Bing Zhe Bai

Study of Ultra-Long Life Fatigue of High Strength Steel with Duplex-Phase of Carbide-Free Bainite and Martensite

The ultra-long life fatigue properties of novel high strength steel with duplex-phase of carbide-free bainite and martensite (CFB/M) were studied in this article. The specimens were heat-treated by the following procedure: austenitizing at 900°C for 20 min followed by air cooling and then tempering at 340°C for 2 h. All the fatigue tests were carried out by ultrasonic fatigue testing equipment, at a cycling frequency of 20 kHz. Experimental results show that there is no conventional fatigue limit for the steel studied in this paper. From the S-N curve it could be suggested that it is more reliable to use the fatigue strength at 10 8 cycles than that at 10 7 cycles for the safe-life design. Based on the observations of fracture surface, it is obvious that the fatigue fracture surface can be classified into two types: initiating from surface and interior. A fish-eye mark and an area with a particular morphology surrounding the fracture origin termed ODA (optically dark area) by Japanese scholars are always found in the latter type, but no obvious inclusion is observed inside it. By calculating the values of ΔK at ODA and fish-eye, it is found that the values of Δk fish-eye and ΔK ODA tend to be constant respectively, independent of the stress amplitude, fatigue life and the distance of the initiation site to the edge of specimen, therefore, it is thought that the contribution of hydrogen to the formation of ODA should not be considered.

The Effect of Si on the Toughness of High Strength Mn-Si-Cr Series Bainitic Steels

The relationship between the toughness and silicon content of high strength Mn-Si-Cr series bainitic steels has been investigated. The results show that with increasing in silicon content, the onset temperature of the steel’s tempered martensite embrittlement (TME) rises; moreover, the minimum value of tested toughness decreases and the tempering temperature corresponding to the minimum value of toughness increases. This phenomenon results from the effect of silicon on the stability of filmy carbon-enriched retained austenite in carbide-free bainite/martensite (CFB/M) microstructure.

The Development of Mn-Series Air-Cooled and Water-Quenched Bainitic Steels in China

The origin and development of air-cooled Mn-series bainite steels are introduced. The invented idea, strengthening-toughening mechanism, mechanical performances, development and application of this kind of steel including granular bainitic steels, FGBA / BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitic steels are presented. The invented idea mechanical performances, development and application of second generation of Mn-series bainitic steels, i.e. water-quenched Mn-series bainitic steels invented by the authors newly are introduced. The water quenched Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure, reduce the amount of alloying content, increase harden capability and improve weldability. It should be pointed out that the application of both air-cold and water- quenched Mn-series bainitic steels are complementary and mutually reinforcing. Some newest technology of Mn-series bainitic steels in China are discussed in this paper. It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as: significantly reducing costs of both raw materials and production; good combination of strength and toughness; excellent weldability; simple procedure; large savings in energy resources and environmental pollution is reduced.

The Influence of Deformation Condition on Microstructure and Properties of GH4169 Alloy

The influence of temperature, strain rate, strain on mechanical parameters and microstructure evolution in isothermal deformation of GH4169 alloy were investigated. Lower resistance is obtained under higher temperature and or lower strain rate. However, coarser microstructure also is gained under such condition. The tensile property at room temperature and elevated temperature, permanent properties at elevated temperature of the GH4169 alloy with the treatment of “direct aging after isothermal deformation” are better than those of the same alloy with the treatment of solid-solution+direct aging after isothermal deformation. Isothermal forming was undertaken for a 1/4 size simulated part of a kind of turbo disk of an aviation engine. Its external is fine and the microstructure of the material was fine and homogeneous for both straightly after forming and after direct aging.

Superplastic Behaviors of a Ti-Alloy with Ultrafine Grain Size

The superplastic deformation behavior of a Ti alloy with submicron grain size was investigated. The ultrafine grained (UFG) Ti alloy exhibits good superplasticity at lower temperature (800°C) and at higher strain rates (not lower than 1.8×10 -2 s -1 ). Severe deformation inhomogeneity(DI) could occur at certain conditions. To lower deformation temperature or to raise strain rate could ease or eliminate inhomogeneity. The influence of deformation conditions and grain size on DI was analyzed. Based on examining on the m value and deformation activation energy of the UFG Ti alloy, SPD mechanisms were proposed to be grain boundary sliding accommodated by dislocation movement at lower temperature and by diffusion at higher temperature.

