High Performance Steel And Its Application Engineering Essay

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INTRODUCTION

High Performance Steel (HPS) is the designation given to steels that offer higher performance in areas such as tensile strength, toughness, weldability, cold formability and corrosion resistance compared to the mild steel grades that are more traditionally used. In the past 15 years there have been significant improvements in steel making technologies that have made it possible to develop HPS for the construction and engineering industries.

Traditional structural steels incur more expenses arising for several reasons. Maintenance costs such as painting and rust-removal mean repeated work must be carried out on the steel, where it possibly may need replacing if its condition is deemed unsatisfactory. Also traditional methods of production, such as the blast furnace, consume enormous amounts of power compared to the electric arc furnace which is commonly employed to manufacture these new HPS's.

A new societal demand for slender light weight structures in construction projects including long span bridges and multistorey buildings has been met with the production of HPS. In such civil engineering structures there is a requirement for the use of high strength materials to ensure the fabrication demands are met. HPS fulfils a great potential for use as a new effective and aesthetic structural solution.

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Our structures are increasingly gaining technical and obscure designs, too, with the design requirements of the steel often needing one or several optimized characteristics. HPS steels are able to have an increased ductility and yield strength, for example, and yet have an excellent weight to strength ratio due to the different properties that can be designated to the flange or web of a beam. Some HPS's do largely concentrate on only one property, such as heat resistance or seismic-activity resistance. In this report, it was found that certain HPS classes have the same or similar properties and chemical composition. Nevertheless, differences can be clearly made for each and shows that HPS is a valid civil engineering construction material.

Current design codes are lacking in their guidelines and support of HPS. HPS has been derived from concept to application in a short period of time. Over 200 HPS bridges have been constructed and put into service over the past 12 years, and another 200 are in the design and planning stages, already HPS is being used in 42 states in America. Also, the construction industry in Australia accounts for a significant percentage of annual national revenue. In 2008-09, this equated to nearly $280 Billion [46], further lending to the argument for HPS to be recognised in the Australian Standards.

TYPES OF HIGH PERFORMANCE STEELS

- Composition and Mechanical Properties

Weathering Class Steels:

HPS 485W (also referred to as HPS 70W) was the first high performance steel grade developed and commercialised in the American program. It is currently presented in the American standard ASTM A709.

HPS 485W is produced by either quenching and tempering or thermo mechanical controlled processing. The U.S. has adopted thermo mechanical controlled processing practices to produce HPS 485W since the quenching and tempering process limits plate lengths to 15.2 m. TMCP on the other hand can produce plates up to 50 mm thick and 38 m long, depending on the weight.

The chemical composition of HPS 485W and the older version of 485W are compared in the table below. The big difference observed is that the carbon level was greatly reduced, thereby increasing weldability and toughness capabilities. It can be seen that the carbon content is now only 0.11 % dropped from 0.19% in the earlier composition. The strength is maintained through alloy adjustments, micro-alloy additions, and processing changes.

Element

Percentage Composition

Old 485W

HPS 485W &

HPS345W

HPS 690W

Carbon (C)

0.19

0.11

0.08

Manganese (Mn)

0.8 - 1.35

1.1 - 1.35

0.95 - 1.5

Phosphorous (P)

0.035

0.02

0.015

Sulphur (S)

0.04

0.006

0.006

Silicon (Si)

0.25 - 0.65

0.3 - 0.5

0.15 - 0.35

Copper (Cu)

0.2 - 0.4

0.25 - 0.4

0.9 - 1.2

Nickel (Ni)

0.5

0.25 - 0.4

0.65 - 1.0

Chromium (Cr)

0.4 - 0.7

0.45 - 0.7

0.4 - 0.65

Molybdenum (Mo)

-

0.02 - 0.08

0.4 - 0.65

Vanadium (V)

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0.02 - 0.1

0.04 - 0.08

0.04 - 0.08

Aluminium (Al)

-

0.01 - 0.04

0.02 - 0.05

Nitrogen (N)

-

0.015

0.015

Niobium(Nb)

0.01 - 0.03

Following the successful introduction of HPS 485W industry requests were made for a high performance version of the standard 345W grade. Hence HPS 345W (or HPS 50W) grade was established from the exact chemistry of the HPS 485W, as shown in the table above. However this was produced using conventional hot rolling or controlled rolling.

