This defect is mainly observed in castings made of flake graphite cast iron, but this phenomenon may also occur in nodular graphite cast iron and cast steels. They are independent of any production processes but are more frequently encountered in sand casting (bentonite-bound and synthetic resin-bound molds). Dispersed shrinkage and pin holes are related with regard to their causes of formation and therefore often occur together (Figs. 1 to 3).
The defects may occur both at the surfaces (often perpendicular to the surface) or edges of the mold and the surfaces of the core(s). They are predominantly encountered in large-area parts (slabs of plates), in which the iron spreads widely and thus provides a large surface for reaction.
Formation of dispersed shrinkage (by the way, also formation of pin holes and/or edge blistering) is influenced by the solidification rate so that only certain, mostly medium wall thicknesses are affected. Brake disks for example, are very prone to defects. Directly underneath the casting skin small cracks or regularly shaped cavities are present with an orientation in the form of commas perpendicular to the casting surface and penetrate the casting by a few millimeters.
These crack-like cavities do not have such a glossy surface as (hydrogen) pin holes; they are predominantly dendritic on the inside. In the cross sections behind the defect the castings are completely dense. Althogh these defects often give reason for complaint and produce rejects, the generally only affect the visual appearance and not so much the function of a casting. The defect is only considered an actual impairment at functional surfaces that must be absolutely dense and tight.
The static load-bearing capacity, particularly with ductile materials and low load levels, is practically not affected, at all. However, fatigue strength is impaired so that this defect causes rejects particularly among safety components.
The defects originate from gray diffusion of nitrogen during solidification; with simultaneous hydrogen diffusion may even intensify this defect. The crack-like appearance of dispersed shrinkage defects can be explained by the fact that the nitrogen gas diffusion only takes place at a later stage of solidification; by this time, a number of iron dendrites are already present in the material so that a longish cavity with dendritic surface is formed instead of a spheroid gas blister growing in the melt similar to a pin hole. When viewed from this point, dispersed shrinkage is a preliminary pin hole stage. Only the nitrogen dissolved in the melt is capable of diffusing in the form of gaseous nitrogen during solidification and thus cause blisters; the limit of error for dispersed shrinkage but also for pinholes in flake graphite cast iron is > 95 ppm of nitrogen.
The solubility of nitrogen in iron is influenced by the various alloying elements. Vanadium and chromium increase solubility, while silicon and carbon decrease it. Moreover, comparatively high contents of nitrogen may cause decrease of titanium and carbon equivalent (degree of saturation) contents below the specified limits. Great proportions of steel in the charge may also cause significant increase of the nitrogen content in the melt. To this end, Figure 4 reveals interesting results for EN-GJL-250 acc. to DIN EN 1561. The limit value for nitrogen content (here: 110 ppm according to K. H. Caspers) was determined by examinations of critical castings with low defect rates. When using oil coke and FeSi 75 this limit value is already exceeded from a steel content above 30 %; i.e. there is a distinctly adverse combination of charge components present in the melt. In contrast to this, the use of silicon carbide and electrode graphite achieves great protection against gas diffusion.
By the wax, this has a positive effect on the supercooling behavior of the melt; the low degree of supercooling during eutecticsolidification results in improved graphite diffusion and decreases the tendency for white solidification (s. Graphite degeneration).
The comparison of the various curves in Fig. 4 reveals that the use of oil coke and FeSi 75 with a steel content of 20% causes nitrogen contents that correspond to a steel content of 70 % when used with silicon carbide and graphite (which do not represent a higher risk for rejects despite the high steel contents). This makes clear that eventually it is not only the steel content that causes and increase in nitrogen but it is also a matter of the additives required. The correctly identified cause is easily attributed to the wrong effect.
Nitrogen is introduced into the furnace by means of the charge material; in cupola furnaces also through the air blast. Moreover, it must be taken into account that higher steel contents entail higher melting temperatures. This is inevitably accompanied by a higher nitrogen solubility in the melt (Fig. 4).
