Microstructure Rating Atlas and Evaluation Method for Normalized and Isothermally Normalized Carburizing Steel (100× Magnification)

Microstructure Rating Atlas and Evaluation Method for Normalized and Isothermally Normalized Carburizing Steel (100× Magnification)

Carburizing steel is widely used in high-precision mechanical transmission parts, gear forgings and dynamic load-bearing components due to its excellent surface hardening performance and core toughness after carburizing and quenching. Normalizing and isothermal normalizing are essential pre-heat treatment processes for carburizing steel forgings, which refine grains, homogenize microstructures, eliminate forging residual stress, and stabilize hardness uniformity. The normalized microstructure quality directly determines the subsequent carburizing distortion, quenching performance and final component fatigue resistance. Based on GB/T 13320 standard specifications and industrial metallographic evaluation systems, this document systematically elaborates the morphological characteristics, 1–8 grade rating atlas rules, standardized specimen preparation, microscopic observation specifications and engineering qualification judgment criteria of normalized and isothermally normalized carburizing steel microstructures under 100× magnification, forming a complete standardized metallographic evaluation technical system.

1. Technical Overview of Carburizing Steel Normalized Microstructure

Conventional low-carbon and low-alloy carburizing steels (such as 20Cr, 20CrMnTi, 12Cr2Ni4) obtain typical ferrite + pearlite dual-phase microstructures after conventional normalizing and isothermal normalizing. Isothermal normalizing realizes uniform phase transformation at constant temperature, featuring finer grains, more stable tissue distribution and less residual stress compared with conventional air-cooled normalizing.

Unreasonable heat treatment parameters such as insufficient austenitizing temperature, inadequate holding time, unreasonable cooling rate and unbalanced isothermal temperature will lead to abnormal microstructures, including uneven grain size, coarse ferrite, massive pearlite segregation, granular bainite mixed structure and residual banded structure. These defective microstructures will cause inconsistent hardness, increased subsequent heat treatment distortion and reduced fatigue performance of carburized parts.

Microstructure rating evaluation is uniformly implemented under 100× metallographic microscope magnification. The grading is based on grain fineness, structural uniformity, phase composition and abnormal tissue proportion, covering a complete 1–8 grade classification system for engineering quality control.

2. Standard 1–8 Grade Microstructure Morphological Characteristics (100×)

In accordance with the second group rating atlas of GB/T 13320 for carburizing steel normalized microstructures, the typical morphological characteristics of each grade under 100× magnification are defined as follows, covering qualified fine structures, slightly uneven structures and severely abnormal defective structures.

2.1 Grade 1: Excellent Uniform Normalized Structure

The microstructure consists of uniform equiaxed ferrite and fine flaky pearlite with extremely consistent grain size. The phase distribution is uniform without coarse grains, banded segregation, granular bainite and abnormal mixed structures. The tissue boundary is clear and the structure stability is optimal. This grade represents the highest-quality normalized state of carburizing steel, with minimal heat treatment distortion and excellent subsequent carburizing adaptability, suitable for high-precision gear and fatigue-sensitive components.

2.2 Grade 2: High-Quality Uniform Structure

The matrix is composed of equiaxed ferrite and uniform pearlite. Individual local grains are slightly larger, but the overall structural uniformity is good. No harmful abnormal tissues exist. The grain size difference is within a tiny range, meeting the high-standard heat treatment quality requirements of most precision mechanical components.

2.3 Grade 3: Qualified Slightly Uneven Structure

The main structure is ferrite + pearlite dual-phase tissue. A small number of fine and scattered uneven grains appear locally, and the pearlite distribution is slightly fluctuating. No massive coarse grains and abnormal phase transformation tissues are observed. The overall structure is stable, and the heat treatment performance is qualified for conventional engineering components.

2.4 Grade 4: General Qualified Structure

The grain uniformity decreases slightly, with coarser individual ferrite grains and partial fine-grained aggregation areas. The pearlite bands are locally slightly dense, without continuous segregation and bainite mixed tissues. It belongs to the allowable conventional qualified grade in industrial batch production, meeting the basic processing and service requirements of ordinary structural parts.

2.5 Grade 5: Slightly Defective Mixed Structure

The microstructure uniformity decreases significantly. Coarse ferrite grains increase obviously, and sporadic granular bainite appears in local areas. The pearlite distribution is uneven with partial banded aggregation. The structural stability is weakened, and slight hardness fluctuation occurs. This grade is allowed for non-key auxiliary components and requires process optimization in batch production.

2.6 Grade 6: Moderately Abnormal Structure

A large number of coarse ferrite grains and uneven pearlite bands appear. The proportion of granular bainite increases obviously, and the dual-phase structure presents obvious regional segregation. The material hardness uniformity deteriorates, and the subsequent carburizing quenching distortion tendency increases. It is judged as a borderline unqualified grade and is prohibited for key load-bearing and precision transmission components.

