The Effects of Prior Austenite Grain Refinement on Strength and Toughness of High-Strength Low-Alloy Steel

The effects of prior austenite grain (PAG) refinement on the mechanical properties of bainitic/martensitic steels not only come from itself, but also have more complex effects by affecting the substructure formed by coherent transformation. In this study, the samples of a low-alloy steel were water quenched from different austenitizing temperatures and the bainitic/martensitic microstructures with different PAG sizes were obtained. Electron back-scattered diffraction was used to characterize the microstructure and different types of boundaries were identified and quantitatively analyzed. The tensile tests and series temperature Charpy impact tests of different heat treatment were also carried out and comprehensively analyzed with microstructure characterization works. The results show that the uniform refinement of prior austenite grain can increases the density of packet boundary and block boundary, which leads to microstructure refinement with higher density of high-angle grain boundaries with misorientation >45°. The contribution of this microstructure refinement to toughness is significant, but its contributions to strength and elongation are relatively limited. Compared to uniform refined PAG, if the PAGs are mixed crystal, the density of block boundary will be reduced, which leads to a lower density of the high-angle boundary with misorientation >45° and the positive effects of microstructure refinement on toughness improvement are weakened. The observation of fracture surface of impact specimens indicates that refining the PAG can delay the tendency of brittle fracture with the decrease in test temperature, and even in the case of brittle fracture, the cleavage facet of the fracture surface is relatively smaller. This result also verifies that PAG refinement can effectively improve toughness by inhibiting cleavage fracture.

Mechanism of FIB-Induced Phase Transformation in Austenitic Steel

The transformation of unstable austenite to ferrite or α′ martensite as a result of exposure to Xe+ or Ga+ ions at room temperature was studied in a 304 stainless steel casting alloy. Controlled Xe+ and Ga+ ion beam exposures of the 304 were carried out at a variety of beam/sample geometries. It was found that both Ga+ and Xe+ ion irradiation resulted in the transformation of the austenite to either ferrite or α′ martensite. In this paper, we will refer to the transformation product as a BCC phase. The crystallographic orientation of the transformed area was controlled by the orientation of the austenite grain and was consistent with either the Nishiyama–Wasserman or the Kurdjumov–Sachs orientation relationships. On the basis of the Xe+ and Ga+ ion beam exposures, the transformation is not controlled by the chemical stabilization of the BCC phase by the ion species, but is a result of the disorder caused by the ion-induced recoil motion and subsequent return of the disordered region to a more energetically favorable phase.

Visualization of Prior Austenite Grain Boundaries in Low-Carbon Steels

Knowledge of the size of the prior austenite grain is of key importance. If abnormal grain growth occurs during austenitization, the resultant inhomogeneous microstructure may negatively affect the strength and toughness properties of the final product. The visualization of prior austenite grain boundaries with an etchant based on picric acid has been applied for years. Despite this long-time experience, it is often challenging to achieve sufficiently good visualization of prior austenite grain boundaries in many steel grades, especially low-carbon steels. This work will study the effect of the cooling rate from austenitizing temperature down to room temperature, of the subsequent tempering treatment and the etchant on the visualization of prior austenite grain boundaries in a low-carbon microalloyed steel. All these parameters have an impact on the etching result. A suitable etchant for the visualization of prior austenite grain boundaries in a low-carbon microalloyed steel could be found.

Fatigue Performance of Low Pressure Carbonitrided 20MnCr5 and SAE 8620 Steel Alloys

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

On a Modified Approach of Measuring Quench Severity and its Application

Instrumented methods for measuring the coefficient of heat transfer are difficult to implement in industrial quench systems. In 1985 Roy Kern presented a simple empirical method for calculating the quench severity of commercial quench systems using measured Jominy hardenability and a mid-radius (r/R=0.5) hardness of a 3-inch diameter 8640 or 4140 steel bar. A more general approach using the Kern methodology is presented here with hardness profile matching to determine the quench severity. Experiments were performed using 2-inch diameter bars of 8620 with a length to diameter ratio of 4. Test bars and Jominy bars were heat-treated following ASTM A255. Test bars were quenched using an experimental draft tube with a water velocity of 6 ft/s. An excel workbook was programmed to calculate the quenched hardness profile as a function of quench severity using prior austenite grain size and steel chemistry. Measured Jominy hardness and calculated hardenability were in good agreement provided the prior austenite grain size was incorporated into the calculations. Both the Kern method and hardness profile matching produced a quench severity equal to 1.45.

Effect of Initial Microstructure on the Toughness of Coarse-Grained Heat-Affected Zone in a Microalloyed Steel

Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.