When a drawing calls out A106 Gr B or A333 Gr 6, the number is not a part code, it is a statement of intent: how the pipe was made, what service it is qualified for, and what it was tested to. Get the spec wrong and the chemistry can look fine while the pipe is wrong for the job. This guide walks the carbon specs first, then stainless, then the alloy and duplex grades.
A106 vs A53: the Most Common Mix-Up
Both are carbon steel pipe, and A106 Grade B and A53 Grade B have nearly the same room-temperature strength (60 ksi tensile, 35 ksi yield). The difference is intent:
- A106 is seamless only and qualified for high-temperature pressure service: boilers, refineries, process steam.
- A53 can be welded or seamless and is a general-service spec for structural work, fire protection, and lower-pressure conveyance.
Because the strength matches, people substitute one for the other. On a code job that is a mistake; the specification, not the strength chart, governs.
ASTM Pipe Spec Decoder
Decode ASTM pipe specifications: A106 (high-temperature seamless carbon), A53 (general carbon, welded or seamless), A312 (austenitic stainless), A333 (low-temperature impact-tested), A335 (chrome-moly for creep service), and A790 (duplex). Returns the service intent, manufacturing method, chemistry, and mechanicals, and flags the A106-vs-A53, low-temperature, and P91 post-weld heat treatment distinctions that bite on procurement.
A333: Bought for the Cold
A333 Grade 6 has essentially the same chemistry and room-temperature strength as A106/A53 Grade B. What you are paying for is the Charpy V-notch impact test at low temperature, commonly -50 F. Carbon steel loses toughness as it gets cold and can fail in a brittle manner; A333 verifies it will not. For LPG, refrigeration, and cold-climate piping, never substitute a non-impact-tested carbon pipe.
A312 and A790: the Stainless Pipes
A312 covers austenitic stainless pipe: TP304 for general corrosion resistance, TP316/TP316L (with molybdenum) for chlorides and chemical service, and the L grades for welded corrosive work. A790 covers duplex stainless (2205) and super duplex (2507), which carry roughly twice the yield of 316 and much better chloride stress-cracking resistance. The catch on duplex is a temperature ceiling: above about 600 F it embrittles, so it is a strength-and-corrosion pipe, not a high-temperature one.
A335: Chrome-Moly for Heat
A335 is ferritic alloy pipe for high-temperature creep service. P11 (1.25Cr-0.5Mo) and P22 (2.25Cr-1Mo) need controlled preheat, a matching low-alloy filler, and post-weld heat treatment. P91 (Grade 91, 9Cr-1Mo-V) is the modern high-efficiency grade, and it is unforgiving: its creep strength comes from a precise normalize-and-temper and a tightly controlled PWHT. Welding P91 like a P22, skipping PWHT, or overshooting the tempering temperature ruins the very property it was chosen for.