API 5L X65 and X70 are the dominant grades for long-distance gas transmission pipelines. This guide compares the two grades on strength, toughness and welding, and explains how to pick the right one for onshore and offshore gas line projects.
API 5L line pipe is the standard material for oil and gas transmission pipelines. The two grades that dominate onshore long-distance gas line procurement are X65 (minimum yield 65,000 psi / 450 MPa) and X70 (minimum yield 70,000 psi / 485 MPa). Higher grades (X80, X90, X100) are used in specific high-pressure projects, but X65 and X70 are the workhorses for the majority of gas transmission systems built today.
Strength and Weight Savings
Moving from X65 to X70 increases the minimum yield strength by about 8%, which allows the pipe wall thickness to be reduced for a given design pressure. For a 48-inch (DN1200) gas line at 1200 psi design pressure, switching from X65 to X70 typically allows a wall thickness reduction of about 1–2 mm. The thinner wall saves steel weight, lowers transportation cost, and reduces welding time and consumable cost in the field, which often offsets the higher per-tonne price of X70 plate.
When X65 Is the Standard Specification
X65 line pipe is the default specification for most onshore gas transmission projects at design pressures up to about 1500 psi. It has well-understood field welding procedures, a mature supplier base worldwide, and proven performance in service. For projects with moderate diameter (NPS 16 to NPS 36), moderate pressure, and standard operating temperatures, X65 is the right balance of strength, toughness and cost.
When X70 Is the Required Upgrade
X70 line pipe is required for high-pressure gas transmission (above about 1500 psi), for large-diameter trunk lines where the wall thickness reduction gives significant tonnage savings, and for projects where the operator wants to reduce the number of field girth welds by using longer pipe joints. X70 is also specified for sour service applications when paired with the appropriate supplementary requirements (SSC testing per NACE TM0174). The trade-off is more demanding field welding procedure qualification, with tighter control of heat input and interpass temperature.
Toughness, Welding and Sour Service
Charpy impact toughness at the design temperature is at least as important as the strength grade for line pipe. For cold-climate pipelines, specify API 5L PSL2 with supplementary requirement for Charpy at the lowest expected operating temperature (typically -20 °C, -30 °C, or -46 °C depending on location). For sour service, the pipe must comply with NACE MR0175 / ISO 15156, with controlled hardness, sulfur and microstructure. For offshore and Arctic service, additional requirements for strain-based design and low-temperature toughness may apply.
What to Specify on the Line Pipe RFQ
State the standard (API 5L, latest edition), the product specification level (PSL1 or PSL2), the grade (X65 or X70), the delivery condition (normalized, quenched and tempered, thermomechanical rolled), the pipe type (seamless, DSAW, ERW, or UOE depending on diameter), the diameter and wall thickness, and the supplementary requirements (Charpy temperature, NACE, hydrostatic test pressure). For high-pressure transmission, default to PSL2; for lower-pressure gathering systems, PSL1 may be acceptable.
Related Resources from Baobin Steel
- Carbon steel pipe catalog – API 5L and structural pipe
- Mild steel overview – grades and standards
- Request a quote – send your line spec for API 5L pipe pricing