Hong-Sheng Fang

Unified rationalization of the Pitsch and T-H orientation relationships between Widmanstätten cementite and austenite

Abstract This work demonstrates that understanding the habit planes of cementite plates is an important step to gain an insight into the irrational orientation relationships (ORs) between Widmanstatten cementite and austenite, i.e. the Pitsch and T–H ORs. A reproducible irrational OR in this system is attributed to a unique correspondence between the OR and the habit plane, under the condition that the habit plane is a quasi-invariant plane. The OR is constrained by two parallelism conditions: parallelism of [0 1 0] C and 〈1 1 0〉 A ; parallelism of a group of Δ g ’s in the zone axis of [0 1 0] C . The calculated ORs and habit planes are fully consistent with the experimental results from the literature.

The mechanism of bainite transformation in steels

Abstract Sympathetic nucleation and ledgewise growth are competitive with each other throughout the bainite formation in steels. Many evidences are provided to support this mechanism as follows: ultra-fine structures of bainite and its surface relief, and carbide nucleation inside austenite at the interface of ferrite/austenite, etc.

Creation of Air-Cooled Mn Series Bainitic Steels

The development and mechanical performances of new type air-cooled Mn series bainitie steels including granular bainitie steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitie steels invented by the authors are summarized. The novel series of bainitie steels are alloyed with Mn, and several series bainitie duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type air-cooled Mn series bainitie steels are presented. Furthermore, the applications in different fields of these Mn series air-cooled bainitie steels with different strength level are also introduced. It is suggested that the significance of the development of the air-cooled Mn series bainitie steel can be summarized as follows: reducing costs of both raw materials and production; good combination of strength and toughness; self-hardening with high bainitie hardenability by air cooling from hot working without additional quenching-tempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.

A study on wear resistance of the white layer

The white layer has received some consideration, but there are two different opinions on its wear characteristics. It is considered to be of tribological advantage in some instances. However, the microcracks and voids formed by adiabatic shearing are precursors to fracture and should decrease the wear resistance. As to the white layer previously generated by impact wear, the wear characteristic of the white layer is studied by a pin on disc machine in the present paper. It is found that the white layer causes delamination of material, which increases the wear loss significantly. The delamination of the white layer, being prior to the cutting wear, is the primary wear mechanism for materials with the white layer.

The failure models induced by white layers during impact wear

Abstract The characteristics of impact wear of high hardness steel have been studied by repetitive impact testing. The subsurface microstructure and wear debris have been investigated, and both wear resistance and failure models are discussed. Experimental results indicate that there are two zones beneath the wom surface (WS): namely a white layer (WL) and a deformation zone (DZ). The WL is an intensely deformed layer. Microcracks which form originally in this WL ultimately cause the failure of the steel samples. Based on the experiment, two failure models related to the WL are proposed. (1) Delamination occurs when microcracks propagate along the path parallel to the WS. (2) Spalling occurs when microcracks propagate along the flow lines in the DZ.

Performance comparison of AlTiC and AlTiB master alloys in grain refinement of commercial and high purity aluminum

Abstract For further knowledge about the refining performance of AlTiC master alloys, A15.5Ti0.25C and A16.5Ti0.5C master alloys containing high Ti and C content were prepared and used in grain refining experiments of 99.8% commercial pure aluminum(CPAl). Their performance was compared with two types of Al5Ti1B refiners whose performance was nowadays considered to be the best. These two types of master alloys show similar refining efficiency at the addition level of 0.2%. However, at the addition level of 0.5%, there still exists great performance difference between AlTiC and Al5TiB alloys in grain refinement of 99.98% and 99.995% high purity aluminum(HPAl). The growth of columnar grains is fully suppressed due to the refinement of AlTiC at the addition level of 0.5%. Also, at the same addition level, the grain refining experiments of A13Ti0.15C and A15Ti0.2C master alloys which have found initial commercial applications are conducted in the above-mentioned three types of pure aluminum. According to the experimental results, these two refiners of different compositions are both nonideal. The second phase particles extracted from each refiner were observed through TEM, while the nuclei of grains after grain refinement were observed through SEM. The results were analyzed through computation and comparison of the constitutional-supercooling parameter and the growth-restriction parameter whose values were determined by solute element in aluminum melt with different purity. Apparently, AlTiC master alloys with high content of Ti and C element have great refining potential.