HPS 690W (or HPS 100W) was also included as part of the original research into high performance steels. HPS 690W is a relatively new, copper nickel based, low alloy steel that has been manufactured for use in bridge construction. It is produced by quenching and tempering techniques to be made into plates of thickness up to 100mm.

Testing done on the 345W, 485W and 690W HPS grades has identified excellent performance of higher than minimum specifications in areas such as tensile strength, toughness, weldability, cold formability and corrosion resistance. The mechanical properties and requirements of the three weathering class high performance steels are identified in the table below.

HPS 345W

HPS 485W

HPS 690W

Yield Strength (MPa)

345

485

690

Ultimate Tensile Strength (MPa)

485

585 - 760

760 - 895

Elongation (%)

18%

CVN

41 J at -12 o C

48 J at -23 o C

48 J at -34 o C

To date HPS 485W has been most extensively researched, tested and used. HPS 345W and 690W have only been recently developed and made available for use in the engineering and construction industries.

Seismic Activity Resistant Steels:

Under the American system the class of ASTM A913 steels have been designed with varying chemical composition and characteristics which make them highly suitable as earthquake resistant steels [40]. Much like other high performance steels, the optimized aspects of its properties allows the steel to cope with exceptional circumstances; in this case dealing with seismic forces placed on steel structures.

In Australia, its availability is nil or limited mainly due to the low seismic activity recorded in the region. However, High Performance Steels (HPS) with earthquake abilities are required in Australian civil engineering and this is supported by the existence of AS1170.4 and other sections of Australian Standards which are concerned with seismic activity design [41].

The ASTM A913 classification covers several grades of steel, ranging from Grade 50 to Grade 70, respectively increasing in Yield Strength from 345MPa to 485MPa. For the purpose of this report, Grade 50 and Grade 65 will be focused on, and to maintain compliance with Australian Standards Grade 50 will be referred to as Grade 345 and Grade 65 will be referred to as Grade 450.

While strong similarities hold between the weathering steels of A709 and the seismic resistant steels of A913, clear differences arise in the chemical composition and also in the production process of the A913 class. The Carbon content, or carbon equivalent, has been lowered and the result is that no pre-heating before welding is necessary. As a comparison, the carbon equivalent of A913 is to be no more than 0.38% for Grade 345 whereas for A992 the carbon equivalent has a maximum value of 0.47%. These characteristics make A913 a slightly more ductile material than others [44], adding to its usefulness in seismic regions as a structural steel. The quantities of alloys are toward the higher end of the spectrum in terms of HPS, and for A913 this may be, in part, to counter the presence of larger amounts of sulphur to offset for its strength and toughness. Full chemical composition is shown in Figure 400.

Maximum Content (%)

Grade 345

Grade 450

Carbon

0.12

0.16

Manganese

1.6

1.6

Phosphor

0.040

0.030

Sulphur

0.030

0.030

Silicon

0.40

0.40

Copper

0.45

0.35

Nickel

0.25

0.25

Chromium

0.25

0.25

Molybdenum

0.07

0.07

Vanadium

0.06

0.06

Similarities in the mechanical properties of A709 and A913 can be noted namely higher yield and ultimate tensile strength, improved weldability and good toughness at low temperatures. Details of mechanical properties are given in the table below. Note that ductility (% Elongation) is significantly less than that for common 1020 Steel [44]. Toughness is again similar to A709 however A913 maintains its toughness at lower temperatures and is also designed to a required minimum value of 54J at 21°C.

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Despite the fact that A709 does have an experimental toughness of 94J at 20°C [43], no minimum requirement is set. For Grade 345 the steel holds its toughness down to a temperature of -20°C, and for Grade 350 a temperature of -50°C [42].

Grade

Yield Strength Min.

(MPa)

Tensile Strength Min. (MPa)

Elongation % Min.

200mm 50mm

345

345

450

18

21

450

450

550

15

17

The defining characteristic of A913, however, lies in several parameters which are built in to the material to help it manage seismic activity. These are what separates' this class from the other types also mentioned in this report, and revolve around lesser known parameters.

