How do you tell blowholes from hydrogen gas porosity in sand castings, and which controls prevent each?

What do blowholes and hydrogen porosity look like on a machined casting?

Blowholes are one or a few relatively large, smooth, rounded cavities, usually near the top (cope) surface of the casting or next to a core. Hydrogen porosity is many small, round pores spread through a section, often more of them in the thicker areas.

Both are gas defects, and both have smooth, rounded walls because the gas pushed the metal outward while it was still liquid. A technical note from the Non-Ferrous Founders’ Society describes gas pores as often smooth and rounded, and points out that pores near a core, an upper surface, a blind pocket or a poorly vented region suggest mold gas or trapped air, while broadly distributed fine pores suggest dissolved gas (NFFS, 2026). That location and distribution clue is the most useful single test a buyer can apply at receiving inspection.

The radiograph standard for aluminum and magnesium castings draws the same line in its own words. ASTM E155 separates gas holes, which appear as round or elongated, smooth-edged dark spots occurring individually, in clusters or throughout the casting, from gas porosity, which is minute voids usually distributed through the whole casting. In practice, “gas holes” on a film is what the shop floor calls a blowhole, and “gas porosity” is what metallurgists mean by hydrogen porosity.

Neither should be confused with shrinkage. Shrinkage cavities are irregular, angular or branching and collect in the heavy sections that freeze last; gas pores are round. When a cavity is round but sits in a hot spot, both mechanisms may be acting together, and the NFFS note warns that shape is a clue, not proof.

How do you diagnose which gas defect you have?

Map where the holes are across several castings, then match the pattern to the process record: holes in the same place in every casting point to the mold or the cores, while holes that come and go with the melt point to hydrogen. Confirm with a radiograph or a section before agreeing on a corrective action.

A short diagnostic checklist for the buyer or quality engineer:

1. Mark the location on a print. Take five to ten rejected parts, mark each hole on a copy of the drawing and note its size by eye. Clusters at one spot mean a local source.
2. Note the pouring position. Ask the foundry which face was the cope (top) in the mold and where the cores sat. Holes just under the cope face or above a core are classic blowholes.
3. Sort by heat or pour date. If the foundry marks heat numbers or pour dates, sort the rejects. Fine, scattered pores concentrated in particular heats point to melt hydrogen.
4. Look inside a hole. A smooth, shiny or lightly oxidized wall is gas. Visible dendrites or a jagged, branching edge is shrinkage.
5. Image the unmachined sections. Radiography graded against ASTM E155 (aluminum and magnesium) or ASTM E446 (steel) shows whether the holes on the machined face are isolated or the visible edge of distributed porosity. Sectioning, metallography and computed tomography show cavity shape and connectivity, and the NFFS note recommends that the physical evidence and the process record agree before a fix is chosen.
6. Ask for the melt-quality record. For aluminum, ask for the degassing record and the reduced pressure test result for the heats involved. The reduced pressure test solidifies a small sample under partial vacuum so that any dissolved gas expands into visible pores, and the density index compares that sample with one solidified in air. Dispinar and Campbell’s assessment of the test found that folded oxide films (bifilms) start the pores and hydrogen only adds to them, and concluded that the test is of little use for estimating the hydrogen content itself (International Journal of Cast Metals Research, 2004). Read a poor result as poor melt quality in general, oxides as well as hydrogen, and ask for both the degassing and the melt-cleanliness practice to be reviewed.

What should the drawing, RFQ and PO say so gas porosity does not recur?

Write a separate, zone-based limit for each kind of gas defect, name the inspection method that enforces it, and ask for the melt-quality and pressure-test records that prove the process stayed in control. A drawing that only says “no porosity” cannot be met or enforced.

• Zones. Mark critical zones on the drawing (sealing faces, O-ring grooves, threads, bearing bores, highly stressed sections) and leave the rest as general areas with a looser limit.
• Radiographic limits. State the reference standard and a severity level for each discontinuity type and zone, for example gas holes and gas porosity graded separately per ASTM E155 for an aluminum casting. Grading them separately is what lets the report tell you which control has slipped.
• Machined-surface limits. State the maximum visible pore size and the number allowed in a stated area on each machined zone, with zero allowed across a seal land if the part must seal. Choose the values from your own function; there is no universal figure.
• Pressure or leak test. If the part holds a fluid, specify the test medium, pressure, hold and pass criterion after machining, and whether impregnation is permitted.
• Records on the PO. Ask for degassing and melt-check records by heat, traceability from each casting to its heat, and radiographs or section photos for first articles.
• Pouring orientation. If one face is critical, note on the RFQ that it should be poured in the drag, and ask the foundry to confirm or propose an alternative before tooling.