Yankang Zheng

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.

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.

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.

Formation mechanism of bainitic ferrite and carbide

Superledges on the broad faces of bainitic ferrite plates have been observed with transmission electron microscope (TEM). The observed superledges, ranging from less than 1 to 24 nm in height, are imaged in three dimension by way of tilt operation under TEM. Also, an array of smaller superledges are observed to pile up in front of a secondary phase. Pileup of superledges in front of a barrier is indicative of the mobility of an individual superledge. The precipitation of carbide associated with bainitic ferrite is also studied. It is observed that a carbide of a wedgelike shape may exist in front of a superledge with its tip(i.e., thinner end) penetrating the austenite and its root (the other end) originating at α:γ boundary. This condition indicates that the observed carbides may nucleate at the austenite side of α:γ phase boundary and grow toward austenite matrix.

Application of atomic-force microscopy to metallography

Atomic-force microscopy (AFM) was applied to study the surface undulations (reliefs) resulting from the martensitic transitions in Fe-Ni-C and Cu-Zn-Al alloys. Both the morphology of the surface undulations determined by a profile line across the surface and the height of the undulation could be measured directly and easily with AFM. The results show that AFM can be a powerful tool in metallography.

Use of scanning tunneling microscopy in metallography

Abstract Scanning tunneling microscopy was used for the first time to study the bainitic microstructure of an austempered Fe-1.0C-4.0Cr-2.0Si alloy when certain experimental procedures were taken to minimize surface absorption and oxidation. Specimen preparation methods and observation procedures required for the study of easily oxidized materials by scanning tunneling microscopy, and the interpretation of the scanning tunneling image, including microstructural contrast, phase morphologies, etc., are discussed. The results demonstrate that scanning tunneling microscopy, which is used mainly in studying special materials, such as pure gold, single crystal silicon, etc., is another powerful metallographic tool.

Study of ledge structures of bainites in the Cu-base 27.1Zn-3.6Al alloy

Abstract The fine structure of bainite ( α 1 platelets) in the Cu-27.1Zn-3.6Al alloy was studied by means of transmission electron microscopy. It was found that no stacking faults appear during the earliest growth stage and there exist three-dimensional ledges on the broad faces and (or) at the edges of the α 1 -plates at the different formation temperatures. The results demonstrate that the growth of bainites is governed by a diffusion-controlled ledgewise mechanism.

Surface relief associated with martensite and bainite in a Cu–Zn–Al alloy measured by atomic force microscopy

Atomic force microscopy was employed to quantitatively study the surface relief accompanying martensite and bainite in a Cu–Zn–Al alloy. It is demonstrated that the surface relief angle associated with martensite is 14.3°, in good agreement with the theoretical result deduced from the phenomenal theory of martensite crystallography (PTMC). However, the surface relief angle associated with bainite is 2.0°–3.2°, which disagrees with the PTMC result. This indicates that the transformation mechanism of bainite is different from that of martensite. The fine structures of the surface relief associated with martensite and bainite are also investigated. The surface relief of martensite is composed of the small parallel relief caused by small martensite plates, and that of bainite is composed of small cells induced by subunits in bainite.