Welding of low-temperature steel pipes (A333-6) in LPG projects

In liquefied projects, pipelines generally use low-temperature pipes (A333-6). The process for welding this material is complex, and there is limited experience with welding these steel pipes domestically. To ensure the quality of welding for this material, we need to select welding materials based on its characteristics, establish welding procedures, and inspect and repair weld seam quality to meet design and standard requirements, ensuring the quality of field construction. Therefore, the following points are summarized:

1. Preparations before welding:
1.1. Steel A333-6 belongs to low-temperature steel with a minimum usage temperature of -101°C. The steel contains a high amount of Ni, which does not form carbides but can form solid solutions with iron. The grain size in the heat-affected zone may coarsen, reducing toughness. Therefore, it is essential to strictly control the heat-affected zone during construction.
1.2. Welding procedure qualification
1. The methods and contents of welding procedure qualification comply with the regulations of “Welding Procedure Qualification for Steel Pressure Vessels” (TB4708-2000).
2. Selection of welding materials should follow the principle of compatibility with base metal. The selection of welding materials is shown in the table below:

3. Welding method: Gas tungsten arc welding (GTAW) is used for root pass, and shielded metal arc welding (SMAW) is used for fill and cap passes, also known as TIG-SMAW combined welding. For pipe diameters ≤2”, full GTAW is used.
4. Welding layers: Multi-layer multi-pass welding is adopted.
5. Welding parameters:

Welding method | Welding layer | Power supply type and polarity | Welding current/A | Welding voltage/V | Ar flow rate/L/min | Welding speed/cm*min^-1 | Preheat temperature/°C | Interpass temperature/°C | Welding line energy/J*cm^-1
GTAW | 1 | DC,
straight polarity | 60～110 | 8～12 | 6～8 | 8～12 | ≥7.5 | 100～150 | 100～150 | ≤16000
SMAW | 2 | DC,
reverse polarity | 60～120 | 20～28 | | | ≥7.5 | 100～150 | 100～150 | ≤16000
SMAW | Others | DC,
reverse polarity | 60～120 | 20～28 | | | ≥7.5 | 100～150 | 100～150 | ≤16000

1.3 Preparation of materials:
1. Pipes and fittings must have a certificate of conformity (quality inspection certificate). Inspection items and technical indicators should comply with national and industry standards and technical requirements. There should be no cracks, shrinkage holes, folds, or heavy skins on the surface.
2. Welding materials:
(1) The selection of welding materials should comprehensively consider the chemical composition, mechanical properties, usage conditions, and welding conditions of the welded pipes. For low-temperature weld pipes of material A333-6, to ensure welding quality, we adopt TIG-SMAW combined welding. TIG rods use the TIG-50 brand; electrodes use E5015.
(2) Materials used for welding low-temperature steel pipelines should come with a certificate of conformity. Electrodes should comply with “Electrodes for Low-Alloy Steels” (GB5118-85).
(3) Storage and management of welding materials should comply with regulations. The welding material warehouse must be dry and ventilated, free from harmful gases and corrosive media. Different types, brands, batches, and specifications of welding materials should be stored separately. Welding materials should be placed on shelves, at least 300 mm away from the ground and walls to prevent moisture.
(4) Issuance, storage, and recovery of welding materials should be managed by designated personnel with detailed records.
1.4 Equipment and personnel preparation:
Equipment: GTAW machines, DC welding machines, grinding machines, and other auxiliary equipment.
Personnel: Welders engaged in pipeline welding must take examinations according to the welding procedures specified in the “Rules for Examination and Management of Welders for Boilers, Pressure Vessels, and Pressure Pipelines” issued by the Ministry of Labor and Social Security or the “Code for Construction and Acceptance of Welding Works of Field Equipment and Industrial Pipelines” (GB50236-98), Chapter Six. Only welders who pass the examination can undertake corresponding welding tasks.
2. Welding process:
2.1. Bevel preparation and inspection
Bevels should be cut and polished according to Table 2-1.

