Key Points of Metallographic Testing for CNC Machining Services
Sample Preparation for Metallographic Testing
Selection of Sampling Locations
The accuracy of metallographic testing results largely depends on the rationality of sampling locations. For CNC-machined components, sampling should be carried out in areas that can truly reflect the material’s performance or failure characteristics. For example, in parts with cracks, the crack source area and the crack propagation area should be included. In welded components, samples should be taken from the base material, heat-affected zone, fusion line, and weld seam area to comprehensively evaluate the welding quality. Avoid areas affected by cutting, processing, or environmental factors, such as overheated zones, oxide layers, or deformed layers.
Sampling Directions
The sampling direction should be selected according to the testing purpose. For rolled or forged parts, a longitudinal section can be used to observe the deformation direction of grains or fibrous tissues. A transverse section is suitable for analyzing grain size, the distribution of non-metallic inclusions, or surface treatment layers like carburized layers and coating thickness. For anisotropic materials such as composite materials or 3D-printed parts, a normal section can be used to study their anisotropic properties.
Sampling Methods
Different materials require appropriate cutting methods to minimize damage. For hard materials like quenched steel, wire cutting or slow-speed cutting is preferred to reduce heat accumulation and prevent tissue changes due to high temperatures. For soft materials such as copper and aluminum, excessive extrusion during cutting should be avoided to prevent deformation. During the cutting process, mechanical stress should be controlled to prevent sample deformation or cracking. For materials prone to oxidation, such as magnesium alloys, rapid sample preparation or sealed storage is necessary.
Microstructure Observation Techniques
Optical Microscope Observation
Optical microscopes are commonly used in metallographic testing. When observing samples, the appropriate magnification should be selected based on the sample’s tissue characteristics and testing requirements. For example, to observe the overall distribution of grains, a lower magnification (e.g., 100x) can be used. To analyze the fine structure of phases, a higher magnification (e.g., 500x or 1000x) is more suitable. The light settings of the microscope should be adjusted to make the sample tissue present the best contrast, allowing for clear observation of grain morphology, phase distribution, and non-metallic inclusions.
Scanning Electron Microscope Observation
Scanning electron microscopes (SEMs) offer higher resolution and can observe the surface morphology and microstructure of samples in greater detail. SEMs can be used to analyze the fracture morphology of failed components, helping to determine the failure mode, such as ductile fracture, brittle fracture, or fatigue fracture. They can also observe the fine structure of phases and the distribution of non-metallic inclusions at a higher magnification, providing more detailed information for material performance evaluation.
Electrolytic Etching and Chemical Etching
To observe the microstructure of samples under a microscope, etching is usually required. Chemical etching uses chemical reagents to selectively corrode the sample surface, highlighting grain boundaries and phase boundaries. The choice of etchant depends on the material composition. For example, a 4% nitric acid alcohol solution is commonly used for carbon steel, while Keller’s reagent (a mixture of HF, HCl, and HNO3) is suitable for aluminum alloys. Electrolytic etching is used for materials with high chemical stability, such as precious metal alloys. It applies a direct current to the sample, and the different phases corrode at different rates due to their different electrode potentials, thus revealing the microstructure.
Quality Evaluation Based on Metallographic Testing Results
Grain Size Evaluation
Grain size is an important indicator of material properties. Generally, smaller grain size leads to higher strength, hardness, and better toughness and fatigue resistance. The grain size can be evaluated using image processing technology to automatically measure the grain size distribution. According to relevant standards, the average grain size or grain size grade can be determined, and the material’s mechanical properties can be predicted based on the relationship between grain size and properties.
Phase Composition Analysis
The phase composition of an alloy has a significant impact on its performance. By analyzing the phase composition in the metallographic structure, the material’s chemical composition and phase transformation process can be evaluated. For example, in steel, the proportion of ferrite, pearlite, bainite, and martensite affects its strength, hardness, and toughness. In non-ferrous alloys, the type and distribution of phases also determine their performance characteristics. Through phase composition analysis, the heat treatment process can be optimized to obtain the desired phase structure and improve material performance.
Non-Metallic Inclusion Detection
Non-metallic inclusions in metals come from various sources during the melting and casting process, such as oxides, nitrides, and refractory residues. These inclusions can act as stress concentration points, reducing the material’s mechanical properties, especially fatigue strength and corrosion resistance. According to relevant standards, non-metallic inclusions can be classified and detected, such as sulfides (A-type), alumina (B-type), silicates (C-type), and nitrides (D-type). By analyzing the type, quantity, size, and distribution of non-metallic inclusions, the material’s quality can be evaluated, and measures can be taken to reduce their harmful effects during the smelting and processing process.
