Why does C90 cap hardness instead of just requiring strength?

In H₂S service, hard spots are where sulfide stress cracks initiate. Capping hardness at 25.4 HRC (and proving it by through-wall surveys and SSC tests) is what makes the grade sour-safe.

Grades · Pipe & Tube Steel · API C90 Casing

API C90 Casing

Data verified

C90 is a controlled-hardness API 5CT grade for sour (H₂S) wells: quenched-and-tempered to a 90 ksi yield window while the hardness is capped at 25.4 HRC so sulfide stress cracking cannot start. It is specified where N80-type material would be an SSC risk.

Hardness≤25.4 HRC (sour-service cap)

AISIAPI 5CT C90

GBC90 (GB/T 19830)

CategoryPipe & Tube Steel

Chemical composition

Element	Content (%)
P	≤0.030
S	≤0.030
Note	API 5CT controls C90 by strength, hardness and SSC testing — no C/Mn composition limits

Weight percent, per the governing standard ranges. Actual values per heat are on the mill certificate.

Properties

Yield strength	621–758 MPa (90–105 ksi)
Tensile strength	≥689 MPa (100 ksi)
Hardness	≤25.4 HRC (sour-service cap)
Density	7.85 g/cm³

Heat treatment

Quenched and tempered by the mill; hardness through the wall is verified and SSC (sulfide stress cracking) qualification applies for sour service.

Equivalent grades

Standard	Designation
aisi	API 5CT C90
gb	C90 (GB/T 19830)

Compare side by side: API C90 Casing vs API T95 Casing · API C90 Casing vs API L80 Casing

Equivalents overlap in chemistry and use but are not always identical — see our equivalents guide before substituting in certified designs.

Applications & Processing

Applications: Sour-service production casing and tubing, H₂S wells under NACE MR0175/ISO 15156, deep gas wells with corrosive production.

Processing: A tested product, not a commodity: orders must call up API 5CT with SSC test requirements and the hardness survey — plain “90 ksi pipe” is not C90.

FAQ

Why does C90 cap hardness instead of just requiring strength?: In H₂S service, hard spots are where sulfide stress cracks initiate. Capping hardness at 25.4 HRC (and proving it by through-wall surveys and SSC tests) is what makes the grade sour-safe.

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Source: API 5CT published ranges · Last updated 2026-11-03. Data is indicative — verify critical values against the governing standard and mill certificate before use.