Feature 2 | METHODS AND MATERIALS
Shipyard fabrication and quality control
Following is part one of a three part paper written by Lloyd’s Register’s David J Howarth and John Durkin (parts two and three will appear in the September and October issues of Te Naval Architect). Te paper demonstrates the need for the maritime industry to learn the lessons, not only to improve quality and ultimately safety, but also to reduce costs and improve efficiency.
Introduction Class requirements are intended to ensure that the vessel operates safely and successfully during its design life by ensuring the integrity of the hull envelope. Construction of a new vessel involves the control of materials, shot blasting and priming, fabrication of hull panels, fabrication and erection of blocks within a dry dock. Several of the above activities involve the use of a variety of welding processes which have been selected for a specific reasons – usually high productivity, low cost and fitness for purpose or application. If the fabrication inspection methods
are not suitable for the detection of fabrication flaws, it is possible that structurally unsound welds will be installed in the vessel. Welds in the above condition may be considered critical in the context of fatigue during service. This paper looks at the basis for
quality control in a shipyard and the effects of fabrication processes on the mechanical properties of the materials.
Principles of welding qualtiy control Steel grades intended for the vessel’s hull are specified by the classification society’s rules. The chemical composition and
mechanical properties for all grades and strength levels are today unified by the International Association of Classification Societies [1], [2]. Plates and sections are shot blasted
and primed before profile cutting to a variety of shapes for a particular part of the hull structure. The forming of plates is either carried out cold by roll
The Naval Architect July/August 2010
Consumable grade
1N 2N 3N
1Y 2Y 3Y 4Y
2Y40 2Y40 3Y40 4Y40
A
B, D E
A
B, D E -
AH32 DH32 EH32 FH32
Suitable for steel grades
AH32 DH32 EH32
AH 32 DH32 EH32 FH32
AH36 DH36 EH36 FH36
AH36 DH36 EH36
AH36 DH36 EH36 FH36
AH40 DH40 EH40 FH40
Table 1: Consumable grades appropriate to structural steel grade.
forming or pressing, or warm by line heating. Issues with both cold and warm techniques are discussed later with respect to their possible effects on mechanical properties.
Welding processes Today the welding processes used in the fabricating of the hull structure involve the following welding processes:
• Shield Metal Arc welding (SMAW) • Gas Metal Arc Welding (GMAW) • Flux Cored Arc Welding (FCAW) • Submerged Arc Welding (SAW)
• Electro Gas Welding (EGW). Currently, the most common welding
process used for hull construction is the gas shielded flux cored arc welding. This process is capable of depositing
large volumes of weld metal quickly and efficiently. Electro gas and submerged arc welding are also commonly used. They are both automatic high heat input processes, but submerged arc welding is restricted to horizontal flat panels such as found in panel lines and decks, whereas electro gas welding is used in welding flat vertical joints and therefore is generally restricted to the joining of block structures. The use of ceramic tiles for semi-automatic and automatic processes for single side joints is widely used at every stage during hull construction. Hybrid laser welding is not included in the list but is used extensively in thin materials (6 – 10mm) where a low distortion process is necessary to help reduce distortion [3]. The selection of welding consumables
43
CVN test temp. o
C
+20 0
-20
+20 0
-20 -40
+20 0
-20 -40
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