ASTM A707 L5 Class3 Flanges

ASTM A707 L5 Class3 forged flanges are manufactured to major dimensional standards including ASME B16.5, ASME B16.47 Series A & B, and NORSOK L-005, making them suitable for high-pressure oil & gas, offshore, and cryogenic applications, even in demanding sour service conditions. Their strength stems from a unique copper precipitation hardening mechanism within an ultra-low carbon Ni-Cu-Nb alloy system. This provides an exceptional combination of high strength (min. 415 MPa yield strength) and guaranteed impact toughness at -62°C (-80°F), along with excellent welding performance.

Chemical Composition Requirements

Element	Composition, %
C	≤0.07
Mn	0.40-0.70
P	≤0.025
S	≤0.025
Si	≤0.35
Ni	0.70-1.00
Cr	0.60-0.90
Cu	1.00-1.30
V	≤0.05
Nb	≥0.03
Mo	0.15-0.25

*Columbium and Niobium are alternate names for element 41 in the Periodic Table of the Elements.
*The carbon equivalent CE shall be determined by CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15.
*CE shall be ≤0.47 for thickness ≤ 2 in. (50 mm); CE shall be ≤0.48 for thickness ≥ 2 in. (50 mm).

Mechanical Properties at Room Temperature

Tensile Properties	Requirements
Tensile Strength min. MPa [ksi]	515 [75]
Yield Strength min. MPa [ksi]	415 [60]
Elongation in 2", min. %	20
Reduction of area, min. %	40
Hardness, HBW	156-235
Cv energy absorption, min, avg, ft-lbf [J]	50 [68]
Cv energy absorption, min, ft-lbf [J]	40 [54]

*Tensile & hardness requirements of ASTM A707 L5 Class 3 respectively. *0.2% offset.
*For a set of three full-size [10 by 10 mm] Charpy V-notch specimens. Acceptance values for sub-size specimens are reduced in proportion to the reduction in width of the specimen.
*These requirements are intended to minimize fracture initiation. They are not intended to give assurance against fracture propagation. If minimization of fracture propagation is of interest, consideration should be given to specifying Supplementary Requirement S4 at the operating temperature. S4: the impact specimens shall exhibit a minimum of 50 % shear fracture appearance at the temperature specified on the order.
*Minimum impact energy permitted for one specimen only of a set three specimens.

Heat Treatment

The ASTM A707 L5 Class 3 material can undergo various heat treatments, including annealing, normalizing, quenching and tempering, normalizing and tempering, normalizing with precipitation hardening, or quenching and precipitation hardening. However, Precipitation Hardening is the central and most critical process for achieving its defining properties.

The precipitation hardening treatment specifically involves heating the forging to a temperature range of 538°C to 677°C (1000–1250°F), holding it for a minimum of 30 minutes, and then cooling at any convenient rate. The fundamental purpose of this process is to significantly increase the yield strength (reaching 75-100 ksi for modified grades) through the fine precipitation of epsilon-copper particles within the ferrite matrix, while largely preserving toughness.

This unique heat treatment mechanism enables the material to simultaneously meet the requirements for high strength, good low-temperature toughness, and excellent welding performance. It is particularly suited for manufacturing large, critical forgings intended for demanding service environments, such as those containing hydrogen sulfide (sour service).

Applications

ASTM A707 L5 Class 3 flanges are engineered primarily for petroleum and natural gas pipeline systems, particularly where excellent low‑temperature toughness (down to –80 °F / –62 °C) is required. This grade guarantees toughness at –62 °C, delivers a robust minimum yield strength of 415 MPa, offers excellent welding performance with little or no preheat, and is available in chemistry‑modified variants for sour service applications. It is widely employed in demanding offshore oil and gas environments, low temperature pressure vessels, LNG facilities, and Arctic or cold climate pipelines. As large diameter forgings, ASTM A707 L5 Class 3 is the material of choice for critical junctions in severe service conditions, where high strength (minimum 75 ksi / 515 MPa for Class 3) must be preserved at low ambient temperatures. This combination makes it a premier, cost effective engineering solution for large diameter forged flanges where safety, reliability, and ease of fabrication in extreme environments are paramount.

Sour Service Performance and the SSC Variant

The L5 alloy grade offers excellent welding performance. Its low carbon equivalent minimizes cold cracking susceptibility, frequently allowing fabrication without preheat. However, hardness in the weld zone can increase as a result of copper aging, necessitating specific controls for use in sour environments.

The standard high-strength A707 L5 (“MOD” variant) provides good base metal hardness (<240 Hv30), making it suitable for moderate sour environments. However, welded HAZ hardness often exceeds 280 Hv10, which may not meet typical sour service hardness limits (e.g., <22 HRc or ~250 Hv). For severe H₂S service, specially modified A707 SSC grades have been developed. These variants feature adjusted chemistry (e.g., reduced Mn, Nb) and optimized aging/overaging treatments to control HAZ hardness below 260 Hv10, thereby providing superior sulfide stress cracking resistance as demonstrated in NACE Method A tests.