The impact of Nb on bearing steel GCr15 martensitic phase transformation in situ observation

The impact of Nb on bearing steel GCr15 martensitic phase transformation in situ observation of 2019 - 01 - 31 as the aerospace, energy, and the rapid development of rail transportation equipment manufacturing industry, high-end equipment, including bearing steel, high quality special steel demand is strong, and with a high quality requirements. As a kind of high carbon chromium bearing steel GCr15, is the basis of the important mechanical parts, its quality including the use of performance, life and reliability, have strict requirements. The micro alloying technology is one of the commonly used method to improve the quality of bearing steel. In view of this, the work of bearing steel GCr15, choose different Nb content by high temperature laser confocal microscope, the martensitic transformation process for in situ observation, understand the impact of Nb element of martensitic transformation, for niobium microalloying technology in high carbon steel and provide the basis for the application. Test materials from domestic special steel plant production of cold working with bearing steel GCr15 hot rolled & Phi; 28 mm bar, its chemical composition as shown in table 1. Among them, the no. 1 experimental steel adding Nb, 2 and 3 test steel added 0, respectively. 018% and 0. 040%Nb。 Test the chemical composition of steel (table 1 The mass fraction, %) From the rod cross section 1/2 circle interception & Phi; 7. ×5毫米 3 mm small cylindrical specimens, bottom up and down with one side polished, smooth and select as observation. The VL2000DX - SVF17SP type high temperature laser confocal microscope equipment, three kinds of test steel are at a rate of 2 ℃ / s heated to 700 ℃, with 0 again. The speed of 3 ℃ / s heated to 1300 ℃, thermal quenching after 10 min. In situ observation of martensite during the cooling process form nucleus and the growth process and behavior. Results show that in the process of martensite phase transformation, plate embossed strip priority along austenite grain boundary and the martensite plate strip to form, and martensite lath was 60 & deg; 120度; Or parallel distribution, number of martensite rising sharply, shift speed from fast to slow, after the formation of the martensite lath length shorter than the first. Add 0. 018% and 0. 040% Nb micro alloy can improve 8 the Ms point of GCr15 separately. 3 ℃ and 11. 5℃。 That Nb elements will increase Ms points, promote the martensite phase transformation.