Summarize the inspection of common springs:
1 Overview
With the progress and development of science and technology, metallographic microscopes have been greatly improved and improved in both the structure and the optical system. The spring product materials require good hardenability to ensure that the core of the entire section is in About 85% of the martensite structure is obtained after quenching and before tempering. It is especially important to ensure good hardenability for spring products with a diameter of more than 15mm. For this reason, metallographic inspection has developed rapidly in the spring industry. Metallographic inspection is not only to study the internal structure of metal materials with the help of a metallurgical microscope, but also to conduct macroscopic inspections by the naked eye or under a low-power magnifying glass.
Metallographic samples were mostly prepared by mechanical polishing in the past, but with the rapid development of metallographic inspection technology, the electrolytic polishing method for preparing metallographic samples has highlighted incomparable advantages, and this technology has been widely used abroad. Electrolytic polishing not only makes the surface of the material smooth and bright, but also has other benefits. For example, surface stress is reduced, surface carbon and oxides are removed, friction is reduced, etc.; it is necessary to revise GB/T 13298-1991 “Metal Microstructure Inspection Method” to include this technology.
It is understood that the newly revised GB/T 13298-201X “Metal Microstructure Inspection Method” standard will be applicable to the electrolytic polishing of metallographic samples. The main technical content includes: the characteristics and applications of electrolytic polishing; the working conditions, safety protection measures, common problems and improvement methods of electrolytic polishing of some common electrolytic polishing liquids; the use and regulations of electrolytic polishing equipment; test methods and inspection rules.
The relationship between the performance, chemical composition, metallographic structure and mechanical properties of each corresponding spring product, generally speaking: as the grain size decreases, the strength and hardness of the material increase; as the inclusion content increases Increasing, the toughness of the material tends to decrease; the mechanical properties of anisotropic grains are related to the preferred orientation of the grains. Therefore, commonly used metallographic inspection methods such as identification of metallographic structure, grain size and evaluation of inclusions have become important tools for material process control, quality assurance and failure analysis. The following is a brief description of the interpretation and application of common metallographic inspection standards for springs.
2. Macro inspection of steel
In the process of steel smelting or hot processing, due to the influence of certain factors (such as non-metallic inclusions, gases, and process selection or improper operation, etc.), the internal or surface of the steel is defective, which seriously affects the material or product. Quality will sometimes lead to scrapped materials or products. Porosity, bubbles, shrinkage cavity residues, non-metallic inclusions, segregation, white spots, cracks and various abnormal fracture defects in steel can be found through macroscopic inspection. Macro inspection usually includes sulfur mark test, acid etching test, tower test, fracture inspection, etc. In production inspection, an appropriate macro inspection method can be selected according to the inspection requirements.
2.1 Acid etching test
The acid etching test is a test method for showing the macrostructure of steel materials. The test method is carried out in accordance with the national standard GB/T226-1991 “Steel Microstructure and Defect Inspection Method”. This method is simple in equipment, convenient to operate, and clear It shows various defects in steel materials, such as cracks, inclusions, porosity, segregation and bubbles.
The acid etching test uses the difference in the corrosion of various parts of the steel material by the acid solution, thereby clearly showing the macrostructure and defects of the spring material. According to the distribution of macrostructure and the existence of defects, the metallurgical quality of the steel can be known. By inferring the causes of defects, feasible measures can be taken in the process to achieve the purpose of improving product quality.
This standard specifies the hot and cold acid etching method and electric corrosion method for testing the macrostructure and defects of steel. This standard applies to the inspection of the macrostructure and defects of steel. During the arbitration inspection, if the technical conditions have no special regulations, the hot acid etching method shall prevail. This standard has been implemented for 20 years, and the low-power etching method and the etching solution of steel have been greatly updated. At present, my country is striving to incorporate the electro-corrosion method into the ISO standard. It is necessary to revise the GB/T226 standard and provide technical support for the revision of ISO 4969:1980.
The evaluation scope and evaluation rules of the macrostructure and defects of steel can be in accordance with GB/T1979-2001 “Structural Steel Macrostructure Defect Rating Chart”. This standard is applicable to carbon structural steel, alloy structural steel, spring steel steel (forging, rolling Blank) Defect evaluation of cross-section specimens. There are 6 sets of the rating chart, which are respectively applicable to stipulating the macrostructure and defects of different sizes of steel. Common macro defects in steel ①Generally loose; ②Porous center; ③Ingot segregation; ④Center segregation; ⑤Subcutaneous bubbles; ⑥Internal bubbles and so on.
When evaluating various types of defects, the pictures listed in the appendix of the standard shall prevail. When evaluating various types of defects, they shall be limited by visual inspection. In order to determine the types of defects, a magnifying glass no larger than 10X is allowed. When its degree of severity is between two adjacent levels, it can be rated at half level. For defects that do not require an assessment level, only the defect category is determined.
2.2 Other
Fracture inspection (GB/T1814-1979 “Steel Fracture Inspection Method”), Sulfur Mark Test (GB/T4236-1984 “Sulfur Mark Inspection Method of Steel”), Tower Test (GB/T15711-1995 “Steel Tower Hairline” Acid Leaching Inspection Method), etc., are also macro inspection techniques for steel. In recent years, due to the technological progress of my country’s steel industry, the control ability of steel in the smelting or hot working process has improved, and the internal or surface defects of steel have become less and less. In order to reduce inspection costs, most companies in the spring industry no longer do this 3 types of project inspection. This article will not go into details.