GB/T 8731-2025 PDF English
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GB/T 8731: Historical versions
| Standard ID | USD | BUY PDF | Delivery | Standard Title (Description) | Status |
| GB/T 8731-2025 | 335 | Add to Cart | Auto, 9 seconds. | Free-cutting structural steels | Valid |
| GB/T 8731-2008 | 85 | Add to Cart | Auto, 9 seconds. | Free-cutting structural steel | Valid |
| GB/T 8731-1988 | 285 | Add to Cart | Auto, 9 seconds. | Free-cutting steel--Technical requirements | Obsolete |
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GB/T 8731-2025: Free-cutting structural steels
---This is an excerpt. Full copy of true-PDF in English version (including equations, symbols, images, flow-chart, tables, and figures etc.), auto-downloaded/delivered in 9 seconds, can be purchased online: https://www.ChineseStandard.net/PDF.aspx/GBT8731-2025GB NATIONAL STANDARD OF THE PEOPLE’S REPUBLIC OF CHINA ICS 77.140.01 CCS H 40 Replacing GB/T 8731-2008 Free-Cutting Structural Steels Issued on: OCTOBER 31, 2025 Implemented on: MAY 1, 2026 Issued by. State Administration for Market Regulation; Standardization Administration of the People’s Republic of China.
Table of Contents
Foreword... 4 1 Scope... 6 2 Normative References... 6 3 Terms and Definitions... 8 4 Expression Method of Designation... 8 5 Ordering Contents... 10 6 Manufacturing Process... 10 6.1 Smelting method... 10 6.2 Processing method... 10 6.3 Delivery condition... 10 7 Technical Requirements... 10 7.1 Designation and chemical composition... 10 7.2 Mechanical properties... 12 7.3 Low magnification... 14 7.4 Tower shape... 14 7.5 Decarburized layer... 14 7.6 Non-metallic inclusions... 14 7.7 Microstructure... 14 7.8 Ultrasonic testing... 14 7.9 Iron oxide scale... 14 7.10 Machinability... 15 7.11 Surface quality... 15 7.12 Dimensions, shape, and weight... 16 7.13 Other requirements... 17 8 Test Methods... 17 9 Inspection Rules... 19 9.1 Inspection and acceptance... 19 9.2 Batching rules... 19 9.3 Sampling quantity and location... 19 9.4 Reinspection and judgment rules... 19 9.5 Numerical rounding-off... 19 10 Packaging, Marking, and Quality Certificate... 19Foreword
This Document was drafted as per the rules specified in GB/T 1.1-2020 Directives for Standardization – Part 1.Rules for the Structure and Drafting of Standardizing Documents. This Document replaced GB/T 8731-2008 Free-Cutting Structural Steel. Compared with GB/T 8731-2008, the major technical changes of this Document are as follows besides the structural adjustments and editorial modifications. a) Change the definition of free-cutting structural steel (see Clause 3 of this Edition; Clause 3 of the 2008 Edition); b) Change the expression method of the designation (see Clause 4 of this Edition); c) Change the ordering contents (see Clause 5 of this Edition; Clause 5 of the 2008 Edition); d) Change the designations of Y08, Y15, and Y45, and the chemical composition ranges of Y15Mn, Y35Mn, and Y40Mn (see Table 1 of this Edition; Table 1 of the 2008 Edition); and add the designations of Y15MnS, Y40MnS, Y50, and Y100S (see Table 1 of this Edition); e) Change the designation of Y45MnSPb (see Table 2 of this Edition; Table 2 of the 2008 Edition); f) Add the designation of Y08MnBi, a bismuth-series free-cutting structural steel (see Table 5 of this Edition); g) Add the permissible deviations in the chemical composition of finished products containing lead, bismuth and calcium (see 7.1.3 of this Edition) h) Add inspection requirements and methods for sulfide inclusions (see 7.6, Appendix C of this Edition); i) Add the requirements for microstructure, ultrasonic testing, iron oxide scale, and machinability (see 7.7~7.10 of this Edition); j) Add surface quality requirements for hot-rolled wire rod, hot-rolled steel bar, and forged steel bar (see 7.11 of this Edition); k) Change the regulations regarding dimensions, shape, and weight (see 7.12 of this Edition; Clause 6 of the 2008 Edition); l) Add inspection methods for the machinability of steel (see Appendix D of this Edition). Please note some contents of this Document may involve patents. The issuing agency of this Document shall not assume the responsibility to identify these patents. Free-Cutting Structural Steels1 Scope
This Document specifies the expression methods of designation, ordering contents, manufacturing processes, technical requirements, test methods, inspection rules, packaging, marking, and quality certificate for free-cutting structural steel. This Document applies to free-cutting structural steel bars, wire rods, wires, sheets, and strips (hereinafter referred to as steels) for machining. The chemical composition also applies to steel ingots, billets, and their products.2 Normative References
