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Metal material defects can be classified into surface defects and internal defects based on their location； Based on their morphology and formation characteristics, they are referred to as cracks, scars, peeling, double skin, rejoining, inclusions, surface bubbles, subsurface bubbles, and exposed bubbles, respectively. Defects in metal materials mainly include cracks, central porosity, shrinkage cavities, and excessive inclusions. The main defects are internal defects, followed by uneven hardness, surface defects, cracking, etc.; defects at the end of the ingot due to risers account for 6.52%. The main defect among them is cracking; followed by porosity. In some severe cases, this leads to numerous defects. Overall, these defects may be related to riser feeding, pouring procedures, holding temperature practices, and ingot shape. The effect of defects in metal materials on their mechanical properties: 1. Defects that reduce the tensile strength of a material cause stress concentration to occur during stretching, which in turn reduces the fatigue life of the material and weakens its tensile strength. 2. Reduced corrosion resistance: Surface defects can serve as initial points of corrosion for metal materials, accelerating their rate of corrosion and reducing their corrosion resistance. 3. Reducing the fatigue life of metal materials: Surface defects can cause stress concentration in metal materials, thereby reducing their fatigue life. 4. Pores and voids affect the density and electrical conductivity of materials; they can reduce the material’s density, while foreign substances and oxide scales may impair its electrical conductivity. Methods for detecting defects in metal materials. 1. Visual inspection: Visual inspection is one of the simplest and most commonly used methods. It involves observing the surface for defects with the naked eye or with the help of certain auxiliary devices, such as microscopes. This method is applicable to various materials and is low-cost; however, it relies on human judgment and may be subjective. 2. Magnetic particle testing: Magnetic particle testing is a non-destructive testing method commonly used to detect surface defects in metal materials. Its principle relies on the magnetic field distribution in ferromagnetic materials; if there are defects such as cracks or scratches on the surface of the metal material, these will affect the magnetic field distribution, causing the magnetic particles on the ferromagnetic material to gather around those defects and form magnetic particle spots. This method is not applicable to non-ferromagnetic metals and non-metallic materials, but it yields good detection results. 3. Ultrasonic testing: Ultrasonic testing is a highly precise non-destructive testing method suitable for detecting various defects on the surface and within metal materials, such as cracks, pores, and inclusions. Its principle relies on the acoustic properties of high-frequency ultrasound waves as they propagate and reflect within the metal to carry out detection. This method is widely used in fields such as aerospace, shipbuilding, nuclear energy, and the automotive industry. 4. Coating inspection: Many metal materials are coated with a protective coating during the production and processing stages. Coating inspection helps to detect any defects in the coating, such as cracks, pitting, or bubbles, thereby determining whether there are any defects on the surface of the metal material. Some commonly used coating inspection methods include defect detection, corrosion detection, and luminescence detection, etc. 5. Eddy current testing: Eddy current testing is a method that utilizes the principle of eddy current induction for inspection. An alternating electric field is generated by an induction coil, resulting in eddy currents, which are used to detect defects, foreign objects, and other issues within metal materials. This method is suitable for detecting various metal materials and can identify defects on the surface layer. Methods to improve surface defects in metal materials 1. Surface polishing: Polishing can remove pores and bubbles, making the surface smoother and reducing the negative effects on fatigue life, corrosion resistance, and electrical conductivity. 2. Electrochemical polishing: Electrochemical polishing can remove oxide scales and rust, providing a clean surface quality for subsequent surface treatments. 3. Chemical cleaning: Chemical cleaning includes methods such as acid washing and alkali washing, which can remove foreign substances attached to the surface and improve the purity of the metal surface. 4. Electrodeposition technology allows for the creation of a uniform coating on the surface through electrodeposition, thereby enhancing the material’s corrosion resistance and mechanical properties. Standard methods for detecting defects in metal materials: Standard EN10163-3 is applicable to the assessment of surface defects in hot-rolled steel plates, hot-rolled narrow strips, hot-rolled sheets, or hot-rolled steel strips. GB/T 226—2015 specifies the acid etching method for examining the microstructure and defects of steel at low magnification. GB/T 1979—2001 provides a grading chart for microstructural defects in structural steel at low magnification. GB/T 14999.1—2012 covers test methods for superalloys, Part 1: Acid immersion testing of longitudinal microstructure and defects. GB/T 14999.2—2012 covers test methods for superalloys, Part 2: Acid immersion testing of transverse microstructure and defects. GB/T 14999.3—2012 covers test methods for superalloys, Part 3: Inspection of longitudinal fractures in bars