African Fusion June 2018

New approaches to NDT

Presenting at the NDT Conference hosted by SAINT earlier this year, Tony Paterson high- lights the need for adopting new approaches to NDT in response to newmaterials and new operational imperatives, such as life extension drives and the health issues associatedwith the triple bottom line: planet, people and profit. NDT challenges relating to new materials, health and life extension projects

N on-destructive testing (NDT) is a wide group of analysis tech- niques used in the science and technology industries to evaluate the properties of a material, component or system without causing damage. Because NDT does not permanently alter the article being inspected, it is a highly valuable set of techniques that can provide assurance and save both money and time in product evaluation, troubleshooting, and research. Welding is a natural discontinuity as a cast structure is used to connect wrought parent materials. There is also scope for procedure and human errors. Failure in the heat-affected zone (HAZ) is a more common mode of failure. Hence NDT considerations paymuch attention to welds. The NDT role of providing assurance faces challenges into the future and may require some reassessment. This arises from operating circumstance changes. Also engineering materials have changed significantly, along with the late 20th and 21st century effects of globalisation, this including health and global sourcing.

results in larger deflections.

In no way restricting NDT, the future may well require amore systematic and regular assessment as residual life is as- sessed or as life extensionmodifications are considered in the face of ‘sweating the assets’. NDT will become more not less important. Consider engineeringmaterials. As a re- sult of various pressures, discussed be- low, the method of achieving structural (and other) steel mechanical properties changed significantly over the last 20 years. Whilst it has beenwell understood that steels can be strengthened through twomeans, composition and tempering through mechanical breakdown and thermal treatment – the latter includ- ing normalising, normalised-rolled, the thermomechanical rolling process (TMR) and Quenched and Tempered steels – hot rolled steels remained dominant. The new stronger materials have enableddesigners todesign lighter structures. However, there is a cost. As the Youngs Modulus remains constant, the lower sectionmodulus, Z, achievable Engineering material development

The motor industry, a very signifi- cant steelmarket, led theway to change. By the late 1980s Californian envi- ronmental legislation forced change on the motor industry by placing a premium on fuel efficiency, effectively placing a premiumon vehiclemass. This initially led to a motor industry move towards aluminium– aluminiumhas in- creased eight fold per vehicle from1975 to present to around 200 kg/vehicle. The steel industry responded by developing new, stronger steels andwonparts of the market back. From a structural engineering point of view, new stronger tempered steels were introduced. These included the commonly used S235, S275 and S355 grades, the S designating steel, the numbers minimum yield strengths for sections less than 16 mm thick. As the composition of all grades is near identi- cal, it is clear that grain refinement and/ or tempering determines the difference in performance. One question we seek to assess is the impact of welding on themoremod- ern materials and whether current NDT methods address these, whether it is suf- ficient to check for discontinuity alone when the welding temperature rises well above the transition temperature. Another area that has seen signifi- cant change includes the creep-resistant steels where the newest steels require both pre-weld and highly structured post-weld heat treatment. In addition the product life cycle for structures is often in excess of 50 years, with over 100 years not uncommon. Creep only shows up after extendedperiods of exposure to high temperatures and high pressures. Globalisation has enabled purchase worldwide. The steel industrydeveloped separately in different parts of a then isolated world. It developed differently in different countries. The products de- veloped for specific purposes were not identical. Many different countries developed local steel compositions,

Figure 1: Process plants are typically made up of tanks, heat exchangers, distillation columns, pumps and interconnecting thin wall pipes.

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June 2018

AFRICAN FUSION