Low Temperature Weld Neck Flange: The Trusted Choice for High-Quality Industrial Applications

author: Qishine Industry

2023-10-02

One-Minute Overview

Low-temperature grades are proven by a charpy impact test at the service temperature, not just tensile strength.
A350 LF2 is rated to about -46 deg C; LF3 goes down to about -101 deg C.
The weld neck hub is butt-welded to the pipe, lowering stress at the joint.
Welding needs low hydrogen, controlled preheat, and sometimes post-weld heat treatment.

What a Low-Temperature Flange Means

A low-temperature weld neck flange is graded so it stays tough below freezing instead of turning brittle. Forged to ASTM A350 (LF2 or LF3) and finished to ASME B16.5, it is the joint used on lines that see cold, cyclic, or impact load. This article explains what the grade means, how it is made and tested, how the gasket seats against the matched flange, and where it is specified.

Grade and Service Temperature

What makes a flange 'low temperature':

The forging must pass an impact (charpy) test at the design temperature with a minimum absorbed energy.
Carbon is kept low and grain is fine so the steel resists brittle fracture.
The grade is named with a class - LF2 CL2 is the common low-temp carbon steel forging.
Without that impact proof, a standard carbon flange can crack when cold-shocked.

Grade	Min service temp	Use
A350 LF2 CL2	About -46 deg C	General low-temp carbon steel
A350 LF3	About -101 deg C	Deeper cold, higher notch toughness
A350 LF6	Low temp, higher strength	Where strength and cold both matter

Forging and Heat Treatment

How the part is controlled:

Open-die forging refines the grain and follows the neck contour.
Normalizing (and sometimes tempering) gives a fine, even structure for toughness.
Chemistry is balanced for low carbon equivalent so it welds without cracking.
Each lot is impact-tested and traced to the heat number on the cert.

Weld Neck Design for Cold Service

Why weld neck for cold service:

The long hub is butt-welded to the pipe, spreading load away from the bore.
That reduces stress concentration where a cold line flexes or vibrates.
Bore matching avoids a step that would trap strain at the weld.
Face type (RF / FF / RTJ) sets the gasket and the make-up of the joint.

Welding and Inspection

Welding low-temp flanges takes care:

Use low-hydrogen electrodes and keep them dry to avoid cold cracking.
Preheat within the procedure; do not let the joint cool too fast.
For thicker sections, apply post-weld heat treatment to relax the weld.
Verify with NDT (UT, MT, or PT) and re-check impact if the WPS changes.

Typical Applications

Where it is specified:

LNG, cryogenic, and refrigeration lines that run well below zero.
Offshore and onshore oil and gas, including instrument and utility lines.
Petrochemical plants with cold separation or liquefaction trains.
Power and air-separation units where brittle fracture is not an option.

Specification Checklist

Specify with these points clear:

ASTM A350 grade and class with the impact test temperature stated.
ASME B16.5 dimensions and the facing that matches your gasket.
EN 10204 3.1 certificate with charpy values tied to the heat.
Welding procedure (WPS/PQR) and any NDT level the line requires.

FAQ

What is the difference between LF2 and LF3?

Both are A350 low-temperature forgings; LF2 is proven to about -46 deg C and LF3 to about -101 deg C, with LF3 giving higher notch toughness for deeper cold.

Is a low-temperature flange just a stronger flange?

No. Strength matters, but the defining proof is the charpy impact test at the service temperature, which shows the steel stays tough instead of brittle.

Can I use A105 instead of A350?

Only if the service stays warm enough; A105 is not impact-tested for low temperature, so a cold line should use A350 LF2 or LF3.

Do I need post-weld heat treatment?

For thicker walls or high restraint, yes - PWHT relaxes the weld and lowers cold-crack risk. Follow the welding procedure for the line.

Why weld neck and not slip-on?

The butt-welded hub spreads stress into the pipe wall, which matters on cold lines that flex, vibrate, or see thermal cycles.

How do I verify the grade on delivery?

Match the marking to the EN 10204 3.1 cert and confirm the charpy values at the stated test temperature and heat number.

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