Muntz metal is an alpha-beta brass which is composed of 60% copper and 40% zinc. Since it contains a large portion of beta component (zinc), it is stronger, harder, and more rigid than brasses of lesser zinc content. Furthermore, it is relatively less expensive than other brass. Because of these and its distinctive color, it is usually used in architectural projects. This alloy hardens rapidly which makes it not suitable for cold working and is widely used in hot-stamping and extrusion process instead. Muntz metal was named after George Frederick Muntz of Birmingham England after commercializing the alloy in 1832.Muntz metal is also known as C28000, Alloy 280, malleable brass and yellow metal(sequoia-brass-copper.com; http://www.brass-extrusions.com).
The nominal composition of Muntz metal is 60/40 brass. However, its composition can range from 59-63% copper, 0.09% lead and 0.07% iron and the balance zinc (shown in Table 1). It has a machinability rating of 40, which is relatively good. Welding, soldering and brazing are rated as excellent, oxyacetylene welding, spot welding and butt welding as good, gas shielded arc welding as fair, and coated metal arc welding and seam welding as not recommended. The hot forging rating (at T=11500-1450 ºF) for this alloy is 90. Furthermore, this alloy has a very good hot malleability, fair cold malleability, high strength and low ductility. Its natural color is reddish yellow but eventually turns red-brown or gray-brown when weathered. The annealing temperature is between 800-1100ºF. Table 3 shows the physical characteristic of Muntz metal (sequoia-brass-copper.com; http://www.brass-extrusions.com).
Table 1. Composition of Muntz metal.
| Cu | Zn | Fe | Pb | |
| Min/Max | 59-63% | Balance | 0.07% | 0.09% |
| Nominal | 60% | 40% | – | – |
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Table 2. Fabrication characteristics of Muntz metal.
| Joining Technique | Suitability |
| Brazing | Excellent |
| Butt Weld | Good |
| Capacity for Being Cold Worked | Fair |
| Capacity for Being Hot Formed | Excellent |
| Coated Metal Arc Welding | Not Recommended |
| Forgeability Rating | 90 |
| Gas Sheilded Arc Welding | Fair |
| Machinability rating | 40 |
| Oxyacetylene Welding | Good |
| Seam Weld | Not Recommended |
| Soldering | Excellent |
| Spot Weld | Good |
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Table 3. Physical characteristics of Muntz metal.
| Product Property | US Customary | Metric |
| Coefficient of Thermal Expansion | 11.6 ·10.6 per F (68-572 F) | 20.9 · 10 6 per °C (20-300 C) |
| Density | 0.303 lb/in3 at 68 F | 8.39 gm/cm3 @ 20 C |
| Electrical Conductivity | 28 %IACS at 68 F | 0.164 MegaSiemens/cm at 20 C |
| Electrical Resistivity | 37.0 ohms-cmil/ft at 68 F | 6.15 microhm-cm at 20 C |
| Melting Point Liquid US | 1660 F | 904 C |
| Melting Point Solid US | 1650 F | 899 C |
| Modulas of Elasticity in Tension | 15000 ksi | 103400 MPa |
| Modulas of Rigidity | 5600 ksi | 38610 MPa |
| Specific Gravity | 8.39 | 8.39 |
| Specific Heat Capacity | 0.09 Btu/lb/°F at 68 F | 377.1 J/kg · °K at 293 K |
| Thermal Conductivity | 71.0 Btu · ft/(hr · ft2 ·°F) at 68 F | 1229.9 W/m · °K at 20 C |
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The fabrication of muntz is generally divided into two steps: forming and joining. Forming in terms of metal forming is the processing of metal through mechanical deformation wherein the object is reshaped without the addition of removal of material. Forming of Muntz metal can be done by bending, brake forming, roll forming, extrusion, casting, forging, stamping, hydroforming, spinning, laminating and explosive forming. Bending is the simplest forming method used to create simple curves and is usually conducted at either ambient or elevated temperature. Unlike bending, brake forming can create simple curves or angles in straight lengths of sheet, strip and plate. However, the section length is limited to the size of the brake and longer materials are processed by roll forming. The corners of the products of the roll forming is generally not as sharps as those from brake forming. Extrusion is done by forcing the heated metal, with the use of pressure, through a die opening shaped to the desired cross-section of the end product. Stamping is done with use of shaped dies and power press or hammer. In hydroforming, the malleable sheet is placed over a male punch or die and hydraulically pressed. Spinning is used to from circularly symmetric shells by working sheet stock over a mandrel as the mandrels spins. Laminating is used to form composite panels. Lastly, explosive forming are usually used to form very large architectural elements to exempt the use of large equipment (Copper Development Association [CDA], 2013).
Joining of Muntz metal and any other alloy can be done in three ways namely seaming, fastening and metallurgical bonding. Seaming is suitable for interior use where the material is attached to the underlying substrate with concealed fasteners, providing a smooth look. Fastening utilizes mechanical fasteners such as screw, bolts and rivets to connect pieces, either to each other or to an underlying structure. Metallurgical bonding is a process of joining which includes soldering, brazing and welding. The difference among the three metallurgical bonding processes is the temperature with which they are conducted, the filler they used and the strength of the end product. Solderingis done at relatively low temperature with the use of filler metals that melts at 350-800 ºF. Since soldering is a low-strength joining process, it is often reinforced with clinch, rivets or screws. Brazing is an intermediate-temperature and intermediate-strength joining process that used nonferrous filler materials that melts at 1100-1500 ºF. Welding is done at high temperature and/or pressure with or without the use of filler(Copper Development Association [CDA], 2013).
After fabrication, another step is employed to the product called finishing or metal finishing. Metal finishing is done to remove debris or unwanted objects on surface of the fabricated product and to make it corrosion resistant. Finishes for Muntz metal can be done with use of mechanical, chemical, protective coating and laminated finishes. Mechanical finishes involves mechanical operation and can be further classified into as fabricated, polished and buffed, directionally textured, nondirectional texturedand patterned finishes. Chemical finishes includes conversion coatingswhich are done to mimic natural weathering. Protective coatings are finishes applied for protection but may also provide visual effects. They are divided into two general forms. This includes transparent coating that preserve natural color, texture and metallic luster of copper metals, and opaque coatings that used to prevent corrosion and abrasion while retaining formability. Laminated finishes, like protective coatings, impart corrosion dn abrasion resistance to the metal as well as long-time resistance to degradation by sunlight (Copper Development Association [CDA], 2013).
The demand of Muntz metal is high in countries such as India and United States. Muntz is relatively cheap because of its high zinc content. One of the suppliers is the Continental Steel & Tube Co. that supplies the alloy in plates or sheets in wide variety of width, gauges and tempers.
Muntz metal is originally used as replacement for copper sheathing on the bottom of boats beacause it maintained anti-fouling characteristics of copper at lower price. Today, it is industrially used as condenser tube, support sheets, heat exchanger baffles, heat exchanger plates, engraving plates, condenser tube plates, and main tube sheets for condensers and heat exchangers. It is also used as architectural panels, large architectural trims, structural sheets, door frames, decorative hardware and valve stems, brazing rods and signage (www.brass-extrusions.com/). Because it is highly resistant to corrosion, it is usually employed as construction material for equipment used in highly corrosive chemicals.
References:
Copper Development Association. (2013). Copper brass bronze design handbook: Architectural applications. Retrieved on September 7, 2015 from http://www.copper.org/applications/architecture/arch_dhb/technical-discussion/copper_alloys/.
http://www.brass-extrusions.com/c28000-muntz-metal-2102816.html
http://www.sequoia-brass-copper.com/365FPU.htm
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