Alloy Steel vs Carbon Steel: What to Select for Your Investment Cast Products
Foundry experts will tell you alloy steels are more challenging to cast than carbon steels, but this shouldn’t influence your choice of metal. In fact, investment casting alloy steel yields bigger benefits than using the same process for carbon steel. If this seems like a contradiction, the explanation lies in understanding the benefits of investment casting.
The Difference Between Alloy and Carbon Steel
Steel is iron containing between 0.0002 and 2.1% carbon. Such carbon steels carry the 10XX designation under the SAE classification system, where XX represents the carbon percentage. These steels have good strength and ductility, making them suitable for a range of applications. In addition, steel with a carbon content of around 0.2% has good machinability too.
Alloy steels are those where additional elements are incorporated. Manganese, nickel, silicon and chromium are some of the most common. When the proportion of alloying elements is between 5 and 20% of the total the resulting high alloy steel is considered a tool steel.
Unlike carbon steels, alloy steels are readily hardened and/or toughened by heat treatment. However, they are more challenging to machine and to cast, with specifics depending on the actual alloy composition.
Benefits of Investment Casting
Investment casting works well for steel parts. This is because the ceramic molds handle the high temperatures involved, and the process yields near net shape geometries that need little final machining.
Molds are made by coating a wax pattern with a ceramic slurry. This is built up in layers, and layer thickness is used as a way to control solidification rate. Once the slurry is dry, the wax is melted out to leave the cavity. The ceramic shell that remains may be heated to reduce its chilling effect on molten metal as it’s poured in.
Investment casting is capable of producing fine detail and thin sections. Surface finish is a reflection of the original wax pattern with the addition of granularity from the slurry. Using a very fine slurry for the first layer helps create a smooth finish on the cast part.
Geometric accuracy is very good. Investment casting can typically maintain tolerances of better than 0.005” per linear inch.
Minimize Machining With Investment Casting
High accuracy, inclusion of fine detail, and excellent surface finish, mean parts may be cast very close to the final dimensions. In addition, draft angles aren’t necessary, unlike in sand casting. Together, these minimize the amount of machining needed to create mounting surfaces, holes and bores.
Alloy or Carbon Steel for Investment Casting?
Foundry experts report that alloy steels are usually a little more difficult to cast than carbon steels. This is due to the properties of the alloying elements and how they behave when molten. However, the impact is small and should not be a reason for compromising on cast part performance. If a design calls for alloy steel, (usually for reasons of strength or the ability to raise hardness by heat treatment,) then that’s the metal to use.
The bigger issue is the ability to minimize machining through investment casting. Cutting the amount of machining needed can save a lot of time, tool expense and scrap.
Depending on composition, alloy steels can pose significant machining challenges. Some high alloy steels can only be machined by slow and costly processes like EDM and grinding. A near net shape process like investment casting reduces the need for machining, and so can save time and money.
Ask Impro About Saving Time and Money With Investment Casting
The investment casting process is a good choice for producing complex parts in both ferrous and nonferrous metals. It’s particularly beneficial with ferrous metals as the ceramic molds can tolerate very high temperatures. Furthermore, as it’s capable of producing near net shape forms it’s a way of minimizing secondary machining.
Both carbon and alloy steels are good candidates for investment casting. Carbon steel grades typically have slightly better castability, but alloy steels realize greater benefits as they present greater machining challenges. For customers who need the material properties of alloy steels, investment casting reduces the amount of machining needed. For more information, contact us to schedule a discussion with a casting specialist.