Understanding Limitations and the Power of Knowledge
By Richard Gundlach
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What matters most to management is the ability to stay in business and stay cost competitive. A good knowledge of casting technology can be a powerful asset to produce better, more robust products. It is imperative that foundries educate their customers on the design of castings, and possibly provide design services to customers. Alternately, they should offer design improvements to customers to take advantage of the properties of quality metal and the advantages of design only afforded in the casting process. They also need to learn how to reduce defect sizes and quantities in their castings by using the many new technologies available today.
By using the power of advanced casting technology coupled with efficient and effective design the metalcasting industry will have a bright future.
The kinetics of the solidification process produces microshrinkage porosity, coarse solidification structures and oftentimes, undesirable inclusions and second phases that develop as a result of alloy segregation during solidification. The decrease in the solubility of gases during solidification also can produce internal porosity. The casting process also can introduce inclusions due to reoxidation of the molten metal during pouring, as well as cause the entrainment of foreign debris that further adds discontinuities to castings. The various voids, inclusions and microstructural features reduce strength and ductility and cause castings to suffer reduced monotonic and cyclic properties compared to wrought and forged metals.
The microstructures in castings are also much coarser than the grain size and second phase particles in wrought metals, further separating the properties of castings from wrought and forged parts. Heat treatments can be applied to castings to refine the grain structure and improve the mechanical properties in castings, causing them to approach those of wrought metals. However, the sizes of the various discontinuities and second phase particles in castings are not generally changed by heat treatment.
The size and shape of inclusions, porosity voids and second phase particles have a major influence on the properties of castings. These damaging features have a limited effect on yield strength, but they have a major effect on tensile strength and elongation. The ways in which these features influence the fracture mode are discussed.
For many applications the casting is subjected to cyclic loading, and fatigue strength is the most important mechanical property of a casting. Discontinuities are even more damaging to fatigue strength than to tensile properties.
Types of Discontinuities in Castings
Discontinuities in iron castings include microshrinkage porosity, gas porosity, oxide films, slag inclusions, coarse graphite particles, nodule clustering, degenerate graphite, flake graphite at the cast surface, and other issues.
The Al–Si casting alloys are subject to gas porosity, microshrinkage porosity, coarse eutectic silicon phase and several secondary phases resulting from elevated levels of iron and copper in the alloys. In ductile iron, discontinuities consist of degenerate graphite particles, dross inclusions, and microshrinkage porosity. In steel castings, the discontinuities consist of microshrinkage porosity, slag and oxide inclusions, unfavorable distributions of oxysulfides and grain boundary aluminum nitride inclusions.
Stress Concentration at Discontinuities
It is well known that geometric notches and fillets produce stress concentration. Internal discontinuities also produce stress concentration (Figure 1). Locally around the notch or discontinuity, the stress increases upon approaching the edge of the discontinuity. Eventually, the local stress exceeds the yield strength of the metal and yielding occurs. Thus, in softer metals, the theoretical peak stresses predicted at a notch of discontinuity are never attained because yielding occurs; however, in higher strength metals the full effect of the stress concentration may be realized.
When the shape of the discontinuity is severe and its size large enough, the metal surrounding the tip of the discontinuity plastically deforms. Plastic deformation will lead to crack formation.
The stress concentration at the very tip of the discontinuity or crack is described by the stress intensity factor K-a term used in fracture mechanics. Fracture mechanics is a study of the stress state associated with cracks in metal. Fracture mechanics allows one to predict the fracture strength and even the fatigue life in components with a flaw. Fracture strength is predicted from the flaw size and location; the geometry of the part; externally applied loads and the material properties of the component. When the stress intensity at the perimeter of a flaw reaches the critical stress intensity factor Kc, the crack advances and crack growth can become unstable, leading to rapid crack propagation and failure.
Crack growth can occur by different modes—tensile and shear. Probably the most common mode encountered in structural components is the tensile mode, or ‘Mode I’ loading. Hence, the most commonly determined and reported critical stress intensity factor for metals is the KIc, which is also a material property called the plane strain fracture toughness.
Embrittlement in Castings
Embrittlement could be defined as any mechanism that causes both tensile strength and tensile elongation to be reduced. Such embrittlement was displayed in a series of aluminum alloy 356 test bars with various sized oxide inclusions.
A recent investigation of gray cast iron has shown that gray iron can also display a form of embrittlement. In that study, the manganese and sulfur concentrations were varied in order to determine the optimum combination of manganese and sulfur. When optimized, the tensile strength, response to inoculation (in terms of cell count), and chill tendency coincided with a specific level of sulfur for each of three levels of manganese. The strength diminished significantly at higher levels of sulfur, even though the sulfur levels were in a commercially accept- able range.
Precision stress–strain curves were generated for the optimally alloyed and the high-sulfur versions from the study, and the results showed that the metal was embrittled.
