A Study On Thermo-Mechanical Treatment For Thin-Wall Tube Workpieces Of30CR3Steel

30Cr3is a microalloyed super-strength steel for which the traditional heat treatment is quenching and tempering. However, such treatments may often cause large distortion and lower mechanical quality in the workpiececs, especially when they are in the thin-wall tube form, due to the dramatic temperature gradient. A thermo-mechanical treatment is proposed to minimise the distortion and the strengthening mechanism of the treated30Cr3steel is explored in the present thesis.In order to construct the TTT diagram of30Cr3steel, the variation in the sample diameter was measured using dilation method. The microstructure evolution during thermo-mechanical process was observed and the transformed fractions were measured in optical micrographs. The microstructure observations were conducted by using OM, SEM and TEM to explore the strengthening mechanism.The plotted TTT diagram indicates30Cr3steel having a good hardenability so that almost100%austenite would transform into martensite during quenching. Microstructure observation shows that the formed martensite further decomposes into ferrite and carbide during the following tempering, whose softened matrix provides a guaranty for large plastic deformation. Meanwhile, the tempering produces the uniformly dispersed carbide particles enhances the strength, the carbide is identified as M23C6with a size of300nm existing within martensite lath where had high density of dislocation. After spinning process, a great number of dislocations produced in deformation provide possible nucleation sites and driving force for carbide’s nucleation and growth.In order to reduce the internal stresses, a second tempering was applied so that additional spherical carbides with an average size of300nm are dispersively precipitated in the matrix. Selected area electron diffraction (SAED) revealed that they are M6C. In the meantime, the previously formed M23C6during first tempering further grew to that with average size of500nm. These two types carbide play the key roles in the improvement of strength of30Cr3steel after thermo-mechanical treatment, i. e. they act as the barrier to dislocation movement.Obviously, the size of precipitated carbide particles influences the mechanical property of tempered steel. The larger particle decreases the ferrite-cementite phase boundary area and consequently results in softer and weaker steel yet one that is tougher and more ductile. The temperature determines the size of the cementite particles. The temperature dependence of strength was carried out for oil-quenched steel of30Cr3, with increasing temperature the strength decreases, which corresponds to the growth and coalescence of the cementite particles, at400℃, the microstructure having cementite uniformly embedded within the continuous ferrite phase displays maximum value for strength and in the range of temperature200℃-450℃. Higher temperature would let cementite grow over size.The proposed thermo-mechanical treatment consists of austenizing at920℃for30min and quenching in oil, then tempering at590℃for2h followed with a spinning deformation at room temperature, and finally tempering at400℃to remove the internal stress.Finally, it is proposed that the thermo-mechanical route for30Cr3steel is920℃for30min quenched in oil or by high pressure nitrogen gas, tempering at590℃for2h to form ferrite matrix and carbide, spinning at room temperature and followed by a tempering at400℃for lh to remove internal stresses. Measurement of mechanical property shows that the treated workpiece meets the design requirement.