Two Novel Methods to Realize the Superplastic Forming of Ultrahigh Strength Steels

Two novel methods of obtaining microduplex structures, ferrite plus spherical carbides, in ultrahigh strength steels (~2000MPa) are introduced. One is through an adequate deformation just below the austenite-ferrite equilibrium transformation temperature (i.e. Ae3 temperature, ~983K) followed by water quenching. The adequate deformation directly leads to the formation of a (ferrite plus spherical carbides) microduplex structure. The microstructure evolution during the deformation includes pearlite transformation, cementite spheroidization and ferrite recrystallization. The other is through an adequate deformation above Ae3 temperature (~1003K) followed by water quenching to produce martensite firstly and then obtain a (ferrite plus spherical carbides) microduplex structure during warm deformation of martensite. Microstructural analysis on the microduplex structure shows that submicron carbides are located at ferrite grain boundaries while nanometer ones are dispersed inside ferrite grains. This kind of carbide distribution may suppress the coarsening of ferrite grains and form a dynamic equilibrium of ferrite grain size on a specific deformation condition. The strain rate sensitivity of the (ferrite plus spherical carbides) microduplex structures is about 0.4 at 973K and strain rate of 10-4s-1.

Effect of Final Cooling Temperature on Mechanical Properties of a Water Cooled Mn-Series Low Carbon Bainitic Steel as 8Mn2SiNb

The effect of final cooling temperature on the mechanical properties of a water cooled Mn-series low carbon bainitic steel as 8Mn2SiNb has been investigated in this paper. The results indicate that the optimum final cooling temperature is 450 °C, followed by air cooling to room temperature. Compared with air cooling, the condition of water cooling to 450 °C increases the tensile strength and yield strength about 13.3% (From 805MPa to 929MPa) and 59.0%(From 464MPa to 741MPa) respectively, remaining 21.5% elongation and 151J toughness. SEM observation reveals that the microstructure of the steel after water cooling to 450 °C is mainly granular bainite +lath martensite +refined grain boundary allotriomorphic ferrite (FGBA). Compared with air cooling, the condition of water cooling to 450 °C increases the volume fraction of strengthening phase (M-A island) from 28.2% to 38.1%.

Effect of 0.06%Nb on the Microstructure and Mechanical Properties of Mn-Series Low Carbon Air-Cooling Bainitic Steels

The effect of 0.06%Nb on the microstructure and mechanical properties of grain boundary allotriomorphic ferrite (FGBA) / granular bainite (Bg) air-cooling bainitic steels has been investigated in this paper. The results indicate that the steel acquires superior mechanical properties by adding 0.06%Nb. Compared with Non-Nb steel, the addition of 0.06%Nb increases the tensile strength and yield strength about 37.1% (From 780MPa to 1070MPa)and 26.6%(From 557MPa to 705MPa) respectively, remaining 18.3% elongation and 97J toughness. The addition of 0.06%Nb not only promotes the nucleation of intragranular ferrite but also refines the allotriomorphic ferrite grain , both of which in turn contribute to the refinement of granular bainite cluster including its ferrite platelets and M-A islands. Under the synthetic roles of the microstructure refinement and precipitation strengthening, 148MPa yield strength improvement has been acquired in the low carbon air-cooling bainitic steel by the adding of 0.06%Nb.

Study on Heavy Section Forging Steel for Reactor Pressure Vessel Applying Simulation Methods

The composition and heat treatment of heavy section forging steel for reactor pressure vessel were optimized applying physical and numerical simulation methods, including numerical simulation to calculate temperature distribution during quench of model forging, artificial neural network to predict CCT diagrams of steels and small sample control cooling to simulate specific heat treatment. And the influence of compositon and heat treatment on microstructure and properties were discussed. Results showed that the experimental steel obtained satisfactory properties based on optimization of chemical composition and heat treatment. It is estimated that the hardenability and temper stability of experimental steel were improved by tungsten alloyment and higher temperature temper was good for superior microstructure, proper strength and better toughness. In the present work, application of simulation methods is proved to be reasonable for study on heavy section forging steel.

Mn-Series Low Carbon Air Cooling Bainitic Steels Containing Niobium

The effect of four different niobium(From 0-0.1%) addition on the mechanical properties of allotriomorphic ferrite (FGBA)/ granular bainite (BG) air cooling bainitic steels has been investigated in this paper. The results show that (1) The 0.06%Nb steel acquired superior strength and toughness combination by applying 1250°C×60min solution treated, finish rolling at 850°C, and air cooling. The corresponding mechanical property of the thick plate (30mm) is: σb>1050MPa, σ0.2>700MPa, δ5>17%, Akv>90J. (2) The addition of niobium refine the grain size of FGBA, and promoted the transformation of bainite structure. With the increase of niobium content, the refinement of ferrite grain and bainitic cluster is improved. (3) More refined M-A island is acquired by the small addition of niobium. According to M-A Analysis tools and transversal methods, with the rise of niobium content, the volume fraction of M-A island increase from 21% to 35%, and the average size of M-A island decrease from 1.1μm to 0.7um. (4) It is suggested that 0.02-0.06% niobium can improve the mechanical properties of the steel obviously. However, excess addition of Nb (0.1%) deteriorates the impact toughness obviously. (5) Under the synthetic roles of the microstructure refinement and precipitation strengthen, 60-160MPa yield strength improvement has been acquired in the low carbon air cooling bainitic steel by the small addition of niobium. (6) This steel is with low production cost since the alloying element Mn is cheap.