Microstructure and mechanical properties of Co-28Cr-6Mo-0.22C investment castings by current solution treatment

This study examined the impact of current solution treatment on the microstructure and mechanical properties of the Co-28Cr-6Mo-0.22C alloy investment castings.The findings reveal that the current solution treatment significantly promotes the dissolution of carbides at a lower temperature.The optimal conditions for solution treatment are determined as a solution temperature of 1,125°C and a holding time of 5.0 min.Under these parameters,the size and volume fraction of precipitated phases in the investment castings are measured as6.2μm and 1.1vol.%.The yield strength,ultimate tensile strength,and total elongation of the Co-28Cr-6Mo-0.22C investment castings are 535 MPa,760 MPa,and 12.6%,respectively.These values exceed those obtained with the conventional solution treatment at 1,200°C for 4.0 h.The findings suggest a phase transformation of M_(23)C_(6)→σ+C following the current solution treatment at 1,125°C for 5.0 min.In comparison,the traditional solution treatment at 1,200°C for 4.0 h leads to the formation of M_(23)C_(6)and M_(6)C carbides.It is noteworthy that the non-thermal effect of the current during the solution treatment modifies the free energy of both the matrix and precipitation phase.This modification lowers the phase transition temperature of the M_(23)C_(6)→σ+C reaction,thereby facilitating the dissolution of carbides.As a result,the current solution treatment approach achieves carbide dissolution at a lower temperature and within a significantly shorter time when compared to the traditional solution treatment methods.

Effect of annealing treatment on microstructures and properties of austenite-based Fe-28Mn-9Al-0.8C lightweight steel with addition of Cu

The mechanical properties of an austenite-based Fe-Mn-Al-C lightweight steel were improved by co-precipitation of nanoscale Cu-rich and κ-carbide particles.The Fe-28Mn-9Al-0.8C-(0,3)Cu (wt.%) strips were near-rapidly solidified and annealed in the temperature range from 500 ℃ to 700 ℃.The microstructure evolution and mechanical properties of the steel under different annealing processes were studied.Microstructural analysis reveals that nanoscale κ-carbides and Cu-rich particles precipitate in the austenite and ferrite of the steel in this annealing temperature range.Co-precipitation of nanoscale Cu-rich particles and κ-carbides provides an obvious increment in the yield strength.At the annealing temperature of 600 ℃,both the yield strength and ultimate tensile strength of Fe-28Mn-9Al-0.8C-3Cu (wt.%) steel strip are the highest.The total elongation is 25%,which is obviously higher than that of Cu-free steel strips,for the addition of Cu reduces the large sized κ-carbides precipitated along austenite/ferrite interfaces.When the annealing temperature rises to 700 ℃,the strength and ductility of the two steel strips deteriorate due to the formation of massive intergranular κ-carbides precipitated along austenite/ferrite interfaces.It can be concluded that a proper co-precipitation of Cu-rich particles and κ-carbides would improve the properties of austenite-based Fe-Mn-Al-C steel.

Solidification characteristics of Fe-Ni peritectic al oy thin strips under a near-rapid solidification condition

This paper is an experimental investigation of the structure evolution and the solute distribution of 2 mm thick strips of Fe-(2.6, 4.2, 4.7, 7.9wt.%)Ni peritectic alloy under a near-rapid solidification condition, which were in the regions of δ-ferrite single-phase, hypo-peritectic, hyper-peritectic and γ-austenite single-phase, respectively. The highest area ratio of equiaxed grain zone in the hyper-peritectic of Fe-4.7wt.%Ni alloy strip was observed, while other strips were mainly columnar grains. The lowest micro-segregation was obtained in the Fe-7.9wt.%Ni alloy strip, while micro-segregation in the Fe-4.7wt.%Ni alloy was the highest. As opposed to the microsegregation, the macro-segregation of all the Fe-Ni strips was suppressed due to the rapid solidification rate. Finally, the structure formation mechanism of Fe-Ni alloy strips was analyzed.

An accelerated aging assisted by electric current in a Fe-Mn-Al-C low-density steel

An aging method assisted by electric current was applied to a Fe-18Mn-9Al-1C(wt.%)low-density steel.It improves the microstructure and therefore significantly increases both the yield strength and ductility of the steel.This current-assisted aging method can increase the yield strength by 178 MPa and elongation by 1.16 times in only 0.5 min at 450℃.However,the yield strength is increased only 90 MPa by the traditional aging method(heat conduction)at 450℃ for 180 min,and the elongation is even decreased from 42.0%to 31.6%.The obvious improvement in yield strength by the current-assisted aging for a short time is resulted from the fact that the current-assisted aging promotes a rapid precipitation of nano-scaleκ-carbides inγ-austenite by reducing the thermodynamic barrier and accelerating the atomic diffusion.This work demonstrates that this current-assisted aging method is significantly time saving and cost-effective for low-density steels,with potential for various industrial applications.

Melt flow,solidification structures,and defects in 316 L steel strips produced by vertical centrifugal casting

Vertical centrifugal casting can significantly enhance the filling capability of molten metals,enabling the production of complex thin-walled castings at near-rapid cooling rates.In this study,the melt flow,solidification structures,and defects in 316 L steel cast strips with a geometry of 80 mm×60 mm×2.5 mm produced by vertical centrifugal casting were numerically and experimentally analyzed under different rotation speeds.With gradually increasing the rotation speed from 150 r/min to 900 r/min,the simulated results showed the shortest filling time and minimum porosity volume in the cast strip at a rotation speed of 600 r/min.Since a strong turbulent flow was generated by the rotation of the mold cavity during the filling process,experimental results showed that a“non-dendritic”structure was obtained in 316 L cast strip when centrifugal force was involved,whereas the typical dendritic structure was observed in the reference sample without rotation.Most areas of the cast strip exhibited one-dimensional cooling,but three-sided cooling appeared near the side of the cast strip.Moreover,the pores and cracks in the 316 L strips were detected by computed tomography scanning and analyzed with the corresponding numerical simulations.Results indicated the existence of an optimal rotational speed for producing cast strips with minimal casting defects.This study provides a better understanding of the filling and solidification processes of strips produced by vertical centrifugal casting.

