IIW White Paper

9 Needs and challenges of major industry sectors for future applications

New methods of repair and maintenance including the use of composites. In-line inspection of butt fusion welded plastic pipe. Mechanised welding of external and internal joints on mainstream pipelines. High-pressure water-jet cutting of damaged concrete support structures for low cost maintenance of in-service pipelines. High integrity mechanised welding of titanium. 9.12.1 Hot topics The specific aspects of challenges where welding and joining technology transfers are needed and contribute to meeting the national objectives include the following: Urgent need for a country to upgrade its water catchment, storage, treatment and distribution and waste water infrastructure in both urban and rural applications. Minimisation of resource wastage and the risks of serious health and supply breakdown due to failing pipes/distribution. Maintenance of aging infrastructure. 9.13 Advanced steels sector The use of advanced steels with enhanced properties can contribute to the improvement of ordinary life in many aspects. Welding of advanced steels in similar and dissimilar configurations poses challenges, however. For example, High Strength Low Alloy (HSLA) steels with ultimate tensile strengths not less than 780 MPa were developed by many steel companies, but their welding is faced with the following problems: Lack of knowledge about the metallurgical factors of the weld metal necessary to obtain required mechanical properties. Requirement of preheat at temperatures rising with the base metal strength. Welding residual stress that increases with the base metal strength and limits the tolerable ultimate and fatigue strength of the weld to levels much lower than the base metal. 9.13.1 Metallurgical challenges With respect to the metallurgical factor, the acicular ferrite, which forms through γ → α transformation intragranularly nucleated at oxide inclusions, is generally accepted to be a desirable one with sufficient ultimate strength and toughness for the weld metal for HSLA steels of 580 MPa class or less. The acicular ferrite, however, is thought to be insufficient to bear the mechanical load required for the steel of more than 780 MPa classes, and so a microstructure harder than the ferrite, like a bainite and martensite (B+M) microstructure, is expected to be suitable for the weld metal of the steel with higher strength. An important difference between the weld metals of acicular ferrite and B+M microstructure is the effect of oxide inclusion that is indispensable for the formation of the acicular ferrite as a nucleation site. For the B+M structure, however, the oxide inclusion is unnecessary for their formation, and has significantly harmful effects on the toughness at volume fractions introduced during conventional arc welding processes except for tungsten inert gas (TIG) welding and metal inert gas (MIG) welding. Although TIG and MIG welding can produce weld metals with much lower oxide contents, they are difficult to carry out at high heat inputs, and so inferior in the welding productivity to the other arc welding processes.

137

Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life