The first one is the fact that they are limited in their Yield point, that is, a restriction is designed into the steels' Yield point. As Gaxmann states "The upper limit of the yield strength...helps the designer to control the formation of the plastic hinge in the beams under earthquake loads".

This would allow a structure to bear the force of an earthquake and result in less or minimal damage, and, coupled with the 'strong column-weak beam' design especially for seismic activity [40], further enhance a structures seismic ability. This satisfies AS1170.4, where the collapse of the structure will be avoided [41].

The second important parameter for the A913 class is that a ratio of 0.85 can be set as the maximum value for the ratio of the Yield Strength to Tensile Strength, should a further or guaranteed safety margin be required. This should be seen as an optimum or required level between ductile transformation and capitalising on the Yield Strength available (leading to a higher ratio) which is a result of modern manufacturing techniques [45].

Stainless Steels

Stainless Steels are corrosive resistant steel alloys that contain a high percentage of the element Chromium, which is no less than 10.5%. The high percentage of Chromium reacts with oxygen to provide a thin protective layer over the steel, but unlike a galvanising coat over steel, the protective layer reforms and continues to protect the steel [20].

There are four main types of stainless steels:

Austenitic Stainless Steel - this stainless steel alloy is formed when adequate amounts nickel are combined with stainless steel so that the crystal structure changes to austenite. The general quantity of Chromium and Nickel are 18% and 8% respectively [21].

Duplex Stainless Steel - this alloy is formed like the Austenitic Stainless Steel but more complicated and harder, where the quantity of Chromium is greater than or equal (approximately between 18% and 28%), but the amount of nickel is less than that of Austenitic Stainless Steel (approximately between 2.5% and 4%). This amount of nickel does not change the crystal structure to austenite but also does not change it so that it is ferrite. This blend of austenite and ferrite crystal structures is called Duplex [20, 21].

Ferritic Stainless Steel - this alloy is a normal stainless steel as it has main compositions of Chromium, which varies between 12% and 18%, and carbon, which is quite low compared to other stainless steels [20].

Martensitic Stainless Steel - this is almost the same as ferritic stainless steels except that it has relatively high carbon content, which is approximately between 0.1% and 1.2%. The Chromium content is the same as ferritic stainless steel [20].

The two types of stainless steel alloys that are used in the construction and civil industry are the Austenitic and Duplex stainless steels, which have different grades. The different grades also have different chemical composition for their specific applications.

The different grades for Austenitic and Duplex stainless steels used in construction are [23]:

304 - This grade is an Austenitic Stainless Steel and has a couple of sub-grades, which are high carbon and low carbon (304H and 304L respectively).

316 - This grade is also an Austenitic Stainless Steel and has some sub-grades, which are high carbon and low carbon (316H and 316L respectively).

2304 - This grade is a Duplex Stainless Steel. It is an alternative stainless steel compared to Grade 316 as it is more economical in it applications.

2205 - This is a Duplex Stainless Steel and it the most common grade of Duplex Stainless Steel that is being used at the moment. Its chemical composition was constrained in 1996 hence the reason why there are two of the same grade showing similar chemical compositions.

2507 - This is a super-duplex stainless steel which has excellent strength and corrosion resistance.

According to Atlas Steel, the chemical composition of the grade and sub-grades of 304 are similar with the exceptions of Carbon, Chromium and Nickel. Table 1 shows the chemical composition of 304 in the form of minimum and maximum percentage of elements.

The chemical composition of the grade and sub-grades of 316 have the same elemental differences as in grade 304 with the addition of Molybdenum. Table 1 shows the percentage of chemical composition in grade 316.

Grade

C

Mn

Si

P

S

Cr

Mo

Ni

N

304

0.07

2

0.75

0.045

0.03

17.5 - 19.5

-

8 - 10.5

0.1

304L

0.03

2

0.75

0.045

0.03

17.5 - 19.6

-

8.0 - 12.0

0.1

304H

0.04 - 0.1

2

0.75

0.045

0.03

18 - 20

-

8 - 10.5

-

316

0.08

2

0.75

0.045

0.03

16 - 18

2.0 - 3.0

10.0 - 14.0

0.1

316L

0.03

2

0.75

0.045

0.03

16 - 18

2.0 - 3.1

10.0 - 14.1

0.1

316H

0.04 - 0.1

2

0.75

0.045

0.03

16 - 18

2.0 - 3.2

10.0 - 14.2

-

Table 1: Chemical Composition of Austenitic Stainless Steels in Construction and Civil Engineering [23].

Table 2 shows that the chemical composition of duplex and super-duplex stainless steels are relatively similar in elemental percentages with the exception that grade 2304 and 2507 have added a miniscule amount of copper into the stainless steel. The other main difference is the percentage of Molybdenum, Nickel and Chromium gradually increasing with the different grades.

Grade

C

Mn

Si

P

S

Cr

Mo

Ni

N

Cu

2304

0.03

2.5

1

0.04

0.03

21.5 - 24.5

0.05 - 0.6

3 - 5.5

0.05 - 0.2

0.05 - 0.6

2205

0.03

2

1

0.03

0.02

22 - 23

3.0 - 3.5

4.5 - 6.5

0.14 - 0.2

-

2507

0.03

0

0.8

0.035

0.02

24 - 26

3.0 - 5.0

6.0 - 8.0

0.24 - 0.32

0.05

Table 2: Chemical Composition of Duplex and Super-Duplex Stainless Steels [23].

Table 3 shows the minimum requirement of mechanical properties in Austenitic and Duplex Stainless Steels.

Grade

(Austenitic)

Tensile Strength (MPa) (min)

Yield Strength, 0.2% Proof (MPa) (min)

Elongation (% in 50mm) (min)

Hardness

Rockwell B (HR B) (max)

Brinell (HB) (max)

304

515

205

40

92

201

304L

485

170

40

92

201

304H

515

205

40

92

201

316

515

205

40

95

217

316L

485

170

40

95

217

316H

515

205

40

95

217

 Grade

(Duplex)

 

 

 

Rockwell C (HR C) (max)

Brinell (HB) (max)

2304

600

400

25

32

290

2205

655

450

25

31

293

2507

795

550

15

32

310

Table 3: Mechanical Properties of Austenitic and Duplex Stainless Steels [23].

ADVANTAGES OF HIGH PERFORMANCE STEEL

Advantages of high performance steels come mainly from its ability to offer higher performance in areas such as tensile strength, toughness, weldability, cold formability and corrosion resistance compared to the mild steel grades that are more traditionally used in construction and engineering industries. Use of HPS generally results in smaller members and lighter structures.

Strengths of high performance steels are greater than the equivalent conventionally used mild steel grades. HPS 345W, 485W and 690W all display higher yield and tensile strengths which achieves the goal of developing superior steels with higher strength and higher levels of safety. Stainless steels have high tensile strengths but some stainless steels have low yield strength. The strength of stainless steel depends on how it was formed and treated. The tensile strengths of stainless steels are relatively high. Although, Duplex Stainless Steels have higher tensile strength and yield stresses than the Austenitic Stainless steels.

Fracture toughness of high performance steels is much greater than conventional steels. The brittle to ductile transition occurs at a much lower temperature allowing the HPS to remain fully ductile at lower temperatures where conventional steel would start to show brittle fracture. Industry requirements specify that brittle failure is to be avoided in steel bridges above the "lowest anticipated service temperature" (Book). The tests done for HPS 485W steel shows that it still remains ductile at extreme service temperatures of -60oC. This is an important break through and accomplishment in controlling brittle fracture and failure.

With the higher fracture toughness higher performance steels have a much higher crack tolerance than conventional grades. "Full scale fatigue and fracture tests of I girders fabricated of HPS 485W in the laboratory showed that the girders were able to resist the full design overload with fracture even when the crack was large enough to cause 50% of loss in the net section of the tension flange" (Book). This increase in crack tolerance allows for greater time in the detection and repair of fatigue cracks.

A main aim of the HPS program is to develop bridge steels with significantly improved weldability. Improving weldability helps to eliminate hydrogen induced cracking in the weldment. Development of HPS has successfully led to the reduction or elimination of preheat in various cases. (FHWA). The Austenitic stainless steels are easily formed and have excellent weldability.