2.7 Grade 7: Severely Uneven Defective Structure

The structure is extremely disordered, with alternating distribution of massive coarse ferrite, clustered pearlite and a large amount of granular bainite. The grain size difference is huge, and banded segregation defects are widely distributed. The heat treatment stability is poor, which will cause obvious performance fluctuation of subsequent processed parts and belongs to unqualified microstructure.

2.8 Grade 8: Failure Overheating and Abnormal Structure

Typical overheated normalized structure with extremely coarse ferrite grains, serious pearlite banded segregation and a large number of abnormal bainite mixed tissues. The structural compactness and uniformity are completely lost, resulting in severe material performance attenuation and uncontrollable heat treatment distortion. This grade is completely unqualified and the components must be re-normalized.

3. Standardized Metallographic Inspection and Preparation Procedures

3.1 Specimen Sampling Requirements

Sampling shall be carried out at typical representative positions of carburizing steel forgings, avoiding forging deformation zones and surface decarburization layers. For thick-specimen components, samples shall be taken at 3–5 mm from the surface to ensure the sampled tissue can truly reflect the core normalized state of the component. Batch sampling shall adopt random sampling principle to ensure the representativeness of inspection results.

3.2 Specimen Grinding, Polishing and Etching

The specimens are processed by standard mechanical grinding and fine polishing to obtain scratch-free mirror surfaces. Low-concentration nitric acid alcohol solution is used for uniform etching. The etching time is strictly controlled to ensure clear display of ferrite and pearlite grain boundaries, no over-etching or under-etching defects, and complete and real presentation of microstructure morphology.

3.3 Microscopic Observation Specifications (100×)

All rating evaluations must be completed under standard 100× metallographic microscope magnification. Multiple representative fields including edge, transition and core areas of the specimen shall be observed. The worst tissue field is taken as the final grading basis in accordance with specification requirements to avoid evaluation deviation caused by local optimal tissue.

4. Core Standard Evaluation Rules

4.1 Grading Benchmark Principle

Adopt standard atlas comparison method for grading evaluation. Take grain fineness, structural uniformity, banded segregation degree and abnormal bainite content as the four core evaluation dimensions. The grade is determined by the worst single defect index to ensure the rigor of quality evaluation.

4.2 Half-Grade Evaluation Rule

If the actual microstructure morphology is between two adjacent standard grades, half-grade evaluation (such as 2.5, 3.5) can be adopted to improve the grading accuracy, which is applicable to refined quality inspection and batch process optimization evaluation.

4.3 Batch Judgment Principle

For batch production inspection, if the single-piece microstructure grade exceeds the qualified range, random re-sampling and re-inspection shall be carried out. Unqualified batches shall be subject to process adjustment and re-normalization treatment to eliminate tissue defects.

5. Engineering Qualification Classification and Application Criteria

High-Precision Key Components (Gear, Shaft, Fatigue Parts)
Qualified grade: Grade 1–3. Require uniform fine dual-phase structure without obvious segregation and abnormal mixed tissues to ensure low distortion and high fatigue resistance after carburizing and quenching.

Conventional Load-Bearing Components
Qualified grade: Grade 1–4. Slight uneven grain distribution is allowed, and the structural performance meets the requirements of conventional heat treatment and service conditions.

Non-Key Auxiliary Components
Conditional allowable grade: Grade 5. It is allowed to be used for low-load auxiliary parts without high precision requirements, and batch process optimization is required to reduce defective rate.

Unqualified Rejection Standard
Grade 6–8 microstructures are unqualified. Components with such grades have unstable heat treatment performance and large performance fluctuation, and must be reworked by re-normalization or scrapped.

6. Common Evaluation Errors and Correction Specifications

Local Optimal Field Misjudgment
Avoid grading based on partial fine tissues. The final grade shall be determined by the worst abnormal field of view to ensure sufficient structural safety margin.

Confusion Between Process Defects and Structural Grades
Distinguish surface decarburization, artificial polishing scratches and etching defects from inherent normalized microstructures to avoid incorrect grade downgrade caused by external interference.

Ignoring Isothermal Normalizing Structural Characteristics
Isothermal normalizing has finer and more uniform tissues than conventional normalizing. Higher grading standards shall be appropriately adopted for isothermal normalized products to match the process advantages.

7. Technical Summary

The microstructure rating evaluation of normalized and isothermally normalized carburizing steel under 100× magnification is a standardized and quantitative heat treatment quality inspection technology. The ferrite + pearlite dual-phase uniformity, grain fineness and abnormal tissue content are the core symbols of qualified normalized structures. Reasonable grade classification directly affects the precision control and service performance of subsequent carburizing and quenching processes.

In strict accordance with GB/T 13320 standard atlas and evaluation procedures, standardized specimen preparation, multi-field observation and worst-point grading judgment can accurately identify normalized tissue defects of carburizing steel, effectively control batch heat treatment quality, optimize normalizing process parameters, and provide reliable technical guarantee for high-quality manufacturing of carburized and quenched precision components.