High casting soundness is necessary for meeting the elongation and impact requirements. The target is ASTM severity level 2 or less radiographic soundness throughout all sections. No gross shrinkage can be tolerated. This unsoundness is restricted to scattered microporosity that might occur in the matrix structure cell boundaries or at the mid center of any section. Other requirements are good as-cast surface finish and freedom from other types of casting defects. Surface finish is important because the only machining that is required is for the nose cone insert, threading for aircraft suspension lugs and an aft V-groove where the flight guidance steel wings are attached during final bomb casing assembly. The balance of the OD and the ID remain with as-cast finishes.  The production 500 pound CDIB body is illustrated below.
 

 
Figure 2.  Production Unit
 

 
Development Program
 
Using an outside contractor, Tolo, Inc., the NAWC at China Lake, California, divided development into three phases for the 500 pound bomb.
 
Phase I: Computer modeling of casting design, solidification, gating, mold rigging and projected mechanical properties.
 
Phase II: Metal process development and prototype manufacture using test blocks and 30 CDIB bodies.
 
Phase III: Process verification and a 75 unit production of prototype bodies meeting all requirements for flight and mission performance testing.
 
Figure 3 illustrates the final mold design developed in Phase I. Sample castings had been produced in a iterative process throughout Phase I and tested, leading to best possible mold design to meet all final casting requirements,
 
The work determined that:
 
•  Casting is best in the vertical position in no-bake resin silica sand split molds, to minimize distortion and provide required surface finish.
 
•  The long center core, supported only at each end, must be metal arbor reinforced and top vented. Chaplets may not be used to prevent local defects.
 
•  A large top ring riser and blind side risers are needed to feed heavier sections.
 
•  Zircon sand is necessary in the heavier suspension lug area (where side risers are located).
 
•  Two CDIB’s on were the most efficient mold design.
 
•  With this design, mold yields around 30-35% are obtained. (In commercial production, this might be improved.)
 
The total down sprue on the 500 pounder is over 7.5 feet tall, necessitating metal fill speed trap. Molds are clamped in a pouring jacket and poured vertically. Two Keel blocks are incorporated in each mold for subsequent determination of tensile, Charpy impact, chemical and microstructural properties. All properties need to be met after the final anneal heat treatment, so test bars must be heat treated in the same furnace load as the castings.
 
During mold design work in Phase I, a long run of casting Keel blocks was used to determine metal properties and make process changes to meet target metal properties.
 
Experimentation looked at different silicon levels, carbon equivalents, alloying with nickel, treatments, inoculants, and pouring temperatures to optimize practices. All tests were of sufficient scope to forecast expected 6 sigma control ranges for commercial manufacture. The bottom line is meeting a 3 sigma lower statistical scatter limit. Keel blocks will be required when production is commercialized.
 
During Phase II, three-fourths of the bomb bodies were destructed to gain additional statistical property and casting integrity data, check out dimensional control and develop machining and heat treatment practices. Included were tests from heavy and light sections, top and bottom, and lug areas.
 

 
Figure 3.  Production Mold Design
 
The plan followed was:
 
Produce thirty 500 pound bomb bodies with...
 
• 24 bodies for property testing.
 

 
• Simultaneously cast Keel blocks from poured molds.
- Tensile, impact and chemical properties.
 
• Six CDIB for arena fragmentation and structural tests at the Naval Air Warfare Center, China Lake, California.
 
Simulated Bomb Material Tests
Throughout Phases I and II, parallel projects were contracted to two government development foundries and two commercial ductile iron foundries. Each produced 30" long, 12-14" OD simple right cylinders with wall thicknesses similar to the 0.50-0.60" minimal walls in 500 and 1000 pound bombs. The majority of these and Keel blocks cast simultaneously were destruct tested after anneal heat treatment for tensile, impact chemical and metallographic properties. The purpose was to verify if expectations of final properties are realistic and can be met by any well controlled commercial producer. Some cylinders were also fragmentation tested at the NAWC, at China Lake, as shown in Figure 4.
 

 
Figure 4.  Cylinder Fragmentation Test
 
These results are then verified in actual CDIB fragmentation tests shown in an arena testing in Figure 5.
 

 
Figure 5.  Bomb Test in  Arena
 
In the arena, explosive charges fragment the DI castings, Fragments are captured in Celitex and wood screening for size, weight and distribution analyses. Fragment velocities, distance, impact forces, etc., are either measured or calculated. Results need to duplicate the acceptable patterns of existing commercial steel bodies. These analyses show annealed CDIB bodies do the job. A walnut to marble size appears desirable.
 