High performance grades of steel have better atmospheric corrosion resistance. Higher alloy levels in the HPS 690W, HPS 485W and HPS 345W steels allow for better performance of the steels even in chloride containing environments. Due to the higher quantities of chromium and copper in HPS 690W a protective coating on the outside of the steel is formed. This helps to prolong corrosive behaviour, by up to 4 times that of normal steel [4]. "The HPS 485W grade satisfies the composition requirements listed in ASTM specification G-101 to allow classification as a "weathering" steel suitable for use in the unpainted condition" (Book). This can reduce or even eliminate the need for painting in many bridge locations. The elimination of painting can greatly reduce the life cycle costs of a steel structure and so this is considered an important factor in deeming the steel 'high performance'.

In terms of stainless steels, all stainless steels have substantially high corrosive resistance. This is due to the Chromium reacting with the oxygen to form a protective oxide layer that continually protects the alloy [20].

Corrosion is split into two types, which are general corrosion and localised corrosion. The general corrosion is atmospheric and aqueous corrosion, this is when corrosion is spread all around the metal. The austenitic and ferritic stainless steels are resistant against this type of corrosion. The localised corrosion is when corrosion has affected a specific area in the steel, whether it is pitting corrosion or surface staining. The type of stainless steel that can resist this category of corrosion is dependent on the environment and other elements that are combined with the alloy [21].

Most stainless steels also have high temperature resistance. The Austenitic Stainless Steels have excellent heat resistance with most grades being able to resist temperatures of 350°C and up to 950°C, however constant usage at this temperature is not recommended. The high carbon austenitic stainless steels can resist those temperatures for longer periods of time.

A further advantage of stainless steels are that they are made from recycled materials (approximately 60%) and are completely recyclable since the elements (Chromium, Nickel and Molybdenum) can be recovered after the stainless steel has been through its long service life [20].

Use of HPS can reduce the first cost of steel bridges by reducing the weight of steel in the structure despite the fact that the current costs of HPS steel may be higher than its conventional equivalent. For example cost estimates prepared by the Tennessee Department of Transportation for their demonstration bridge program indicated a reduction in steel weight by almost 25% compared to the original grade 345W design. Even though HPS 485W held slightly higher current costs than the conventional steel grade, 16% reduction in the total cost to fabricate and erect steel for this bridge was still achieved. Estimates indicated a total savings of approximately $78 000 for this project. (Article)

Cost savings are also being taken advantage of through processes involved in production of HPS. With production making use of the Quenching and Self-Tempering techniques, costs are able to be kept lower in comparison to traditional and general purpose structural steels. Less power is needed to manufacture and, therefore, costs see a reduction.

Cost savings are being realized even though the recent nature of HPS development means that many of the projected cost savings associated with fabrication efficiency are not yet available. Therefore it can be assumed that even greater savings are projected for the future when welding and fabrication procedures are totally optimized. It is difficult to identify specific cost saving figures as this will vary with bridge type and span length. (Article)

DISADVANTAGES OF HIGH PERFORMANCE STEEL

As with any other application, HPS as a construction material in the engineering industry must be considered for its disadvantages.

Although the Duplex Stainless Steels have good heat resistance but are not as suitable as the Austenitic Stainless Steels at high temperatures. At temperatures above 300°C, duplex stainless steels become brittle hence the reason why their recommended use is not above 300°C [23].

Duplex stainless steels are weldable but should be done carefully as welding may result in excess ferrite changing the crystal structure and very hard to form, it cannot be bent easily for different applications [23].

The greatest disadvantage of HPS as a construction material in civil engineering is that there is not an extensive understanding of the properties of HPS yet. It is a new concept still being developed. In the early days few bridge owners were willing to risk potential problems in fabrication as they were unaware of the behaviour of the material.

Current design codes are lacking in their guidelines and support of HPS. This is a disadvantage when trying to push a new material onto the market, and gain industry acceptance.

When considering the seismic resistant A913 steels an important disadvantage is that Arcelor Mittal is the only known producer of the ASTM A913 class which leads to a monopoly of the sector, and is trademarked under the name HISTAR® [42].

Also, with this class only relatively new to the field (approximately ten years), long-term experience is absent and can be seen as a set-back for a product which is focused on improving the reliability of structures. Due to its lower carbon content, the ability to resist weathering of A913 may also be questionable. Its seismic characteristics may have resulted in some negative alterations for it to be able to defend against the elements; painting or coating of sorts may be an option, however these would incur extra costs and increased construction times.