2.2. Assembly and positioning:
(1) When assembling pipes and fittings with the same wall thickness, the inner walls should align. The misalignment should not exceed 10% of the thickness and should be no more than 1 mm.
(2) Before welding, clean oil, paint, dirt, and rust within a range of no less than 20 mm around the bevel and its edges.
(3) Positioning welds should be evenly distributed. The starting point of the actual welding should be between two positioning welds. Positioning welds should penetrate fully without weld spatter and should have good fusion.
(4) If preheating is required for formal welding, positioning welds should be preheated under the same conditions.
2.3. General welding regulations.
Welding environmental conditions should comply with the following requirements; otherwise, effective protective measures should be taken. The ambient temperature should not be below 0°C.
A. For SMAW, wind speed <8 m/s; for GTAW, wind speed <2 m/s;
B. Relative humidity <90%;
C. No rain or snow.
2.4. Welding:
(1) Welding of pipelines should be carried out according to the Welding Procedure Specification (WPS).
(2) After reaching the preheat temperature, proceed immediately with root pass welding and complete it continuously. The root pass should penetrate fully with good formation. The weld bead thickness for thick-walled pipes should not be less than 3 mm.
(3) After completing the root pass, the face pass should be welded immediately while maintaining the preheat temperature, and each weld should be completed continuously. If welding is interrupted, post-heating and slow cooling measures should be taken. Before resuming welding, check for cracks and continue welding according to original procedures if none are found.
(4) During multi-layer welding, interpass temperatures should equal or slightly exceed preheat temperatures. Weld joints should be staggered between layers.
(5) In multi-layer welding, the number of layers (n) should generally conform to the formula below:
N = S / 3.0 + 1 (where n = number of layers, S = wall thickness (mm))
(6) After welding, perform heat treatment at 300-350°C for 15-30 minutes, followed by insulation and slow cooling.
(7) The diameter of the tungsten electrode used in GTAW should be selected based on the current strength.
3. Weld inspection and repair:
3.1 Weld inspection:
(1) First, conduct visual inspection of the welds after welding. Clear slag and spatter from the weld surface before inspection. The appearance of the welds should meet the requirements specified in the table below:

Table 3-1
Thickness S (mm) of the workpiece | Weld height (mm) | Weld width (mm)
>4 | ≤0.2S and ≤2 | 10-20

(2) The number of non-destructive tests and internal quality standards for pipeline welds should be executed according to design specifications. Non-destructive testing should use radiographic testing methods. The qualified level is Grade II.
3.2 Weld repair:
(1) Repair welding procedures should be the same as formal welding procedures.
(2) Repairs should be done before heat treatment. If repairs are needed after heat treatment, reheat treatment of the repaired welds is required.
(3) For internal defects in the weld, remove them using grinding wheels or carbon arc air gouging, then repair according to repair procedures.
(4) The number of repairs to the same location should not exceed twice. If more than two repairs are needed, analyze the cause, formulate measures, and obtain approval from the project’s technical director.
4. Post-weld heat treatment:
4.1 Overview
For A333-6 low-temperature steel pipelines, post-weld heat treatment is necessary to ensure construction quality. This helps reduce residual stress, soften hardened areas, improve the microstructure and properties of the weld and heat-affected zone, enhance the plasticity and toughness of the joint, and stabilize structural dimensions.
4.2 Heat treatment measures:
(1) Perform heat treatment only after non-destructive testing is qualified.
(2) Use electric heating for heat treatment of pipeline welds. Maintain uniform heating temperature and distribution, with temperature measurement points located at the edges of the even heating area.
(3) The heating range for heat treatment should extend at least three times the weld width on either side of the weld center, with a minimum of 25 mm. Areas outside the heating zone up to 100 mm should be insulated. The insulation layer thickness is 100 mm.

(4) Temperature control should be precise, with a tolerance of ±25°C. During the heating phase, test power supply first without abnormalities, then apply power for heating, strictly following the heat treatment curve and recording instrument readings.

4.3 Hardness testing:
(1) Conduct hardness spot checks after each batch of heat treatments, with a sampling ratio of at least 10%.
(2) Measure hardness once in the weld, base metal, and heat-affected zone, taking three measurements per location and averaging the results for the final test result.
5. Non-destructive testing (RT):
After the welds pass visual inspection, non-destructive testing can be performed after at least 24 hours. Pipeline butt welds should comply with the provisions of “Radiographic Testing and Quality Grading of Fusion Welded Butt Joints of Steel” (JB4730-2005).

In summary, only through a systematic and comprehensive understanding of the characteristics of low-temperature steel A333-6 can we implement effective methods to successfully complete the welding of A333-6 low-temperature steel pipelines.

Welding of low-temperature steel pipes (A333-6) in LPG projects