WT Steel: When Should you Use it?
8 April, 2019 | Blog
In Canada, as in other northern countries, both industrial and infrastructure projects must often be carried out in harsh winter conditions. Structures installed in these conditions can be subject to cyclical loads from dynamic or mobile load impacts.
In this context, the right structural steel must be determined in the design phase to ensure the structure’s integrity. By choosing the right type and grade of steel, structural elements will be protected against the brittle fracture that can occur in winter. This blog post clarifies the reasons for choosing WT steel and examines the phenomenon of brittle fracture by steel.
In a northern environment—apart from ductility, a basic mechanical property that enables steel to undergo significant deformation before rupturing—impact strength at low temperatures is a critical property. The steel must be resilient to brittle failure. WT steel is steel with improved toughness. This type of steel must meet specific testing requirements, based on the daily mean minimum temperature in the location where it is to be used.
Under certain conditions of use, it has been observed that metal structures may be sensitive to a type of destruction called brittle fracture. An impact test is used to measure brittle fracture strength. It consists of fracturing a notched test specimen at the prescribed temperature. This type of test is intended to measure the energy absorbed by the test specimen at the point of rupture as well as to assess the impact strength of a notched test specimen and its propensity to brittle fracture. A Charpy pendulum impact test apparatus is used to conduct the test. Using a standardized test specimen (Figure 1) with a V-notch resting on two supports, the test determines the energy needed to rupture the specimen with a hammer. The speed (5.0–5.5 m/s) and size of this hammer are also standardized.
Figure 1: Standardized test specimen for the Charpy test
Toughness is the term for the fracture energy applied or not to the section below the notch on the test specimen: it is expressed in joules or joules/cm2.
Figure 2: Results of an impact test using a Charpy V-notched test specimen
The two curves (Figure 2) are an example of results for a Charpy V-notch test depending on whether the speed of impact is high (impact, dynamic test) or slow (static test). Thus, we can observe that the toughness of the structural steel is a function of temperature and speed of loading. Consequently, the choice of steel grade depends on the specimen’s gauge, the type of structural element, the steel’s strength and resistance, the determining service temperature and thetype of loading applied..
A low temperature, as in northern conditions, makes the steel brittle. A high temperature, as in a fire, reduces the steel’s rigidity and resistance or causes it to dilate.
Fatigue is induced in structural elements when they are subject to repeated loads (e.g., frequent hoisting activities, impacts and/or dynamic loads from vibrating machinery, mobile loads on a bridge). Fatigue can cause certain elements to crack, particularly in areas located at right angles to fixation points or welded joints. The principle of assessing fatigue strength is based on the concept of stress difference, the number of load cycles during the structure’s service life and the type of construction detail considered.
For example, when designing a structure to support vibrating machinery (e.g., a crusher for a mining project) that will be built in northern conditions, the number of loading cycles set by the manufacturer must be properly assessed for the equipment’s entire service life. For example, a crusher with a speed of 18 to 60 RPM that operates two hours a day for 25 years will produce 20 to 65 million cycles. When the number of cycles exceeds one million, the structure’s fatigue strength must be verified, while the Charpy test category and the type and grade of steel must be classified based on the CSA G40.21 and CSA S16 standards. Moreover, it is necessary to confirm with suppliers that WT grade steel members are in fact available. This type of steel is not always readily available and may require a longer procurement period.
Finally, WT steel is used on bridges for members that are subject to bending or tensile loads. This means that the members undergo repeated impacts throughout their service life—consider the impact you feel when a vehicle drives over the expansion joints of a bridge deck.
If you have any questions about what type of construction steel to use or are facing challenges in terms of planning construction projects in the winter months, our experts from the Structural team can help.
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