Determination of Average Grain Size of Metals is a microscopic analysis method used to evaluate the grain size of metallic materials. Grains are the crystalline structural units inside metals. Finer grains result in higher strength and better toughness of the material; coarser grains make the material more prone to brittle fracture. Therefore, grain size is a critical indicator for evaluating the quality of steel products, especially steel for pressure vessels. The following is a systematic explanation of grain size determination:
1. Why Determine Grain Size? (Engineering Significance)
- Strength and Toughness: According to the Hall-Petch Equation, finer grains mean more grain boundaries, which provide stronger resistance to dislocation movement, leading to higher yield strength and tensile strength of the material. Meanwhile, fine-grained materials also exhibit better impact toughness.
- Heat Treatment Control: Grain size indicates whether the material’s heat treatment process (such as normalizing, annealing, quenching) is properly performed. Excessively high heating temperature causes grain growth (overheating), resulting in deteriorated material properties.
- Welding Quality: Inspect whether grains in the weld heat-affected zone (HAZ) have become coarse due to high temperature.
2. Testing Standards
In China, the determination of average grain size of metals is mainly carried out in accordance with the national standard: GB/T 6394-2017 Determination of Average Grain Size of Metals This is the most widely adopted standard at present, equivalent to the American standard ASTM E112.
3. Classification of Determination Methods
Based on specimen preparation and observation methods, the main approaches are as follows:
A. Comparison Method – Most Commonly Used
Principle: Visually compare the prepared microstructure photograph or image with standard rating charts (series of images in the appendix of GB/T 6394).Steps:
- Observe the microstructure at 100× magnification.
- Find the standard rating chart that most closely matches the grain size in the field of view.
- Read the corresponding grain size number (G).Features: Simple and fast operation, suitable for routine inspection.
B. Intercept Method – Most Accurate
Principle: Pass a straight line (or circle) of known length through the microstructure and count the number of intersections between the line and grain boundaries.Formula: The grain size number G is proportional to the number of intercepts per unit length.Features: High objectivity, good data reproducibility, suitable for scientific research or arbitration analysis.
C. Area Method
Principle: Measure the number of grains per unit area.Features: Relatively cumbersome calculation, now usually performed automatically by image analyzers.
D. Image Analysis Method
Principle: Use an automatic image analysis system to directly capture metallographic photos and calculate grain size.Features: High degree of automation, fast speed, suitable for large-batch testing.
4. Grain Size Number (G)
The grain size is expressed by the grain size number as specified in the standard. A higher number indicates finer grains:
- Grade 1–3: Coarse grains (usually unacceptable, or only used for certain special heat-resistant steels).
- Grade 4–6: Medium grains (general structural steels).
- Grade 7–10: Fine grains (high-quality steels, steels for pressure vessels).
- Above Grade 10: Ultra-fine grains (high-performance steels).
Conversion rule: Each increase of 1 in the grain size number approximately doubles the number of grains.
5. Test Procedure
1. Sampling
Cut specimens from representative locations of the steel product or weld.
2. Specimen Preparation (Critical Step)
- Mounting: Embed small specimens in resin for easier handling and grinding.
- Grinding and Polishing: Grind stepwise with abrasive papers from coarse to fine, followed by mechanical polishing to obtain a mirror-bright, scratch-free surface.
- Etching: Immerse the polished specimen in a specific chemical etchant.Purpose: Remove the passivation film on the metal surface and reveal grain boundaries.Common etchants:
- Nital (alcoholic nitric acid solution): Most widely used, suitable for carbon steels and low-alloy steels.
- Alcoholic picric acid solution: Suitable for revealing grain boundaries of austenitic stainless steels (electrolytic etching may sometimes be required).
3. Microscopic Observation
Use an optical microscope, typically at 100× magnification (adjustable to 50× or 200× depending on grain size).
4. Rating
Determine the grain size grade using the comparison method or intercept method.
6. Special Requirements in the Pressure Vessel Industry
In pressure vessel manufacturing, clear requirements are specified for grain size control:
- Steel plate requirements: For vessel plates such as Q345R, the grain size grade is generally required to be no coarser than Grade 5 (i.e., finer than Grade 5, preferably Grade 6–8).
- Welding requirements: Grains in weld metal and the heat-affected zone must not be excessively coarse, otherwise the impact toughness of the joint will be reduced.
- Quenched and tempered steels: Steels after quenching and tempering must have refined grains to achieve excellent comprehensive mechanical properties.
Summary
The determination of average grain size of metals involves etching the metal surface and examining the size of the “cellular structure” under a microscope. Core logic: The smaller the cells (the higher the grade number), the stronger and more durable the material. Mnemonic:Sample, mount, grind and polish,Etch grain boundaries to show their truth,Compare with standards to read the grade,Higher number means finer grain.