Microstructure and Mechanical Properties
Several years ago, researchers investigated the tensile properties at numerous locations in a large ductile iron casting. The tensile elongation and strength varied significantly. When the fracture faces were examined, multiple types of discontinuities were observed, including oxide film inclusions and graphite nodule clustering.
With ductile iron fractures, cracking begins with void formation around the graphite nodules. Voids form more easily in castings with coarse graphite nodules. The stress intensity around the nodules increases with increased nodule size, and the fracture strength of the pearlite ligaments between the nodules is reached at a lower bulk stress, resulting in lower tensile strength and tensile elongation. The tensile properties in 1-in. (2.5 cm) Y-blocks and 3-in. (7.6 cm) Y-blocks of a recent study illustrate the reduction in strength and elongation with section size. As nodule count is increased and nodule size is reduced, tensile strength and elongation both increase.
Heat treatment can be used to enhance the strength and elongation in ductile iron with coarse graphite nodules. Through refinement of the grain structure the mechanical properties can be improved. A recent study on ductile iron samples from 3-inch (76-mm) Y-blocks showed that normalizing just below the upper critical temperature can produce a fine-grained ferritic–pearlitic microstructure. The refinement of the microstructure resulted in a significant increase in strength and elongation.
Another form of degradation in properties is regularly displayed in ductile iron castings. In pearlitic–ferritic irons, failure occurs by uniform deformation during tensile elongation. As long as the pearlite is the continuous microconstituent and ferrite is discontinuous, uniform deformation continues.
When ferrite is confined to individual graphite nodules, that is, in the form of bullseye ferrite, voids form at the poles of the nodules. Voids between neighboring nodules increase the local stress in the surrounding pearlite matrix and eventually, the voids link up when cracks propagate through the pearlite.
When ferrite is continuous, forming a path linking nodule to nodule, local deformation in the ferrite regions occurs. As a result, premature fracture through the ferrite regions causes a somewhat reduced ductility.
Many castings are subjected to cyclic loading and consequently fatigue strength is the most critical design parameter.
Most laboratory testing programs measure cyclic properties and the fatigue strength in fully reversed loading. The testing is performed on machined and, smooth-walled specimens, not castings. For many wrought materials the fatigue strength or endurance limit generally equals one- third to one-half of the tensile strength. In castings, due to the presence of discontinuities, the fatigue strength can be lower, approaching a quarter of the tensile strength.
Until recently, the fatigue properties of the various cast metals have been unavailable. AFS recognized this and several years ago began to build a database on the graphitic irons. The database includes the strain-life properties that the Finite Element Analysis (FEA) models require to predict the fatigue strength of a part. The database is being expanded to include aluminum and steel casting alloys. Designers are using the database to design castings for structural applications.
Design of Castings
In his Hoyt Lecture of 1961, J.B. Caine has said ‘‘the advantage of castings is derived from the ability of liquid metal to assume any shape, shapes that cannot be formed efficiently by any other forming process. This flexibility enables the design of shapes as castings that will uniformly distribute the load … no part of the casting is overloaded … and stress concentration is at a minimum.’’
The means to compete with wrought metals of higher strength and internal integrity comes from careful analysis of the design of a part. Modifications of the design usually can result in the use of a lower strength cast metal and arrive at a cast part with equal or higher strength.
Castings can be designed with the most favorable geometries to reduce stress concentration by adding stock at high stress areas, increasing radii, using curved surfaces and adding metal where needed to reduce stress. Often, metal can be removed at locations where it is not needed to maintain or even reduce the gross weight of the part.
Many defect indications can be reduced or eliminated. The facility engineer can use chills to refine or eliminate porosity in critical areas of the casting. Addition agents can be utilized for grain refinement. Extra care can be employed to improve surface finish. Controlled cooling can help reduce residual stresses. Shot-peening, fillet rolling and other surface treatments can be applied to promote beneficial compressive stresses to resist fatigue failure. All these activities contribute to increased fatigue strength.
While porosity and other discontinuities reduce the monotonic and cyclic strength of castings, the tremendous freedom of design can make cast parts competitive with wrought parts and weld assemblies. Due to limitations in the design of the part, wrought parts often contain local regions with stress concentrations. Welded joints produce elevated residual stresses that further reduce the fatigue strength of a part. By flexibility in design, both types of features can be substantially reduced or removed by redesigning of the part as a casting.
Investigators have shown the design stresses can be substantially reduced by redesigning a part, thus making castings quite competitive with wrought parts. A good understanding of the power of design will make a casting stronger and more tolerant of the imperfections in castings. Sharing this understanding with the customer will help the facility sell more castings to its customers.
This paper was adapted from the Hoyt Lecture delivered at the 2016 CastExpo in Minneapolis.