In liquid-state iron the solubility of nitrogen is higher than in solid-state iron; consequently the nitrogen content is lowered in solidivied areas while it is increased in the residual melt. Thes differences in concentration are built up very quickly and if the limit of solubility in the melt is exceeded, a nitrogen bubble is precipitated which can be found in solid-state iron in the form of a gas blister.
Commonly used carburization agents contain up to one percent of nitrogen, i.e. every addition of 1 % of carburization agent increases the nitrogen content in the iron by 10 ppm, this is per 1/10 % nitrogen content of the carburization agent. In this way, the error limit of around 95 ppm of nitrogen can be easily exceeded.
Moreover, nitrogen absorption from mold and core binders is of considerable importance. Additional nitrogen loads originating from introduced lustrous carbon formers and synthetic resin residues may occur in bentonite-bound molding materials used in iron foundries. In contrast to that, it is required to establish an atmosphere with a highly reducing effect. The formation of oxide nuclei or CO blisters significantly facilitate diffusion of gasses (H2, N2) due to the considerably lower pressure required for that. It is particularly cold-setting (furan resin) but also hot-setting binders, e.g. in cores produced with the Croning method (s. Resin-coated sand and Hexamethylenetetramine!), that frequently contain ureaformaldehyde resins with high nitrogen contents which, as a sum exceed the
interference threshold of approx. 95 ppm of nitrogen, particularly with poor gas venting from the mold or the core.
A higher nitrogen content in bentonite-bound recirculation molding material that was introduced by higher amounts of used core sand for example, requires a lower nitrogen content in the cast iron with otherwise equal production conditions so that the use of charge materials with uniformly low nitrogen content is also important in this respect.
Although nowadays the chemical binder systems have distinctly lower nitrogen contents, than several years ago, insufficient quality of the recirculated material (e.g. in furan resin sands) or excessive supply of Croning core residues with no or low thermal loads (core prints, etc.) the error limit for green casting sands, which is at approx. 0.08 % nitrogen, may be exceeded.
Influences by bentonite-bound molding material such as high moisture and excessive bentonite content and resulting increased gas absorption in the melt will also result in this defect. Molding water must be thoroughly embedded into the intermediate bentonite layers. Excessive contents of “free” water bear the risk of oxygen or hydrogen formation due to dissociation of vapor. Oxygen promote development of oxide nuclei and additionally impairs the efficiency of the lustrous carbon formers. The hydrogen may diffuse into existing nuclei and expand them to form edge blisters.
Measures for prevention (acc. to S. Hasse, FT&E):
1. Control of the nitrogen content in the melt and lowering it below 95 ppm, if required.
2. If possible, increase the degree of saturation in gray cast iron and increase the titanium content to 0.03 %, if required, to bind the nitrogen in the form of titaniumcarbonitride. Aluminum is also suitable for bonding of nitrogen, but must be used under strict control of the formation of graphite spherulites, equal to titanium in nodular graphite cast iron. Moreover in nodular graphite cast iron there is a risk of increased hydrogen absorption from the vapor of the molding material and thus the risk of hydrogen pin holes, mainly caused by aluminum.
4. Use low-nitrogen carburization agents and regularly check them for nitrogen contents.
6. Use low-nitrogen binders for synthetic resin-bonded molds and cores and optimize binder contents.
7. Reduce the lustrous carbon former contents in the molding material, use low-nitrogen carbon carriers. The lustrous carbon former used must form a sufficient amount of gas at a preferably early stage to ensure displacement of oxygen from the mold cavity.
8. Check recycled mold and core molding materials for nitrogen contents and improve the recycled material quality, if required.
9. If possible, reduce water and bentonite contents in the molding material to a minimum amount.
10. Avoidance of free (unbonded) water.
12. Check exothermic riser inserts for nitrogen contents.