NPT Pipe Thread Fittings manufactured in Grade 12.9 Steel (class 12.9) are dimensioned to ASME B1.20.1. The sections below give the mechanical and physical properties of the grade, its behavior in corrosive and elevated-temperature service, and the full size table.
Why Choose Grade 12.9 Steel for NPT Pipe Thread Fittings
The highest common strength class, for high-load machine assembly. For npt pipe thread fittings, this matters when requires plating or black-oxide protection is the primary requirement.
Mechanical & Physical Properties
| Grade designation | class 12.9 |
|---|---|
| Magnetic response | Magnetic |
| Service temperature | up to approx. +300 °C |
| Corrosion characteristic | Requires plating or black-oxide protection |
Corrosion Behavior in Service
Carbon and alloy steel npt pipe thread fittings require a coating for corrosion protection. Zinc plating (with or without passivation) is the most common; hot-dip galvanizing is used for outdoor structural service; black-oxide is mainly a tarnish-prevention finish for tool-room use.
Comparable Grades
For NPT Pipe Thread Fittings, class 8.8 is the comparable strength step. Move up when the joint preload approaches the proof load of the current class; move down when the joint is over-specified. For corrosion-critical service, switch to stainless A2 or A4.
Sizes of NPT Pipe Thread Fittings Available in Grade 12.9 Steel
| Size | Nominal size (in) | Major diameter (mm) | Pitch (mm) | Threads per inch (TPI) | Taper | Reference standard | Notes |
|---|---|---|---|---|---|---|---|
| 1/16 NPT | 1/16 | 7.938 | 0.941 | 27 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1/8 NPT | 1/8 | 10.287 | 0.941 | 27 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1/4 NPT | 1/4 | 13.716 | 1.411 | 18 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3/8 NPT | 3/8 | 17.145 | 1.411 | 18 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1/2 NPT | 1/2 | 21.336 | 1.814 | 14 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3/4 NPT | 3/4 | 26.67 | 1.814 | 14 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1 NPT | 1 | 33.401 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1-1/4 NPT | 1-1/4 | 42.164 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1-1/2 NPT | 1-1/2 | 48.26 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 2 NPT | 2 | 60.325 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 2-1/2 NPT | 2-1/2 | 73.025 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3 NPT | 3 | 88.9 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3-1/2 NPT | 3-1/2 | 101.6 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 4 NPT | 4 | 114.3 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 5 NPT | 5 | 141.3 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 6 NPT | 6 | 168.275 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |