Determination of nitrided case depth and metallographic microstructure examination for steel and iron parts
1 Scope
This standard specifies the method for determination of nitrided case depth and metallographic microstructure examination before and after nitriding and relevant technical requirements of steel and iron parts.
This standard is applicable to the determination and evaluation of the depth, brittleness and porosity of nitrided case and nervation and wave like nitride on the surface of steel and iron parts after gas nitriding, ion nitriding and nitrocarburizing.
2 Normative references
The following documents contain provisions which, through reference in this text, constitute provisions of this standard. For dated references, subsequent amendments to (excluding correction to), or revisions of, any of these publications do not apply. However, parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. For undated references, the latest edition of the normative document referred to applies.
GB/T 7232 Terminology of metal heat treatment
GB/T 9450 Steels - Determination and verification of the depth of carburized and hardened cases (ISO 2639: 2002, MOD)
GB/T 9451 Determination of total or effective thickness of thin surface hardened layers of steel parts (ISO 4970: 1979 MOD)
3 Terms and definitions
For the purposes of this standard, the terms and definitions given in GB/T 7232 and the following apply.
3.1
prior metallographic structure (original structure)
metallographic structure of the workpiece before nitriding
3.2
brittleness of nitrided case
degree of fracture of Vickers hardness indentation at edges and corners on the surface of nitrided parts under a certain test force
3.3
porosity of nitrided case
density of micropores in compounds on the surface of nitrided parts
3.4
nervation and wave like nitride
nervation and wave like nitride in the diffusion layer of nitrided parts, which runs parallel to the surface
4 Prior metallographic structure examination
For nitrided parts with different service conditions and different nitrided steels, different pre-heating treatments may be adopted before nitriding to obtain corresponding prior metallographic structure.
4.1 Grading of prior metallographic structure before nitriding
The grade of prior metallographic structure of quenched and tempered steel before nitriding may be classified into five grades according to the percentage of free ferrite in sorbite, as shown in Table 1 and Figure 1.
4.2 Examination method and requirements of prior metallographic structure
The prior metallographic structure is examined before nitriding (large workpieces may be examined within 2mm depth of the surface), magnified by 500 times under the microscope, and evaluated with reference to the prior metallographic structure grade diagram. Generally, parts of Grades 1 to 3 are qualified, and important parts of Grades 1 to 2 are qualified.
The working face of nitrided workpiece is not allowed to have decarburized case or coarse sorbite structure.
Table 1 Grade of prior metallographic structure before nitriding and its description
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Prior metallographic structure examination
5 Specimen conditions
6 Method for determination of nitrided case depth
7 Examination of nitrided case brittleness
8 Examination of porosity of nitrided case
9 Examination of nitride in nitrided case
10 Examination report
Annex A (Informative) Recommended erosion agents
Annex B (Informative) Partial test data
