Overview
The GB 5216-85 is the Chinese national standard for Quenched and Tempered Structural Steels. It contains minimum requirements for chemical and mechanical properties, testing, and inspection practices. Under this standard, steels are classified and evaluated in service depending on their condition. In particular, the chemical composition and mechanical properties required for special applications are specified.
Chemical Composition
Chemical composition requirements of GB 5216-85 are based on the structural steel composition of the steel being tested or produced. The following parameters are required to meet the standard: carbon equivalent, manganese, silicon, sulfur, phosphorus, and residual elements. Carbon equivalent is one of the most important criteria for a successful quenching and tempering process and is specified at a maximum of 0.52%.
The maximum of other elements is as follows: manganese is 0.8 to 1.2%, silicon is 0.15 to 0.25%, sulfur is 0.025%, phosphorus is 0.03%, and the residual elements are 0.04%. In addition, the typical steel is “free of niobium”, which is defined as nickel plus molybdenum content 0.06%. The main alloying elements are encouraged to be tested per GB/T6394-1986.
Mechanical Properties
The mechanical properties of GB 5216-85 quenched and tempered structural steels are typically evaluated at room temperature using either the charpy or impact test. This test establishes the energy absorbed by the sample upon fracture and is reported in joules (J). For steels with a grade of P245 to P355, the impact energy must be at least 40 J at 20°C (68°F), and none of the specimens can have an impact energy below 27 J at 10°C (50°F).
The tensile test is also utilized for these steels, measuring the tensile strength, yield strength, and elongation at break. The requirements for tensile strength and yield strength are based on the steel grade and thickness. The minimum allowable elongation at break is based on the steel grade, thickness, and shape. For P245 to P355 steels, the minimum allowable elongation at break is generally 18%.
Testing and Inspection
GB 5216-85 utilizes both destructive and non-destructive tests. Destructive tests are typically used to determine the mechanical properties of the finished steel and include the tensile test, impact test, and various other tests. Non-destructive tests commonly used for quality assurance and inspection include ultrasonic testing, radiographic testing, and magnetic particle inspection.
Ultrasonic testing is used to determine the integrity of parts and welds. This testing method can locate cracks and other discontinuities, as well as measure wall thickness. Radiographic testing is used for welds, making this a critical test for welded steels. The magnetic particle inspection is most commonly used for detection of cracks on the steel surface.
Conclusion
GB 5216-85 provides a comprehensive set of requirements for Quenched and Tempered Structural Steels in China. This standard requires a minimum chemical composition, specified mechanical properties, and a set of tests and inspections to ensure the quality of the steel product. It ensures that steels are suitable for their intended use in various applications.
