Good Mid-Range Corrosion Resistant Alloy
- Mid-range corrosion resistant alloy
- Decent mechanical strength to 1000°F
- Alloy 20 chemistry and specifications
- Alloy 20 sulfric acid performance
- 12 point screw features and benefits
Also known as Carpenter 20 and 20CB-3, Alloy 20 12-point screws offer users a combination of nickel, chromium, molybdenum, and copper which provides good general corrosion resistance. Alloy 20 screws are best suited for mid-range corrosion use – better than steel alloys but not at the level of high performance nickel based alloys like Hastelloy C276. Alloy 20 was designed specifically to withstand the harsh environment of hot sulfuric acid but also performs well in a variety of other acids. Alloy 20 12-point screws are an ideal choice when you don’t need the corrosion resistance power and price-tag of high performance nickel alloys like Hastelloy C276; but you need something that will give you more protection than 316 stainless steel or even a Super Duplex 2507 stainless steel.
Alloy 20 12-point screws provide good performance in hot sulfuric acid, specifically they offer good resistance to stress-corrosion cracking in boiling sulfuric acid at specific range of 20% to 40% concentration, as well as general sulfuric acid resistance. Alloy 20 offers solid acid protection and will perform better than Duplex or even Monel, but for high temperature and high concentration acids, you may need to look to Inconel or Hastelloy fasteners. In addition, Alloy 20 12-point screws also show useful resistance to phosphoric acid and nitric acid.
Key Corrosion Resistant Features:
- Good sulfuric acid protection at 20% - 40%
- Decent stress corrosion cracking, pitting and crevice corrosion resistance in chloride
- Resonable general corrosion protection
- Good intergranular corrosion resistance in the as-welded condition
In addition to their corrosion resistance, Alloy 20 12-point screws also perform well at moderately high temperatures, with the ability to maintain their mechanical strength up to 1000°F.
Alloy 20 12-point screws are stabilized with Columbium to minimize carbide precipitation during welding. In addition it offers intergranular corrosion resistance in the as-welded condition.
• Power plants rely on Alloy 20 12-point screws for flue-gas desulfurization
• Alloy 20 12-point screws are used to connect heat exchangers, mixing tanks, metal cleaning equipment, pickling tanks and racks, and piping in mid-range corrosive chemcial processing environments.
• Alloy 20 12-point screws help solve corrosion issues in food and dye production.
• Manufacturers of plastics and synthetic rubber also utilize alloy 20 fasteners.
The driver of an Alloy 20 12-point screw uses two overlapped hexagon shapes, creating 12-points and a flanged underside. These are also referred to as ferry cap screws or 12-point flange screws. The advantages of this head style include:
• Higher torque capability compared to a socket head cap screw
• Lack of recess to trap fluid or debris.
• As the heads are generally smaller than a hex, Alloy 20 12-point screws are often used situations where installation space is tight and saving weight is critical.
• The overall disadvantage is the extra cost involved in forming the heads.
The Controversy of 6 vs. 12 point Bolts
There is much debate over which bolt is better. Some say that the more points a nut or bolt has – such as a 12-point bolt - the less chance you have to round it off. And that the additional contact points of a 12 point screw give you more surface to apply load. While the jury is out as to the accuracy of this, it’s the application that should dictate whether the benefits of a 12 point screw out weight the additional costs of making them.
Alloy 20 Specifications: UNS N08020, ASME SA240, , ASME SB366, ASME SB462, ASME SB463, ASME SB464, ASME SB473, ASME SB729, ASTM A240, ASTM B366, ASTM B463, ASTM B464, ASTM B473, ASTM B729, ASTM BB462, ANSI/ASTM A555-79, EN 2.4660, , Werkstoff 2.4660, CN7M, DIN 2.4660
Alloy 20 Sulfuric Acid Comparison