African Fusion March 2017

welding by flame cutting and grinding. Weld joints need to be slightly wider than for conventional steels to ensure good root access and to accommodate the sluggish behaviour of the nickel- based welding consumable. The most commonly applied welding methods are SMAW, SAW and FCAW and depend- ing on the region, GTAW. The strong magnetic nature of 9% Ni can pose arc blowproblems, which can be controlled by applying alternatingwelding current (ac) and by demagnetising on site. Pre- heating of the weld zone is not neces- sary although the maximum inter-pass temperature should be limited to a maximum of 150 °C. In tank construction, stick elec- trodes and flux-coredwire are predomi- nantly used for vertical (PF, 3G) welds and submerged-arc welding for hori- zontal (PA, 1G) and horizontal-vertical (PC, 2G) welds. Because the 9%nickel steels contain a certain amount of austenite, there is little risk of hydrogen cracking. Filler metals that give an austenitic or nickel- based weld metal absorb hydrogen easily. Nonetheless, to minimise the width of the heat-affected zone (HAZ), the heat input during welding is usu- ally restricted to 2.0 kJ/mm. The HAZ is kept as narrow as possible because the favourable structure of the parentmetal would typically be destroyed here. Anotherwelding recommendation is applying multi-pass welding to achieve ‘weld normalisation’ of the HAZ. For the weldmetal to acquire the desired tough- ness at low temperatures, nickel-based filler metal must be used. For welding of 5% and 9% Ni steel, the most com- monly used consumables are covered

or so-called double nor- malised and tempered. The latter involves heat treat- ment in which the steel is heated to the two-phase region several times and thus undergoes partial aus- tenitising. During this heat treatment, the small aus- tenite areas absorb, through diffusion, large amounts of the available carbon and nitrogen. Consequently, carbon and nitrogen con- tents in the martensite/ ferrite are reduced and both hardness andbrittleness are decreased. The high carbon and nitrogen contents also contribute to the austenite areas remaining stable at lower temperatures. This gives the steel its excel- lent toughness at cryogenic temperatures. Martensite

that is not converted into austenite is, of course, also tempered during heat treatment, thereby also contributing to the increased toughness of this phase of the structure’s material. When it comes towelding 9%nickel, the options are matching and non- matching filler metals. For nearly all actual fabrication, non-matching fillers are chosen. This is due to the need to match the thermal expansion of the par- ent material, while optimising the weld metal strength. Nickel alloys match this requirement closer than stainless steels. Welding must be done with the par- ent plates fully restrained to prevent weld strength being lost after distortion. 9% Ni steel can be prepared for

describing low temperature toughness behaviour of a metal. Simply explained, lateral expansion is a measure of the plastic deformation of a material during impact testing. The most common requirement for lateral expansion is a minimum of 0.38 mm. There is in general a linear correlation between impact toughness and lateral expansion. The higher the impact tough- ness (joules, J), the higher the value of lateral expansion. This correlation is also influenced by the welding process and slag-systems. Welding of 9% Ni Steel The 9% nickel steels used in LNG stor- age tanks are quenched and tempered

Steel

Welding 5% Ni steel

Welding 9% Ni steel

DIN EN ISO 15609-1 SMAW process 111

AWS- Norm

Product (and current)

AWS-Standard

Product (and current)

E316L-15 (Mod)

Thermanit 19/15

E NiCrMo-6 E NiCrMo-3 E NiCrMo-6

UTP Soudonel D (AC/DC+) UTP 6222 Mo (DC+) UTP 7013 Mo (AC/DC+) UTP UP 6222 Mo + UTP UP Flux 6222 Mo (AC/DC+) Thermanit 625 + Marathon 104 (AC/DC+)

E NiCrMo-6 E NiCrMo-6

UTP Soudonel D (AC/DC) UTP 7013 Mo (AC/DC)

n.a.

Thermanit 17/15 TT + Marathon 104 Thermanit 19/15 + Marathon 104

ER NiCrMo-3

SAW process 12 Wire/flux

ER NiCrMo-4 Thermanit Nimo C276 +Marathon 104 (AC/DC 1,6 mm and AC 2,4 mm)

ER NiCrMo-4 Thermanit Nimo C276 +Marathon 104 (AC/DC+ 1,6 mm and AC 2,4 mm)

GMAW process 135 FCAW process 136

n.a.

Thermanit 17/15 TT (DC+)

ER NiCrMo-3

UTP A 6222 Mo (DC) Thermanit 625 (DC+)

ENiCrMo-3 T1-4

UTP AF 6222 Mo PW (DC/AC)

ENiCrMo-3 T1-4

UTP AF 6222 Mo PW (DC+/AC)

Consumables for welding 5% and 9% Ni tanks.

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March 2017

AFRICAN FUSION