•The dissolved Nb contents in austenite and acicular ferrite were measured.•The JMAK equations of Nb precipitation in austenite and ferrite were established.•The Nb carbonitride precipitation in austenite affects the phase transformation.•Nb carbonitride precipitation in ferrite exhibits high strengthening potency.The dissolved Nb contents in four high-Nb steels were quantitatively measured after hot deformation at 925 °C and reheating at 600 °C. The Nb carbonitride precipitation kinetics in austenite and in acicular ferrite were studied, and the effects of Nb carbonitride on the precipitation hardness were discussed. The results show that the nucleation rate of Nb precipitation in austenite is strongly depended on the dissolved Nb content in matrix. The parameter n of the JMAK equation is increased with the increase of the dissolved Nb content in matrix. The n value of the factor of nucleation rate of Nb carbonitride precipitated in acicular ferrite is related to the dissolved Nb and C contents in matrix. Nb carbonitride precipitated in austenite decreases the hardness of the final products by accelerating the phase transformation. By contrast, Nb carbonitride precipitated in acicular ferrite exhibits high precipitation strengthening potency.

Fatigue properties are important parameters for the safety design and security evaluation of gas transmission pipelines. In this work, the fatigue life at different stresses of full-thickness X80 pipeline steel plates with a ferrite/bainite dual-phase microstructure was investigated using a MTS servo-hydraulic universal testing machine; the fatigue crack propagation rate was examined with CT specimens by using an INSTRON 8874 testing machine. Results indicate that fatigue life increases as maximum stress decreases; as the maximum stress decreases to the maximum operating stress (440 MPa), the fatigue life is approximately 4.2×105 cycles. The fatigue crack of the full-thickness fatigue life specimens is generated at the surface of rolled steel plates and then the crack propagates and grows inward until a fracture is formed. During fatigue crack growth, a transitional turning point appears in the curve of da/dN with ΔK in the Paris region. The transitional turning point that divides the Paris region to two stages is approximately ΔK≅30 MPa m1/2. The change in the growth rate (da/dN) is related to the variation of the crack path and in the fracture mode because of the possible microstructural sensitivity of fatigue crack propagation behavior. This study also discussed the effect of duple phase ferrite/bainite microstructure on fatigue crack initiation and propagation.

This study designed post-weld heat treatments, including reheating and tempering, associated with hot bending to investigate the microstructures, toughness, and hardness of two weld metals with different Ni contents (<1 wt pct level). The results indicated that a high Ni content decreased the ferrite transformation temperature and increased the proportion of acicular ferrite (AF). Furthermore, a high Ni content promoted the martensite/austenite (M/A) constituent formation after reheating. The promotion of the M/A formation increased the number of cementite particles, and accelerated cementite coarsening during tempering. The large-angle grain boundary density from the AF improved the toughness despite the negative effect of cementite. The strengthening contributions were calculated, and the grain refinement was the greatest. The high Ni content decreased the effective grain size with a 2 deg tolerance angle, thus enhancing the grain refinement contribution.

To research the effect of large precipitates (size > 0.2 μm) on strain-induced dynamic transformation, the variation of V contents in large precipitates has been investigated quantitatively in two V–Ti micro-alloyed steels. The results showed that high N content promoted V precipitation on the surface of Ti large precipitates rapidly. Subsequently, large precipitates containing V induced the formation of intragranular ferrite, which accelerated the dynamic transformation process remarkably, promoted the occurrence of continuous dynamic recrystallization of ferrite and improved the refinement effect.

To obtain necessary information for the simulation of high speed wire production process, the effect of grain refinement and precipitation strengthening on two high speed wire rod steels with different vanadium and nitrogen contents was investigated by continuous cooling transformation (CCT) characteristics. CCT curves were constructed by the dilatometer test and microscopic observation. Results showed that the formation of intra-granular ferrite (IGF) could refine grain remarkably and accelerate the ferrite transformation. Schedules for high speed wire production process focused on the effect of cooling rate. Ferrite grain was refined by increasing cooling rate and the formation of IGF. The microhardness calculation revealed that the steels were strengthened mostly by a combined effect of grain refinement and precipitation hardening. Degenerated pearlite was observed at lower transformation temperature and the fracture morphology changed from cementite lamellar to nanoscale cementite particle with increasing cooling rate. Based on the analysis above, an optimal schedule was applied and the microstructure and microhardness were improved.

Mechanical properties of weld metal are the key factors affecting the quality of heavy-wall X80 hot induction bends. The effects of bending parameters on the mechanical properties of weld metal for hot bends were investigated by simulation conducted on a Gleeble-3500 thermal simulator. Continuous cooling transformation (CCT) diagrams of the weld metal were also constructed. The influences of hot bending parameters (such as reheating temperature, cooling rate, and tempering temperature) on the microstructure and mechanical properties of weld metal were also analyzed. Results show that the strength of all weld metal specimens is higher than the value indicated in the technical specification and increases with the increase of reheating temperature, cooling rate, and tempering temperature. The impact toughness is apparently related to the variation of reheating temperature, cooling rate, and tempering temperature.

