What Is ASTM A333 Grade 6?

Quick Answer

ASTM A333 Grade 6 is a killed, fine-grain carbon-manganese steel pipe designed for low-temperature service down to -46°C (-50°F). It is the most widely specified grade within ASTM A333 and the standard choice for carbon steel piping in LPG facilities, cold-climate pipelines, and depressurization systems where Joule-Thomson cooling occurs.

Grade 6 is available in both seamless and welded forms. Its chemical composition and mechanical properties are similar to ASTM A106 Grade B, but the killing practice (fully deoxidized with silicon and/or aluminum) and fine-grain treatment give it the toughness required to pass mandatory Charpy V-notch impact testing at -46°C.

Chemical Composition

Element	A333 Grade 6 (max %)	A106 Grade B (max %)
Carbon	0.30	0.30
Manganese	0.29-1.06	0.29-1.06
Phosphorus	0.025	0.035
Sulfur	0.025	0.035
Silicon	0.10 min	0.10 min
Chromium	-	0.40
Copper	-	0.40
Molybdenum	-	0.15
Nickel	-	0.40

Note the tighter limits on phosphorus and sulfur compared to A106 Gr. B. These “tramp elements” reduce impact toughness at low temperatures and are therefore restricted.

Mechanical Properties

Property	A333 Gr. 6	A106 Gr. B
Yield strength (min)	240 MPa (35 ksi)	240 MPa (35 ksi)
Tensile strength (min)	415 MPa (60 ksi)	415 MPa (60 ksi)
Elongation (min)	30% (in 50 mm gauge)	30% (in 50 mm gauge)
Charpy impact at -46°C	18 J avg (13 ft-lbs)	Not required
Impact test temperature	-46°C (-50°F)	Not specified
Hardness (max)	197 HBW (typical)	197 HBW
Grain size	Fine grain (ASTM 5 or finer)	Not mandatory

Impact Testing Requirements

The defining feature of ASTM A333 Grade 6 is the mandatory Charpy V-notch impact test at or below -46°C.

Requirement	Value
Test temperature	-46°C (-50°F) or lower
Specimen orientation	Longitudinal
Specimen size	Full-size (10 x 10 mm) when WT permits
Minimum average (3 specimens)	18 J (13 ft-lbs)
Minimum single specimen	14 J (10 ft-lbs)
Sub-size specimen	Proportionally reduced values

When to Specify A333 Grade 6

• Any carbon steel piping service at or below -29°C per ASME B31.3 (the typical threshold below which impact testing is required)
• LPG and propane storage and handling (operating temperatures to -42°C)
• Cold-climate pipeline stations and process plants
• Depressurization and blowdown systems where rapid gas expansion causes significant cooling
• Air cooler outlet piping in cold ambient conditions

Pro Tip

ASME B31.3 requires impact testing for carbon steel below -29°C (curve A materials) or when the MDMT falls below the exemption temperature. Specifying A333 Gr. 6 satisfies this requirement because the specification includes mandatory impact testing at -46°C. However, if the design minimum temperature is between -29°C and -46°C, some EPC contractors still accept A106 Gr. B with supplementary impact testing (supplementary requirement S5). Verify the project specification before defaulting to A333 Gr. 6.

A333 Grade 6 vs. Other Low-Temperature Options

Material	Min. Temp	Specification	Cost Factor
A333 Gr. 6 (CS)	-46°C	ASTM A333	1.0x
A333 Gr. 3 (3.5% Ni)	-101°C	ASTM A333	2.5-3.5x
A333 Gr. 8 (9% Ni)	-196°C	ASTM A333	5-8x
A312 TP304L (SS)	-196°C	ASTM A312	4-6x
A312 TP316L (SS)	-196°C	ASTM A312	5-7x

Grade 6 is the most economical option for temperatures down to -46°C. Below that threshold, the cost increases significantly as nickel alloy or stainless steel becomes necessary.

Read the full guide to pipe types