APPLICATIONS OF HPS IN CIVIL ENGINEERING

High-performance steel was originally developed for use by the military in submarine construction. "In 1992, the Carderock Division, Naval Surface Warfare Center partnered with AISI and the Federal Highway Association (FHWA) to research ways HPS could be transferred from military technology to civilian applications and develop the new and improved steel alternative for use in bridge construction" (Article). HPS is now commercially available for primarily highway bridge construction.

The AASHTO HPS Guide encourages the use of hybrid girders, combining the use of HPS 485W with HPS 345W and HPS 690W steels. Combined use of HPS 485W in the negative moment regions and HPS 345W in other areas results in optimum and economical use. This hybrid combination of steels results in 20% reduction in steel weight, and enables the girder sections to be constant depth instead of haunched. By eliminating the variable web depth, a costly longitudinal bolted web splice can be avoided. An example of this is the Ford City Bridge in Pennsylvania that is depicted here.

Many State Departments of Transportation and other agencies have designed and constructed HPS bridges. Nebraska DOT was the first to use HPS 485W in the design and construction of the Snyder Bridge - a welded plate girder steel bridge. This bridge is shown in the image below.

When HPS first became available, Nebraska DOT replaced the conventional grade 345W steel with HPS 485W steel of equal size. They wanted to use this case to gain an understanding and experience on the HPS fabrication process. "The fabricators concluded that there were no significant changes needed in the HPS fabrication process" (Article).

The DOT then undertook research to develop an innovative concept optimizing the use of HPS. The result of this initiative is a two-box girder bridge with full depth composite deck system.

Continued research and demonstration bridges tested by the various states transport authorities across America concludes that the 40% higher yield strength of HPS 485W over Grade 345W "gives the engineers liberty to design longer, shallower spans when strength is the controlling limit state" (FHWA).

This is found to be beneficial when replacing simply- supported, multi-span structures with continuous-span structures. The development bridge projects across states including Nebraska, Tennessee, Pennsylvania, and New York led to the following conclusions for the optimum techniques of constructing with HPS.

Use uncoated HPS steels.

Use HPS 485W steel for flanges and webs over interior supports, where moments and shears are high.

Use hybrid girder sections for composite sections in positive bending, where moments are high, but shears are low.

Use under matching fillet welds with HPS 485W to reduce cost of consumables.

Use constant width plates to the greatest extent possible.

Consider waiving live load deflection limits for lane loads.

Use TMCP plates to greatest extent possible.

Guidelines sources from Tennessee Department of Transportation (TNDOT)

High performance steel fibres can also added to concrete for several applications. These include High flexural fatigue strength, high wear resistance, heavy duty floors, road and harbor pavements, slope stabilization and precast applications. The steel fibres act as other reinforcing steel would, increasing tensile strength but to a greater extent due to the nature of the steel. The steel fibres and typically made of a basic high strength-low alloy steel usually in the range of 345MPa.

Use of the seismic resistant HPS of A913 is similarly used for bridge construction mentioned above however specifically in areas prone to seismic activity. The A913 class would also be suitable for use in structures that are offshore. Oil rigs, jetties and similar structures are all subjected to the cyclic loading caused by ocean movement, presenting these steels as an obvious option.

An example of the off shore oil rig is shown in the figure below.

The tension leg platform is a relatively recent development in the construction of offshore oil rigs. The design consists of a platform connected by tethers to a set of foundation templates. These tethers experience a high mean stress, which high strength steel was found to be able to withstand. The steels used for the tethers have yield strength from a range of 600MPa to 800MPa. The platform itself is also made of high strength low alloy steel plates. The steel is used for its high strength to volume ratio; that is it requires less material to provide the same support and hence less weight which is a critical concern for an offshore structure. The steel itself is a titanium manganese high strength low alloy steel; the titanium manganese increases the strength of the welding joints. The steel has strength of 485MPa.

Stainless steel is also currently being used in the construction of bridges and structures around the world. The consideration of stainless steel in construction is increasing due to the effects of the environment on normal carbon steels, in specific areas, causing it to corrode quicker reducing its design life [24].

