Last edited 27 Nov 2016

Stainless steel in construction

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Contents

[edit] Introduction

Steel is one of the basic materials used in today’s civil engineering industry due to its proven high strength and durability.

Though commercial carbon steels do not corrode in pure, dry air at room temperature, they do corrode in moist and contaminated environments. Corrosion of steel used in buildings causes structural failures that result in safety hazards. The relatively newly emerged stainless steels, which contain a high proportion of alloy elements such as chromium, generally do not form rust on their surfaces and do not discolour at normal atmosphere. As stainless steel possesses similar desirable properties to normal steel and eliminates their disadvantages, stainless steel has been widely used as a construction material. The rate of growth of stainless steel use in the civil engineering sector is rapid. It is estimated that in 2006, approximately 14% of the world’s total stainless steel production went into the construction industry (Baddoo, 2008).

[edit] History

[edit] Early History

The use of iron in human history can be traced to around 4,000 years BC when early civilizations in Asia and Africa used iron ore to make tools for agricultural purposes (Swank, 1892; Todd, 2003). In about 1400BCpeople learned to strengthen iron by heating iron ore and charcoal in simple furnaces (Todd, 2003). In early 300BC, crude steel, which is stronger than iron was made by reheating iron with other metals, however, it was not until the 8th century that cast and wrought iron were first introduce to civil engineering (Sharp, 1993). The earliest application on record is a suspension iron chain bridge in China.

[edit] 15th to the 19th Century

Britain started to manufacture large amounts of iron in the early 15th century (Swank, 1892). During the 15th century, improvements of furnaces accelerated the development of the iron industry in England; between the 16th and 18th centuries, the lack of charcoal in England gave rise to a large amount of iron import from Sweden, America and Russia.

Europeans used iron for roofing members in the 18th century (Sharp, 1993).

The construction of the clear span “Ironbridge” at Coalbrookdale in 1777-9 marked an advance in the use of cast iron.

The industrial revolution in the 19th century led to rapid development of the steel industry as the demand for machinery and transportation increased (Todd, 2003). The use of modern day steel in Britain began in 1856 (Sharp, 1993). The Forth Railway Bridge built in 1890 was the first large scale application of modern day steel in Britain. In the construction of the bridge, riveted and bolted work was used to put together the tapered cantilevered trusses into multiple spans.

[edit] 20th Century to Present

The mass manufacture and use of steel in the 20th century brought great advance to urbanisation; from street lights to telephones, from typewriter to railways, the world benefited greatly from steel (Sharp, 1993).

The most remarkable application of steel was the construction of the skyscraper which led the architectural world to a new era. Steel frames became commonly used in Britain from 1909 as their strength was proven. Though there were many attempts to develop iron alloys from the mid-19th century, applicable stainless steel was not developed until the first decade of 20th century in the United Kingdom and Germany (Gunn, 1997; Beddoes& Parr, 1999).

Manganese was among the earliest metals added to steel to increase its heat resistant properties in the 1930s and the new alloy was used commercially in the early 1950s (Beddoes& Parr, 1999). In the mid-20th century, it was found that steels could be hardened by adding metals such as titanium. Duplex stainless steel emerged in the early 1980s (Gunn, 1997). Today, stainless steel is used extensively in roofing, cables of suspension bridges and flood barriers.

[edit] Properties

[edit] Chemical Properties

Sedriks (1979) defines stainless steel as iron alloys containing a minimum of approximately 11% chromium; this amount of chromium prevents corrosion in unpolluted atmosphere and this is the reason that the steel is named “stainless”. As its name suggests, the corrosion resistance is one of the most important factors. However unlike its physical and mechanical properties which have measured values or derived equations, there are no established chemical equations or simple tests for corrosion resistance. The knowledge about corrosion resistance is therefore solely from recorded experience.

[edit] Physical Properties

Only a few physical properties are important to stainless steels used for engineering purpose; specific gravity, electrical conductivity, magnetic susceptibility and thermal conductivity (Beddoes& Parr, 1999). Stainless steels are more resistant to high temperature than carbon steels are (Baddoo, 2008). Though the physical properties are not the most important, they may have an effect on the selection of the material.

[edit] Mechanical Properties

[edit] Tensile Properties

According to Biggs(1993), Figure 1 shows the tensile test of steel. The linear part OA is the elastic region where the material will return to its original shape under the given stress. Point A is called yield strength and beyond A is the plastic region where further stress will cause permanent deformation. If a stress is given from A to C and then unloaded, the length of the material will increase by OD. When the material is reloaded, the stress/ strain curve becomes linear again and the new yield strength increases to point C. At point F, the material breaks under tension and this point is slightly lower than the maximum stress at point E (ultimate tensile strength). In civil engineering practice, safety factors are used since the real distribution of stresses, especially in complex structure, may not be exactly the same as in this test.

Figure 1: (Image deleted - copyright free image required)

[edit] Fracture Toughness

According to Beddoes & Parr (1999), the toughness of a material is the resistance to fracture when small defects raise the stress locally to the breaking stress. Biggs (1993) points out that the stress to cause fracture is inversely proportional to the length of the crack; the speed of propagation of a crack could reach the speed of sound in the material and this implies that the cracking cannot be stopped once it starts even if the load is removed.

[edit] Fatigue

When a metal fractures under repeated stress at a level much lower than the ultimate tensile stress, it is called failure by fatigue (Biggs, 1993). Beddoes & Parr (1999) suggest that as fatigue always starts at the surface, great care should be taken during surface treatment. Fatigue is sometimes overlooked by designers even though there have been incidents of failure by fatigue in recent history. The collapse of the offshore platform Alexander Kielland in 1980 which caused the death of 123 people was due to fatigue failure of one leg of the platform (BBC, 1980). Fatigue failure may also be one of the reasons for the collapse of the overhead walkway across the atrium of Kansas City, Hyatt Regency Hotel in 1981 (Roe, 1981).

[edit] Types

[edit] Introduction

There are over 200 kinds of stainless steel composition and there are new ones being developed each year (Beddoes & Parr, 1999). Each stainless steel can be distinguished by its response under heat treatment and the mode of fabrication.

Stainless steel can be classified into three main categories based on their metallurgical structure: austenitic, ferritic and martensitic (Cochrane, 1993).

The Schaeffler diagram (Figure 2) relates metallurgical structure with the composition of stainless steels (Sedriks, 1979). The nickel (Ni) and chromium (Cr) equivalents based on weight percentage are the y and x axis of the Schaeffler diagram respectively. The equations of the two equivalents are written as:

%Ni equivalent = %Ni + %Co + 30 (%C) + 25 (%N) + 0.5 (%Mn) + 0.3 (%Cu), (1)

where Co is cobalt, C is carbon, N is nitrogen, Mn is manganese and Cu iscopper;

%Cr equivalent = %Cr + 2 (%Si) + 1.5 (%Mo) + 5 (%V) + 5.5 (%Al)+1.75 (%Cb) + 1.5 (%Ti) + 0.75 (%W), (2)

where Si is silica, Mo is molybdenum, V is vanadium, Al is aluminum, Cb is columbium, Ti is titanium and W is tungsten.

Figure 2 : (Image deleted - copyright free image required)

[edit] Austenitic Stainless Steel

Austenitic stainless steels are the most widely used category for building applications (Cochrane, 1993). Austenitic stainless steel is considered more corrosion resistant than ferritic and martensitic stainless steels (Sedriks, 1979). Austenite is a form of iron that contains trace amounts of carbon at 900 to 1400C (Beddoes & Parr, 1999). To retain austenite structure in room temperature, some common austenitisers such as nickel, manganese and nitrogen are added. Austenitic stainless steel with Molybdenum is resistant to sea water and chloride-bearing solutions; this kind of stainless steel is therefore used extensively in aggressive marine and industrial environments (Blair & Pankiw, 1999; Cochrane, 1993). Though austenitic stainless steels cannot undergo heat treatment, they have tensile strengths of the order of 1000N/mm­­­­2 and can be used as reinforcing bars with concrete (Cochrane, 1993).

[edit] Ferritic Stainless Steel

Ferritic stainless steel is given its name because the crystal structure of the steel is the same as that of iron at room temperature (Beddoes & Parr, 1999). This class of stainless steel is magnetic unless it is heated to above 750 . The Schaeffler diagram (Figure 1) suggests that ferritic stainless steel should contain more than 12% chromium and very little Nickel (Sedriks, 1979). Though sometimes small amount of other elements such as aluminium, titanium and molybdenum are added, ferritics are considered as binary alloys; therefore ferritic stainless steel has a reduced corrosion resistance and is cheaper than austenitic stainless steel (Beddoes& Parr, 1999; Cochrane, 1993 ). The application of ferritic stainless steel in buildings is limited to the interior where corrosion resistance is not so much of a factor (Cochrane, 1993). Ferritics do not respond to heat treatment and are more difficult to weld and shape than austenitic stainless steels (Blair& Pankiw,1999; Beddoes& Parr, 1999).

[edit] Martensitic Stainless Steel

Martensitic stainless steel has approximately 11.5% to 18% chromium content and has an austenitic structure when it undergoes heat treatment and can be quenched to martensite with an increase in hardness (Sedriks, 1979). Although martensitic stainless steels are extremely strong and tough, Beddoes & Parr (1999) point out that the strengthening of the material brings some disadvantages: they are more brittle and are harder to weld and shape. Precipitation-hardened martensitic stainless steels are formed by adding alloying elements into solid solution at high temperature without the formation of coherent solution, followed by rapid cooling which retains the elements in the solution (Blair& Pankiw,1999). The desired mechanical properties are achieved by reheating the material to different temperatures.

