API 5CT Casing & Tubing Grades

Q: What does the number in an API 5CT grade like N-80 or Q-125 mean?

In API 5CT (ISO 11960), the number is minimum yield strength in ksi and the letter is an arbitrary designator with no chemistry meaning. N-80 = 80 ksi minimum yield (552 MPa); Q-125 = 125 ksi (862 MPa). Yield across the grade range spans 40 to 125 ksi minimum.

The letter-number grade naming is the language of oilfield tubulars — N-80, L-80, P-110, Q-125. Here's what each grade actually means in minimum yield, hardness limit, heat-treatment regime, and H₂S sour-service status per NACE MR0175.

Spec

API 5CTISO 11960

Yield range

40–125ksi min

Sour-service

L-80 · C-90 · T-95 · C-110

PSL tiers

1 · 2 · 3

How grades are named Letter-number: the letter is an arbitrary designator (H, J, K, L, N, C, T, P, Q) with no chemistry meaning on its own — the number is minimum yield strength in ksi. N-80 = 80 ksi min yield (552 MPa); Q-125 = 125 ksi (862 MPa). L-80 and N-80 share nominal yield but differ in metallurgy and maximum hardness. Casing (4.5–20 in) and tubing (1.5–4.5 in) share the same grade designations.

Master Grade Table

Grade	Min Yield (ksi / MPa)	Max Yield (ksi / MPa)	Min UTS (ksi / MPa)	Max HRC	Heat treat	Sour service	Typical application
H-40	40 / 276	80 / 552	60 / 414	—	As-rolled / N	No	Conductor, shallow surface
J-55	55 / 379	80 / 552	75 / 517	—	As-rolled / N	No	Surface & shallow production
K-55	55 / 379	80 / 552	95 / 655	—	As-rolled / N	No	Same depth as J-55, higher UTS
N-80 Type 1	80 / 552	110 / 758	100 / 689	—	N or N&T	No	Sweet production casing
N-80 Type Q	80 / 552	110 / 758	100 / 689	—	Q&T	Limited*	Same strength, better toughness
L-80 Type 1	80 / 552	95 / 655	95 / 655	23	Q&T	Yes	Sour production casing/tubing
L-80 9Cr	80 / 552	95 / 655	95 / 655	23	Q&T	Yes	CO₂ + limited H₂S (9% Cr)
L-80 13Cr	80 / 552	95 / 655	95 / 655	23	Q&T	Yes	CO₂ corrosion tubing (13% Cr)
C-90	90 / 621	105 / 724	100 / 689	25.4	Q&T	Yes	Deep sour, moderate H₂S
T-95	95 / 655	110 / 758	105 / 724	25.4	Q&T	Yes	Deep sour, higher strength
C-110	110 / 758	120 / 827	115 / 793	30	Q&T	Yes (restricted)	HPHT sour
P-110	110 / 758	140 / 965	125 / 862	—	Q&T	No	Deep sweet, high pressure
Q-125	125 / 862	150 / 1034	135 / 931	—	Q&T	No	Ultra-deep HPHT sweet

*N-80 Type Q is not NACE-qualified by grade; sometimes accepted under project-specific hardness control. N = normalized, N&T = normalized & tempered, Q&T = quenched & tempered. Chemistry is carbon-manganese for most grades; L-80 9Cr and L-80 13Cr are martensitic stainless steels. Values paraphrased from API 5CT Table C.5 — not a purchase spec.

Sour-Service Qualification — NACE MR0175 / ISO 15156

Sulfide stress cracking (SSC) is the failure mode that drives sour-service grade selection. H₂S dissolved in produced water releases atomic hydrogen that embrittles high-strength steel. Hardness control and Q&T heat treatment are the main compliance levers.

