[MOD NOTE: the following comment was originally posted by member Patrick Nowak. The original was split into its own thread, but its text is copied back here because of its relevance to this discussion.]
I'm going to jump in on this thread because there are several things that have been shared above which are not metallurgically correct. My apologies to the original poster as I know this material goes well beyond the question that was originally raised, but first I will address those questions.
The original poster asked how to tell the difference between ductile and grey cast iron. Ductile iron will ring much like steel when struck while grey cast iron tends not to do that. that is not an absolutely definitive method as you could still have a steel part. A spark test can also be useful in making this determination. None of the various forms of cast iron discussed below is typically forged either hot or cold. The entire reason for casting the part to begin with to get the shape you want. Ductile iron could be forged, though that is quite rare. If you want to try it you need to heat the metal to a very dull red and use very light blows. Due to the high carbon content, the melting temperature is much lower than that of steel so it is easy to over heat. Additionally, the microstructures are not really suited to forging. If I had something I new was ductile iron I would probably try to adjust it without heat it.
Now to address the incorrect information:
1. Cast Steel Tools: Tools Stamped "CAST STEEL" were made from steel which was first cast into ingots and then forged or rolled to shape. This started well BEFORE the Bessemer process was introduced. Bessemer (and William Kelly in the US) both invented essentially the same process in the mid-1800s, but that was NOT the first time liquid steel as used (See The History of Metals in America by Simcoe). The process was commercialized (but not invented) by Benjimen Huntsman in about 1750. There are at least two documented references to the same process that predate 1700. See work by Cyril Stanly Smith. The use of steel made by the cast steel process for tools was common and these were stamped CAST STEEL to separate them from tools made from Blister or Shear steels. I am not aware that Bessmer steels were used for these applications. Note that I have in my own collection plane blades which are stamped "CAST STEEL" and which I have examined metallographically. These are wrought iron blades with very small amounts of steel forge welded to them. They were made by Ohio Tool which went out of business in 1920.
2. Forgings vs. Castings: Before dealing with this question please know that I am a degreed metallurgical Engineer with 20 years of experience, 18 of which have been in the open die forging industry. The terms that have been used so far in this thread are not metallurgically precise and therefore there are some misunderstandings. The "strength" of a metal in metallurgical terms is usually one of several specific types of strength: Ultimate tensile strength, yield strength or impact strength. Assuming we are comparing a casting and a forging of the same steel alloy, size and shape they could have the exact same ultimate tensile strength and yield strength because these properties are a function of heat treatment, not manufacturing method. Where forgings are superior to castings is in impact strength and fatigue properties. These properties are a function of several different variables including heat treatment, grains SIZE, and grain flow. In carbon and low alloy steels the GRAIN SIZE is a function of heat treatment, so I can make a casting with the same grain size as that in forging if I choose to do so. What I cannot do with a casting is create GRAIN FLOW around corners, shoulders or other features which I can do in a forging. The grain flow is due to small micro inclusions and non-metallic particles being elongated during rolling or forging and then being bent around features during final hot shaping. This gives forgings characteristics somewhat similar to wood. When you split a log you always put the wedges in the end and try to separate the wood at the boundary of one growth ring and another. When done correctly the wood splits easily. But if you drive a wedge into the side of the log such that the force is applied parallel to the length of the log it will not split. Likewise if you are trying to split around a knot or series of knots you will have difficultly splitting the wood. Forgings are just like that. loaded in one direction they will have greater toughness than loaded at 90 degrees to that direction. Castings tend to have uniform properties no matter how they are loaded.
