Pinholes 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. A number of measures have proven effective in the prevention of pinholes, and the most important ones are listed below.
An overview of the key measures for preventing pinholes
- Reducing hydrogen carriers, i.e. using dry charge materials and avoiding highly rusty, oily feedstock.
- Reducing nitrogen carriers (e.g. scrap steel) in the charge make-up; taking the minimum content of titanium (0.02%) into account in the case of GJL. Using returns that are blasted as far as possible.
- Increasing the chromo-saturation level if possible, setting the manganese content to no more than 0.4% Si, reducing the aluminum content (not ≥ 100 ppm). The negative effect of aluminum is intensified in the presence of titanium, pinholes occur increasingly in the case of a Ti content of 0.02% and an Al content of 0.01% to 0.2%.
- Applying inoculants with reduced aluminum content and ensuring that the casting temperatures are correspondingly high. Low casting temperatures increase the defect tendency, which is seen as a sign that the formation of pinholes is caused by a surface oxidization of the melt during casting and solidification.
- Changing the gating system so that the melt has little opportunity to oxidize; shortening flow paths, avoiding turbulence
- Keeping the hydrogen content of the melt to a minimum by means of intensive boiling during the refining period in steel casting. Ensuring careful primary deoxidation and short casting times, avoiding long flow paths
- Reducing the formation of slag
- Reducing the moisture content in the mold material, reducing the amount of inflow of core sand (nitrogen, urea), possibly refreshing the new sand
- Checking and, if necessary, optimizing the content of lustrous carbon of the mold material, possibly changing the material that forms lustrous carbon.
- Possibly reducing the amount of binders in synthetic resin-bonded molds and cores or applying a different binder and/or other catalysts.
- Coating molds and cores (nitrogen-adsorbing blocking layers, for example from the ASK Chemicals product range), drying water-based coatings, checking and optimizing the venting.
- Avoiding the input of feeder systems into the mold material system.