Foundry slag is the non-metallic by-product that collects on the surface of molten metal during melting, consisting of oxides, silicates, and impurities. Because slag is lighter than liquid metal, it floats on top; removing it before pouring is essential for the internal quality of the casting.
As Ahkemak Metallurgy, active in the casting sector since 1973 and operating a 4000 m² facility in Konya, we treat slag control as a core step of our quality process across steel casting, gray cast iron, ductile iron casting, and stainless steel casting. In this article, we explain what slag is, how it forms, and how it is managed, based on half a century of foundry experience.
How Does Foundry Slag Form?
Slag forms from three main sources during melting: oxidation of impurities in the metal, erosion of the furnace refractory lining, and reactions of added flux materials. Sand, rust, and oil residues on the charge material directly increase slag volume.
The process works as follows:
- Charge materials such as scrap and pig iron are melted in the furnace.
- Elements in contact with oxygen (silicon, manganese, aluminum) form oxides.
- Flux materials such as limestone bind these oxides into a fluid slag layer.
- The low-density slag rises to the surface and is skimmed off before pouring.
What is Foundry Slag Made Of?
The composition of foundry slag varies with the metal being melted and the furnace type. A typical slag contains:
- Silicon dioxide (SiO₂): Comes from molding sand and oxidized silicon; usually the largest component.
- Calcium oxide (CaO): Originates from lime added as flux.
- Iron oxide (FeO) and manganese oxide (MnO): Products of oxidation reactions in the metal bath.
- Aluminum oxide (Al₂O₃) and magnesium oxide (MgO): Come from refractory wear and alloying elements.
Why Does Slag Affect Casting Quality?
When slag is not managed correctly, defects known as slag inclusions form inside the casting. These defects reduce part strength, cause tool breakage during machining, and lead to part rejection in critical sectors such as the defense industry and automotive.
How Are Slag-Related Casting Defects Prevented?
- Using clean, sorted charge material
- Maintaining slag fluidity through correct temperature control
- Using slag-retaining filters and siphon systems in the pouring ladle
- Designing the gating system so slag cannot enter the mold cavity
At Ahkemak Metallurgy, every casting passes through a detailed quality control process covering all of these measures, so our customers receive parts with a clean internal structure and a long service life.
Is Slag Different in Steel Casting, Gray Iron, and Ductile Iron Casting?
Yes. The amount, chemistry, and management of slag differ across the three casting types because melting temperature and metal chemistry change slag behavior.
- Steel casting slag: More reactive due to melting temperatures reaching 1,600°C. Since sulfur and phosphorus removal is performed through the slag, its chemistry (basicity) must be precisely controlled.
- Gray cast iron slag: Less reactive thanks to the lower melting point (1150-1200°C) and high carbon content; higher in volume but relatively easier to manage.
- Ductile iron casting slag: The magnesium added for nodularization creates a sticky, MgO-rich slag known as dross. Because dross increases inclusion risk, ductile iron requires the strictest slag control of the three.
Knowing these differences allows us to select the correct flux and skimming practice for each casting type.
How is Foundry Slag Recycled and Reused?
Slag is not waste; when processed correctly, it is a by-product with economic value. According to the U.S. Geological Survey (USGS), the majority of iron and steel slag worldwide is recovered and reused, primarily in construction.
The main reuse areas are:
- Fill and aggregate material in road construction
- Additive in cement and concrete production
- Recovery of metal droplets through magnetic separation
- Soil conditioner in agriculture
Committed to sustainable production principles, Ahkemak Metallurgy is developing its recovery processes for slag and all other production by-products in line with its target of raising its waste recycling rate to 60% by 2030.
Frequently Asked Questions – FAQ
Are slag and dross the same thing?
Not exactly. Slag refers to the fluid layer formed by flux and oxides, while dross refers to the more solid, sticky oxide mixture caused mainly by magnesium in ductile iron casting. Both must be removed from the metal before pouring.
What happens if slag gets into the casting?
A slag inclusion forms inside the part. This defect reduces tensile strength and fatigue life and appears as cavities or hard spots during machining. In critical parts, it is a cause for rejection.
How can slag volume be reduced?
Using clean scrap, selecting the right flux, controlling temperature, and maintaining the refractory regularly are the main methods. Removing sand and rust from the charge material alone makes a significant difference.
Is foundry slag harmful to the environment?
Not when stored and processed correctly; on the contrary, it reduces natural resource consumption in the construction and cement sectors. Uncontrolled storage, however, can create dust emission and leaching risks.
In which casting types does Ahkemak Metallurgy apply slag control?
Slag control is applied across all of our production lines: steel casting, gray cast iron, ductile iron casting, and stainless steel casting. Every part delivered to the defense, automotive, machinery, ship, transportation, and construction sectors goes through this process.
People Also Ask – PAA
Why does slag float on molten metal?
Slag density (roughly 2.5-3 g/cm³) is far lower than that of liquid iron and steel (around 7 g/cm³). This density difference makes slag rise to the surface, where it can be skimmed easily.
Does slag form in induction furnaces?
Yes. Induction furnaces generate less slag than arc furnaces, but depending on charge quality, regular skimming and the use of slag conditioners are still required.
What does the color of slag indicate?
Slag color gives clues about its chemistry. Greenish, glassy slag indicates high silica content, while dark, dull slag can indicate high iron oxide content, meaning increased metal loss.
How is a slag inclusion in a casting detected?
Irregular dark spots on the surface, cavities revealed after machining, and non-metallic indications found in ultrasonic or radiographic testing all point to slag inclusions in the part.
You can find our contact address via the link.
If you enjoy the What is Foundry Slag?, please visit our News page.
