High Strength Steels - Cost or Economy?

In a time of ever increasing cost consciousness, there is a growing need to consider whole of project costs when specifying materials. High strength quenched and tempered (Q&T) steels illustrate how the up-front cost of materials can be more than offset by other factors.

In the half century since these steels, typically in the 500 to 700 MPa range, first became commercially available, they have found their way into many structural applications where high strength and light weight are critical. These Q&T steels offer the design engineer many advantages which, in the right circumstances, can generate significant cost savings. Key design benefits include longer or lighter spans and greater load carrying capacity. Financial benefits can also be realised through reduced transport and lifting costs and reduced welding requirements.

In general, Q&T steels offer substantial weight savings over traditional steel grades, and designers are increasingly using this to advantage in "hybrid" applications. Common examples of this practice include beams with high strength flanges and standard strength web, and steel tanks with higher strength steel for the more heavily loaded lower sections, thereby maintaining a constant wall thickness for simplified fabrication. This hybrid approach can give steel a crucial cost advantage.

The case for Q&T steels is much the same as the case for structural steel over heavier materials, but in this case the comparison is with ordinary steels rather than with other materials. Q&T steels' light weight can really come into its own in remote locations, on difficult-to-access sites, and in city locations with time limits on heavy lifts.

Australia's producer of such steels is the Bisalloy plant at Unanderra, NSW, which produces high strength plate ranging from 5 mm to 100 mm thick, with a maximum width of 3200 mm and maximum length of nine metres. Feed XLERPLATE® is supplied from BlueScope Steel's Port Kembla works, and is closely controlled for gauge and shape. It is levelled and, when specified, ultrasonically tested.

Bisalloy's process begins by heating the plates to 900 degrees celsius, then water quenching it to room temperature. The plate is then tempered at temperatures ranging from 200 to 650 degrees celsius to achieve the right combination of strength and toughness for each application. Finished plates are abrasively blast cleaned, levelled and sampled for mechanical testing prior to despatch.

Structural benefits

The resulting steels exhibit high strength and hardness coupled with good formability and weldability. They also maintain excellent crack propagation resistance and exceptional low temperature notch toughness despite their high strength.

The advantages of using high strength steels are generally a result of reduced weight and dimensions. Design stresses can be increased and plate thickness may be reduced, resulting in significant weight savings. Reduced plate thickness can also save on welding costs. Simplified structural components and construction techniques are often possible, particularly for large structures, and foundation costs may also be reduced due to lower dead weight.

The most economical and efficient use of Q&T steels is in members stressed in tension where the high strength can be fully exploited, and in projects where the dead weight of the steel is the predominant load, such as in long span bridges. In compression they are most effective in heavily loaded, stocky columns or in stiffened compression elements where buckling is not the controlling criterion.

In major buildings, high strength steel can be used to good effect in the more highly stressed elements of a project. In one American example, the designers of a 28 storey office tower used Q&T steel selectively to achieve spacious floor areas and slim, uniformly dimensioned columns throughout the height of the building. The high strength steel was used on the lower levels where the total load was greatest. Columns were fabricated from 690 MPa yield strength plate up to the 15 storey level and in the core of the building, with 350 MPa steel for the mid levels and 250 MPa for the top floors.

One of Australia's more prominent buildings to incorporate Q&T steel is the Northern Territory Parliament House, where the high strength steel is used for its columns and beams. This project utilised several hundred tonnes of Q&T steels, all fabricated locally in Darwin.

Bridges offer great potential for the use of high strength steels. A typical application is in an Italian motorway bridge spanning almost 700 metres with the deck supported by a deep steel box girder. With individual spans of 95 metres, the girder's highly stressed parts are fabricated from 690 MPa yield strength steel, with 250 and 350 grade plate elsewhere in the structure.

Similarly, a German cable stayed bridge with a main span of 350 metres uses high strength steel in the twin 48 metre tapered box section towers which anchor its cable stays. In a typical hybrid construction, 690 MPa yield strength steel is used in the lower sections and 350 grade plate in the upper areas and the deck.

Cost benefits

To quantify the cost benefits of such designs, consider the 980 metre Minato Ohashi Bridge in Osaka, Japan, which uses 690 MPa yield stress steel in its fabricated box sections. When the cost of this design was calculated for different grades of steel, it became clear that, although high strength steel cost more, this was more than offset by reduced fabrication and erection costs. Such examples are not uncommon when the application is well suited to the specification of Q&T steels.

One application where the light weight of Q&T steels can dramatically affect performance is in the telescopic jib of a mobile crane. In one such design, the substitution of high strength steel in place of 250 grade plate produced dramatic improvements. By reducing the plate thickness required, the mass of the boom was reduced by more than 30 per cent, but more significantly, the load capacity was increased by 118 per cent.

Persuasive benefits

High strength steels offer very persuasive benefits in pressure vessels, in particular the large spherical vessels used for storing pressurised fuel gases. By enabling the practical construction of much larger vessels, high strength steels offer savings in materials, ground area and fittings required for a given storage capacity. The savings accrue mainly from the need for fewer vessels, and the economies of scale increase significantly with the strength of the steel, with savings in all aspects of the project.

Few applications take more complete advantage of Q&T steels than do offshore drilling rigs which are towed to the drilling site and jacked up on location once their legs have been lowered to the sea bed. The use of 690 MPa yield strength plate in the jacking mechanism offers more than just weight savings. Excellent low temperature toughness becomes a major issue in the bleak marine environment, as does the steel's good weldability in adverse conditions.

Other structural applications for high strength Q&T steels are as diverse as pressurised road tankers, above-ground and und

erground mining equipment, semi-trailers and low loader frames, sophisticated lifting equipment and the pressure hulls of submarines.

Although the economics will be influenced by many factors, it is increasingly common to find that the cost of the material is offset at least in part by the reduced material requirement, and frequently is more than offset when total project costs are calculated.

For more information on quenched and tempered steels:

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