Improving Global Quality of Life
Improving Global Quality of Life
Improving Global Quality of Life
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Welding thick walled steel components (such as pressure vessels) generates residual stresses that can be the<br />
cause <strong>of</strong> brittle fracture and stress corrosion cracking. Current codes for the fabrication <strong>of</strong> pressure vessels,<br />
boilers and piping specify that PWHT is required if the thickness <strong>of</strong> the components being welded exceeds<br />
a specified value. The use <strong>of</strong> fracture mechanics approaches can provide the decision whether or not PWHT<br />
is necessary to avoid the risk <strong>of</strong> failure by fracture or plastic collapse. The results <strong>of</strong> the fracture mechanics<br />
assessment (using FITNET FFS or BSI 7910 or R6 or API 579 procedures) can demonstrate that the assumed<br />
flaw(s) in the as-welded condition may be acceptable (i.e. are non-critical in terms <strong>of</strong> fracture or plastic<br />
collapse). This kind <strong>of</strong> approach can technically justify the avoidance <strong>of</strong> costly PWHT.<br />
Base and Filler<br />
Materials<br />
Welding / Joining<br />
Process<br />
Weldability /<br />
Joinability<br />
Service<br />
Behaviour<br />
Load / Environment<br />
Design /<br />
Construction<br />
Figure 4.1 Schematic showing the interrelationship between key factors<br />
<strong>of</strong> welded structures (Reproduced courtesy: M. Koçak)<br />
4.1.1 New materials and weldability<br />
The term weldability (joinability) is treated in this chapter in accordance with the German Standard DIN<br />
8528 and to the ISO Technical Report 581 as a component property influenced by the material, the joining<br />
process and the respective design/fabrication methods.<br />
Steels<br />
As in the past, and in future, the increase <strong>of</strong> the strength levels will be the most important goal in the<br />
development <strong>of</strong> all weldable steel types to achieve further economic benefits with respect to weight and<br />
cost reduction. The decrease in dimensions <strong>of</strong> the components and hence mass <strong>of</strong> material to be handled<br />
permits the use <strong>of</strong> smaller handling equipment (machine tools, cranes, heat treating furnaces, quenching<br />
equipment etc.). Additionally, a reduction in the amounts <strong>of</strong> welding consumables can be achieved with the<br />
use <strong>of</strong> higher strength steels in structures. Such well-known advantages, however, can only be exploited<br />
if the cracking resistance <strong>of</strong> the respective joints remains an acceptable level in the weld metal and in<br />
the HAZ. It can only be emphasised that existing guidelines, specifications and standards cannot easily be<br />
transferred to welds <strong>of</strong> novel high strength steels. The correct evaluation <strong>of</strong> the fracture toughness as well<br />
as the corrosion and hydrogen cracking resistance <strong>of</strong> the joints will thus represent a major challenge for<br />
welding <strong>of</strong> high strength steels in all industrial branches in future.<br />
In shipbuilding and submarine fabrications, for instance, thin steel plates with strength levels <strong>of</strong> up to<br />
700 MPa have recently been introduced to increase the topside carrying capacity. The respective welds<br />
should provide excellent fatigue resistance and high rate loadings and no strength reduction during flame<br />
straightening or post weld heat treatments. Hence, there is a need for high strength/high toughness/high<br />
fatigue life weldable steels with matching consumables for structural, maritime and naval applications. This<br />
scientific challenge needs to be tackled by material scientists, design and welding engineers together.<br />
20 <strong>Improving</strong> <strong>Global</strong> <strong>Quality</strong> <strong>of</strong> <strong>Life</strong> Through Optimum Use and Innovation <strong>of</strong> Welding and Joining Technologies