API 5L X70 Pipeline Steel

API 5L X70 is a medium-hardness (~230 HV) HSLA pipeline steel with a density of 7.85 g/cm³. The microstructure consists of acicular ferrite and bainite with fine microalloy precipitates. The relatively low carbon content (0.07%) and fine-grained microstructure make this a straightforward material to prepare. Banding assessment requires examination in the longitudinal (rolling) direction. Weld zone analysis (HAZ characterization) is also common for pipeline steels.

Use an abrasive cut-off wheel designed for carbon/low-alloy steel (aluminum oxide abrasive). Standard wheel thickness (1.0-1.5 mm). Cutting speed: 200-300 RPM with generous coolant. The moderate hardness (~230 HV) allows reasonable cutting rates. For banding assessment, section in the longitudinal (rolling) direction to reveal the banding pattern. For weld analysis, section perpendicular to the weld to reveal the weld metal, HAZ, and base metal in one cross-section. Leave 2-3 mm allowance for grinding.

Hot compression mounting with phenolic or epoxy-phenolic resin is standard. The material is thermally stable at mounting temperatures (150-180°C). Cold mounting with castable epoxy is also acceptable. For pipeline weld specimens, which may be large, ensure the mount is large enough to include the full weld profile (weld metal, HAZ, and base metal on both sides). Standard mounting practices apply.

Standard grinding procedure for low-alloy steels. Use SiC grinding papers with abundant water lubrication. Disc speed: 250-300 RPM. Apply moderate pressure (25-35 N per 30 mm sample).

Grinding sequence:

• 120 grit: Remove sectioning damage (20-40 seconds). Moderate pressure.
• 240 grit: Remove previous scratches (20-40 seconds).
• 320 grit: Refinement (20-40 seconds).
• 400 grit: Further refinement (20-40 seconds).
• 600 grit: Final grinding (20-40 seconds).

Rotate specimen 90° between steps. Use complementary rotation. The relatively uniform hardness of the acicular ferrite/bainite structure grinds evenly.

Standard polishing for low-alloy steels. The fine-grained microstructure polishes well.

Diamond polishing sequence:

• 6μm diamond: 3-5 minutes on a medium-hard synthetic pad with moderate pressure (25-30 N per 30 mm sample).
• 1μm diamond: 2-4 minutes on a synthetic pad (20-25 N).

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad (15-20 N). Colloidal silica final polishing is particularly effective for revealing the fine acicular ferrite and bainite microstructure in HSLA steels.

Standard Nital etching is the primary method for revealing the acicular ferrite and bainite microstructure in HSLA pipeline steels.

2-5% Nital (Chemical Etching) - Primary choice:

• Composition: 2-5 ml HNO₃, 95-98 ml ethanol
• Application: Immerse for 5-15 seconds. Start with 2% Nital; increase to 5% if more contrast is needed.
• Reveals: Acicular ferrite laths, bainite packets, prior austenite grain boundaries (with longer etching), and compositional banding. Excellent for banding assessment per ASTM E1268.
• Rinse: Ethanol, then dry with warm air.

4% Picral (Chemical Etching) - For phase differentiation:

• Composition: 4 g picric acid, 100 ml ethanol
• Application: Immerse for 10-30 seconds.
• Reveals: Pearlite colonies and bainite with better selectivity than Nital. Useful for distinguishing bainite from pearlite in the banded regions.
• Safety: Picric acid is shock-sensitive when dry. Keep wetted at all times. Store properly.