Papers by Author: Xin Lai He

Abstract: The influence of prior austenite grain size on the crystallography of allotriomorphic ferrite is investigated in a low carbon steel. The results show that as the prior austenite grain size decreasing, the fraction of allotriomorphic ferrites that do not keep K-S orientation relationship with any surrounding prior austenite grains is increased. It is observed that such ferrites usually form at the grain edges or grain corners. It is known that with the grain size decreasing, the fraction of grain edges and corners increases. It is suggested that the free energy of the defects at such nucleation sites is higher than that at grain faces, and the nucleation barrier of ferrite is lower. As a result, the possibility for the ferrite to form that does not have orientation relationship with all surrounding austenite grains is increased at such sites.

Abstract: The relaxation-precipitation-controlling phase transformation (RPC) technique after deformation at non-recrystallization zone to refine the intermediate transformation microstructure has been simulated on a Gleeble-1500 thermo-simulator. The optical microscope, SEMTEMPTA(particle tracking autoradiography) technique to reveal the boron distribution were employed to study the variation of austenite grain size and subgrain size, the features of microstructure after RPC, precipitation and the evolution of dislocation configuration during the relaxation and the boron distribution. The results show that after relaxation at non-recrystallization zone, the subgrain formed inside an original austenite grain. With the relaxation time increasing, the size of the subgrains increased and the misorientation also increased. During the cooling after the relaxation the boron can also segregate at the boundaries of subgrains and the boron segregation can reveal the subgrains forming in deformed austenite before phase transformation. It has been found that during the relaxation strain induced precipitates occurs and these precipitates can pin the subgrain boundary and make it more stable. Comparing the subgrain size demonstrated by PTA with the optical microstructure a conclusion can be drawn that the packet of bainite generally cannot break through the boundaries of subgrains, so the subgrain appearing at the relaxation stage can confine the growth of the microstructure during the transformation in succeeding and the final bainite is refined.

Abstract: The low carbon steels were smelted with special oxide introduction technique and the HAZ properties has been studied with thermal simulation. The optical microscope, SEM and TEM were used to analyze the composition, size and distribution of the inclusions, and the mechanical properties after thermal simulation were also investigated. The influence of oxide inclusions on the austenite grain size was also studied. The results show that after the smelting the inclusion is complex, in the core is Ti oxides about 1-3 micron and around it is MnS. When the reheat temperature is below 1000, the size of austenite grain is the same for experimental steel and base steel. However, when the reheat temperature is over than 1100, the size of austenite grains in experimental steel is one third of that in base steels. After thermal simulation, with the t_8/5 increasing the toughness of HAZ decreased. The austnite grain size also increased. The microstructure is composed of intergranular ferrite and intragranular acicular ferrite. Therefore by introducing the fine oxide inclusion to the steel the austenite grain was refined and during the phase transformation the acicular ferrite formed at inclusions at first. These two factors are the main causes to improve the toughness of heat affected zone for steels produced by oxide metallurgy technique.

Abstract: The influence of the oxide inclusions on the microstucture, properties and the Heat-affected-zone (HAZ) toughness of low alloy steels has been investigated. The low alloying steels were smelted with special oxide introduction technique and the properties of HAZ have been studied with thermo-simulation. The type, size and distribution of the non-metal inclusions in steels has been studied by optical microscope and scanning electronic microscope (SEM). The microstructure and mechanical properties of base metal and samples undergoing welding thermo-simulation were also analyzed. The results show that the inclusions in experimental steel are mainly complex inclusions composed of oxide containing Ti and MnS, these complex inclusions are distributed homogenously and their size is between 1 and 3 micron. The microstructure of HAZ consists of intragranular acicular ferrite (IAF), intergranular ferrite and small amount of lath bainite while the cooling time during the phase transformation between 800°C and 500°C (t_8/5) is short. After the thermo simulation with t_8/5 increasing the toughness of HAZ decreased and the size of prior austenite grain increased. If the size of the austenite grain decreased the amount of IAF also decreases.