[edit] Duplex Stainless Steel

Steel with 28% chromium and 6% nickel and containing both austenite and ferrite is called duplex stainless steel (Sedriks, 1979). This type of steel has both beneficial and harmful characteristics of the two phases (Beddoes & Parr, 1999). By adding other austenite and ferrite stabilizers, the composition of the two phases can be varied (Sedriks, 1979). A lot of effort has been put into developing the properties of this relatively recently emerged steel (Blair & Pankiw, 1999).

Duplex stainless steels are normally used when corrosion resistance and strength are equally important (Beddoes & Parr, 1999). Duplex stainless steel has been a suitable alternative to carbon steel, other types of stainless steel and nickel based alloys; it has been used for civil engineering applications such as blast and weather walls (Gunn, 1997).

[edit] Fabrication

[edit] Cutting, Joining and Weldability

When stainless steels are used as structural steelwork, they need to be cut into the required shapes or joined together. The cutting process involves sawing and burning while connecting involves welding and bolting (Pope, 2003). Today cutting is monitored and carried out by computer generated “virtual templates”. Welding is carried out in manufacture shops while bolting usually takes place on the construction site.

Air carbon-arc cutting is the use of high-velocity streams of air to remove melted metal behind the arc; it is often used for cutting irregular shapes. Plasma-arc cutting directs gas through an electric arc and ionises it. As the positively charged ions combine with electrons at the steel surface, the temperature rises significantly and this cuts the steel. Electron and laser beams are usually used for cutting thin steel when precision is required. Laser beams are commonly used to drill fine holes.

Making a bolt joint is generally cheaper than making a welded joint (Schollar, 1993). Bolt joints are like pegs in holes and tension and shear both need to be considered. Welded joints require the fusion of the connected faces by heating and allow the use of the cross section of a member rather than having holes for bolts (Schollar, 1993).

There are two types of welds, fillet welds and butt welds. Fillet welds do not require shaping for the edges of plates. Butt welding is used when the full strength of the plates needs to be developed and the weld metal is put across the whole thickness of the plates. Cracks may occur in the weld itself or the parts being connected due to the contraction of the weld when it is cooled down.

[edit] Cold Forming and Heat Treatment

The process of increasing the yield strength of stainless steel by plastic deformation at room temperature is called cold forming (Cruise & Gardner, 2007).

In ancient times, copper and bronze were hardened by hammering to form edges; the cold forming of stainless steel works by using the same principle but with much greater control (Beddoes & Parr, 1999). Today cold forming is achieved by rolling stainless steel into plates and sheets or drawing them into rod and wire. While the strength of stainless steel increases, the ductility decreases.

Heat treatment is used to change the mechanical, physical and chemical properties of stainless steel (Beddoes & Parr, 1999). It usually involves heating or cooling at extreme temperatures to achieve the desired level of hardening or softening. There are many different methods of heat treatment. Annealing is the most common method which is used to soften stainless steels to make them easier for cold working. For ferric and austenitic steel, the annealing process heats them until recrystallization occurs and the size of the steel is increased. For martensitic stainless steel, the annealing process heats them until they are austentic and then cools them very slowly; after the process the steels become ferric with dispersed carbide.

[edit] Surface Finishing and Treatments

[edit] Polished Finish

Internal and external decorations such as stairs and window trims are usually polished by applying abrasives by means of wheels, belts and pads (Cochrane, 1993). During the polishing process, rough cut edges, welds and markings are cleaned and dressed (Beddoes& Parr, 1999). Electro-polishing is used when mechanical methods of polishing are unable to perform the job, for example chequer plate flooring (Cochrane, 1993). Electro-polishing is achieved by immersing the stainless steel in buckets with hot acids and applying an anode current to the steel by the electrochemical cell. After the process, the surface of the steel becomes smooth and can be easily cleaned (Cochrane, 1993).

[edit] Coloured Stainless Steel

Coloured stainless steel is widely used in civil engineering applications such as revolving doors, wall panels and lifts. Coloured surfaces are obtained by producing a coloured film through chemical processes. First, the stainless steel is immersed in chromic and sulphuric acids at a temperature a little below the boiling point of the mixture. The colour of the steel is changed from bronze to blue, followed by gold, red, purple and finally to green. Second, the stainless steel is rinsed with water and the film of colour is hardened by cathodic treatment. The appearances of the final product depends on the starting material: a metallic sheen is obtained for a bright and polished surface and matt colour is obtained if the surface was not originally polished. Corrosion resistance of the stainless steel is also increased by this colouring process.

