Furen Xiao

Continuous cooling transformation of undeformed and deformed low carbon pipeline steels

The continuous cooling transformation (CCT) behaviors of three low carbon pipeline steels containing the different carbon and alloy additions such as Mn, Nb, V, Ti and/or Mo were investigated in the undeformed and deformed conditions, respectively. The corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed, which almost involved the formation curves of bainitic ferrite, acicular ferrite, polygonal ferrite, and pearlite. It was found that with the exception of V, the aforementioned alloy additions played a significant role in suppressing the formation of polygonal ferrite and promoting the formation of acicular ferrite. Furthermore, hot deformation could also strongly promote the formation of acicular ferrite, that is, the temperature zone of acicular ferrite transformation was enlarged from 400-600 degreesC in the static CCT diagrams to 450-700 degreesC in the dynamic CCT diagrams. The corresponding cooling rate range of acicular ferrite transformation was significantly increased, and the island constituents in acicular ferrite became finer due to hot deformation

Effects of contents of Nb and C on hot deformation behaviors of high Nb X80 pipeline steels

The behavior of the flow deformation and the effects of contents of Nb and C on deformation behaviors of high Nb X80 pipeline steels during hot compression deformation were studied by thermal simulation test. The content of solid solution Nb was quantificationally studied during the reheating and hot deformation process, and the effects of change of solid solution Nb in steels on hot deformation behaviors were discussed. The results show that the contents of Nb and C have great effects on the flow stress behaviors of high Nb X80 pipeline steels. When the C content in steels is constant, the recrystallization activation energy increases from 387 to 481 kJ/mol with increasing the Nb from 0.082% to 0.13% (mass fraction). However, the effect of Nb is correlative to the C content, i.e. w(Nb)/w(C). When w(Nb)/w(C) decreases from 3.61 to 2.18, the recrystallization activation energy decreases from 481 to 434 kJ/mol.

Synthesis of Bulk Nanocrystalline CoNi Alloys and Study of Their Microstructure and Magnetic Properties

Bulk nanocrystalline materials (BCNMs) of cobalt-nickel alloys have been prepared by high-speed jet electrodeposition (JED). Furthermore, the influence of the concentration of additive in electrolyte on deposition rate, current efficiency, as well as the morphology, chemical composition, microstructure, and properties of CoNi alloys has been investigated. The experimental results show that the amount of the additive has little effect on the deposition rate, current efficiency and Co-content of the deposited alloys, but they have a remarkable effect on the microstructure and properties. As the concentration of additive increases from 0 to 2.5 g/l, the grain-size of deposition layer decreases from 14 nm to 5 nm, the microhardness increases from HV495 to HV675, and the soft magnetic properties of the deposition layers are sharply improved. However, when the amount of the additive is over 2.5 g/l, the influence of additive is significantly weaker. The results indicate that the JED method with additive in electrolyte is a very effective technique for the manufacture of the nanocrystalline CoNi alloy.

Ripening behavior of M 23 C 6 carbides in P92 steel during aging at 800 °C

The rapid coarsening of the M23C6 carbides has been held responsible for the creep fracture in 9–12Cr martensitic heat resistant steels. A commercial P92 steel was subjected to thermal aging at a high temperature of 800 °C to investigate the ripening behavior of the M23C6 carbides. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the microstructure evolution, especially the ripening process of the M23C6 carbides. The new concept of the effective mean size, dependent on the critical radius, was introduced to correct the measured mean size and then the Ostwald theory was applied to describe the ripening behavior of the M23C6 carbides. The ripening of the M23C6 carbides was revealed to be grain boundary diffusion controlled.

Effects of Chemical Composition and Hot Deformation on Continuous Cooling Transformation Behavior of Acicular Ferrite Pipeline Steels

As an optimal microstructure of pipeline steels, acicular ferrite is widely found in steels used in oil and gas pipeline transportation because it possesses both high strength and good toughness. In this paper, the microstructure of acicular ferrite and its continuous cooling transformation (CCT) diagrams of six steels with different carbon and alloy additions have been studied by using dilatometry, optical metallography. And the effects of different hot deformation processes on the CCT diagrams and microstructures have also been studied. Furthermore, the effects of microalloyed elements and hot deformation on continuous cooling transformation have been discussed. The results show that lower carbon content and alloy additions such as Mn, Nb, Ti, Mo, Ni and/or Cu in steels will promote the formation of acicular ferrite. The hot deformation promotes the acicular ferrite transformation and refines the microstructures of final products.

