Weld metal and heat affected zone (HAZ) properties of tanker hull structures

TWI Bulletin, September/October 2002

Jack Still worked in the steel industry as a metallurgist, which involved advising customers on welding of clad steels. He transferred to the fabrication industry and worked for Motherwell Bridge, Babcock and Wilcox and eventually Redpath Dorman Long. During this period he was involved with the fabrication of pressure vessels, tanks, boilers, offshore structures and modules. In 1978 he joined the offshore oil and gas industry and during the last 24 years worked for Britoil/BP, Shell Expro and eventually Amerada Hess on projects involving jacket and production modules, construction of an FPSO and refrubishment of mature structures.

Julian Speck joined TWI in 1998 with a BSc Eng in Materials Engineering from the University of Cape Town. He now undertakes a variety of structural integrity and risk-based assessment projects in the oil, gas and power industries. He is involved in the development of TWI's fitness-for-service (FFS) assessment and risk-based inspection programmes.

With the introduction of floating, production, storage and offloading vessels (FPSOs) for processing hydrocarbons from remote subsea wellheads, concerns have been highlighted regarding the integrity of tanker hull structures for FPSO applications. Although construction of hull structures are in accordance with class rules, FPSOs are intended to remain on location during their field life without having to dry-dock for inspection or repair. Jack Still and Julian Speck provide an overview of a test programme conducted to assure the fracture resistance of a recently completed Afromax tanker hull destined for conversion to an FPSO.

It is common practice for the ship builder to specify the hull materials in accordance with the classification society rules. The materials normally selected for an Afromax tanker consist of Grades A and AH32 carbon-manganese steel. Owing to the anticipated service conditions of the vessel, the outer hull materials were changed to Grades D and DH32. In addition, the thickness of the deck and the bottom of the hull, in Grade DH32, was increased by 3mm.

This upgrade in material was intended to provide improved resistance against fracture, ie resistance to the initiation of brittle cracks (fracture toughness) and crack arrest properties. Specific details of the composition and properties of hull steels are summarised in Tables 1 and 2, respectively. Figure 1 illustrates the location of the material grades within the hull structure.

Table 1: Chemical properties and product analysis

Table 2: Mechanical properties for hull materials

Selection of welding consumables

The selection of welding consumables, was based on the weld metal Charpy impact toughness requirements matching the minimum Charpy impact toughness levels specified for the hull materials. Ship classification societies in this instance specify either Grade 2Y or 3Y consumables for welding Grades D and DH32 steels. The Charpy impact toughness requirements for 2Y consumables is 34J at 0°C which also meets the requirements for Grades A and AH32 steels. Grade 3Y consumables specify Charpy impact toughness requirements of 34J at -20°C, which satisfy the toughness requirements for Grade D and Grade DH32 steels.

Welding procedure test programme of work

Gas shielded flux cored arc (GSFCAW) and electro-gas (EGW) are the two main welding processes used during the construction of a tanker hull structure. Ceramic backing tiles are extensively used in conjuction with the above processes to enable single-sided welds to be produced, reducing the fabrication time. Figure 2 outlines the areas where GSFCAW and EGW are applied. Based on the use of the welding processes during construction of the hull structure, the weld test programme involved both Grades D and DH32 steels. The test plates included two GSFCAW consumables (AWS A5.29 E80T1-K2 ¹ from two consumable manufacturers (hereafter referred to as consumables A and B), and one EGW consumable, ie consumable C (AWS A5.26 EG70T-2 ²).

The welding procedure test plates were welded as single-sided welds in the vertical position, using a ceramic backing strip. Examples of the welding parameters recorded for both GSFCAW and EGW are presented in Table 3. No preheat was applied, ie material temperature was recorded as ambient. The maximum interpass temperatures recorded were 88 and 21°C, for the GSFCAW and EGW, respectively. Typical weldment macro-sections for both welding processes are shown in Fig.3.

Table 3: Electro gas welding and gas shield flux cored arc welding procedures

Bead	Diameter mm	Polarity	Amps	Volts	Weld speed, cm/ min	Heat input, KJ/ cm	Bead 5 layers	Diameter  mm	Polarity	Amps	Volts	Weld speed, cm/ min	Heat input, KJ/ cm
1	1.6	DCRP	410	38	9.6	97.4	1	1.2	DCRP	210	27	11.6	29.3
							2	1.2	DCRP	210	29	19.6	18.6
							3	1.2	DCRP	220	32	14.8	28.5
							4	1.2	DCRP	200	27	15.7	20.6
							5	1.2	DCRP	190	27	13.2	23.3
Electro gas welding (EGW) shielding gas Shielding gas flow rate 25 litres/min using 99% CO 2							Gas shield flux cored arc welding (GSFCAW) shielding gas Shielding gas flow rate 25 litres/min using 99% CO 2

Chemical analyses of plate materials and weld metals

The base metal and weld metal chemical analysis results are presented in Table 4. The GSFCAW welds contained additions of nickel, titanium, and boron. Both the nickel and the boron elements were in sufficient quantities to improve fracture toughness. The titanium is available for the dual purpose of promoting fracture toughness, while also acting as a sacrificial element to combine with nitrogen and thereby protect the boron addition. The weld metal chemical analysis of the EGW contained a relatively high manganese and molybdenum content.