Caviation erosion of bainitic steel

Abstract The cavitation erosion of a bainitic steel in distilled water has been investigated by means of a rotating-disc erosion apparatus. It was found that the bainitic steel specimen with higher value of impact toughness has better cavitation resistance and the increase of the relative content of lower bainite can improve the cavitation resistance considerably. The cavitation mechanism of bainitic steel was explored by examining the eroded surface and the cross-section of the specimens with uptical microscope and scanning electron microscope. The cavitation damage process of bainitic steel consists of three stages. First, the cavitation pits and cracks are formed by the detachment of small chips in the surface; second, the pits and cracks are enlarged and penetrated by the loss of small chips in their inside surfaces; finally, the stepwise propagation of the pits and cracks results in the loss of massive chunks. It was discovered that lower bainite phase in steel can resist the enlargement and extension of erosion pits and cracks because of its higher toughness and effective impact resistance. The surface hardening of bainitic steel during the cavitation inhibited the formation and growth of the pits and cracks, consequently the cavitation resistance of bainitic steel increased.

Formation of bainite in ferrous and nonferrous alloys through sympathetic nucleation and ledgewise growth mechanism

The subunits constituting a bainitic sheaf in an Fe-C-Cr-Si alloy were discovered by scanning tunneling microscopy (STM) to consist of sub-subunits, and sub-subunits were also composed of sub-sub-subunits. Detailed investigation shows that a bainitic relief is composed of many smaller reliefs, which correspond to a different structure of bainite,i.e., subunits, sub-subunits, and sub-sub-subunits. It is determined by STM that the surface relief arising from the formation of bainite in an Fe-C-Cr alloy istent shaped rather than an invariant plane strain (IPS) type of surface relief. Careful observation shows that the relief obtained from a sub-sub-subunit is also tent shaped. It is discovered by STM that an α1 plate,i.e., bainite formed in Cu-Zn-Al alloys, is composed of subunits. This is also demonstrated by transmission electron microscopy (TEM). The preceding results indicate that bainitic plates in Cu-Zn-Al alloys and bainitic subunits in steels are not the smallest structural units. Based on the preceding results on the ultrafine structure and the nature of surface relief accompanying bainite, it is proposed that the bainitic structure forms through a sympathetic nucleation and ledgewise growth (SNLG) mechanism. This article shows that the SNLG mechanism can be successfully applied to interpret the complicated structure of bainite.

Mn-Series Low-Carbon Air-Cooled Bainitic Steel Containing Niobium of 0.02%

A new hot rolled low-carbon air-cooled bainitic steel containing Nb of 0.02% has been developed based on alloying design of the grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) duplex steel. The as-rolled microstructure and mechanical properties of bainitic steel containing Nb of 0.02% were investigated by tensile test, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that adding 0.02% of Nb obviously improves the strength without sacrificing toughness of the FGBA/BG steel. Adding 0.02% of Nb not only refines the grain boundary allotriomorphic ferrite grains but also promotes the refinement of granular bainite including its bainitic ferrite and M/A island. Any Nb(C, N) has been hardly observed in the steel containing Nb of 0.02%. It is suggested that the strengthening mechanism of Nb of 0.02% can be mainly attributed to the effect of Nb in solution (solute drag-like effect) on the phase transformation rather than the precipitation strengthening of Nb(C, N) particles.

CCT Curves of Low-Carbon Mn-Si Steels and Development of Water-Cooled Bainitic Steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content, and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore, water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300 mm was cooled by water, and a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40 mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength, yield strength, and toughness (AKU at −20 °C ) of bar with diameter of 300 mm after water cooling were higher than 850 MPa, 620 MPa, and 65 J, respectively, and those of plate with thickness of 40 mm after water cooling were higher than 1000 MPa, 800 MPa, and 50J, respectively.