About martensite

When high carbon steel is quenched at normal temperature, the fine austenite grains and carbides can cause it to obtain a fine needle-like martensite structure. This structure cannot be distinguished under an optical microscope and is called hidden needle martensite. Martensite transformation of steel When the cooling rate of austenite is greater than VK and is supercooled below MS, martensite transformation begins. Because the martensite transformation temperature is extremely low, the degree of supercooling is large, and the formation speed is very fast, the transformation of austenite to martensite only occurs the lattice reorganization of r-Fe to a-Fe, without the diffusion of iron and carbon atoms. Therefore, the carbon content of martensite is the carbon content of austenite before transformation. Since the maximum dissolved carbon content in a-Fe is 0.0218%, martensite is a supersaturated interstitial solid solution of carbon in a-Fe. There are two main types of martensite structures, namely lath martensite and lamellar martensite. The form of martensite formed in quenched steel is mainly related to the carbon content of the steel. Lath martensite is a typical martensite structure formed by iron-based alloys such as low carbon steel, maraging steel, stainless steel, etc. Because the three-dimensional shape of the unit is lath, it is called lath martensite. Because its substructure is mainly composed of high-density dislocations, it is also called dislocation martensite; lamellar martensite is common in high and medium carbon steels. The thickness of each martensite crystal is very small compared with the radial size, and its cross-sectional shape is needle-like, so it is called lamellar martensite or needle-like martensite. Because its substructure is mainly composed of fine twins, it is also called twinned martensite. Generally, when Wc1.0%, it is almost all lamellar martensite; When Wc=0.3%-1.0%, it is a mixture of lath martensite and lamellar martensite. As the carbon content increases, the amount of lath martensite in quenched steel decreases and the amount of lamellar martensite increases. When high carbon steel is quenched at normal temperature, the small austenite grains and carbides can cause it to obtain a fine needle-like martensite structure. This structure cannot be distinguished under an optical microscope and is called hidden needle martensite.

I have never understood the knowledge, thank you to the original poster.

Martensite is the product of rapid cooling of supercooled austenite. It is supersaturated ferrite and has changed from the original face-centered cubic lattice to a body-centered cubic lattice.

Ms and Mf are the starting and ending points of martensite transformation respectively, also known as upper and lower martensite points. When supercooled austenite is rapidly cooled to Ms (230 degrees), martensitic transformation begins to occur until the transformation of Mf (-50 degrees) is completed. If it is only cooled to room temperature, some austenite will remain untransformed and retained. The transformation mechanism is that because the temperature is very low, carbon has no time to diffuse, and all remains in ɑ-Fe, forming a solid solution in which carbon is supersaturated in ɑ-Fe. This transformation is a non-diffusion type transformation.

I also learned some professional knowledge. The processing performance of martensitic stainless steel is good.

It is a structure formed after steel is heated to a certain temperature and then quickly cooled.