Welding process for ABS AH36 marine steel plates

1. Carbon Equivalent: A Key Indicator of Weldability

Carbon equivalent (CEV) is a crucial parameter for assessing the susceptibility of steel to cold cracking during welding. For ABS Grade AH36 marine steel plates, ABS specifications require that the carbon equivalent be typically controlled below 0.38% (based on the International Institute of Welding (IIW) formula).

The formula for calculating CEV is:

CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

Through low-carbon design (C ≤ 0.18%) and microalloying technology, AH36 maintains a carbon equivalent within a range suitable for conventional welding whilst ensuring a yield strength of 355 MPa. This means that ABS Grade AH36 marine steel plates possess good weldability; however, the risk of cold cracking persists when plate thickness increases or constraints are heightened.

2. Low-hydrogen welding processes: The key to preventing cold cracking

Cold cracking (delayed cracking) is the defect that requires the most vigilance in AH36 welding, characterised by its appearance several hours or even days after welding. The core measure for preventing cold cracking is the adoption of low-hydrogen welding processes.

Recommended low-hydrogen welding methods include:

1) Shielded Metal Arc Welding (SMAW): Use low-hydrogen electrodes such as E7016 or E7018, which must be dried in accordance with specifications prior to use (typically at 350°C for 1–2 hours)

2) Flux-cored arc welding (FCAW): Select low-hydrogen flux-cored wires such as E71T-1C or E71T-9C

3) Submerged arc welding (SAW): Use in conjunction with low-hydrogen flux

The ABS Code specifies clear requirements for the diffused hydrogen content of consumables used in the welding of ABS Grade AH36 marine steel plates, typically requiring a grade of H15 or lower (i.e. diffused hydrogen content not exceeding 15 mL per 100 g of deposited metal). For thicker joints or those subject to high restraint, the use of ultra-low-hydrogen welding consumables of H10 or H5 grade is recommended.

3. Preheating Temperature and Interpass Temperature

Preheating is another key measure to prevent cold cracks. It slows the cooling rate of the weld and heat-affected zone, promotes the diffusion and escape of hydrogen, and reduces the tendency for hardening.

For ABS Grade AH36 marine steel plates, the recommended preheating temperature ranges are as follows:

The interpass temperature (between-pass temperature) should be controlled between 150–230°C, and should not exceed 250°C, to prevent overheating which could lead to a reduction in the toughness of the heat-affected zone. Monitoring preheating and interpass temperatures using an infrared thermometer or thermocouple is standard practice when welding ABS Grade AH36 marine steel plates.

4. Control of heat input

Heat input (linear energy) has a significant impact on the mechanical properties of AH36 welded joints. Insufficient heat input may lead to hardening of the heat-affected zone; conversely, excessive heat input may result in grain coarsening and a reduction in toughness.

The recommended heat input range is typically 1.0–3.5 kJ/mm, calculated using the following formula:

Heat input = (welding current × arc voltage × 60) / (welding speed × 1000)

For welded joints in ABS Grade AH36 marine steel plates requiring high low-temperature toughness, a lower heat input (1.5–2.5 kJ/mm) is recommended, combined with a multi-pass welding process. Each weld pass should act as a ‘tempering’ effect on the preceding pass, thereby aiding in grain refinement.

5. Post-Weld Heat Treatment (PWHT)

Generally, post-weld heat treatment is not mandatory for welded joints in ABS Grade AH36 marine steel plates with a thickness of 50 mm or less. However, ABS regulations may require PWHT in the following circumstances:

1) Critical load-bearing welds in plates exceeding 50 mm in thickness

2) T-joints in heavily constrained thick plates or cast steel connection nodes

3) Structures operating in low-temperature environments (e.g. -40°C)

When PWHT is required, a stress-relief annealing process is typically employed: heating to 550–620°C, holding for a duration calculated as one hour per 25 mm of plate thickness, followed by slow cooling in air.

6. Welding Procedure Qualification (WPQ) and Non-Destructive Testing

ABS regulations require that all welding procedures for ABS Grade AH36 marine steel plates must undergo Welding Procedure Qualification (WPS Qualification) prior to formal application on the hull. The qualification includes:

1) Transverse tensile test

2) Transverse bending test (face bend, back bend)

3) Charpy V-notch impact test (weld metal and heat-affected zone; test temperature same as base metal)

4) Macroscopic metallographic examination

5) Hardness testing

Following qualification, rigorous non-destructive testing (NDT) must be carried out during the welding process. Typical weld inspection requirements for ABS Grade AH36 marine steel plates are as follows:

1) 100% ultrasonic testing (UT): all primary load-bearing welds

2) Magnetic particle testing (MT): surface crack detection on all T-joints and fillet welds

3) Radiographic testing (RT): In specific circumstances (e.g. where thickness is significant or UT cannot be performed)

7. Common issues encountered during actual construction

When actually welding ABS Grade AH36 marine steel plates in shipyards, particular attention must be paid to the following issues:

1) Storage of welding consumables: Once opened and exposed to air for more than 4 hours, low-hydrogen electrodes must be re-dried or discarded.

2) Groove Preparation: Oil, rust and moisture must be thoroughly removed from the groove and the 25 mm area on either side.

3) Environmental Conditions: When relative humidity exceeds 80% or wind speed exceeds 2 m/s, shielding measures should be taken.

4) Interpass Cleaning: Weld slag must be thoroughly removed after each pass to prevent slag inclusion defects.

Conclusion

Establishing a suitable welding procedure for ABS Grade AH36 marine steel plates requires comprehensive consideration of multiple factors, including carbon equivalent, preheating temperature, heat input and post-weld inspection. Strict adherence to the welding procedure qualification requirements of the ABS Code, coupled with the implementation of on-site quality control measures, is fundamental to ensuring that the strength of the ship’s welded joints matches that of the base material and meets the design life requirements. In the next article, we will explore surface treatment and coating protection techniques for AH36 marine steel plates.