Pinholes: Defect Pattern & Causes

Pinholes, often also referred to as surface blowholes, occur sporadically and over large areas and can affect all cast piece areas. In many cases, they only become visible after mechanical processing, but they are always visible to the naked eye. They are primarily found on the outside of the cast piece or just below the surface of cast pieces made of cast iron with lamellar graphite, nodular graphite and vermicular graphite and in malleable iron casting and steel casting.
Manifestations

Pinholes can appear in various forms, from spherical blisters with a bare metal surface or covered with small graphite skins to larger, irregularly shaped cavities accompanied by slags or occurrences of oxidation.

 

Intensification of pinhole formation in mold-inoculated cast iron with lamellar graphite due to decreasing casting temperature.

There are hydrogen pinholes and hydrogen-nitrogen pinholes, which cannot be distinguished from each other, and CO-slag reaction pinholes. Known possible root causes of this defect include specific properties of the iron on the one hand and specific properties of the mold material system on the other hand. In a concrete case, the formation of pinholes will often not result from only one cause but rather from the combined effect of several overlapping causes. Pinholes lead to impairments in coatings such as enameling, hot-dip galvanizing, powder coating, etc. The ability to withstand static stress is hardly affected by this defect, in particular in the case of ductile cast iron types and a low stress level. In the case of processed functional surfaces that also have to be leakproof, the defect leads to rejections. In addition, an unfavorable position of the defect can impair the dynamic strength, so that pinholes cannot be tolerated in safety components that are under vibratory stress.

Causes

The principle cause of pinholes is based on the specific properties of the metal (i.e. iron), as well as the mold material system.

Hydrogen pinholes and hydrogen-nitrogen pinholes form in three stages

  • Reaction of the water vapor with the accompanying elements. Metal oxides and atomic hydrogen form, which diffuses into the liquid metal. Nitrogen-hydrogen compounds are split in a similar way and are also able to diffuse into the liquid metal.
  • Micro gas bubbles form due to the reaction between the metal oxides and the carbon of the melt.
  • Hydrogen and possibly nitrogen diffuse into the micro gas bubbles and enlarge the bubbles.

The metallurgic causes of pinhole formation

  • Excessive hydrogen content in the melt that can occur due to hydrogen carriers such as moist charge materials in general, due to fine-grained, unprotected ferro-alloys that often adsorb water, highly rusty feedstock (adsorbed OH groups), oils and emulsions that give off hydrocarbons in particular, and, ultimately, due to the influence of increased air humidity itself.
  • Excessive nitrogen content in the melt that can be introduced through nitrogen carriers such as scrap steel (up to 130 ppm N, max. 200 ppm N), rail steel (up to 170 ppm N), pig iron (10 to 60 ppm N) and carburizing compounds (from 0.11 x 104 ppm to 1.675 x 104 ppm N).

The nitrogen content in the cast iron is in the area between 40 and 140 ppm, so that degassing can occur if the formation of bubbles is induced externally, e.g. though inductive agitation. The critical limit for the formation of pinholes is frequently specified as being between 80 and 100 ppm, although a lower content in combination with CO precipitations can already be critical.
In contrast to steel casting, iron casting is less prone to absorbing gas, since carbon and silicon reduce the solubility and therefore also the readiness to absorb hydrogen and nitrogen. As a result, a cast iron melt with a low chromo-saturation level is far more sensitive, and this is also reflected in the frequency of defects in malleable iron casting, which must have a lower chromo-saturation level by nature.
Chromium, molybdenum, manganese, vanadium and titanium increase the solubility of hydrogen and nitrogen, while aluminum, phosphorous, silicon and carbon reduce it. One case that was examined revealed that cast pieces with the same gas content were completely sound at 70 ppm Al, while the same cast pieces with 380 ppm Al exhibited a significant formation of pinholes. Higher aluminum contents can cause the aluminum in the iron to convert to aluminum oxide and hydrogen with the water vapor of the pouring fumes. This hydrogen then causes the formation of bubbles. This is also where the line should be drawn between gas bubbles with purely metallurgical causes and the interfacial reaction between the mold/core and the cast piece.

The mold material-related causes of pinhole formation

  • Too much nitrogen in the molding sand with excessive moisture content
  • An unfavorable gas atmosphere in the mold cavity, whose causes should be sought in the type and amount of materials that form lustrous carbon
  • Too much nitrogen in core sands or too high nitrogen-hydrogen compounds in the core molding material binders
  • During the pyrolysis of Croning® core binders in particular, large amounts of gas are released before a firm peripheral shell has formed.

The mechanism can be explained as follows:

In a reducing atmosphere, ammonia breaks down according to the equation

2NH3 + N2 + 3H2 .

The ammonia is almost fully dissociated at 600°C and under atmospheric pressure. The gas volume doubles in this process, and 1 mole of nitrogen and 3 moles of hydrogen emerge from 2 moles of ammonia. This hydrogen gas can react with the precipitated carbon monoxide according to the following equation:


CO + H2 + H2 O + C

  • The bentonite-bonded mold material is not sufficiently conditioned, which leads to the presence of free water that is not bound in the bentonite (in the three-layer mineral montmorillonite) and that increases the risk of hydrogen forming.

There are other aspects besides the composition of the iron and the properties of the mold material that influence the formation of pinholes. If the slag is not skimmed completely, this can lead to slag inclusions that may possibly act as seeds for the formation of gas bubbles. Even if the slag has been removed, cast iron with nodular graphite contains numerous oxides that can contribute to the formation of pinholes. The slag that develops during the magnesium treatment also has an impact, even if it is not clear whether it causes a surface defect in a purely mechanical way or promotes the formation of gas. The casting and gating techniques can also exert influence on the formation of pinholes. A gating system with a non-turbulent flow and short casting runs reduces the pinhole tendency. Cold drops can also be a cause for pinholes. They are oxidized and then enveloped by the casting flow. A reaction in which CO forms can occur here and this leads to the formation of gas bubbles.