IIW White Paper

Welding thick walled steel components (such as pressure vessels) generates residual stresses that can be the cause of brittle fracture and stress corrosion cracking. Current codes for the fabrication of pressure vessels, boilers and piping specify that PWHT is required if the thickness of the components being welded exceeds a specified value. The use of fracture mechanics approaches can provide the decision whether or not PWHT is necessary to avoid the risk of failure by fracture or plastic collapse. The results of the fracture mechanics assessment (using FITNET FFS or BSI 7910 or R6 or API 579 procedures) can demonstrate that the assumed flaw(s) in the as-welded condition may be acceptable (i.e. are non-critical in terms of fracture or plastic collapse). This kind of approach can technically justify the avoidance of costly PWHT.

Base and Filler Materials

Welding / Joining Process

Service Behaviour

Weldability / Joinability

Load / Environment

Design / Construction

Figure 4.1 Schematic showing the interrelationship between key factors of welded structures (Reproduced courtesy: M. Koçak)

4.1.1 New materials and weldability The term weldability (joinability) is treated in this chapter in accordance with the German Standard DIN 8528 and to the ISO Technical Report 581 as a component property influenced by the material, the joining process and the respective design/fabrication methods. Steels As in the past, and in future, the increase of the strength levels will be the most important goal in the development of all weldable steel types to achieve further economic benefits with respect to weight and cost reduction. The decrease in dimensions of the components and hence mass of material to be handled permits the use of smaller handling equipment (machine tools, cranes, heat treating furnaces, quenching equipment etc.). Additionally, a reduction in the amounts of welding consumables can be achieved with the use of higher strength steels in structures. Such well-known advantages, however, can only be exploited if the cracking resistance of the respective joints remains an acceptable level in the weld metal and in the HAZ. It can only be emphasised that existing guidelines, specifications and standards cannot easily be transferred to welds of novel high strength steels. The correct evaluation of the fracture toughness as well as the corrosion and hydrogen cracking resistance of the joints will thus represent a major challenge for welding of high strength steels in all industrial branches in future. In shipbuilding and submarine fabrications, for instance, thin steel plates with strength levels of up to 700 MPa have recently been introduced to increase the topside carrying capacity. The respective welds should provide excellent fatigue resistance and high rate loadings and no strength reduction during flame straightening or post weld heat treatments. Hence, there is a need for high strength/high toughness/high fatigue life weldable steels with matching consumables for structural, maritime and naval applications. This scientific challenge needs to be tackled by material scientists, design and welding engineers together.

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Through Optimum Use and Innovation of Welding and Joining Technologies

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