Standards for the Impact Toughness of Steels
The impact toughness of steels is important in a wide range of mechanical engineering applications. It is a measure of the material’s ability to absorb and redistribute energy when subjected to a sudden, high impact load. Impact toughness is usually expressed in terms of the Charpy V-notch impact test, which involves measuring the energy absorbed by a test specimen of steel when it is broken by a falling pendulum.
Impact toughness plays a critical role in the design and application of steel constructions, and thus it is important to know the acceptable minimum levels for impact toughness for each type of steel. These requirements are laid out in specialty codes and standards which are formulated by both international and national standards organizations.
The most widely used international standard for the specification of impact toughness is ISO148, which is based on the results of Charpy V-notch impact tests in a variety of steel compositions. ISO148 outlines the minimum impact toughness requirements for steels of different grades, and also specifies the impact test temperature range in which the test specimen must be tested. The most commonly used temperatures are 20°C and -20°C.
At these temperatures, ISO148 identifies three levels of impact toughness: IV, III, and II. IV is the highest level and II the lowest, with III being an intermediate level. Each level is further divided based on the impact toughness values, which are expressed in terms of the number of joules of impact energy that the test specimen can absorb without breaking.
Some national standards organizations have modified the requirements of ISO148 to better suit their particular needs. For example, the American Society of Mechanical Engineers (ASME) has a set of codes and standards for steel design and fabrication that adds a Level I classification to ISO148’s series of impact toughness levels. Level I is set at a higher minimum value than Level II, and is designed for particularly demanding applications.
The impact toughness of steels also varies significantly depending on the composition and heat treatment of the material. For example, some commonly used alloys such as carbon steel and stainless steel may have different impact toughness requirements based on their precise chemical composition and heat treatment schedule.
It is important to note that the values given in standards like ISO148 and ASME are minimum requirements; higher levels of impact toughness may be specified if needed. For any particular application, impact toughness should be determined by actual testing and compared to the applicable standards. If the actual impact toughness of a steel is found to be lower than the minimum standard value, then it may not be suitable for that particular application.
In conclusion, the impact toughness of steels is an important design consideration in many mechanical engineering applications. The minimum requirements for impact toughness are laid out in specialty codes and standards such as ISO148 and ASME, but these values should always be confirmed by actual testing before specifying a steel for a particular application.
