What GB/T 1979-2001 Actually Covers
GB/T 1979-2001, Standard diagrams for macrostructure and defect of structural steels, is a defect rating atlas. It classifies and describes the macrostructure defects revealed by acid etching, explains how each defect forms, sets out the principles for rating it, defines the diagram series and the range over which each diagram applies, and specifies the content of the inspection report. It applies to carbon structural steel and alloy structural steel products, including billets, blooms and finished hot-rolled round bar, whenever the purchase specification calls for macrostructure examination.
It is a rating document, not a surface-condition specification. Surface defects such as seams, laps, scale pits and decarburization are controlled by the relevant product standard and by surface inspection methods, not by this atlas. An order that cites GB/T 1979-2001 to govern as-rolled surface finish is citing the wrong document.
Defect Types Rated in the Diagram Series
| Defect | Usual location | Main cause |
|---|---|---|
| General porosity | Distributed across the section | Insufficient reduction, residual gas |
| Centre porosity | Axis of the bar | Solidification shrinkage, low reduction ratio |
| Ingot pattern segregation | Mid-radius zone | Interdendritic solute enrichment |
| Spot segregation | Mid-radius zone | Localized solute-rich pockets |
| Subsurface blowholes | Close to the surface | Entrapped gas below the skin |
| Residual shrinkage cavity | Centre | Insufficient discard of the ingot top |
| Flakes | Interior of heavy sections | Hydrogen in slowly cooled product |
| Axial intergranular cracks | Centre line | Segregation combined with cooling stress |
| Non-metallic inclusions | Dispersed or clustered | Deoxidation and refractory products |
Each defect is rated by comparison with a series of graded reference photographs, and the result is recorded as a level together with the position and field observed.
How the Rating Is Used in Round Bar Acceptance
Etching is performed by hot acid attack on a prepared section in accordance with GB/T 226, with sampling and specimen preparation governed by GB/T 2975. The permissible rating levels, the number of samples per heat, the sample position along the bar and the reporting format are fixed by the order or by the product standard for the grade; quality carbon structural steel and alloy structural steel bars are commonly ordered with a stated maximum for general and centre porosity. Normal practice is to record the worst field seen and to state whether the assessment was made per heat or per piece.
Because the atlas is a comparative visual method, the result depends on operator qualification, illumination and a properly etched face. Laboratories should log etching time, temperature, reagent and the diagram edition used, so that ratings can be reproduced between mills when a dispute arises.
Chemical and Microstructural Checks That Support It
Macrostructure rating is normally combined with chemical analysis of the heat, tensile testing and, where agreed, microscopic inclusion rating to GB/T 10561, which follows the same approach as ISO 4967. Metallographic examination of the microstructure is carried out to GB/T 13298. Together these tests separate defects that originate in melting and casting from those introduced by rolling or heat treatment, which is the distinction a purchaser needs when deciding whether to reject a heat or to re-cut the bar.
Revision Status and Companion Documents
The 2001 edition has been superseded by GB/T 1979-2025, Evaluation of macrostructural defects in structural steels: acid etching and ultrasonic testing methods. The new edition keeps the etched-face evaluation and adds ultrasonic testing as a second route for internal defects in bars and billets, which matters for large-section product where sectioning is expensive. Orders placed today should state which edition applies and which method is required.
Practical Guidance for Bar Purchasers
Set out five points on the order: the edition applied, the sample position measured from the end of the bar, the number of samples per heat, the etching method, and the maximum acceptable rating for each defect class. Where the bar feeds a forged or machined safety-relevant part, add ultrasonic testing and a statement of the acceptance level, because a favourable etched section does not exclude internal defects outside the sampled plane.
Frequently Asked Questions
Q: Does GB/T 1979-2001 cover surface quality of round bar?
A: No. It is limited to the classification and rating of macrostructure defects exposed by acid etching, such as porosity, segregation, subsurface blowholes and flakes. Surface condition is governed by the product standard and by surface inspection, so a clause citing GB/T 1979 for surface finish has no effect.
Q: Which products does the standard apply to?
A: Carbon structural steel and alloy structural steel, in the form of billets, blooms, forged stock and finished bars including hot-rolled round bar. It is used whenever the order requires an assessment of internal soundness by etched section.
Q: What etching method is referenced?
A: The hot acid etching method of GB/T 226, with sampling and specimen preparation to GB/T 2975. The etched face is examined visually and each defect is rated against the diagram series in the standard.
Q: Is the 2001 edition still current?
A: It has been replaced by GB/T 1979-2025, which retains acid etching and adds ultrasonic testing. Contracts that reference the 2001 edition remain valid if the edition is named in the order, but new orders should specify the edition explicitly to avoid disputes.
Q: What acceptance level should be specified?
A: There is no single default. Limits depend on the grade, the section size and the end use, and are agreed between purchaser and mill or taken from the product standard. Heavily loaded or highly stressed parts are normally ordered with tighter limits for centre porosity and segregation than general engineering bar.
Q: How many samples are taken per heat?
A: The frequency is set by the order or the product standard rather than by the atlas itself. Many specifications call for one sample per heat or per rolling lot, taken from a defined position along the bar so that results remain comparable between deliveries.