The effects of Nb on strain-induced precipitation and softening were quantified by means of the Gleeble-3500 thermo-mechanical simulator and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The results show that Nb cannot completely dissolve during the reheating processes in the high Nb steels. The undissolved precipitates act as heterogenous nucleation sites for the precipitates induced by the strain, and prolong the strain-induced precipitation start time shown in the stress relaxation curves. The strain-induced precipitation start time is linearly related to the content of supersaturated Nb (the difference between dissolved Nb content after soaking and dissolved Nb content in equilibrium at corresponding deformation temperature). Increasing the content of supersaturated Nb can shift the nose of the precipitation C-curve to shorter time and higher temperatures. The effect of content of supersaturated Nb on precipitation increases along with the decrease of temperature. The softening ratio is linear with dissolved Nb content before the strain-induced precipitation and with precipitable Nb content after the strain-induced precipitation. The precipitates induced by strain have more strong retarding effect on the static recrystallization than the solid solution of Nb atoms. However, with the increase of the average precipitate radius, the retarding effect of precipitate rapidly decreases.Highlights► The effects of Nb in solution or precipitates on static softening were researched. ► The Ps is linearly related to the content of supersaturated Nb. ► The softening ratio is linear with dissolved Nb content before the Nb precipitation. ► The softening ratio is linearly related to the content of precipitated Nb. ► The precipitates have more strong effect on retarding the static recrystallization.

The Na2O–B2O3–SiO2 glass with low thermal expansion coefficient and melting point is suited to manufacture vitrified bond superhard grinding tool. However, its bending strength, fracture mode, and chemical durability are lower. Some additives, such as alumina, calcium and/or zinc oxides, and so on, were added in the vitrified bond in order to improve the property of grinding tool. In this work, a new additive of nano-AlN was added in Na2O–B2O3–SiO2 vitrified bond for improving microstructure and properties of the vitrified bond. The Na2O–B2O3–SiO2 vitrified bonds with and without nano-AlN were sintered at different atmosphere. The effects of nano-AlN and sintering atmosphere on the microstructure and properties of Na2O–B2O3–SiO2 vitrified bonds were investigated. The results show that the highest bending strength, impact strength, and wear resistance were obtained when the Na2O–B2O3–SiO2 with nano-AlN of 6% was sintered at argon, because nano-AlN as crystallization nucleus promotes the crystallization of α-SiO2, β-SiO2, and tridymite during sintering process. The microstructures of the crystalline phases were refined. When the vitrified bonds with nano-AlN were sintered at air, the nano-AlN powder was oxidated and decomposed into Al2O3 and gas. The decomposition of nano-AlN increases slightly the bending strength and wear resistance of the vitrified bond due to the increasing amount of crystalline phase, but the decomposition of nano-AlN increases the amount of small close pores and size of crystalline phase, which results in the decreasing of impact strength of vitrified bond.

The effects of alloying elements in welding wires and submerged arc welding process on the microstructures and low-temperature impact toughness of weld metals have been investigated. The results indicate that the optimal contents of alloying elements in welding wires can improve the low-temperature impact toughness of weld metals because the proeutectoid ferrite and bainite formations can be suppressed, and the fraction of acicular ferrite increases. However, the contents of alloying elements need to vary along with the welding heat input. With the increase in welding heat input, the contents of alloying elements in welding wires need to be increased accordingly. The microstructures mainly consisting of acicular ferrite can be obtained in weld metals after four-wire submerged arc welding using the wires with a low carbon content and appropriate contents of Mn, Mo, Ti-B, Cu, Ni, and RE, resulting in the high low-temperature impact toughness of weld metals.

Thermomechanical treatment as an effective method has been carried out in order to obtain a better combination of high strength and excellent toughness. In this work, the hot deformation behavior of 86CrMoV7 steel has been studied, and then the process of ausforging has been designed. The microstructures and mechanical properties of 86CrMoV7 steel by (1) direct quenching after forging (ausforging) and tempering, (2) quenching-and-tempering and spheroidizing annealing have been investigated. The results show ausforging-and-tempering provides high strength but same toughness as classical quenching-and-tempering.

In the present work, the dispersive stability of nano-SiO2 with coupling agent A-174 has been investigated, then in situ nano-SiO2-MMA/BA/MAA copolymerization has been polymerized. Furthermore, the effects of nano-SiO2 on performance of poly(MMA/BA/MAA)/EP have been studied, too. The results show that the nano-SiO2 is soft agglomerate, which diameter is about 2.08 μm. But when it was oscillated by ultrasonic wave with adapt separating agent in low molecule solution, nanometer grade nano-SiO2 particle could be attained, the average particle diameter is about 45 nm. In addition, the dispersive stability of nano-SiO2 as treated by A-174 in polymerization system increase. As 3% of nano-SiO2 treated by 4% of A-174 is added, the shear strength of the thermosetting acrylic–epoxy resin increase about 68%, up to 20.07 MPa, and its heat decomposition temperature increase from 332 °C to 386 °C, furthermore, the excellent water resistance and transparency have been obtained.