K55 vs J55 — what actually differs?

Only the tensile floor: K55 requires ≥655 MPa tensile against J55’s ≥517 MPa, at the same 379–552 MPa yield. In practice K55 pipe is made from a stronger melt.

Grades · Pipe & Tube Steel · API K55 Casing

API K55 Casing

Data verified

K55 sits one step above J55 in the API 5CT casing ladder: the same yield window but a higher minimum tensile strength (655 vs 517 MPa), bought with a slightly richer mill chemistry that API still leaves to the producer. It is the default step-up for deeper or higher-load strings.

AISIAPI 5CT K55

GBK55 (GB/T 19830)

CategoryPipe & Tube Steel

Chemical composition

Element	Content (%)
P	≤0.030
S	≤0.030
Note	API 5CT sets no C/Mn limits for K55 — chemistry is mill practice

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

Properties

Yield strength	379–552 MPa (55–80 ksi)
Tensile strength	≥655 MPa (95 ksi)
Density	7.85 g/cm³

Heat treatment

As-rolled or normalized at the mill’s option; supplied as casing/tubing with API threads (STC/LTC/BTC).

Equivalent grades

Standard	Designation
aisi	API 5CT K55
gb	K55 (GB/T 19830)

Compare side by side: API K55 Casing vs API J55 Casing · API K55 Casing vs API N80 Casing

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

Applications & Processing

Applications: Intermediate casing, production casing in conventional wells, geothermal and water-well strings needing more collapse margin than J55.

Processing: As J55, strings are threaded, not welded — specify connection type and coupling stock with the order.

FAQ

K55 vs J55 — what actually differs?: Only the tensile floor: K55 requires ≥655 MPa tensile against J55’s ≥517 MPa, at the same 379–552 MPa yield. In practice K55 pipe is made from a stronger melt.

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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.