4. Metallographic inspection 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 used to improve hardenability. Si increases the elastic limit, and V increases the hardenability and refines grains. Commonly used materials are 60, 70, 65Mn, 60Si2Mn, 50CrVA and so on.
The spring uses its elastic deformation to absorb and release external forces, requiring the finished product to have a high elastic limit, a high yield ratio, a high fatigue strength, and sufficient plasticity and toughness. Generally, the quenching temperature is controlled at Ac3+ (30～50℃) and the tempering temperature is controlled at 400～500℃. In actual production, the growth of austenite grains is usually caused by quenching overheating. Although the normal tempering treatment, the steel The influence of strength is not obvious, but the plasticity and toughness are greatly reduced, which affects the normal use of the spring. Therefore, the qualified limit of the martensite structure can be properly grasped according to the actual situation during metallographic inspection. According to daily production experience, the martensite structure of the silico-manganese steel spring after quenching is fine martensite or finer martensite, which is qualified with reference to JB/T9211-2008 standard 1-4 martensite. The austenite grains should be smaller than grade 5, and the martensite structure with grain size greater than grade 5 is coarser martensite or coarse martensite. When inspecting the quenched martensite, it is corroded with a 3% nitric acid alcohol solution and observed and evaluated at 500X.
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. The sample preparation and the corrosion of the sample are the same as the general metallographic samples. The standard stipulates that the martensite structure is graded 1-8, and the magnification of the martensite photos is 500X. During the grading process, only the martensite structure of the sample after 500X magnification can be compared with the standard grading picture. Know the martensite grade of the sample.
Due to different austenitizing temperatures, the shape and size of martensite 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 (volume fraction). The 8th grade belongs to the superheated structure, which is coarse lath martensite + coarse pin martensite. It is controlled at 2-4 level during normal quenching, and its structure is fine lath martensite + flake martensite.
The structure after normal quenching and tempering is tempered troostite, and the non-martensite structure that appears, the undissolved ferrite or upper bainite structure should not exceed 10%.