[edit] Applications

[edit] Roofing

Stainless steel is widely used as a roofing material. In the UK, stainless steel was initially used for church roofing as it was cost effective (Cochrane, 1993). Standing seam and batten roll are two ways of roof laying. Seam joints can be folded and welded to form watertight joints for flat roofs and as a substrate for roof gardens. Batten roll forms a bold line at joints.

The most famous application of stainless steel as roofing material is the roof finial of the Chrysler Building in New York built in 1928-30 which is still intact today. A more recent example is the roof of the Thames Barrier. The flat panels are curved in two directions to make a dimpled shape. The roof of Doncaster Bus Workshop, UK is made of stainless steel with a mosaic pattern to soften the visual compact.

[edit] Window Walls

Stainless steels have been used for window walls since the 1930s. The most famous application is the stainless steel cover strip used for the Daily Express, London (Cochrane, 1993). Individual casements can be made out of stainless steel facings or roll formed stainless steel. Stainless steel has higher heat resistance than that of normal structural steel and it is therefore used for fire-resisting spandrels and panels.

[edit] Building Trim

According to Cochrane (1993), stainless steel is a popular material used in building interior decorations such as railings and staircases. Stainless tubes are mechanically or weld joined to form the hand railings of staircases. Grid and plank stainless steels are used for flooring in raised floors and walkways. Thin plates of stainless steel can also be attached to old floors and staircases for renovation.

[edit] Structural Work

Stainless steel was not initially widely used for structural purposes in buildings due to the high cost (Gedge, 2008). Over the last decade or so however, the advancement of technology has made stainless steel much more accessible with reduced costs; furthermore, corrosion resistance and durability have improved.

Today, stainless steel is considered one of the most important materials for structural work. Stainless steel is used for its aesthetical and functional properties. Reinforcing stainless steel bars together with concrete make structures more durable and save time for inspections and maintenance (Baddoo, 2008).

[edit] Failure

[edit] Corrosion

Though stainless steel is known for its high corrosion resistance, corrosion can still occur. There are two main categories of corrosion: general corrosion and localised corrosion (Gedge, 2008).

General corrosion occurs when the entire metal surface is uniformly covered with a film of red rust or green patina (Beddoes& Parr, 1999). Stainless steel usually does not have visible general corrosion in a dry atmosphere, but at high temperatures, general corrosion may occur. General corrosion occurs when stainless steel is immersed in acids and alkalis (Sedriks, 1979). Failure by general corrosion is not very critical and could be determined by immersion test or corrosion literature.

Localised corrosion includes pitting, crevice, inter-granular corrosion, stress-corrosion cracking (SCC), corrosion fatigue and erosion-corrosion. This type of corrosion occurs on particular parts of the stainless steel and is usually at a small scale not easily detected. Failure by localised corrosion is catastrophic as the corrosion develops very quickly. The resistance to the localised corrosion is different for each type of stainless steel and it depends on the alloy compositions. Pitting resistance equivalent (PRE) is written as

PRE= %Cr + 3.3%Mo +16%N, (3)

which is a method of describing corrosion resistance (Gedge, 2008). This equation can only be used as a rough reference in the selection of stainless steel.

[edit] Seismic Performance

Stainless steel is well-known for its ductility and thus is considered less likely to fail during earthquakes than other building materials such as concrete (Dowrick, 1987), although research on the seismic behaviour of stainless steel frameworks is limited. Though most types of stainless steel are ductile, cold formed steel does not have significant ductility and can only be used in earthquake resistant structures with appropriate measures to ensure that they meet the relevant criteria. Standard deviation of the strengths of members should small and beams should fail before columns.

[edit] Sustainable construction

[edit] Recycling and Re-use

Construction accounts for 10% of the GDP in the UK (Plank, 2003). The great economic and environmental impacts of construction make sustainable construction an important issue. Recycling and re-use of construction materials could reduce environmental damage significantly. As stainless steel is made from ore and the demand for it has increased significantly in recent years, the need for recycling and re-use has become more and more important. Recycling of stainless steel is economically viable as it contains valuable alloy elements. Today, the production of stainless steel involves melting used scrap with other steel alloys. In the UK, 84% of steel components from building are recycled and 10% are re-used (Plank, 2003).

[edit] Renovation

Approximately 45% of building constructions in Europe are renovated each year and repair and maintenance cost 400 billion Euros (Baddoo, 2008). Therefore saving energy for upgrading existing buildings is critical. Today many buildings are renewed by over-cladding and over-roofing, where a new cover is installed over the existing one to improve energy efficiency, fix damage and enhance appearance. Stainless steel can be used as over-cladding roofing panels and brackets for connecting the old and new structures.

[edit] Find out more

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[edit] External references

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  • 2. Sedriks, A. J. (1979) Corrosion of Stainless Steels. Canada, John Wiley&Sons Inc.
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  • 4. Gunn, R. N. (ed.) (1997) Duplex Stainless Steels. Cambridge, Abington Publishing.
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