Metallurgical Considerations
Ductile iron bomb bodies must provide equivalent properties and performance as any steel assembly. Some important characteristics and reasons are:
 

 
During Phases I and II, a series of composition and processing variations were evaluated. Target being identifying the best potential combination of all mechanical properties. That is highest possible yield strength coupled with highest possible ductility and lowest achievable impact transition temperature, with minimum trade-offs. The following was identified:
 
• All elements except Si, C and Ni need to be as low as practical. Actual limits were set.
 
• Optimum CE and Si should be 4.4 and 2.2 ± 0. 1%. A 1-1.5% Ni level is required at this low Si level to meet yield strength. Si is a ferrite strengthener and contributes to annealability. Ni has one-third the effect of Si on yield strength. Without the Ni, the very critical 40 KSI yield strength is not met, simultaneously with the highest ductility, following full ferritization annealing.

• Additional tests indicate nickel additions can be eliminated if final Si is raised to 2.5%. However, the higher silicon raises the notched bar impact transition temperature. The trade-off study is needed.

• Full ferritizing annealing results in the highest ductility and best impact properties. While subcritical annealing produced equal or slightly higher tensile properties at the sacrifice of ductility and impact. Therefore, current requirement is for full annealing. The reason is full annealing provides increased homogenization of the final matrix structure, reducing the "notching effect" of segregation and cell boundary constituents.
 
• Nodularity approaching 100% and an optimum nodule count of 100-15ON/MM 2 provides best overall properties. Good nucleation in the melt was achieved by use of crystalline graphite and silicon carbide additions during melting and multiple ferrosilicon inoculation steps. Inoculation is enhanced when using a combination of MgFeSi alloy for treatment followed by ladle transfer post inoculation with FeSi or proprietary ladle inoculation plus late stream or mold inoculation at the casting station. With all these silicon additions, a well managed operation is needed to avoid exceeding maximum target final iron silicon levels. Smallest practical additions are necessary at each step. The multiple inoculations also prevent carbides, which affect annealing cycles and have residual property effects.
 
• Complete mold cooling is needed to minimize casting distortion.
 
• Residual Cu and P are particularly detrimental to final ductility and impact properties. These are minimized by careful selection of steel scrap in the charge (slitter steel preferred). A significant charge component of sorel pig iron was also found necessary to maintain low Cu and P levels, manage Mn content to a target of 0.30%, and hold furnace Si low to meet maximum final Si analyses. Cu and P max levels of 0.06 and 0.015 are desirable. The balance of the charge can be returns, but should be only "bomb quality" metal returns.
 
• The minimum properties listed in Table 1 are easily met in Keel blocks poured and heat treated with the castings. Test bars taken from actual castings, although having equivalent tensile and yield strength, had lower ductility and inferior Charpy properties. This tended to occur in all cases and is assignable to isolated centerline microporosity, and perhaps, to micro segregation which is enhanced by the slow solidification in large mass castings. Since these properties necessarily represent n-midsections, and most important strain resisting forces are at the OD, this may not be a significant factor. This is however in no way an acceptance of low radiographic soundness. Keel blocks provide best representation of the true material properties.
 
What’s Next?
The Navy has completed most of the development work on the 500 lb CDIB and is ready to solicit commercial foundries for production contracts. We are told that all development information will be shared with industry. This does not mean the DOD will write prescriptions for production. They are only interested in suppliers meeting the final product properties and quality level specifications. There are of course opportunities for commercial foundries to improve on properties, material costs and processes, such as utilizing cost-saving subcritical annealing, casting mold yield reduction and use of alternative production raw materials and commercial processes.
 
The production of 75 castings in Phase III is underway. These will be furnished for not only use in process verification, but actual flight testing, practice drops and other mission evaluations. Commercial production solicitation could begin by 1999.
 
A very similar program as Phases IV through VI has also been initiated for the 1000 pound bomb. This program should be considerably shorter, having mastered the basics in the 500 pound study. And, we have just learned that the Air Force is now examining the potential for a 2000 pound bomb.
 
There will be a number of opportunities for commercial foundries interested in these types and sizes of castings, Additional information is available from Mr. Joe Etoch, 473320D, NAWC, China Lake, California, 93555-6001.
 
Naval Air Warfare Center - Weapons Division
 

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