• API 5CT coupling types for OCTG casing and tubing pipes used in oilfield

API Coupling, also known as OCTG coupling, refers to the steel couplings that used in connecting casing pipes and tubing pipes. Different casing pipes and tubing pipes need different types of couplings to connect. So couplings for casing pipes have three thread types, and they are couplings for STC (short round thread casing), LTC (long round thread casing) and BTC (buttress thread casing) respectively. So are the couplings for tubing pipes, they are couplings for EUE tubing pipes and NU tubing pipes.

Critical thickness of coupling

Inch NU EU Special clearance BC LC SC
EU BC
1.050 0.169 0.211
1.315 0.211 0.258
1.660 0.239 0.240
1.900 0.196 0.251
2 3/8 0.304 0.300 0.224
2 7/8 0.380 0.358 0.254
3 1/2 0.451 0.454 0.294
4 0.454 0.458
 4 1/2 0.435 0.493 0.259 0.447 0.474 0.462
5 0.266 0.479 0.511 0.491
5 1/2 0.268 0.481 0.514 0.495
6 5/8 0.274 0.469 0.508 0.485
7 0.280 0.530 0.568 0.540
7 5/8 0.348 0.536 0.573 0.546
8 5/8 0.352 0.602 0.647 0.612
9 5/8 0.352 0.602 0.657 0.614
10 3/4 0.352 0.602 0.618
11 3/4 0.602 0.618
13 3/8 0.602 0.618
16 0.667 0.632
18 5/8 0.854 0.819
20 0.667 0.673 0.634

Chemical Composition(%)

Grade C Mn Mo Cr Ni Cu P S Si
H40 ≤0.03 ≤0.03
J55/K55 ≤0.03 ≤0.03
N80-1/N80Q ≤0.03 ≤0.03
R95 ≤0.45ᶜ ≤1.9 ≤0.03 ≤0.03
L80 ≤0.43ᵃ ≤1.9 ≤0.03 ≤0.03
L80-1 ≤0.15 0.30~0.60 0.90~1.10 8.00~10.0 ≤0.03 ≤0.03 ≤0.45
L80-13Cr 0.15~0.22 0.25~1.00 12.0~14.0 ≤0.25 ≤0.35 ≤0.03 ≤0.03 ≤0.45
C90 ≤0.35 ≤1.20 0.25ᵇ~0.85 ≤1.50 ≤0.5 ≤0.25 ≤0.02 ≤0.01 ≤1
T95 ≤0.35 ≤1.20 0.25ᵇ~0.85 0.40~1.50 ≤0.5 ≤0.25 ≤0.02 ≤0.01 ≤1
C110 ≤0.35 ≤1.20 0.25~1.00 0.40~0.15 ≤0.99 ≤0.02 ≤0.01
P110 ≤0.99 ≤0.02 ≤0.01
Q125 ≤0.35 1.35 ≤0.85 ≤1.50 ≤0.99 ≤0.02 ≤0.005

Mechanical properties

Grade Total elongation under load(%) Yield strength(Mpa) Tensile strength

(Mpa)

Hardness

(HRC)

Hardness

(HBW)

H40 0.5 276~552 414
J55 0.5 379~552 517
K55 0.5 379~552 655
N80-1/N80Q 0.5/0.5 552~758/552~758 689/689
R95 0.5 655 724
L80 0.5 552 655 23.0 241
L80-1/L80-9Cr 0.5 552 655 23.0 241
L80-13Cr 0.5 552 655 23.0 241
C90 0.5 621 689 25.4 255
T95 0.5 655 724 25.4 255
C110 0.7 758 793 30.0 286
P110 0.6 758 862
Q125 0.65 862 931 b

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