Effect of Dissolution and Precipitation of Nb on Phase Transformation, Microstructure, and Microhardness of Two High-Nb Pipeline Steels

Niobium (Nb) as an important microalloying element is widely applied in high strength pipeline steels. In this work, the continuous cooling transformation diagrams of two high-Nb steels with and without hot deformation were studied using a Gleeble 3500 thermal simulator. The amounts of dissolved Nb, undissolved Nb, and precipitated Nb were determined by inductively coupled plasma-atomic emission spectrometry. Results show that the increasing of Nb content in the high-Nb steels can restrain the prior austenite grain growth, dynamic, and/or static recrystallization; moreover, it can suppress polygonal ferrite transformation and promote acicular ferrite and bainite transformation, refining the microstructure and increasing the microhardness as a consequence. Nevertheless, the amplified Nb content in steels escalates trends of strain-induced Nb(C,N) precipitation. The increase in the amount of Nb(C,N) precipitates promote the polygonal ferrite and acicular ferrite transformation, while also decrease the microhardness. The results from this work show that the higher Nb content of 0.13% in the tested steel is unnecessary.

Effect of hot deformation and Nb precipitation on continuous cooling transformation of a high-Nb steel

In this work, the effects of hot deformation on continuous cooling transformation of a high-Nb steel were investigated on a Gleeble 3500 thermal simulator. The amounts of dissolved Nb were determined by inductively coupled plasma-atomic emission spectrometry. Furthermore, the effects of hot deformation and Nb precipitation on phase transformation were discussed. Results showed that high-Nb steel is suitable for acicular ferrite pipeline steels because the acicular ferrite microstructure can be obtained in a wide cooling rate range. Hot deformation strongly accelerates the polygonal ferrite transformation and increases the critical cooling rate to obtain a full acicular ferrite microstructure. Moreover, hot deformation markedly refines the final microstructure and improves the mechanical properties of acicular ferrite obtained at a high cooling rate. However, hot deformation can also promote Nb precipitation during holding and even cooling at low rates after hot deformation. Nb precipitation dramatically promotes the polygonal ferrite, weakens the effect of Nb in solution on phase transformation and strengthening, and decreases the microhardness.

Mechanical Properties of the Heat Affected Zone of an X100 Grade Pipeline Steel

The microstructure and mechanical properties of an X100 grade pipeline steel were investigated by the thermal simulation tests performed on a Gleeble-3500 thermal simulator. The results show that the steel has excellent weldability. Even if the weld heat input gets 40 kJ mm-1, the low impact toughness energy in welding coarse grain heat affected zone (CGHAZ) is still higher than 200 J. Yet, an embrittlement zone emerges in inter-critical HAZ (ICHAZ) at the peak temperature of 750 °C, and the lowest strength appears in fine grained HAZ (FGHAZ) at the peak temperature of 950 °C. Moreover, the strength and toughness decrease with the increase of heat input.

Effects of Induction Heat Treatment on Austenitic Transformation, Microstructure and Mechanical Properties of Pipeline Steels

The effects of heating rate, heating temperature and cooling rate on the microstructures and mechanical properties of four pipeline steels for high frequency electric resistance welded pipe have been studied by using a Gleeble-3500 thermo-mechanical simulator. The results show that the heating rates have an effect on austenizing phase transformation temperature (Ac1 and Ac3). It shows that there is a linear relationship between heating rate and austennizing temperature (Ac1 and Ac3) in the range of tested heating rate. With the heating temperature increasing, the strength property goes up, on the contrary, the strength begins to go down when the heating temperature exceeds 900 °C, then a lowest strength point appears on 925 °C in the testing scope. As the further increase of the heating temperature, the strength goes up again. Moreover, the cooling rate has a great effect on the microstructure and the mechanical properties. With the decrease of cooling rate, the strength decrease significantly, meanwhile, the microstructure becomes coarse, even the banded structure can be found. As the conclusion, the optimum heating temperature is 950 °C, and cooling rate is from 8.5 to 13 °C/s.

Effect of Additive on Microstructure and Property of Bulk Nanocrystalline Co-Ni Alloys Prepared by JED

The bulk nano-crystalline cobalt-nickel alloys have been prepared by the double refined methods (DRM) combined the high-speed jet electrodeposition (JED) with the additive added in jet electrolyte, and their microstructure and properties, such as the microhardness and magnetic properties, have been evaluated. The results indicate that the DRM is a very effective technique for grain size refinement and property improvement. The concentration of the additive added in the jet electrolyte has little effect on the Co-content in the deposited alloys, but they have a remarkable effect upon the microstructure and properties.