.20 Crmoh Gear Steel Hardenability And Ribbon Organizations Study

Gear steel is one of the key materials which affects not only the life of vehicle, energy consumption and so on, but also the safety in using. It is required higher quality as it is more largely used in automotive materials. With the development of light weight vehicles, more stringent quality is required to gear steel. It is should to have narrower hardenability and uniformity microstructure to forbidden deformation when heat treatment.20CrMoH is widely used for carburized gears in China. This study focuses on investigation into the prediction model of hardenability and banded structure of its. The prediction model of hardenability is analyzed based on Jominy testing. The formation mechanism of banded structure, the ways to reduce or eliminate, and the effect of banded structure on the performance are discussed by means of microstructure, thermal simulation and heat treatment, as well as heat treatment deformation test.Simulated the hardenability of 20CrMoH steel with Non-linear model and the hardness distribution function, respectively. Results show that the simulation datas of the hardness distribution function are very close to the Jominy experimental dates. It can be used to predict the hardenability of 20CrMoH steel within the error limit-2HRC-+2HRC.Analysis of microstructure and composition show that the selective crystallization dendrite segregation during solidification is the main reason for the banded structure. The rich band and lean band of carbon and alloying elements form respectively to pearlite and ferrite in slow cooling speed after rolling, resulting in banded structure. The Mo element has most effect on banded structure for 20CrMoH steel.Results of thermal simulation experiments show that the final rolling temperature and cooling rate after rolling are the key factors in affecting the banded structure, especially the cooling rate. Microstructure will be smaller and lesser of banded structure when the final rolling temperature is at 900℃and the cooling rate is 0.7℃/s. Results of heat treatment experiments show that the formation of secondary banded structure is the product of phase transition when cooling from austenitic temperature, and not directly related to the prior banded structure. The cooling rate after austenitizing is the key factor to control the prior ferrite formation, so increasing the cooling rate is an important way to reduce the secondary banded structure.Results of carburizing deformation experiments show that the banded structure in steels can lead to some non-martensite formation in quenching, resulting in uneven hardness distribution and stress concentration. Therefore, banded structure increases deformation during heat treatment. Usually the banded structure must be controlled under grade No.3 to alleviate heat treatment distortion.