Grade	NACE status	Max hardness	H₂S envelope
H-40, J-55, K-55	Acceptable (low strength)	—	All partial pressures
L-80 Type 1	Qualified	23 HRC	All partial pressures
C-90	Qualified	25.4 HRC	All partial pressures
T-95	Qualified	25.4 HRC	All partial pressures
C-110	Qualified (restricted)	30 HRC	pH ≥ 3.5, H₂S ≤ 15 psia
L-80 9Cr / 13Cr	Qualified (MR0175 Part 3)	23 HRC	Limited pH / partial-pressure window
N-80, P-110, Q-125	Not qualified	—	Sweet service only

Sour-service trigger NACE MR0175 is invoked when H₂S partial pressure ≥ 0.05 psia (0.3 kPa) in the gas phase. Below that, standard N-80 / P-110 are acceptable. Above it, grade selection is driven by the pH–pH₂S envelope (Region 0/1/2/3 of MR0175 Figure A.2). Never substitute a non-NACE grade in a well with documented H₂S.

Worked Example — Burst Pressure, 7 in 29 lb/ft L-80

Barlow's formula with the API 5C3 / 5CT internal-yield factor of 0.875 (accounts for 12.5% minimum wall tolerance):

P_burst = 0.875 × (2 × Y_min × t) / D

Inputs: OD D = 7.000 in · nominal wall t = 0.408 in · Y_min (L-80) = 80 000 psi.

P = 0.875 × (2 × 80 000 × 0.408) / 7.000 = 0.875 × 9 325.7 ≈ 8 160 psi — matches the API Bulletin 5C2 tabulated internal-yield pressure for this size and grade.

Product Specification Levels (PSL)

PSL-1 — default general service. Standard chemistry, NDE, and testing.
PSL-2 — tighter chemistry, Charpy V-notch impact, expanded NDE. Typical for deep/critical wells.
PSL-3 — highest tier: full hydrostatic, metallographic, and traceability controls. HPHT and premium sour service.

Sources

API Specification 5CT AMPP/NACE MR0175 / ISO 15156 Tenaris OCTG grade datasheets Vallourec VAM OCTG reference SLB Oilfield Glossary

FAQ

At what H2S partial pressure does NACE MR0175 apply for oilfield casing and tubing?

Per NACE MR0175, sour-service requirements are invoked when H2S partial pressure in the gas phase is at or above 0.05 psia (0.3 kPa). Below that threshold, standard N-80 or P-110 casing is acceptable. Above it, grade selection follows the pH-pH2S envelope (Region 0/1/2/3 of MR0175 Figure A.2).

What is the maximum hardness for L-80 Type 1 casing under NACE MR0175?

Per API 5CT and NACE MR0175, L-80 Type 1 has a maximum hardness of 23 HRC and is qualified for sour service at all H2S partial pressures. It carries 80 ksi (552 MPa) minimum yield, 95 ksi (655 MPa) maximum yield, 95 ksi (655 MPa) minimum UTS, and is quenched and tempered.

Which API 5CT grades are NOT qualified for sour service?

Per NACE MR0175, N-80, P-110, and Q-125 are not qualified and are for sweet service only. P-110 has 110 ksi (758 MPa) minimum yield and Q-125 has 125 ksi (862 MPa) minimum yield. C-110 is qualified but restricted to pH >= 3.5 and H2S <= 15 psia at 30 HRC maximum.

What does the number in an API 5CT grade like N-80 or Q-125 mean?

In API 5CT (ISO 11960), the number is minimum yield strength in ksi and the letter is an arbitrary designator with no chemistry meaning. N-80 = 80 ksi minimum yield (552 MPa); Q-125 = 125 ksi (862 MPa). Yield across the grade range spans 40 to 125 ksi minimum.

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API 5CT and NACE MR0175 are copyrighted. Property values above are specification minimums/maximums — facts — cross-checked across Tenaris and Vallourec public datasheets with no conflicts on API-mandated yield, UTS, or hardness. This is not a purchasing specification; tubular procurement must reference the current governing edition and mill-certified MTRs. Mill-proprietary connections (VAM, Blue) and brand-name sour grades (TN95SS, VM95SS) are outside API 5CT and not covered here.