Metallographic inspection and analysis of spring products:
Commonly used spring materials are carbon spring steel or low-alloy spring steel, with a carbon content in the range of 0.55%-0.75%. Alloy elements such as Si, Mn, Cr, and V are often added.Cr and Mn are mainly to improve hardenability. Si improves the elastic limit, and V improves the hardenability and refines the grain. Commonly used materials are 60, 70, 65Mn, 60Si2Mn, 50CrVA, etc.
The spring uses its elastic deformation to absorb and release external forces. The finished product is required to have high elastic limit, high yield ratio, high fatigue strength and sufficient plasticity and toughness. Generally, the quenching temperature is controlled at Ac3+ (30~50℃) The tempering temperature is controlled at 400~500°C. In actual production, the growth of austenite grains is usually caused by quenching overheating. Although the strength of steel is not affected by normal tempering treatment, the plasticity and toughness are greatly reduced. As a result, the normal use of the spring is affected. Therefore, the qualified limit of the martensite structure can be appropriately grasped according to the actual situation during the metallographic inspection. According to daily production experience, the martensite structure of the silico-manganese steel spring after quenching is fine martensite or finer Martensite, refer to JB/T9211-2008 standard 1-4 grade martensite is qualified. Its austenite grain should be less than grade 5, and the martensite structure with grain size greater than grade 5 is coarser martensite or coarse Martensite. When inspecting quenched martensite, it is corroded with a 3% nitric acid alcohol solution and observed and evaluated at 500X.
Due to the different austenitizing temperature, the martensite shape and size are different. Grade 1 belongs to the low austenitizing temperature, and the quenched structure is hidden needle martensite, fine needle martensite and no more than 5% ferrite Body (volume fraction). Grade 8 belongs to the superheated structure, which is coarse lath martensite + coarse sheet needle martensite. It is controlled at 2-4 grades during normal quenching, and its structure is fine lath martensite + Flaky martensite.
The structure after normal quenching and tempering is tempered troostite, and the non-martensitic structure that appears, the undissolved ferrite or upper bainite structure should not exceed 10%.
With the rapid development of materials science and the large-scale use of various spring materials, the requirements for the structural strength and fatigue strength of materials are getting higher and higher, and new expectations are put forward for the precision requirements of quantitative analysis in metallographic testing. For testing spring material organization, ensuring product quality is an indispensable and important means.
The analysis of metallographic organization includes not only organizational identification but also evaluation. It has both qualitative, quantitative and semi-quantitative testing. It is based on relevant national standards or industry standards. The interpretation of this article is the current commonly used standards. Timeliness, please pay attention to the validity of the standard in specific applications.
Standard JB/T9211-2008 “Medium Carbon Steel and Medium Carbon Alloy Structural Steel Martensite Grade” is suitable for the inspection of the martensite structure of medium carbon spring steel and medium carbon alloy spring steel after quenching or tempering below 200 ℃ Method and comparison method of martensite metallographic pictures. Sample preparation and corrosion of the sample are the same as general metallographic samples. This standard specifies the grade of martensite structure is 1-8, and the magnification of martensite photo Both are 500X. In the grading process, you only need to compare the martensite structure of the sample after 500X with the standard grading picture to know the martensite grade of the sample.