Simple Solutions That Work! Issue 11

6 nitrogen (0.030%) produced severe pinholes. This pattern indicated that in type 410 stainless steel, even with nitrogen as low as 300 ppm (0.03%), hydrogen to the extent of 7 ppm level was excessive. This anomaly has come to be known as complex nitrogen/ hydrogen pinholing. Elimination of Sub-Surface Porosity in High Alloy Steels: Research work conducted in the late 1960's identified that the use of ferroselenium can substantially eliminate subsurface porosity in green sand molds (6) . At the time of the research, chemical binder technology was in its infancy so none of binders systems shown in Table 1 were investigated. However, it has since been found that the addition of ferroselenium can also be effective when using chemically bonded sand. One of the primary uses of ferroselenium in the foundry industry is the control of hydrogen porosity. Small amounts of FeSe can virtually eliminate hydrogen porosity (pinholes) in carbon, medium, and high alloy cast steels, wear resistant iron castings such as Ni Resist, and stainless steels poured in green sand or chemically bonded molds. Typical addition rates are 0.005% Se to 0.02% Se (0.10 lbs to 0.40 lbs per ton) but as much as 1 lb. per ton can be added. Because such small additions are used, FeSe is briquetted into a uniform shape to facilitate accurate weight additions. It is generally believed that selenium prevents pinhole porosity by its influence on the surface tension of the melt, so that solid surfaces are not wetted and the probability of heterogeneous gas bubble nucleation is reduced. (6) It is also reported that selenium additions of 0.10% can reduce the rate of nitrogen absorption in liquid steel. molten steel from moisture in atmosphere and refractory materials, deoxidation and alloying elements, and slag additives. Because hydrogen is such a small atom, it can diffuse rapidly once it's absorbed, creating pinholes. Hydrogen absorption can also result from the decomposition of water vapor in green sand molds, from chemical mold or core binder decomposition or from high humidity conditions on the melt deck. Increased usage of recycled metallic borings containing residual cutting fluids will also contribute to hydrogen pick-up. Nitrogen can also readily be absorbed in molten steels from gaseous decomposition products from mold and core binders as well as certain charge materials. Roach and Simmons (3) reported that all stainless steels will tend to pick up nitrogen when melted in air. The ability to retain nitrogen has been shown to be dependent on chrome and manganese levels in steel. Higher chromium and manganese will permit retention of great amounts of nitrogen. Nascent or mono-atomic hydrogen and nitrogen are readily soluble in molten irons and steels. While the first four reactions are likely to generate both surface and subsurface porosity defects, the last reaction usually results only in surface defects, such as surface pockmarking or more often, lustrous carbon laps and surface wrinkles (4) . Sources of Gas from Alloying Materials: The solubility of nitrogen in chromium alloys can be quite high, unless the foundry specifies the need for a low nitrogen ferrochrome grade. In one reported instance, a stainless steel foundry was experiencing severe subsurface porosity and upon further investigation, it was found that the low carbon (0.05% carbon) ferrochrome that was being used contained over 10,000 ppm (1.0 pct) nitrogen. The foundry had no idea that they should put a nitrogen specification on the low carbon chrome that they were purchasing. Nitrogen and hydrogen are also very soluble in manganese alloys so great care must be taken by the foundrymen to specify low hydrogen and nitrogen grades, particularly with electrolytic grades of manganese metal. Hydrogen has also been found to be present in electrolytic nickel cathode squares. The observation that hydrogen and nitrogen are additive in promoting porosity is supported by some analytical data. Rassbach, Saunders, et al (5) found in experimental heats of type 410 (11 to 13% Cr) stainless steels containing 230 ppm nitrogen (0.023%) and 5 ppm hydrogen (0.0005%) were sound. Increased nitrogen levels of 300 ppm nitrogen (0.030%) and 4 ppm hydrogen were also sound. However, an increase of 3 ppm in hydrogen (7ppm or (0.0007 total) on the heat containing 300 ppm