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Impact of non-metallic inclusions on high-strength bolts

Apr 23, 2018

Non-metallic inclusions exist mainly in the form of oxides and sulphides in steel. According to GB/T 10561-2005 “Standard Test Method for Rating of Non-metallic Inclusions in Steels”, non-metallic inclusions are evaluated according to JK diagram (A-type sulfide, B-type alumina, C-type silicic acid Salts, D-type spherical oxides, and DS-type single-particle spherical types).


Inclusions in steel are generally detrimental to their properties and can cause non-uniform microstructure of the steel. Inclusions are often the source of cracks. Therefore, inclusions in steel are important factors influencing the fracture toughness of steel, among which strip MnS has the most significant influence. Its existence cuts off the continuity of the metal. After peeling off, it becomes a pit or a crack. When the cold crucible is formed, it is easy to form a crack source. When the heat treatment is performed, stress concentration is easily caused and a quench crack occurs. Therefore, high-strength fasteners should be strictly controlled on inclusions, given that China's steel standard GB/T 3077, GB/T 6478 has not explicitly required inclusions.


Type C (silicates) and Type D (spherical oxides) have the greatest impact on heat treatment. Silicate inclusions should not exceed 1.5 grades. Spheroidal oxide inclusions should be no greater than Grade 2; oxides and The sum of sulphide inclusions should not be greater than grade 3.


The shape and distribution of inclusions in steel can affect the overall performance of high-strength bolts. Heat Treatment If the shape and distribution of the inclusions in the steel are changed, the inclusions will have different effects on the material before and after the heat treatment. If the heat treatment does not change the shape and distribution of the inclusions and only cause changes in the matrix, the influence of the inclusions on the changes in the matrix during the heat treatment needs to be considered. In the heat treatment process, the influence of inclusions on the structure mainly promotes the nucleation of ferrite, which effectively divides the grain and refines the microstructure.


Practice has proved that, after heat treatment, although the size, grade, and state of inclusions have changed, the situation of poor raw material organization cannot be fundamentally changed and will continue to be left behind. Therefore, the impact of high-strength bolt impact tests is large, often resulting in The problem of low temperature impact energy is unqualified. This problem is critical and the reconditioning heat treatment cannot meet the technical requirements.


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