Deoxidisation refers, in foundry technology, to the removal of dissolved oxygen from molten metals in order to prevent quality defects in castings.[1] Agents such as aluminium, silicon, manganese or calcium are added to the melt, reacting with the oxygen to form oxides that settle out as slag. Without sufficient deoxidisation, porosity and oxide inclusions result.
What does deoxidisation mean in foundry technology?
Deoxidisation means the targeted reduction of the oxygen content in molten metals, in particular in steel and iron alloys. Oxygen normally enters the melt through contact with the atmosphere or through oxides present in the raw materials used. A high oxygen content can cause casting defects during solidification, in the form of porosity and oxidic inclusions. The aim of deoxidisation is to ensure a homogeneous melt quality and to markedly improve the mechanical properties of the final product.
Deoxidisation methods
To remove oxygen effectively, deoxidising agents are added to the melt. Commonly used agents are aluminium, silicon, manganese and calcium, either individually or combined as alloys. These elements have a higher affinity for oxygen and react with it quickly to form stable oxides, such as manganese oxide (MnO), silicon dioxide (SiO2) and aluminium oxide (Al2O3). These oxides either settle out as slag on the surface or are removed from the melt by other process routes. The choice and amount of deoxidising agent used depend on the base material and the requirements placed on the casting.
Importance of deoxidisation for casting quality
Successful deoxidisation substantially raises the quality of the final product, especially in the industrial production of high-value components such as automotive parts, machine housings or plant components. Pores and oxide inclusions reduce the strength and service life of a workpiece and increase the scrap rate at the same time. Deoxidisation is therefore an essential step in modern foundry technology, needed to meet demanding requirements for quality, safety and cost-effectiveness.
Practical use: what the foundry engineer needs deoxidisation for
In day-to-day melting operations, deoxidisation decides whether a casting comes out of the mould free of porosity or ends up as scrap. Too little deoxidisation lets the dissolved oxygen react with carbon during solidification to form carbon monoxide, which becomes trapped as a gas bubble in the part, visible as pinholes just below the casting surface. Too much deoxidisation, or the wrong agent, produces non-metallic oxide inclusions, which act as hard particles that impair machinability and as notches that lower fatigue strength. In practice, the amount of deoxidising agent is therefore not judged by feel, but dosed according to an analysis of the base material, the target steel grade and the intended degree of deoxidisation (killed or unkilled/rimming steel). Before casting, the technician often checks with a quick test or an oxygen probe whether the target level has been reached before releasing the heat.
Deoxidising agents at a glance
| Agent | Oxygen affinity | Typical addition range | Particularity |
|---|---|---|---|
| Aluminium | very high | approx. 0.02–0.05% | standard for fully killed steel, gives a fine-grained microstructure |
| Silicon | high | approx. 0.15–0.30% | forms SiO₂ slag, common with cast iron and cast steel |
| Manganese | moderate | mostly as a ferro-silicon master alloy | also has a desulphurising effect |
| Calcium | high, but hard to dose | ppm range (Ca treatment) | modifies oxide inclusions into a globular rather than angular shape |
These are guideline values for steel melts and vary noticeably with steel grade, starting analysis and furnace type.[4] A binding dosage is set only by the plant’s own melt-control practice.
Frequently asked questions about deoxidisation
What is the difference between killed and unkilled steel?
Killed steel is fully deoxidised, so no gas reaction occurs during solidification and the melt solidifies “quietly”. Unkilled (rimming) steel is only partially deoxidised, so carbon monoxide still escapes during solidification.
How is insufficient deoxidisation recognised in the finished casting?
Typical signs are pinholes near the surface, visible on X-ray or ultrasonic inspection, and an increased scrap rate during machining caused by hard oxide inclusions.
Why is deoxidisation not always taken to the maximum?
Because every deoxidising agent leaves residues of its own. Too much aluminium, for example, increases the number of fine alumina inclusions, which can clog nozzles during continuous casting. Dosage is therefore always a trade-off between removing oxygen and limiting the amount of inclusions.
Standard reference
The designation system for steels under DIN EN 10027-1[2] can indicate the deoxidation practice as an additional symbol, for example a note on killed casting. The classification of steel grades itself is set out in DIN EN 10020[3]. Both standards matter to the technician whenever an ordered steel grade requires a specific degree of deoxidisation.
References
- Metalltechnik-Lexikon: Desoxidation, Stahl (German only, no English edition available). Specialist glossary, accessed 27 August 2026.
- DIN EN 10027-1:2017-01, Designation systems for steels – Part 1: Steel names.
- DIN EN 10020:2000-07, Definition and classification of grades of steel.
- Total Materia: Steel Deoxidation. Specialist article, accessed 27 August 2026.