3. Steel castings: Steel castings were not widely used prior to the implementation of the Bessemer process. Though liquid steel was available prior to that point, it was made is small batches, typically of less than 100 lbs and was generally used for tool steels and other specialty applications. There are records of large ingots, multiple thousands of pounds in fact, being cast in both Sheffield and Germany, but not until the mid-to later 1800s. (See Sheffield Steel by K.C. Barraclaugh and Steel, Iron and Cast Iron Before Bessemer by Buchwald). Wide spread use of steel castings (that is solidified in the near final shape) was not occurring until the early 1830s in the US and the 1850s in Germany and England (see History Cast in Metal). The reason for this is because the molding material used for steel has to withstand much higher temperatures that the green sand used for cast iron. It took quite a while for steel workers to figure this out and identify the correct mold material. The melting temperature of cast iron is about 2100 F while that of pure iron is about 2800 F. The cast steel being made by the Huntsman process was usually poured into cast iron ingot molds so there was no concern about reaction between the liquid metal and mold material as there is when making a near-net-shaped casting using sand molding techniques.
4. Cast Irons: There are 4 types of cast iron: White Cast iron, Grey Cast Iron, Malleable Iron and Ductile Iron. All cast irons have much higher carbon than steels, usually well in excess of 2%, often approaching 3-4%. In white cast iron the carbon is mostly in the form of iron carbide, making the metal extremely hard, brittle and wear resistant. this was the earliest form of cast iron, likely because the alloying elements needed to promote the formation of graphite flakes (primarily silicon) was not present in early cast irons. Grey cast iron is a much more recent development. In this form, there is much less iron carbide and much more graphite. The graphite is in a flake like form. Because graphite is so soft, this form of cast iron can be considered "pre cracked" since the flakes act just the same as if they were internal fractures. This is why gray cast iron cookware can be so fragile. Malleable cast iron is made from white cast iron which is subjected to very long heat treatments that force the iron carbide to transform to graphite having a spheroidal form. this eliminates the brittleness of white cast iron and grey cast iron giving product which is actually quite tough. However, due to the very long thermal cycles needed to achieve these properties, this material has been replaced by a modern alternative-Ductile cast iron. this material was developed around 1950 and is dependent on the addition of magnesium to the liquid metal just prior to pouring it into the mold. When done correctly this forces the graphite to take on a spheroidal form which results in very tough, crack resistant material. When combined with the proper heat treatment, this material can have some properties similar to that of forgings.
5. Dislocations: A dislocation is not the same thing as a grain or a refined grain. A dislocation is a disruption in the crystal lattice at the atomic level. It is caused by any plastic deformation. During hot forging, dislocations form, but they go away quickly because they are not thermodynamically stable. They are removed by the nucleation of new grains (which are austentite grains by the way). Dislocations are only an issue (for good or bad) in parts which have been deformed at a low enough temperature that they remain in the structure. If you are not sure what a dislocation is, what it looks like or how it behaves, look up the Bubble demonstration by Bragg on you tube. It is from the 1950s and does an fantastic job of providing a visual demonstration of this topic.
6. Control of Grain size: When we talk about grain size we are really talking about the grain size of the austenite prior to cooling to room temperature. Actual grain size is a function of the temperature reached (it really doesn't have lot to do with forging). The hotter you heat the metal, the larger the grains will grow. In modern steel making, alloying elements are used to prevent grains from growing very large as long as the temperatures are below about 1750 F. This is true for castings and forgings.
7. As cast structures: When dealing with large cross sections, such as the big ingots I deal with (some on the order of 5 feet in diameter and weighing 100,000#) the as cast structures will be very different from the forged structures because the solidification time is so long. In these cases, you can indeed have as-cast grains or crystals which are very large and which are broken down by the forging or rolling process. But if we are talking about the fairly small casting the original poster asked about, there is no reason to think the grains would have to be larger than those in an item made by forging.
8. Info from the Milwaukee Forge website: While that information is factually correct it is not complete. they have intentionally selected pieces of data that promote forging over casting as a method of manufacture. they have not included anything about the grades, section sizes, types of forgings vs. casting etc. While I am 100% in favor of forgings (that's been my business for the last 18 years) those statements are extremely broad and are not qualified in any way. That makes them less than reliable in my book. Castings are often a fine way of making something. It really depends on the needs of the application. In general, forged or rolled metals will have better fatigue life, impact toughness (in certain directions) and ductility (in certain directions) than castings but all those things are highly dependent on a wide array of variables.