Toughness not the same thing as strength

The inherent toughness XLERPLATE® steel 250 grade offers potential benefits to distributors, fabricators and end-users, but to make use of such benefits it is important to understand exactly what is meant by 'toughness'.

Toughness and strength are two different properties; strength is the ability of a material to resist being pulled apart, while toughness is the ability to resist crack growth in a material.

A material may have low strength, but good toughness, i.e. plasticine, while conversely a material such as glass has high strength and low toughness. These two materials represent extremes, but illustrate the difference between strength and toughness.

Toughness is an important property in material design because good toughness minimises the potential for catastrophic failure. A material with good toughness may bend and buckle as a result of an impact or overloading situation, but it will resist rupture minimising the damage that may otherwise occur.

In the past, when impact tested grade 250 XLERPLATE® was specified, a special mill order was necessary to meet the customer's requirement. BlueScope Steel has further improved the toughness of its standard AS/NZS 3678 - 250 XLERPLATE® offering a steel that will meet required Australian standard impact toughness properties down to minus 15°C (designated L15) in plate thicknesses up to and including 32 mm. The BlueScope Steel 250 grade is therefore suitable for use as grade AS/NZS 3678 - 250L15 XLERPLATE® in thicknesses up to 32mm. In the thickness range specified, BlueScope Steel's 250 grade XLERPLATE® not only meets, but comfortably surpasses the "L0" (or 0°C) requirement that is commonly specified in the construction industry.

Measuring Toughness

Numerous factors affect the toughness of steel. Principal among them are the chemical composition, microstructure grain size and level of impurities (or inclusions) in the steel. In terms of composition, increasing amounts of carbon will lower toughness while increasing amounts of manganese and silicon will tend to improve toughness. Micro-alloying elements, such as niobium, titanium, vanadium and aluminium, will also improve toughness principally by producing a smaller grain size in the steel. Other elements such as sulphur will tend to form elongated inclusions known as stringers which have a marked detrimental effect on toughness.

As mentioned previously, a finer grain size improves the toughness of steel. Several processes have been developed to reduce grain size, in particular normalising and controlled rolling.

Energy Absorption

As toughness is the ability of a material to resist the growth of a crack, the measurement of toughness involves quantifying the force required to propagate (or grow) a crack. Propagation of a crack in a given material requires a certain amount of energy which is characteristic of a particular material at a given temperature. A tough material will absorb a lot of energy before a crack will grow, while a brittle material absorbs very little energy. The measure of toughness of a material is in the energy it absorbs during testing.

The toughness of a material also depends on the temperature at which the material operates. Tests that measure toughness must specify the temperature at which testing is to be carried out and the minimum level of impact energy required at that temperature.

The most common type of testing is known as Charpy Impact Testing. This test is known as an impact test because the test piece is struck by a hammer on the end of a pendulum. Upon impact a certain amount of energy is absorbed in fracturing the test piece. The amount of absorbed energy determines the height to which the pendulum rebounds, thus providing a measurement of the toughness of the material.

Failure, as a result of fracture occurs where there are flaws or stress raisers in a structure. Hence the test piece for toughness testing (commonly known as a "charpy") is notched to represent this situation. The notch is standardised to ensure a ready comparison between samples.

In considering the effect that temperature has on the toughness of steel, it is important to note the temperature range in which a marked change in toughness occurs. This temperature is referred to as the transition temperature and marks the change from ductile to brittle failure as the temperature is decreased. Above the transition temperature, failure will occur in a predominantly ductile manner, while below the transition temperature failure will be brittle.

BlueScope Steel has modified the manner in which its 250 grade XLERPLATE® (5-32 mm thick) is produced to improve the toughness of the steel. This type of plate is now controlled rolled to improve its toughness, ensuring BlueScope Steel 250 XLERPLATE® is suitable for applications where either 250L15 XLERPLATE® or 250L0 XLERPLATE® is required.

For a steel plate to be certified as meeting the Australian Standard for impact toughness at minus 15°C, the plate must be tested. It is for this reason that test certificates for BlueScope Steel's standard range of 250 XLERPLATE®, stocked by steel merchants, will not show impact certification. However, exhaustive testing has shown that BlueScope Steel 250 XLERPLATE® will meet the 250L15 XLERPLATE® impact requirements if tested. The benefit to the end user is that they can be 100 percent confident that a BlueScope Steel 250 grade XLERPLATE® (in the thickness range 5mm-32mm) will pass such a test if required.

The improved toughness of the 250 grade XLERPLATE® produced by BlueScope Steel offers benefits for all in the supply chain. For distributors it will mean that orders requiring an "L0" or L15 grade can be supplied from standard BlueScope Steel 250 grade XLERPLATE® held in stock. For the fabricator and end user this will mean greater availability of these grades, reducing lead times and costs. For all involved there will be less complexity in terms of order requirements. The introduction of a 250 grade XLERPLATE® with increased toughness by BlueScope Steel will make a wider availability of a higher quality steel.

By John Dryden