ASTM A106 Grade B vs Grade C: Seamless Carbon Steel Pipe Comparison

Two Grades Inside One Specification

ASTM A106 covers seamless carbon steel pipe for high-temperature service, and it defines three grades: A, B and C. Grade B is the grade the market runs on, and Grade C sits above it as the higher-strength option within the same standard. Both are seamless, both are intended for hot service, and both are ordered to the same dimensional tables. What separates them is chemistry and, in turn, the guaranteed strength level.

A106 Grade C is produced and stocked far less than Grade B. It exists for cases where the design calls for greater wall strength without moving into an alloy steel such as A335 P11 or P22, and it is usually chosen by specification rather than by preference.

Chemical Composition: Carbon and Manganese

The primary difference between the two grades is carbon content. Grade B is capped at 0.30% carbon, while Grade C is capped at 0.35% carbon. Note that this is a ceiling, not a band: Grade C is limited to 0.35% maximum carbon rather than being specified to a fixed 0.35–0.40% window, and a certificate quoting a carbon content above 0.35% does not meet the standard. Manganese, silicon and the residual element ceilings are effectively the same for both grades.

Element (%)	A106 Grade B	A106 Grade C
Carbon, C	0.30 max	0.35 max
Manganese, Mn	0.29 – 1.06	0.29 – 1.06
Phosphorus, P	0.035 max	0.035 max
Sulfur, S	0.035 max	0.035 max
Silicon, Si	0.10 min	0.10 min

The higher carbon level is what buys the extra strength, and it is also what changes fabrication behaviour. Grade C is more hardenable and slightly less weldable than Grade B, so welding procedures, preheat requirements and any hardness limits imposed by the design code should be reviewed before the grade is used on a heavily welded system. Where the service involves wet sour conditions, a higher carbon level is also less desirable, and many project specifications exclude Grade C for that reason.

Mechanical Properties: Grade B Against Grade C

The strength difference is the reason Grade C exists. The table below gives the minimum room-temperature values written into the standard.

Property	A106 Grade B	A106 Grade C
Tensile strength, min	415 MPa	485 MPa
Yield strength, min	240 MPa	275 MPa
Typical carbon level	Up to 0.30%	Up to 0.35%
Availability	Widely stocked in all schedules	Limited; often made to order

Both grades also carry a minimum elongation requirement, verified on the tension test specimens required by the specification, and both are subject to the same flattening, bending, hydrostatic and nondestructive examination clauses. Strength is the only mechanical property that separates them in any practical sense; ductility and toughness levels are comparable for the same size and wall.

Manufacturing, Heat Treatment and Availability

A106 pipe is seamless and may be furnished hot-finished or cold-drawn. Hot-finished pipe may be shipped without a separate heat treatment, while cold-drawn pipe must be given a heat treatment after the final cold-drawing pass. Where Grade C is ordered, the higher carbon content makes consistent heat treatment more important, because an untempered or unevenly treated structure in a higher-carbon pipe is more prone to hardness variation.

Product form: seamless only, in both grades

Finish: hot-finished or cold-drawn, with heat treatment required after cold drawing

Ends: plain, beveled or threaded as ordered

Testing: hydrostatic test or nondestructive electric test, plus tensile testing per heat

Sizes: covered by the same dimensional tables, typically to ASME B36.10M

Commercial availability is the practical dividing line. Grade B is a commodity item stocked by distributors in a wide range of nominal sizes and schedules, so lead times are short and price competition is strong. Grade C is a niche item: fewer mills roll it, distributor stock is thin, and it is normally produced against a specific order quantity. That availability gap, rather than any technical shortcoming, is the main reason Grade C remains uncommon.

How the Choice Is Made in Practice

Start with the design pressure and temperature. If Grade B satisfies the wall-thickness calculation for the specified allowable stress, there is normally no reason to specify Grade C, because Grade B is cheaper, easier to source and easier to weld. Grade C becomes relevant when the calculated wall for Grade B would be impractical: heavy-wall headers, high-pressure steam service, or situations where a thinner wall reduces weight and welding cost enough to offset the higher material price.

Where Grade C is selected, two checks should accompany the purchase order. First, confirm that the mill certificate reports carbon at or below 0.35% and that tensile and yield results meet the 485 MPa and 275 MPa minimums. Second, confirm the welding procedure covers the higher-carbon base metal, including any preheat and post-weld heat treatment the design code requires. Because ASME SA-106 and ASTM A106 are technically identical, the same limits apply whichever designation appears on the drawing.

Frequently Asked Questions

Q: What is the main difference between A106 Grade B and Grade C?
Carbon content and strength. Grade C allows up to 0.35% carbon against 0.30% for Grade B, and guarantees higher minimum tensile and yield strength.

Q: Is A106 Grade C specified as 0.35 to 0.40 percent carbon?
No. Grade C is limited to 0.35% carbon maximum. A 0.35–0.40% range is not the specification limit and would not comply.

Q: What strength does A106 Grade C guarantee?
A minimum tensile strength of 485 MPa and a minimum yield strength of 275 MPa.

Q: Why is A106 Grade C less common than Grade B?
Because Grade B meets most designs at lower cost and is widely stocked, while Grade C is produced by fewer mills and is usually made to order.

Q: Is Grade C harder to weld?
Its higher carbon content makes it more hardenable and slightly less weldable, so welding procedures, preheat and any required post-weld heat treatment should be reviewed.

Q: Are there welded versions of these grades?
No. ASTM A106 covers seamless pipe only, in Grades A, B and C alike.