Metallographic Inspection and Evaluation of Quenched and Tempered Structure

High-strength bolts should ensure that they are fully austenitized during quenching, the quenched structure is uniform, and there is no undissolved ferrite and non-martensite structure.

The metallographic inspection of the quenched structure must be fully valued. For high-strength bolts of grade 10.9 and above, the uniformity of the quenched structure is particularly important. In the heat treatment of high-strength bolts abroad, great attention is paid to the full austenitization of steel to ensure the uniformity of its structure in order to obtain the best strength and toughness combination and ensure the safety and reliability of the bolts during service. Domestic high-strength bolt manufacturers have not paid enough attention to this. The common problem is that the quenching heating and insulation of the bolts are insufficient and the austenitization is insufficient.

The quality of quenching and tempering treatment is generally evaluated by GB/T 13320-2007 "Metallographic Structure Rating Diagram and Evaluation Method for Steel Die Forgings". This standard is applicable to structural steel parts that have been quenched and tempered; it is not applicable to the evaluation of decarburization, overheating, overburning and other structures. The standard requires that the specimen should be prepared by mechanical methods in a cold state; if hot cutting is used, the heat affected zone must be completely removed. During the sample preparation process, no changes in the structure due to heat should occur. After polishing, the sample is etched with a 2% to 4% nitric acid alcohol solution by volume.

The metallographic structure is rated from 1 to 8, with 1 being the best and 8 being the worst. The third group of rating charts in this standard is applicable to the tempering inspection of structural steel quenched and tempered parts, especially high-strength bolts. When the assessed quenched and tempered structure is between two levels, the next level is the judgment level. For example, when it is greater than level 3 and less than level 4, it is judged as level 4.

The metallographic structure analysis of the quenched and tempered structure is observed under an optical microscope at 500 times. The qualified level is agreed upon by the supply and demand parties through negotiation. If there is no agreement, levels 1 to 4 are qualified. Production practice shows that for bolts serving in low-temperature environments, levels 1 to 3.5 are the acceptance criteria. If there is a dispute during the rating, the mechanical properties test results can be referred to for judgment, especially whether the indicators of low-temperature impact energy meet the technical requirements.

For low-carbon or medium-carbon alloy structural steel bolts of grade 10.9 and above, you can also refer to JB/T7293.3-2014 "Internal Combustion Engine Bolts and Nuts Part 3 Metallographic Inspection of Connecting Rod Bolts", third-level diagram or first- and second-level diagram inspection. According to this standard, the tempered structure of grades 1 to 3 is qualified.

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