Cutting process for ABS Grade AH36 shipbuilding steel plates

I. Heat-Affected Zone: A Key Consideration in Cutting Processes

Regardless of the thermal cutting method employed, when ABS Grade Shipbuilding Steel Plate is locally heated and melted, a region affected by thermal cycling forms near the cut edge, known as the Heat-Affected Zone (HAZ). Within the HAZ, the material may undergo phenomena such as grain coarsening, the formation of a hardened microstructure, or softening; these changes may affect the quality of subsequent welding as well as the fatigue performance of the structure under service conditions.

For ABS Grade AH36 Shipbuilding Steel Plate, controlling the width and hardness of the HAZ is of paramount importance. ABS specifications typically require that the hardness of the cut surface does not exceed HV 350 to ensure that cold cracks are not induced by the hardened edges resulting from cutting. Furthermore, for cut edges requiring critical welds, grinding or machining may be required to remove the hardened layer produced by the cutting process.

II. Plasma Cutting: The Mainstream Choice in the Shipbuilding Industry

Plasma cutting is currently the most widely used method for processing ABS Grade Shipbuilding Steel Plate in shipyards, and is particularly suitable for plates with thicknesses ranging from 6 to 50 mm. The principle involves using a high-temperature plasma arc to melt the steel, whilst a high-velocity gas flow blows away the molten metal.

1. Quality Requirements:

Cutting Surface Flatness: For edges of ABS Grade Shipbuilding Steel Plate intended for subsequent welding, flatness should meet Grade 1 or 2 of the ISO 9013 standard.

Notches and Indentations: Cutting edges should be free from deep notches or undercuts, as these defects can act as stress concentration points.

Slag: Should be easily removable and should not affect subsequent assembly.

2. Recommended Practices: 

For ABS Grade AH36 Shipbuilding Steel Plates exceeding 40 mm in thickness, underwater plasma cutting is recommended, as it effectively reduces the HAZ width and minimises fume emissions. Following cutting, for edges subject to high fatigue loads (such as the corners of hatch coamings), light grinding is recommended to remove approximately 0.5–1 mm of the cut surface layer.

III. Laser Cutting: High Precision and Advantages for Thin Plates

Laser cutting, characterised by its high precision, narrow kerf and minimal HAZ, is finding increasing application in the processing of thin ABS Grade Shipbuilding Steel Plates, particularly for the prefabrication of components for luxury cruise ships, aluminium superstructures and thin-plate hull structures.

1. Suitable Thickness Range

ABS Grade Shipbuilding Steel Plate with a thickness of typically < 20 mm is suitable for laser cutting. For AH36 with a thickness exceeding 25 mm, laser cutting is less efficient and economical than plasma cutting.

2. Advantages

Extremely narrow HAZ width: typically < 0.5 mm, with virtually no impact on the material’s original toughness

High degree of automation: Combined with automated loading/unloading systems and nesting software, high-precision cutting without material allowance can be achieved

Minimal thermal deformation: Suitable for cutting complex shapes and small-sized components

3. Precautions

When laser cutting ABS Grade AH36 Shipbuilding Steel Plate, a very thin remelted layer (metal that has melted and then solidified) forms near the cut seam. For applications with extremely high requirements (such as fatigue-critical components), it is recommended to lightly grind this layer away. 

IV. Flame Cutting: The Classic Choice for Thick Plates

Flame cutting (oxy-acetylene or oxy-propane cutting) is the oldest thermal cutting method, utilising the combustion reaction between oxygen and iron to cut steel. For ABS Grade Shipbuilding Steel Plates exceeding 50 mm in thickness, flame cutting remains an economical and practical option, requiring a relatively low capital outlay.

1. Characteristics

Wider kerf: typically 2–4 mm

Larger HAZ: due to relatively high heat input, the HAZ width may reach 2–5 mm

Risk of thermal deformation: Restraint or pre-bending is required when cutting long strips or thin plates

2. Quality Optimisation

When cutting ABS Grade Shipbuilding Steel Plate, it is recommended to use propane as the fuel gas, as its moderate flame temperature reduces the risk of upper edge collapse and slag build-up. Following flame cutting, a layer of scale is usually present on the cut surface, which must be thoroughly removed by sandblasting prior to painting.

V. Waterjet Cutting: The Ultimate Solution for Cold Cutting

When cutting ABS Grade Shipbuilding Steel Plate and a heat-affected zone must be absolutely avoided, waterjet cutting is the optimal choice. Waterjet cutting utilises ultra-high-pressure water (> 400 MPa) mixed with abrasive media (such as garnet grit) to erode and cut the steel.

1. Key Advantages:

Zero thermal effects: No HAZ, thermal deformation or residual stresses whatsoever

Extremely high precision: Cutting accuracy of up to ±0.1 mm

No material restrictions: Can cut any thickness (commonly AH36 within 100 mm)

2. Limitations:

Slow cutting speed: For 20 mm thick ABS Grade Shipbuilding Steel Plate, waterjet cutting speeds are approximately 50–150 mm/min, far lower than plasma cutting (1000–2000 mm/min)

High operating costs: High costs associated with abrasive consumption and replacement of high-pressure seals

Scope of application: Primarily used for cutting complex-shaped templates, small-batch high-precision parts, and structures in nuclear/special-purpose vessels where zero tolerance for the heat-affected zone is required

VI. Post-cutting Treatment and Inspection

Regardless of the cutting process used, cut parts made from ABS Grade Shipbuilding Steel Plate should undergo the following inspections and treatments before proceeding to the welding stage:

1. Surface cleaning

Removal of oxides, slag and oil contamination

2. Edge inspection

Visual inspection for cracks, notches or delamination.

3. Hardness testing

On-site hardness testing of the HAZ where necessary.

4. Grinding treatment

Light grinding of edges requiring high-quality welds to remove the hardened layer.

For ABS Grade Shipbuilding Steel Plate components requiring classification society inspection, cut test specimens (such as scrap material used for mechanical property verification) must be clearly marked and retained until the project is completed.

Conclusion

Selecting the appropriate cutting process for processing ABS Grade AH36 Shipbuilding Steel Plate requires balancing precision, efficiency, cost and the impact on material properties. Plasma cutting is the optimal choice for most shipyards; laser cutting excels in the field of high-precision thin-gauge plates; waterjet cutting caters to specific requirements where zero tolerance for the heat-affected zone is essential; and flame cutting still holds its place in the rough machining of thick plates. Regardless of the method employed, understanding and controlling the impact of the HAZ on the mechanical properties of AH36, and carrying out the necessary post-cutting treatments, are fundamental requirements for ensuring the quality of the hull structure. In the next article, we will discuss the procurement guidelines and key technical parameters for AH36 shipbuilding plates.