Some examples of stainless steel in the construction and civil industry are shown in table 4.

Structure

Material Type

Stonecutters Bridge Towers, Hong Kong

Duplex

Spencer Street Footbridge, Melbourne

Austenitic

Millennium Bridge, York

Duplex

Siena, Italy

Duplex

Table 4: Bridges that Use Stainless Steel [22, 24].

Duplex Stainless Steels are often the choice used in bridges since they have excellent strength and corrosive resistance compared to the austenitic stainless steels. They are also aesthetically appealing when visible.

The Stonecutters Bridge Tower in Hong Kong uses the duplex type on the two towers, which are approximately 300m high, and they hold up the bridge that has a main span, approximately 1km long, and two side spans, approximately 300m long. The tower is manufactured from concrete and the top parts of the towers have stainless steel coatings protecting the concrete from corrosion in a polluted aqueous environment [24].

Figure 1: Stonecutters Bridge Tower, Hong Kong [22].

The Spencer Street Bridge in Melbourne, the entire bridge was to be constructed from stainless steel but the main truss ended up being produced from normal carbon steel for economic factors. The stainless steel component of the bridge is visible to the community and also has structural benefits [24].

Figure 2: Spencer Street Footbridge, Melbourne

Applications of high performance steel are continually being investigated as the use of this material as a construction material in the engineering industry gains acceptance. Use of High performance steel is also being transferred to other applications such as railroad bridges, buildings, and pedestrian walkways.

CONCLUSION

The existence of High Performance Steels in civil engineering has shown to be highly necessary as a result of extreme circumstances that arise for structures. From weathering capability to earthquake capability, these steels have a sound future. It is also shown that some HPS exist where an optimum level of all or most characteristics, and depending on the application, this fact will be adequate for its intended purpose. Costs may prove a determining factor, both for and against its use. As stated above, some companies have a monopoly on production, however strong justification is needed to offset the fact that production costs of HPS's should be equal or less than that off standard structural steel. Indeed, it could be argued that the high performance properties, the reason for this report, allow for increased sale prices. As a rough guide, it turns out that A36 (standard structural steel in USA) costs $US0.70/kg and heat-treated high-strength low-allow steel costs $US0.80/kg [44].

The applications are borne out of necessity for High Performance Steel, and continue to evolve and adapt to individual situations, and vice-versa. Stainless Steel has shown to possess excellent corrosion resistance; however its immense hardness leaves unsuitable for some structures. It can be concluded that, with any visit to a coastal location as proof, stainless shall only be used as railings and guardrails and the like, with any structure of relative size and/or loading will be made of anything but stainless. Due to alot of the HPS's having a low carbon content, the heat resistance can be mildly reduced, though some grades of stainless may be seen as appropriate choices for heat resistance. The stainless steels containing Austenite have excellent resistance, ranging up to values of 950°C and will suffice for alot of designs where this is a requirement.

The weathering class, or ASTM A709, is suitable for harsh environments. Constant exposure to the elements is a major hazard for all structures, however use of the Grade HPS 50W and HPS 70W steels at least sees a longer structure life span and cheaper maintenance costs.

At present and as mentioned in the introduction, Australian Standards fail to recognise steel's of higher-performing abilities. Much of the classification of the American system has been adopted for the purpose of this report, specifically ASTM A709, A913, A992 and A36. A European system is in place in the Eurocode, however with alot of the US standards also in metric units now and also that major studies of HPS are often sourced from the US results in adoption of their classification. AS1170-2007 does have limit states for steels used in particular circumstances, though, and these are to be adhered for structures in Australia [41].

AS4672.1-2007 deals with some reinforcing steel and wire requirements but this is also only applicable to high carbon steels [47]. On a positive note AS/NZS1554.4-2010 will assist in the construction process using HPS as it deals specifically with welding steels that are high strength and have been quenched and tempered. The parent material that is in compliance with this standard makes reference to several sources, including ISO 4950-3 and ASTM A517 [48].

Nonetheless, HPS are in common use and frequently being used in place of once preferred structural materials the world over. The solutions that HPS's provide to the industry vary from pipelines to giant offshore oil-rigs. As production increases and its benefits gain popularity it appears high performance steels will dominate as the structural steel of choice.