GB/T 13302-1991Steel - Determination of the area content of graphite - Micrographic method using diagrams (English PDF)

GB/T 13302-1991 is the English-translated version of 钢中石墨碳显微评定方法.

China's national method for assessing graphite in steel by microscopic examination against reference diagrams - a short, old and still current standard that addresses a specific and serious defect.

Carbon in steel is normally present as carbide; under certain conditions it comes out of solution as free graphite instead, and where it does the steel loses strength and, far more importantly, loses toughness.

Graphitisation of this kind occurs in carbon and carbon-molybdenum steels held for long periods at temperatures in the region of four to five hundred degrees, which is precisely the condition of steam pipework, headers and their welds in a power station: the graphite forms preferentially in the heat affected zone beside a weld, in a plane, and produces a brittle path across the section.

Pipes have failed catastrophically that way. This standard specifies the micrographic method for determining the area content of graphite in steel by comparison with a series of reference diagrams.

Assessment against standard diagrams rather than by measurement is the right approach for this problem: what matters is the amount, the size and above all the distribution of the graphite - whether it is dispersed or aligned in a chain - and a chart comparison captures the pattern in a way a numerical area fraction alone would not.

The standard sets the sampling and where the specimen must be taken from, which for a weld examination is the whole question, the preparation and polishing, the examination conditions and magnification, the reference diagrams and the rating scale, and how the result is reported. Issued on 2 January 1991 and in force since 1 October 1992.

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This preview omits tables, figures, formulas and parts of the technical clauses. The complete document — 15 pages — is available in the English PDF.

GB/T 13298-2015|GB/T 6394|GB/T 10561|GB/T 4335|GB/T 13299|GB/T 226

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