Table 4: Weld material and parent material chemical analysis

Mechanical properties of the test plate weldments

The mechanical properties of the test plate weldments are presented in Table 5. The Charpy impact toughness values of the weld metal for both welding processes and material grades were found to satisfy the minimum requirements of the classification society. Weld (HAZ) Charpy impact tests on specimens notched one millimetre from the fusion boundary, measured impact energy values well above the minimum specification requirements for the base material. The lowest HAZ Charpy impact toughness results were recorded in test plate No.3, in the Grade DH32 steel. The reason for the low value was not specifically investigated. (It was noted that the recorded welding parameters for this test plate were similar to that of the other test plate welds while the parent material composition showed no irregularities). The HAZ properties of the EGW welds measured comparatively higher values of impact toughness.

Table 5: Weld metal mechanical properties

All weld metal tensile tests recorded 0.2% proof strengths of between 420 to 512N/mm ², while the parent material tensile properties ranged from 304 to 383N/mm ². A Vickers hardness survey carried out on weldments recorded hardness values of between 212-240, and 169-181, in the weld metal and HAZ respectively.

CTOD fracture toughness

Weld metal and weld HAZ CTOD tests were manufactured from all procedure test panels and tested in accordance with BS 7448. ^[4,5] The range and the appropriate value ( δ _mat) of CTOD fracture toughness test results (at -10°C), for each of the test panels, are presented in Table 6. In general, the accuracy of a sample depends on its size and on the variability of the property measured. If a material is microstructurally homogeneous, the sample size can be small. If it is non-homogenous, eg weld HAZ, the sample size must be large to be representative of the material.

Table 6: Summary of measured CTOD values at -10°C

Sets of six CTOD specimens were therefore tested for each weldment region. The 'appropriate' value of CTOD fracture toughness ( δ _mat), for use in an ECA for failure by fracture, is subsequently defined as the second lowest result from the set. ^[6]

δ _mat for all of the test panels was above 0.100mm. The highest were measured in the parent materials, ie δ _mat > 0.500mm. Both parent materials exhibited fully plastic (ductile) behaviour during testing. The failures of the weld HAZ specimens were also ductile, except for a few results from the HAZ of a FCAW panel (Grade D, E80T1-K2 (B), δ _min = 0.332mm) and the EGW panel (Grade D, E70T-2, δ _min = 0.098mm). Post-test examinations revealed that the fracture event of the lowest HAZ CTOD result in the FCAW panel exhibited no tearing. The lowest HAZ CTOD in the EGW panel was found to be due to a significant pop-in, ie the arrest of a crack after it rapidly extends across a brittle microstructural region.

Weld metal and weld HAZ metallography

The weld metal and weld HAZ were metallographically examined from all of the test panels, ranging from 50 to 100X magnification. Examples of these microstructures are shown in Figure 4 (test plate No. 3, 19.5mm thickness, Grade DH32) and Figure 5 (test plate No.4, 17.5mm thickness, Grade D) and comprised the following phases:

Fig.4a. As-deposited and refined weld metal microstructure between two successive passes. The varying microstructural features are typical of multi-pass fusion welds, depending on the location within the weld.

Fig.4b. Weld HAZ, fusion line and weld metal microstructure comprising re-crystallised, transformed and tempered zones within the HAZ.

Fig.5a. As deposited microstructure at the outer edge of the weld, towards the fusion line. These large islands increase in size with increasing peak temperature and increasing residence time.

Fig.5b. Weld metal, fusion line and weld (HAZ). The HAZ microstructure exhibits enlarged areas of polygonal ferrite, a distinguishing feature of this comparatively high heat input process.

Ensuring quality of FPSO welds

The results of the materials test and inspection programs, over and above the class requirements introduced by the operator during construction, ^[6] were used in a fracture mechanics-based assessment to determine the maximum flaw size tolerable in the hull under in-service loading. Although this test programme was applied to a hull structure intended for conversion to an FPSO vessel, the principles could be applied to any vessel particularly tanker hull structures designed to operate in severe operating conditions.

Acknowledgements

The authors acknowledge the guidance of Dr R Razmjoo (Manager, Advanced Materials Structures, TWI) and Dr M Dawes (Principal Consultant, Fracture Testing, TWI) in preparation of this paper. The authors also thank Mr D Ellin, Chief Metallurgist, at The Test House, Cambridge, for undertaking the metallographic examinations.

References