Gas porosity in steel casting occurs when dissolved gases such as hydrogen, nitrogen and oxygen — or steam generated by mold moisture — cannot escape during solidification and remain trapped inside the metal. The result is round, smooth-walled voids inside the part or near its surface.
In this article, the technical team of Ahkemak Metallurgy — specialized in the casting sector since 1973 — explains the causes of gas porosity in steel casting, how to distinguish it from shrinkage cavities, and the preventive steps applied in production.
What Is Gas Porosity?
Gas porosity in steel casting accurs is a casting defect formed when gas bubbles become trapped in the internal structure or near-surface zones of a cast part. Because it reduces the load-bearing cross-section, it lowers strength and causes leaks in parts that must remain pressure-tight.
What Does Gas Porosity Look Like?
- Round or oval voids with bright, smooth internal walls.
- Usually concentrated near the upper surfaces of the part, because gas bubbles rise in liquid metal.
- Small, scattered needle-point voids are called pinholes.
What Are the Main Causes of Gas Porosity in Steel Casting?
Short answer: the source is either the liquid metal itself or the mold environment. The most common causes are:
- Dissolved gases in the melt: Hydrogen, nitrogen and oxygen absorbed during melting separate as bubbles when solubility drops during solidification.
- Mold and core moisture: Excess moisture in molding sand or cores turns into steam on contact with hot metal and enters the melt as gas.
- Improper charge materials: Rusty, oily or damp scrap and moist ferroalloys carry gas directly into the melt.
- Insufficient deoxidation: If oxygen is not bound by elements such as aluminum or silicon, CO bubbles form during solidification.
- Turbulent pouring: Fast, uncontrolled pouring entrains air into the liquid metal; gating system design directly affects this risk.
- Poor mold venting: Low-permeability molding sand and inadequate vents prevent generated gases from escaping the mold.
- Damp ladles and refractories: Undried ladles and refractories become a moisture source at the moment of contact with liquid metal.
Gas Porosity vs Shrinkage Cavity: How Do You Tell Them Apart?
These two defects are often confused, yet their causes and remedies are completely different. Correct diagnosis is the first condition of the correct corrective action.
Distinguishing Features of Gas Porosity
- Rounded shape with smooth, shiny internal surfaces.
- Since the source is gas, it typically appears in upper zones and near the surface.
- The remedy lies in degassing, deoxidation and moisture control.
Distinguishing Features of Shrinkage Cavities
- Irregular, rough, dendritic internal surfaces.
- Caused by volume loss during solidification; found in thick sections and last-to-solidify zones.
- The remedy lies in riser design and directional solidification.
| Note from the Foundry FloorThe experience of our quality control teams shows that most gas porosity complaints arise not from a single cause but from a combination of small oversights — slightly damp scrap, marginally high sand moisture and a rushed pour. Prevention therefore requires controlling the entire chain from melting to molding, not a single parameter. |
How Is Gas Porosity Prevented in Steel Casting?
Preventing gas porosity is a matter of process discipline. The core steps applied on the Ahkemak Metallurgy production line are:
- Select dry, clean, rust-free charge materials and store ferroalloys in dry conditions.
- Apply proper deoxidation to the melt, binding oxygen with aluminum or silicon.
- Measure the moisture content and permeability of molding sand regularly; dry cores completely.
- Provide sufficient vents in the mold so generated gases can escape.
- Control pouring temperature and speed; use gating designs that reduce turbulence.
- Preheat and dry ladles before use.
- Verify parts with non-destructive tests such as radiographic and ultrasonic inspection.
Frequently Asked Questions – FAQ
How does gas porosity affect the strength of a casting?
Voids reduce the load-bearing cross-section and create stress concentrations. In parts under fatigue loading, they act as crack initiation points and shorten service life.
Can gas porosity be detected by non-destructive testing?
Yes. Internal voids are detected by radiographic and ultrasonic inspection, while near-surface porosity is revealed by visual inspection and machining. At Ahkemak Metallurgy, every part passes through detailed quality control processes.
Are pinholes the same as gas porosity?
Pinholes are the small, scattered form of gas porosity, usually needle-sized and located just below the surface. Their source is mostly hydrogen and mold moisture, and the prevention methods are the same.
Why does mold moisture cause gas porosity?
When hot metal contacts moisture in the mold, the water evaporates and dissociates, releasing hydrogen. This gas becomes trapped in the solidifying metal, which is why sand moisture and core drying are critical control points.
Can a casting with gas porosity be repaired?
It depends on the size and location of the voids and the operating conditions of the part. Weld repair may be applied where the relevant standard and the customer permit; critical parts are recast.
People Also Ask – PAA
Does pouring temperature affect gas porosity?
Yes. Excessively high pouring temperature increases the gas solubility of the metal, while too low a temperature causes solidification before gases can escape. The correct range is set according to the alloy and part geometry.
Which gases create porosity in steel casting?
Mainly hydrogen, nitrogen and oxygen. Hydrogen usually comes from moisture, nitrogen from the atmosphere and charge, and oxygen from insufficient deoxidation.
Where does gas porosity appear most on a part?
Because gas bubbles rise in liquid metal, voids concentrate mostly near the upper surfaces of the part and in the upper half of the mold.
What is deoxidation and why does it matter?
Deoxidation is the removal of dissolved oxygen from liquid steel by binding it with elements such as aluminum or silicon. Without it, oxygen combines with carbon to form CO bubbles, causing gas porosity.
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| About the AuthorThis content was prepared by the technical team of Ahkemak Metallurgy, specialized in the casting sector since 1973 and producing steel, stainless, gray and ductile iron castings in its 4000 m² facility founded in Konya in 2008. Serving the defense, automotive, machinery, ship, transportation and construction industries, the company passes every part through detailed quality control processes. For casting defect literature, refer to ASM International (www.asminternational.org). |
