, the Casting Modulus is used to estimate when expansion will begin, expressed as a percentage of complete solidification. DAVID C. SCHMIDT Vice President FINITE SOLUTIONS, INC. ARTICLE TAKEAWAYS: • Risers are design to feed initial metal shrinkage • Gates and Contacts should freeze off as graphite expansion begins • One riser per feeding zone – Too many risers CAUSE shrinkage in cast irons RISER DESIGNBASICS FOR CAST IRONS 32