Abstract: A Fe-0.05C-2.94Mn-1.87Si steel is heat treated using a two-stage isothermal holding process to obtain allotriomorphic ferrite and bainite. Two kinds of allotriomorphic ferrite are obtained, one with only carbon partitioning and the other, alloying element partitioning. It is observed that the allotriomorphic ferrite stimulates the adjacent bainite to select the similar variant on the side where near K-S relationship is maintained between ferrite and prior austenite. The longer the border length of the allotriomorphic ferrite, the larger the stimulated bainite area. The statistical measurement shows that the alltriomorhpic ferrite with alloying element partitioning stimulates such bainite variant selection as well as that with only carbon partitioning.

Abstract: By testing the mechanical properties, microstructure observation, accelerated corrosion test, the effect of alloying element nickel on microstructure and properties of weathering steels was studied. The results showed that nickel content from 0 to 0.44%（mass percent）have little influence on microstructure and mechanical properties of weathering steels.The corrosion resistance enhanced with increasing nickel content, and nickel content of 0.11% or more had significant enfluence on corrosion resistant performance of weathering steels.

Abstract: Back propagation (BP) neural network model was established, using the corrosion data of five kinds of recently developed bainite weathering steel and the commercial weathering steel 09CuPCrNi exposed in the offshore platform in Wanning. The influences of elements P, Cu, C and Cr on the corrosion behavior of weathering steel were studied according to the model. The experimental results indicate that the corrosion depth of bainite weathering steel corroded for 1 year could decline owing to the increasing contents of P, Cu and Cr in steel while C has little effect on the variation of the corrosion depth.

Abstract: . A low carbon steel is austenitized and isothermally held at 680°C to form allotriomorphic ferrite and followed by a holding at lower temperature to form bainite. The morphology of allotriomorphic ferrite/bainite interfaces is studied using optical microscope. Three kinds of combination are observed: Type I: interface on one side is clear while on the other side, unclear; Type II: unclear on both sides; Type III: clear on both sides. Clear interface indicates a large difference in the orientation between the bainite and the ferrite, and unclear interface, a very small difference. The statistical counting shows that the ratio of Type I is about 80-82%, and that of Type II, 7-8%, and Type III, 9-11%. It is observed that this ratio does not change with the austenite grain size and bainite forming temperature. And the clear and unclear side of allotriomorphic ferrite may have different influence on the nucleation rate of bainite at allotriomorhic ferrite/prior austenite interface.

Abstract: The properties and precipitation behavior of Cu-bearing steels have been investigated. The optical microscope and transition electron microscope were employed to study the influence of interrupted cooling and quenching temperature on the precipitation behavior. Also, the properties of samples with different quench processes were tested. The results show that when the steel is interruptedly cooled and quenched from 650-700°C, with the quenching temperature increasing the volume fraction of martensite becomes larger and the hardness becomes higher. When the microstructure is ferrite the second-phase precipitates occurs and they are proved copper-rich particles. However there are no obvious precipitates in martensite. The copper-rich second phase forms by the way of inter-phase precipitation.

Abstract: To improve the toughness and weldability, the carbon content of the steels has to be deduced, and more and more attention has been attracted to the low carbon and ultra-low carbon steels. To strengthen the microstructure Cu and Nb-bearing steels are developed. However, the knowledge on influence of combined addition of Cu and Nb is still in lack. The microstructure and mechanical properties are studied in the 6-mm thick as-rolled and tempered ultra-low carbon steel plates with varied copper and niobium content. The microstructure and mechanical properties are studied in the 6-mm thick as-rolled and tempered ultra-low carbon steel plates with varied copper and niobium content. The experimental results show that if niobium is added without copper, the increase of niobium addition does not have a significant influence on the phase transformation and mechanical properties before tempering. The strength and toughness of those copper-free niobium steels do not vary significantly after tempered at different temperatures, while the strength of niobium steels with 1.8% copper added increases after tempered in the range of 450-650°C and reaches a peak at 500-550°C. If combined with 1.8% copper, the increase of niobium addition from 0.08% to 0.16% improves the hardenabililty and strength significantly, and the strength peak after tempering moves to a lower temperature. The strength of air-cooled niobium steels with 1.8% copper added is usually higher than those water-cooled, while after tempered at a proper temperature, the strength of the latter becomes higher than the former.