The provisions in following documents become the essential provisions of this Document through reference in this Document. For the dated documents, only the versions with the dates indicated are applicable to this Document; for the undated documents, only the latest version (including all the amendments) is applicable to this Document. GB/T 222 Permissible tolerances for chemical composition of steel products GBT 223 (all parts) Methods for chemical analysis of iron, steel and alloy GB/T 224 Determination of the depth of decarburization of steels GB/T 226 Test method for macrostructure and defect of steel by etching GB/T 228.1 Metallic materials - Tensile testing - Part 1.Method of test at room temperature GB/T 229 Metallic materials - Charpy pendulum impact test method GB/T 231.1 Metallic materials - Brinell hardness test - Part 1.Test method GB/T 247 General rule of package, mark and certification for steel plates (sheets) and strips GB/T 342-2017 Dimension, shape, mass and tolerance of cold-drawn round, square and hexagonal steel wires GB/T 702-2017 Hot-rolled steel bars - Dimensions, shape, weight and tolerances GB/T 905 Dimension, shape, weight and tolerance of cold-drawn round, square and coupled plasma atomic emission spectrometric method GB/T 20126 Unalloyed steel - Determination of low carbon content - Part 2.Infrared absorption method after combustion in an induction furnace (with preheating) GB/T 20566 Terminology of steel and alloy GB/T 22368 Low-alloy steel - Determination of multi-element contents - Glow discharge optical emission spectrometry (Routine method) GB/T 28300 Surface quality classes for hot-rolled bars and rods technical delivery conditions GB/T 34474.1 Determination of banded structure of steel – Part 1.Micrographic method using standards diagrams3 Terms and Definitions
For the purposes of this Document, the terms and definitions given in GB/T 20566 and the following apply. 3.1 Free-cutting structural steel Structural steel with good machinability due to the addition of one or more high contents of sulfur, lead, tin, calcium, bismuth and other free-machining elements.4 Expression Method of Designation
The designation of free-cutting structural steel consists of the following three parts. a) First part. The symbol “Y” for free-cutting structural steel; b) Second part. The average carbon content (in parts per ten thousand) is represented by two Arabic numerals; c) Third part. The material series is represented by the symbols for free-cutting elements. sulfur, lead, tin, calcium, and bismuth. For sulfur-series free-cutting steels (see EXAMPLE 1), if the average sulfur content (in parts per ten thousand) is less than 0.20%, the “S” mark is omitted to indicate a lower sulfur content (see EXAMPLE 2); for lead, tin, calcium, and bismuth-series free-cutting structural steels, if sulfur is added in combination, only the symbols for free-cutting elements Pb, Sn, Ca, and Bi are retained, and the “S” mark is omitted (see EXAMPLEs 3 and 5). In addition, if the average manganese content (in parts per ten thousand) is no less than 1.0%, then Mn is marked before the free-cutting element symbol to indicate a higher manganese content (see5 Ordering Contents
The contract or order placed according to this Document shall include the following. a) Product Name; b) This Document number; c) Designation; d) Nominal dimensions; e) Accuracy Levels of dimensions and shape; f) Delivery weight and quantity; g) Processing method; h) Delivery condition; i) Other special requirements.6 Manufacturing Process
6.1 Smelting method Unless otherwise specified by the purchaser, the smelting method shall be selected by the supplier. 6.2 Processing method Unless otherwise specified by the purchaser, the processing method shall be selected by the supplier. 6.3 Delivery condition Steel shall be delivered in hot-rolled, hot-forged, cold-rolled, cold-drawn, or bright silver condition. The specific delivery condition shall be specified in the contract.7 Technical Requirements
7.1 Designation and chemical composition 7.1.1 The designation and chemical composition (smelting analysis) of the steel shall conform to the provisions of Tables 1 to 5. 7.3 Low magnification Upon the request of the purchaser, the low magnification of the steels may be inspected. The acid-etching low magnification test piece on the cross-section of the steel shall be free of visually visible shrinkage cavities, bubbles, inclusions, cracks, delamination, flaking, and white spots. When assessing the macrostructure level, general porosity, central porosity, and ingot segregation shall not exceed Grade 3. 7.4 Tower shape Upon the request of the purchaser, the tower shape of the steel may be inspected. Specific requirements shall be negotiated by both parties and included in the contract. 7.5 Decarburized layer Upon request of the purchaser, the total decarburized layer depth on one side of the steel with a nominal carbon content greater than 0.30% may be inspected. Specific requirements shall be negotiated between the supplier and the purchaser and specified in the contract. 7.6 Non-metallic inclusions Upon request of the purchaser, the non-metallic inclusions of the steel may be inspected. The thick and thin levels of Classes B, C, and D non-metallic inclusions shall not exceed Level 3. The inspection method for sulfide inclusions shall be in accordance with Appendix C. The acceptable level shall be negotiated between the supplier and the purchaser and specified in the contract. 7.7 Microstructure Upon the request of the purchaser, the steel may be inspected for microstructure and banded structure. Specific requirements shall be negotiated between the supplier and the purchaser and specified in the contract. 7.8 Ultrasonic testing Upon the request of the purchaser, the steel may undergo ultrasonic testing. The testing standards and acceptable levels shall be negotiated between the supplier and the purchaser and specified in the contract. 7.9 Iron oxide scale Upon the request of the purchaser, the steel may be inspected for iron oxide scale. Specific requirements shall be determined through negotiation between the supplier and the purchaser and specified in the contract. ......Source: Above contents are excerpted from the full-copy PDF -- translated/reviewed by: www.ChineseStandard.net / Wayne Zheng et al.
