INVESTIGATION OF TRIP EFFECT IN ZG06Cr13Ni4Mo MARTENSITIC STAINLESS STEEL BY IN SITU SYNCHROTRON HIGH ENERGY X-RAY DIFFRACTION
Shenghua ZHANG,Pei WANG(),Dianzhong LI,Yiyi LI
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
Cite this article:
Shenghua ZHANG,Pei WANG,Dianzhong LI,Yiyi LI. INVESTIGATION OF TRIP EFFECT IN ZG06Cr13Ni4Mo MARTENSITIC STAINLESS STEEL BY IN SITU SYNCHROTRON HIGH ENERGY X-RAY DIFFRACTION. Acta Metall Sin, 2015, 51(11): 1306-1314.
After quenching and proper intercritical tempering, ZG06Cr13Ni4Mo martensitic stainless steel is composed of tempered martensite matrix and reversed austenite. The deformation induced martensitic transformation of reversed austenite occurring during the deformation results in the transformation induced plasticity (TRIP) effect, which is beneficial to the mechanical properties of this steel. However, studies on the TRIP effect of reversed austenite are limited to description of phenomenon and mechanism behind is not clear. In order to reveal the mechanical stability and transformation induced plasticity of the reversed austenite during tension test in tempered ZG06Cr13Ni4Mo steel, a custom-built mini tensile instrument has been designed and installed on Shanghai Synchrotron Radiation Facility to conduct the in situ synchrotron high energy X-ray diffraction (SHXRD) experiment during the uniaxial tension. Three samples, which were tempered at 620 ℃ with different holding times and cooling rates in order to obtain different volume fraction of reversed austenite, were used to investigate the relationship between the deformation induced martensitic transformation and work hardening behavior. The integral intensity and the full width at half maximum of diffraction peaks of the reversed austenite and tempered martensitic matrix under different engineering stress were recorded. The gradual decrease in the integral diffraction intensity of reversed austenite with increase in tensile stress indicates that the reversed austenite has been induced to transform into martensite during the tension deformation. Furthermore, the volume fraction of reversed austenite during tension was quantitatively calculated by fitting the whole diffraction spectra of reversed austenite and tempered martensitic matrix with the Rietveld refinement method. The evolution of the reversed austenite fraction indicates that the deformation induced martensitic transformation initiates at the macro-elastic stage and through the whole deformation, which is different to the retained austenite in TRIP steel. Meanwhile, the work hardening exponents of three samples with different volume fraction of reversed austenite have been compared. It is found that the deformation induced martensitic transformation of reversed austenite increases the dislocation density of martensitic matrix and results in the increase in the work-hardening exponent during the plastic deformation, which enhances the ductility of ZG06Cr13Ni4Mo martensitic stainless steel.
Fig.1 The loader (a) and controller (b) of the custom-built mini tensile instrument
Fig.2 The installation of custom-built mini tensile instrument on diffraction instrument rotated along the normal direction (ND), transverse direction (TD) and loading direction (LD) of tension sample
Fig.3 Schematic of the in situ synchrotron high energy X-ray diffraction experiment (q—Bragg diffraction angle)
Fig.4 Synchrotron high energy XRD spectra of sample 2 (a, b) and sample 3 (c, d) before (a, c) and after (b, d) deformation (Sample 2 and 3 are tempered at 620 ℃ for 1 and 2 h and then air cooled, respectively)
Fig.5 Variations of integral diffraction intensity of (111)g (a), (200)g (b), (220)g (c) and (311)g (d) of reversed austenite in sample 2 with different engineering stresses
Fig.6 Variations of integral diffraction intensity of (111)g (a), (200)g (b), (220)g (c) and (311)g (d) of reversed austenite in sample 3 with different engineering stresses
Fig.7 Variations of volume fraction of reversed austenite in sample 2 (a, b) and sample 3 (c, d) under different engineering stresses (a, c) and true strains (b, d)
Fig.8 Variations of dislocation density of martensite matrix with different engineering stresses for sample 2 (a) and sample 3 (b) during uniaxial tensile test
Fig.9 True stress-strain curves of ZG06Cr13Ni4Mo steel after different heat treatments (sample 1 is tempered at 620 ℃ for 1 h and then water cooled)
Fig.10 Variations of work-hardening exponent and volume fraction of austenite as a function of true strain for samples 1, 2 and 3
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