When a well produces fluids containing hydrogen sulphide (H₂S), every metallic component in contact with the produced fluid must resist sulphide stress cracking (SSC). For production tubing, this means specifying API 5CT Grade L80 — the standard sour-service tubing grade engineered to provide the controlled yield strength and hardness that NACE MR0175/ISO 15156 demands.
What Makes L80 Sour-Service Capable
L80 differs from N80 in two critical ways: a maximum yield strength limit (655 MPa) and a maximum hardness limit (23 HRC / 253 HV). N80 has a minimum yield of 552 MPa with no upper limit, meaning it can be supplied at yield strengths well above 655 MPa. While higher strength is normally desirable, in sour service it is dangerous — steels above the NACE hardness threshold become susceptible to SSC, where atomic hydrogen generated by the H₂S-steel reaction diffuses into the steel and causes brittle cracking under applied stress.
The “L” in L80 stands for controlled yield — both minimum (552 MPa) and maximum (655 MPa). This controlled range ensures the material falls within the safe zone for SSC resistance while still providing adequate mechanical properties for standard completion loads. The hardness limit of 23 HRC further constrains the microstructure, preventing the formation of hard phases (martensite, upper bainite) that are most vulnerable to hydrogen embrittlement.
Heat Treatment Requirements
Achieving the L80 yield-strength window requires precise heat treatment. The standard practice is quench and temper (Q&T): austenitise at 860–920°C, oil-quench to produce a fully martensitic starting structure, then temper at 620–700°C to reduce hardness below 23 HRC while maintaining yield strength above 552 MPa. The tempering temperature must be high enough to adequately soften the martensite but not so high that yield strength drops below the minimum.
Alternative L80 variants exist: L80 Type 1 (standard carbon steel, Q&T), L80 9Cr (9% chromium for CO₂ corrosion resistance in addition to SSC resistance), and L80 13Cr (13% chromium for more aggressive CO₂ and mild chloride environments). Each variant addresses different combinations of sour service, CO₂ corrosion, and chloride exposure.
Testing Beyond Standard API
While API 5CT defines L80’s chemistry, mechanical properties, and hardness requirements, many operators impose additional testing for sour-service tubing: SSC testing per NACE TM0177 (Method A tensile test in H₂S-saturated solution for 720 hours), through-wall hardness surveys on every heat to verify no hard zones exist, and 100% electromagnetic inspection (EMI) to detect surface and near-surface defects that could act as SSC initiation sites. These supplementary requirements should be specified on the purchase order, as they are not automatically included in standard API 5CT production.
Design Considerations
L80 tubing design must account for the yield strength ceiling. Unlike N80 or P110, where the actual yield may significantly exceed the minimum, L80’s actual yield is typically 570–640 MPa — close to the minimum. Casing design software should use the actual yield from the MTC rather than the API minimum for more accurate load rating calculations. For wells where L80’s tensile or burst capacity is marginal, increasing the wall thickness (heavier weight per foot) is the standard approach, since upgrading to a higher-strength sour grade (C90, T95) significantly increases material cost.
Need L80 tubing for sour service? Send us your well and H₂S data — we will confirm grade, NACE testing scope, and delivery.