Designing Thin-Walled Iron Castings
Note: This article is based on "Thinking Thin With Green Sand Cast Iron", first published in Engineered Casting Solutions, Winter 2004.
When design engineers are trying to decide between the use of iron or aluminum in their components, the biggest impediment to using cast iron is that iron components are often thicker than necessary, resulting in added weight and reduced energy efficiency. In automotive applications, where weight reduction is a paramount achievement, this all but eliminates the iron component from contention.
This occurs because molding technology is believed to be inadequate for the production of quality thin-wall iron castings. However, recent achievements, specifically in iron green sand metalcasting, has lead to thin-walled iron components that can maintain structural integrity and compete with lightweight (aluminum, magnesium, etc.) castings without unnecessarily raising the weight of the component.
This article will look at what is achievable in thin-wall iron casting using the green sand molding process.
Casting For Thin Walls
The green sand process is the most common mold technique for casting iron components. Utilizing this method, walls as thin as 0.12-in. are possible. But in order to produce this thickness, a foundry must keep a careful eye on its metalcasting operations.
“A foundry with good sand controls, good melting and metal delivery systems, and molding equipment that can produce good, dense molds is going to be more successful in producing thinner sections than a foundry lacking in sufficient sand and metal controls,” said Don Reimer, foundry operations manager at Farrar Corp., Norwich, Kan.
According to Reimer, the green sand process offers several advantages if quality equipment is used. “As foundries grasp a better understanding of controlling sand, incorporate better methods of producing pattern equipment and invest in better molding equipment, tolerances improve significantly,” Reimer said. “Hard, dense molds produce castings that are uniform in section thickness and better pattern equipment eliminates wider tolerance ranges that were once required for pattern shift and pattern thickness inconsistencies.”
However, any time that a casting includes transitions from one section thickness size to another, there is a potential for problems. First, metal can not feed a relatively heavy section through a thinner section without the probability of shrinkage occurring in the thicker section. If the thick pieces are 1 in. thick and the thin is 1/8 in., a shrinkage problem will occur. Another problem that can occur is that the mechanical properties of each section will vary due to the difference in cooling rates. Also, hot tears (a defect that occurs when the skin formed at the start of solidification is not strong enough to withstand the forces of contraction) may occur in the casting.
“Designers must realize that the same criteria will apply to thin wall sections when using iron and the green sand molding process as in any other casting design,” said Reimer. “If a thinner section of a casting is around a boss area or even a thicker section, the designer must be mindful that metal cannot flow through the thinner section to ensure soundness in the thicker sections. Also, sharp transitions should be avoided, or have adequate fillets to eliminate hot tears.”
When the wall thickness is small, other material properties generally seen in cast iron components are sacrificed. As section thickness becomes thinner, there is a tendency to lose elongation in ductile iron. There also is a tendency toward losing nodule count in thin sections, with the possibility of carbide formations. The thinner the section the more care must be taken in order to avoid the carbides. If the carbides occur, the casting will be more brittle and could cause problems during machining.
“Most of these microstructure problems can be dealt with by adjusting pouring temperatures, maintaining proper sulfur to rare earth percentage and by using proper and adequate post inoculation techniques,” said Reimer. “In some cases, in-mold-stream inoculation or even in-sprue inoculation may be required. However, as in any other casting, mechanical properties will vary within the casting as section thickness varies.”
One option to further upgrade thin-wall iron castings is the use of ductile iron and austempered ductile iron (ADI) to help maintain a greater strength-to-weight ratio. “When using ductile iron and ADI, strength requirements can be maintained while utilizing thinner sections, for an overall reduction in the weight of the component,” said Reimer.
Ductile iron combines the processing advantages of gray iron (low melting point, good fluidity and castability, and ready machinability) with many of the engineering advantages of steel (high strength, ductility and wear resistance). This allows for higher material properties as tensile and yield strength then gray iron in thin walls.
Austempered ductile iron is ductile iron that undergoes austempering, a special heat treatment process that, when applied to irons, produces parts that are stronger and tougher than conventionally heat treated, as cast components. ADI also has a relatively low weight per unit of yield strength, allowing for stronger thin sections.
Figure 1 shows a duralite wheel hub that was converted to an austempered ductile iron design from aluminum, reducing weight by 3% and cost by 30%. Although austempered ductile iron has a higher strength-to-weight ratio than aluminum, this weight savings was achieved by redesigning the hub to take advantage of iron’s properties.
Combating Common Defects
The good news for designers is that several things can be done during the metalcasting process to ensure that the component produced features thin walls that are structurally sound.
- Fluidity—Fluidity, which is the ability of the metal to fill the mold, plays an important role in casting for thin-walled sections. The greatest levels of fluidity are found with dense green sand molds. Pressure-squeezed green sand molds are the most dense, followed by jolt/squeeze molds and hand-packed molds;
- Gating Systems—The flow pattern of the metal fill is largely determined by the geometry of the ingate system. In order to achieve thin-walled castings, the runner system should be kept as short as possible to avoid hydrogen pinholing;
- Sand Type—Synthetic sands that have been mechanically compacted produce more rigid molds, which leads to less variation in the castings;
- Pouring Temperature—When pouring iron castings into a green sand mold, a lower pouring temperature between 2525-2642F (1385-1450C), can yield better control of grain size;
- Solidification Rate—The properties of all metals are influenced by the rate at which they solidify, but gray iron is especially sensitive to cooling rate. The slower solidification and cooling of heavier castings result in the formation of coarser graphite flakes and a softer matrix structure, reducing the strength of the iron. The solidification in thin-walled sections can be so rapid that hard iron carbides (or white iron) can form. One practical approach to reducing the cooling rate between thin and thick sections is to decrease mold-filling time. Some foundries have adapted filling times as short as 7-10 sec for 125 kg poured weight.
This single casting valve exhaust manifold (Fig. 2) by Wescast Industries, Inc., was redesigned to a casting, eliminating all weldments at a cost savings of 50%. Cast in silicon-molybdenum ductile iron using green sand molding, the component features 4-mm thick walls.
Wescast generated several cast designs for the component to optimize the air flow characteristics. A combination of analytical (computational fluid dynamics) and experimental (rapid prototypes and flow bench) tools was also used to optimize manifold runner balancing, sensor positioning and catalytic converter flow uniformity. The final design optimization removed 4.3 lb and lowered the unit price by $1.55 from the foundry’s initial casting design.
Oil Industry Component
The thief hatch cover (Fig. 3) cast by Clay & Bailey Manufacturing Corp., features a wall thickness of 0.18-in. around the outer rim of the casting. The thief hatch cover is a pressure vacuum device used in the oil field industry. After the oil is pumped out of the ground it is sent into holding tanks, and when a sample of the oil is taken, it is commonly referred to as taking a “thief” of the oil. The cover opens up when pressure in the tanks builds up, allowing for a sample to be taken.
The casting, which required two chaplets and two cores to produce, had to be carefully cast to achieve quality thin-walled sections. “Any time you have a long, thin section in an iron casting, it has a tendency to pull together during cooling, which can cause structural defects,” said Brad Holmes, sales manager at Clay & Bailey.
“In order to make sure that this did not occur, it was important that we leave the casting in the mold for a longer period of time to make sure that it cooled properly,” said Holmes. “If we were to remove the casting too soon, warping could occur.”