Cannon Goes Light with Steel Alloy
Steel’s high yield strength and tolerance of deflection helped minimize weight in a 15-piece fabrication to casting conversion.
Gary Burrow, HA Burrow Pattern Works Inc., Silesia, Montana
(Click here to see the story as it appears in February's Modern Casting.)
Emerging designs for U.S. Army cannons are required to be smaller and lighter for integration into faster, more agile military land vehicles. Castings are well established in the private sector to reduce weight in cars and trucks while increasing structural performance, but they are underutilized in military applications. High performance specifications, while challenging, are not the primary reason for this underutilization; rather, it is the long lead time typically required to fully comply with the military’s dimensional, surface integrity and internal integrity requirements. The deflector tray casting for the Army’s XM360 cannon (which is being redeveloped into the XM360E1 for the next M1 Abrams tank upgrade) is a success story illustrating how casting design was integrated with manufacturing engineering to transcend the usual outcomes of delays and cost overruns. The integration of the casting process, casting finishing processes and machining of the rough casting resulted in an approved finished component 110 days from the Army’s order date.
The deflector tray is both structural and functional. The tray guides a new projectile into the cannon’s breech and deflects the spent casing into a holding area after the projectile is fired through the barrel. Due to force and surface wear, a steel alloy is required. Steel castings are renowned for structural toughness in severe service applications but also notorious for difficulty in meeting cosmetic and dimensional requirements within a reasonable lead time. Because of their high yield strength, high material stiffness and tolerance of deflection without damage, steel castings also can be effective in lightweight applications. Pushing the limits of minimum as-cast wall thickness is important in reducing weight in this type of application.
H.A. Burrow Pattern Works (HAB), Silesia, Mont., collaborated with the American Metalcasting Consortium (AMC) sponsored by the Defense Logistics Agency (DLA), the Army’s Research, Development & Engineering Command (RDECOM), Benet Laboratory at Watervliet Arsenal, Watervliet, N.Y., Acme Castings, Huntington Park, Calif., and the Steel Founders’ Society of America (SFSA) to convert an original 15-piece fabricated steel design to a one-piece, lightweight 19.5-lb. cast steel design.
The 15-piece fabrication was difficult and expensive to manufacture and had an unacceptably high rejection rate. One of the first decisions in the new design was to choose a different steel alloy, because the carbon steel used for the fabrication would not be a good casting candidate due to fluidity issues in the thin 0.08-in. wall sections. HAB determined an investment cast 17-4 PH alloy in H1100 condition had the fluidity and strength to be a good cast substitute.
After numerous solid model iterations, additional features previously welded to the fabricated tray were added to the one-piece casting design, while still keeping the component’s total projected weight to 20 lbs. The additional features were added to the model casting requirements, and the technical data package (TDP) was modified accordingly. Benet sent a request for quote (RFQ) and TDP to a number of investment casters with the requirement to provide one prototype casting for testing, and then the project hit a snag. Benet received a few bids that were unacceptably high in price and lead time. Many investment casters who did not bid provided the following reasons:
- The walls were too thin.
- The thin-to-thick cross sections were difficult (0.08-in. walls to 1.5-in. x 1.75-in. mounting areas).
- The casting was too rangy to readily be straightened.
- Grade B and C X-ray requirements were too difficult for the casting configuration.
After careful consideration by the team, it was determined the deflector tray still was a viable casting design, and TDP requirements could be produced. HAB, with 85 years of experience in the tooling casting industry and extensive work with many casting facilities, took the lead and sent out a more detailed RFQ.
HAB selected Acme Castings, Huntington Park, Calif., to produce the part based on an agreement that HAB would supply the stereolithography (SLA) patterns, rigging and design and take total responsibility for the finished casting, paying for it regardless of meeting all requirements.
From Order to Completion
Benet issued a purchase order and the manufacturing process was started. Heat treatment of rangy parts is prone to cause distortion. The first process was to develop a means to achieve the dimensional requirements. HAB added tie bars, which would be cut off prior to straightening, to the casting model to minimize the distortion during the quench. The next issue involved facilitating part inspection during the straightening process. The team decided to design an inspection fixture using go/no-go gages on critical features. A conventional metal inspection fixture was deemed too costly with too great a lead time for a single casting. As an alternative, the check fixture was designed to use the conventional metal go/no-go gages. HAB created the gating design using CAD modeling, but did not perform verification with solidification software due to the high cost and long lead time estimates received. Instead, the company relied on its extensive tooling experience and ability to produce a sound casting.
After inspection, HAB shipped the SLA patterns to Acme Castings, along with a gating design model that included fully dimensioned gating drawings and all wax gate bars cut to size and identified. The metalcaster gated the part according to HAB’s design, dipped the ceramic and fired the shell. The shell was cleaned to remove ash residue from the sintered SLA material, sealed and wrapped with k-wool insulation in the locations on the drawing provided by HAB to ensure proper solidification. The ceramic shell was heated in an oven and the castings poured to HAB-specified shell and metal temperatures.
Upon shakeout and cleanup, the casting exhibited some minor misruns and nonfills along the rim of the 0.08-in. walls. These areas were weld repaired and cleaned, and the casting was sent to hot isostatic pressing (HIP) to help ensure metal quality and meet the nondestructive testing (NDT) requirements. After heat treatment, Acme Castings removed the tie bars and set the casting up for straightening. Upon placing the casting in the printed inspection fixture, the metalcaster discovered the casting had hardly moved at all and minimal straightening was required. After straightening, the casting was inspected in the fixture using go/no-go gages and then sent to final heat treatment. An outside lab performed NDT X-rays to Grade B in critical areas and Grade C in the balance of the casting and performed 100% die penetrant. The entire casting passed except for a minor amount of ash residue that fell into a machined area. Because this was to be machined off, no repair was required.
Finally, Acme performed 100% coordinate-measuring machine (CMM) dimensional inspection. While the casting did not pass every dimension due to shell restriction, the profile tolerance allowed enough adjustment to make the casting usable for machining to ensure form, fit and function in the gun system and possible firing test sample. The collaboration achieved an approved prototype, on the first attempt, and delivered a finished casting in 110 days, thus demonstrating a lightweight, high performance steel casting can replace a 15-piece fabrication.