Stability Of Austenite Grains Research Articles

Tempering influence on microstructural evolution and mechanical properties in a core of CSS-42L bearing steel

The microstructures in the core of CSS-42L bearing steel, including the hierarchical packet-block-lath martensite, residual austenite (RA) and carbide were tailored by employing different heat treatment with the emphasis on tempering and then carefully characterized. The microstructure, strength and toughness were investigated in detail as a function of tempering temperature. The results reveal that the tempering at 490 °C and 520 °C primarily induce fine secondary M6C precipitates which are responsible for the precipitation strengthening. When tempered at 550 °C, the strength further promoted significantly, which could be contributed to the dominating coherent M2C precipitation by RA decomposition. Toughness of this steel is mainly determined by the number density and mechanical stability of austenite grains retained since the propagating cracks can be trapped by them, leading to improved toughness. At 520 °C, both the impact toughness and the fracture toughness reach their peak values, which are attributed to the more RA possessing mechanical stability in the microstructure.

Simultaneous Increase of Both Strength and Ductility of Medium Mn Transformation-Induced Plasticity Steel by Vanadium Alloying

In general, microalloying such as vanadium (V) is considered to deteriorate the mechanical stability of austenite grains in medium Mn steel due to the consumption of C content. In this paper, we show that the mechanical stability of austenite grains could be optimized by V-alloying. This is because the V-alloying will refine the austenite grain size and tends to stabilize the austenite grains. The competition between the grain refinement and the reduced C content results in proper mechanical stability of austenite grains, providing continuous transformation-induced plasticity (TRIP) effect. In addition, the V-alloying suppresses the formation of intergranular cracks, leading to a ductile fracture morphology and a large non-uniform elongation.

Experimental investigation on a novel medium Mn steel combining transformation-induced plasticity and twinning-induced plasticity effects

The additional deep cryogenic treatment process prior to intercritical annealing was employed to tailor the mechanical stability of austenite grains in a new medium Mn steel. As a consequence, the medium Mn steel after intercritical annealing contains the austenite grains with different mechanical stability due to the different grain size and C content. The large austenite grains with low C content transform to martensite prior to the strain of 9.5% due to their low mechanical stability and provide the transformation-induced plasticity (TRIP) effect during tensile test. On the other hand, the small austenite grains with high C content have high mechanical stability and proper stacking fault energy, therefore do not transform to martensite but offer the twinning-induced plasticity (TWIP) effect from 9.5% strain up to fracture. Subsequently some of the twinned austenite grains provide the nucleation site for martensite formation from the strain of 26.2% up to fracture, providing the TRIP effect again in the large strain regime. In summary, the present novel medium Mn steel has TRIP effect firstly, followed by TWIP effect and then TWIP + TRIP effects during tensile test, therefore demonstrating enhanced work hardening behavior and excellent tensile properties.

Mechanical stability of individual austenite grains in TRIP steel studied by synchrotron X-ray diffraction during tensile loading

Abstract The stability of individual metastable austenite grains in low-alloyed TRIP steels has been studied during tensile loading using high-energy X-ray diffraction. The carbon concentration, grain volume and grain orientation with respect to the loading direction was monitored for a large number of individual grains in the bulk microstructure. Most austenite grains transform into martensite in a single transformation step once a critical load is reached. The orientation-dependent stability of austenite grains was found to depend on their Schmid factor with respect to the loading direction. Under the applied tensile stress the average Schmid factor decreased from an initial value of 0.44 to 0.41 at 243 MPa. The present study reveals the complex interplay of microstructural parameters on the mechanical stability of individual austenite grains, where the largest grains with the lowest carbon content tend to transform first. Under the applied tensile stress the average carbon concentration of the austenite grains increased from an initial value of 0.90 to 1.00 wt% C at 243 MPa, while the average grain volume of the austenite grains decreased from an initial value of 19 to 15 µm3 at 243 MPa.

Partial transformation of austenite in Al–Mn–Si TRIP steel upon tensile straining: an in situ EBSD study

The transformation of austenite to martensite in an Al–Mn–Si transformation-induced plasticity steel was investigated with in situ electron backscatter diffraction (EBSD) measurements under tensile straining. The visualisation of the microstructure upon straining allows for an investigation of the stability of austenite grains against strain-induced transformation, with particular focus on the grain size and the location of the austenite grains. The findings confirm that size and location of austenite grains are significant parameters for their stability. Small austenite grains were observed to be more stable than large grains, while austenite grains located beside bainitic ferrite are the most stable. Moreover, it is demonstrated that austenite grains transform gradually.

Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant factor contributing to enhancement of the ductility.

Characterization of individual retained austenite grains and their stability in low-alloyed TRIP steels

In situ three-dimensional (3-D) X-ray diffraction experiments have been performed at a synchrotron source on low-alloyed multiphase TRIP steels containing 0.25 wt.% Si and 0.44 wt.% Al and produced with different bainitic holding times, in order to assess the influence of the bainitic transformation on the thermal stability of individual austenite grains with respect to their martensitic transformation. A detailed characterization of the austenite grain volume distribution at room temperature was performed as a function of the prior bainitic holding time. In addition, the martensitic transformation behaviour of individual metastable grains was studied in situ during cooling to a temperature of 100 K. Both the carbon content and the grain volume play a key role in the stability of the austenite grains below 15 μm 3, while the carbon content exerts the dominant effect in the stability of the bigger grains. Measurements also suggest that the tetragonality of the thermally formed martensite is suppressed.