April 28, 2016
Stainless steel is a highly versatile material, possessing a unique selection of useful properties which can be exploited in load-bearing applications. The main property distinguishing stainless steel from carbon steel is its inherent corrosion resistance, due to the tightly adherent protective layer of chromium oxide which spontaneously forms on its surface in the presence of oxygen. This means that stainless steel components can be exposed to a wide range of environments without the need for protective coatings and is ideal for projects where an exposed metallic finish is required.
Due to differences in chemical composition, a stainless steel structural member subjected to a fire behaves differently than a carbon steel member: stainless steel loses strength at a slower rate for temperatures above 500°C and retains its stiffness better at all temperatures. Because of excellent ductility coupled with significant strain hardening behaviour, stainless steel can absorb considerable impact without fracturing. This has led to its use in structures and equipment required to resist extreme loading, such as fire and blast walls for the oil and gas industry as well as security walls, barriers, doors and gates. The corrosion resistance of stainless steel makes it ideal for components in aggressive environments which are inaccessible or difficult to inspect, maintain or replace. For this reason, stainless steel is a popular choice for load-bearing components in the nuclear industry. Here we take a look at some recent diverse uses of stainless steel in structural components.
Gasholder No. 8 was constructed in the 1850s. Part of the largest gas works in London, UK, it became a well-known landmark until it was decommissioned in 2000. The 25m tall guide frame was dismantled in 2011 and refurbished before being re-erected in 2013. To provide an interesting contrast to the gasholder frame, Architect Bell Phillips designed a 30m diameter polished stainless steel circular canopy within the perimeter of the frame. The canopy is supported by 150 columns, made from 25mm thick mirror polished 316L (UNS S31603) stainless steel, containing around 10% nickel. The columns are positioned at a wider spacing near the canal to encourage people to enter the park and give unobstructed views towards the canal. Away from the canal, the columns are closer together, creating a greater sense of enclosure. The canopy roof is 15mm thick 316L stainless steel plate, and incorporates a geometric pattern of perforations, formed by water jet cutting. The perforation density increases towards the canal to create a sense of openness, and a dappled effect as sunlight filters through.
Bearing the load in the nuclear industry
Load-bearing steel-concrete-steel modular structural units in new nuclear power plants are a popular choice, speeding up construction, simplifying the attachment of equipment support points and reducing costs. When one side of the unit is exposed to a corrosive environment, modules with a carbon steel plate on one side and a stainless steel plate on the other present an economic solution and have been used in the Westinghouse AP1000 pressurised water reactors in China and the US (Sanmen 1 & 2, Haiyang 1 & 2, Vogtle 3 & 4).
Although design codes for modular steel-concrete-steel construction in nuclear power plant exist in Japan, South Korea and the USA, there is no code in Europe. This is being addressed by a major EU-funded project led by the UK’s Steel Construction Institute. The SCIENCE project, due for completion in 2017, includes tests on 9m long beams where one plate is carbon steel and the other 10mm thick Type 304L (UNS S30403), stainless steel which contains about 10 % nickel.
Nearly 300 tonnes of Alloy 2304 (UNS S32304) duplex stainless steel (containing 4% nickel) were used in the fabrication of 13,400 m2 of fire and blast walls, heatshields, wind walls and blast relief panels for the Arkutun Dagi development. This is one of three offshore fields developed by the Sakhalin-1 project, which has combined reserves of 2.3 billion barrels of oil and 17.1 trillion cubic feet of gas. Sakhalin is a remote, sparsely populated island in the icy Okhotsk Sea, just off the east coast of Russia. The fire and blast walls are lightweight, high integrity, fully welded, gastight systems manufactured from profiled stainless steel and insulated to achieve specified fire ratings up to H120 and 120 minute jet fire, and to resist blast pressures up to 1.5 bar. The walls were designed, engineered and manufactured by MTE Ltd in Darlington, UK before being shipped to South Korea, for final assembly. ExxonMobil started production at the Arkutun Dagi field in January 2015.
These are just some examples of how the versatile nature of stainless steel is providing solutions for a wide range of structural challenges.