GB/T 38720-2020Microstructure examination of medium-carbon steel and medium-carbon alloy structural steel after quenching (English PDF)

中碳钢与中碳合金结构钢淬火金相组织检验

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Issued by

SAMR; SAC

Level / Type

National · Recommended

Issue date

March 31, 2020

Implementation date

October 1, 2020

Scope

GB/T 38720-2020 is the English-translated version of 中碳钢与中碳合金结构钢淬火金相组织检验.

China's national standard for examining the microstructure of quenched medium-carbon and medium-carbon alloy structural steels. Quenching a medium carbon steel is meant to produce martensite, and the examination establishes whether it did. What is being looked for is not simply martensite or not: the questions are how fine it is, how much retained austenite remains, whether any of the higher temperature products - bainite, pearlite, ferrite - formed instead because the cooling was too slow at some point, and whether the prior austenite grain grew during heating. Each of those has a specific cause and a specific consequence. Coarse martensite from an overheated austenitising means low toughness; a network of ferrite means the section cooled too slowly at the centre; retained austenite means the part will change dimension later. Hardness testing detects none of them individually, which is why the microstructure is graded against reference images rather than inferred from a number.

Document preview — GB/T 38720-2020

National Standard of the People's Republic of China

ICS: 25.2
Classification: J36

Issued by: State Administration for Market Regulation; Standardization Administration of the PRC

Foreword ..........................................................................................................3
1 Scope ............................................................................................................4
2 Normative references ....................................................................................4
3 Terms and definitions.....................................................................................5
4 Raw materials and requirements ...................................................................5
5 Inspection requirements ................................................................................6
6 Quenching structure level and microstructure evaluation .............................7
14 Annex B (informative) Standard spectrum of microstructure level of quenching + high temperature tempering (600°C) ...........................................................27

Foreword

Microstructure examination of medium-carbon steel and medium-carbon alloy structural steel after quenching

Microstructure examination of medium-carbon steel and medium-carbon alloy structural steel after quenching 1 Scope

This Standard specifies raw materials and requirements, inspection requirements, quenching microstructure grade and evaluation for common medium-carbon structural steel and medium-carbon alloy structural steel. This Standard is applicable to inspection and evaluation of microstructure after parts quenching and tempering of medium-carbon structural steel and medium-carbon alloy structural steel. This Standard is not applicable to evaluation of microstructure that contains decarburization, over-burning as well as microstructure under isothermal quenching after parts quenching and tempering of medium-carbon structural steel and medium-carbon alloy structural steel.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

GB/T 699, Quality carbon structure steels

GB/T 1979, Standard diagrams for macrostructure and defect of structural steels

GB/T 3077, Alloy Structure Steels

GB/T 6394, Metal - Methods for estimating the average grain size

GB/T 7232, Terminology of metal heat treatment

2.0 Coarse series <=2.0 <=1.5 <=1.0 <=1.5

4.5 The raw material band structure is tested according to

GB/T 34474.1. The band structure in Cr-Mo, Cr-Ni-Mo steels shall not be greater than level 3. For other steels, it shall not be higher than level 2.

4.6 The acid-leaching microstructure of raw materials shall be tested according to

GB/T 1979. There shall be no visually visible residual shrinkage holes, bubbles, cracks, inclusions, skinning, white spots, interaxial cracks. The level of ingot segregation, center segregation, center looseness and general looseness shall not be greater than 2.

5 Inspection requirements 5.1 Inspectors shall undergo professional training and obtain corresponding employment certificates. At the same time, they shall have basic knowledge of metal materials and heat treatment, as well as the preparation and use of corrosive agents, inspection quality and other knowledge, in accordance with relevant requirements of GB/T 34895.

5.2 The temperature, humidity, vibration, and noise of the testing environment shall meet the requirements for the use of metallographic inspection equipment, including sample preparation equipment, and related safety requirements.

5.3 In general, the test specimens shall be prepared by mechanical methods under the cold conditions of the representative service area. If prepared by hot cutting method, the heat affected zone shall be completely removed. The effective wall thickness, chemical composition, state before heat treatment and heat treatment batch of the specimen shall be able to characterize the tested parts.

5.4 The metallographic examination of sample preparation, grinding and polishing shall be carried out in accordance with the provisions of GB/T 34895. During the preparation process, there shall be no phenomenon of tissue change due to heat. The etching agent is 4% nitroethanol solution. The waste liquid generated during the inspection process shall be disposed of after neutralization and centralized disposal.

5.5 Quenching and tempering test specimens are randomly observed at 500 times magnification for more than 5 representative fields of view. Compare with the standard spectrum to determine the structure level.

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This preview omits tables, figures, formulas and parts of the technical clauses. The complete document — 31 pages — is available in the English PDF.

Referenced standards

Normative references

GB/T 34474.1

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