Microstructure and mechanical behavior of a low-density Fe-12Mn- 9Al-1.2C steel prepared using centrifugal casting under near-rapid solidification

It is vital for emission reduction and energy saving to lighten the weight of automobile. Low-density Fe-Mn-Al-C steels with high strength and excellent ductility have become a promising type of material in the automotive industry. Thus, a new approach was proposed by using centrifugal casting to produce the low-density Fe-12Mn-9Al-1.2C steel with high performance under near-rapid solidification in a near-net shape. The produced steel strips, with a thickness of 2.5 mm and a density of 6.89 g/cm3, were examined for their microstructures and mechanical properties. The results showed that mechanical properties of as-cast steel strip reached 1182 MPa in ultimate tensile strength and 28.1% in total elongation. Aging treatment at 400 or 600℃ for 3 h enhanced tensile strength of the steel strips, while aging at 800℃ dramatically decreased its elongation. Moreover, Young＇ s modulus of the steel strip improved with the increment of aging temperature. The relationship between the mechanical properties and the microstructures was discussed. The results demonstrated that advanced low-density steels with promising mechanical properties could be directly produced from liquid by this simple process.

Phase modulation of bcc-structured Fe35Mn25Al15Cr10Ni15 high-entropy alloy by interstitial carbon

High-entropy alloys （HEAs） usually contain more than five alloying elements. The ductility of a body-centered cubic （bcc）- type HEA typically is lower than that of their face-centered cubic （fcc） counterpart. And low ductility restricts engineering applications of the bcc-structured HEAs. In engineering materials, improvement in ductility usually results in deduction of mechanical strength. A method to improve both mechanical strength and ductility in a bcc-structured HEA was proposed by adding interstitial carbon. Experimental results showed that replacement of 5 at.% Cr with 5 at.% C in a bcc-structured Fe35Mn25Al15Cr10Ni15 HEA resulted in an increase in fcc phase from 0.3 to 93.7 vol.%. Strength and ductility increased at the same time. The transition of bcc-structure to fcc-structure along with a remaining small amount of bcc phase improved mechanical properties. This work indicates that interstitial carbon can be employed to modulate the fraction of constituent phases in a bcc-structured HEA to enhance engineering mechanical properties.

Precipitation strengthening of nano-scale TiC in a duplex low-density steel under near-rapid solidification

Precipitation strengthening of nano-scale TiC is a promising method to improve mechanical properties of Fe–16Mn–9Al–0.8C (wt.%) low-density steel. This work attempted to introduce nano-scale TiC precipitates by adding 1 wt.% Ti element. The experimental results show that these precipitates with the total fraction of about 2 vol.% were formed and no coarse precipitates were observed despite the high Ti addition. It was interesting that the polygonal and needle-shaped TiC precipitates were observed in γ-austenite and δ-ferrite, respectively. Ti addition also decreased the volume fraction of γ-austenite significantly. Correspondingly, the yield strength was increased, but the elongation was significantly decreased due to the significant decrease of γ-austenite. Comparing with the Ti-free steel, the formation of TiC precipitates was the main reason for the increase in yield strength of Ti-bearing steel, and TiC precipitates also led to a higher strain hardening index at the first deformation stage. TiC precipitates promoted the Orowan strengthening, resulting in a higher strain hardening capability than Ti-free steel reinforced by shearable κ-carbide.

Effect of coiling and annealing temperatures on yield point behavior of low-carbon steel

Low-carbon steel is widely used for household appliance and automotive panel steel because of its excellent plasticity.Unfortunately,yield point phenomena easily appear in the low-carbon steel produced by a continuous annealing process and cause degradation to the surface quality during processing.The effect of the coiling temperature(600-750℃)and annealing temperature(740-820℃)on the yield point behavior is studied.Tensile tests show that coiling temperature has a greater effect on yield point elongation(YPE)and aging index(AI)than the annealing temperature.Microstructure observations show that coiling temperature at 750℃would make the micron-sized carbides appearing at the grain boundary disappear and a number of dispersed nanoscale carbides precipitate in grain interior,corresponding to the highest solid solution carbon content in the matrix of 750℃coiled sample.The experimental results suggest that AI rather than YPE has a positive relationship with the solid solution carbon content of the low-carbon steel.And YPE has a positive relationship with the upper/lower yield strength.

Numerical study of heat transfer of gas-atomized Fe-6.5 %Si （mass and solidification behavior fraction） droplets

During spray atomization process, the heat transfer and solidification of droplets play very important roles for the deposition quality. Due to the difficulties of experimental approach, a numerical model is developed, which integrates liquid undercooling, nucleation recalescence and post-re- calescence growth to present the full solidification process of Fe-6.5%Si （mass fraction） droplet. The droplet velocity, temperature, cooling rate as well as solid fraction profiles are simulated for droplets with different sizes to demonstrate the critical role of the size effect during the solidification process of droplets. The relationship between the simulated cooling rate and the experimentally obtained secondary dendrite arm spacing is in excellent agreement with the well-established formula. The pre-constant and exponent values lie in the range of various rapid solidified Fe-based alloys reported, which indicates the validity of the numerical model.