Materials Science Forum Vols. 654-656

Abstract: The dominant microstructures in low carbon bainite steels such as bainitic ferrite are non-equilibrium phases, which will tend to evolve into equilibrium phases when the steels are subjected to thermal disturbance. In-situ observation by optical and scanning electron microscopy was carried out in this investigation to track the evolution when the steels were isothermally held below A1 temperature. It is found that the primary polygonal ferrite grows slowly during isothermal holding, while bainitic ferrite changes rapidly into polygonal ferrite. Self-tempered bainitic ferrite would recover further and recrystallize. The lower the concentration product of carbon and niobium, the faster is the evolution.

Abstract: We have investigated effect of magnetic field at cryogenic temperature on the stability of austenitic phase in a sensitized SUS304 stainless steel. The sensitization of the steel was made by heat-treatment in vacuum at 973 K for 100h after the solution treatment at 1323 K for 0.5 h. Isothermal holding of the sensitized specimens was made under a static magnetic field of 5 T in the temperature range between 60 and 260 K for various holding times. As a result, the TTT diagram under the magnetic field corresponding to the formation of 0.5 vol. % of -martensite shows a double-C curve with two noses. We found that by the application of magnetic field, the nose temperature and the incubation time of upper part of the C-curve, that is due to the    transformation, become lower and shorter, respectively. On the other hand, the nose temperature of lower part of the C-curve, that is due to the      transformation, does not change under the magnetic field although the incubation time shortens by the application of magnetic field.

Abstract: The Japanese sword is produced from a special kind of steel called tamahagane, using a forge-fold operation repeated several times. The purpose of this study is to clarify the development of microstructures with successive forge-fold operations using tamahagane. Specimens under several stages of sword making have been investigated with optical microscopy, scanning electron microscopy and electron probe micro analysis methods. Microstructures have been found to be a combination of ferrite and pearlite with a lot of nonmetallic inclusions. The ferrite bands become finer and among other inclusions Fe2SiO4 takes a spherical shape with increasing recurrence of forge-fold operations.

Abstract: Field Emission Scanning Electron Microscopy with Electron Back-Scattering Diffraction (SEM-EBSD) and Optical microscopy were used to point out the microstructural features of a Japanese sword prepared from tamahagane steel using traditional method. A lath martensite structure, which is usually characterized by packet and block in a prior austenite grain, existed both on the surface and the cross-section of the sword. SEM-EBSD study revealed that the development of prior austenite grain and packet were not much distinctive but the blocks within the packets were fairly observed. It was found that the packet size increased with the prior austenite grain size but the increment was small. Vickers micro-hardness measurement revealed that the sharp end was comparatively harder than other sections of the sword. EPMA study showed that the average carbon content of the sword was around 1 mass% along with a variety of non-metallic inclusions. Formation of lath martensite structure in such high carbon steel is remarkable but comparable to 0.6 mass% carbon ordinary steel. It was realized that the traditional method of preparation using tamahagane as well as the higher content of carbon provided the extraordinary features to the Japanese sword different from the ordinary steel.

Abstract: Using the technique of colored metallography, the phase compositions of two medium-/low-carbon low-alloy carbide-free bainite steel used for railway crossings are obtained. The etching of the samples was carried out using the Lepera reagent and a mixed reagent of hyposulphite and picric acid respectively. The proportion of the bainite phase was calculated using an image processing system with an optical microscope and the residual austenite content was measured using XRD method. The martensite phase content can then be determined. It is found that the reagents can effectively and quantifiably analyze the phase composition of bainite steel and produce approximately the same results.

Abstract: We have investigated the effect of particle size on martensitic transformation by using single crystalline specimens of an Fe-30at%Ni alloy by scanning electron microscope (SEM) observation, X-ray diffraction(XRD), magnetic susceptibility and magnetization measurements. As a result, we have found that an athermal martensitic transformation changes to an isothermal martensitic transformation with decreasing particle size. Considering the result of the TTT diagram, we have estimated the driving force for the martensitic transformation and the cluster size for nucleation of the isothermal martensitic transformation based on the model previously proposed.

Abstract: The effect of initial microstructure, cold reduction ratio, and annealing temperature on the spherodization rate of SK85 high carbon steel sheet was investigated. High carbon steel sheet fabricated by POSCO was soaked at 800oC for 2 hr in a box furnace and then treated at 570oC for 5 min in a salt bath furnace followed by water quenching to obtain a fine pearlite structure. Cold rolling was conducted on the sheets of fine pearlite by reduction ratios of 20, 30, and 40 % and heat treatment for spheroidization was carried out at 600 and 720oC for the various time intervals from 0.1 to 32 hrs. Area fraction of spheroidized cementite was measured with an image analyzer as a function of cold reduction ratios and duration times.

Abstract: The influence of silicon on the spheroidization of cementite in hypereutectoid 1.0C-1.45Cr bearing steels has been investigated, on the basis of microstructural analysis and thermodynamic calculations. The silicon content was varied 0.25 to 2.00 in weight percent. Annealed at 790∼850°C for 6 hr, the 0.25Si and 1.00Si steels were entirely spheroidized at 790°C, while 1.50Si and 2.00Si steels at 830°C, respectively. This implies that the increase of silicon content in hypereutectoid steels retards the spheoridization of cementite. The thermodynamic calculations revealed that silicon atoms were partitioned into not cementite but austenite at annealing temperatures, and the increase of silicon content can raise the chemical potential of carbon atoms within austenite at austenite/cementite interfaces, causing the decrease of driving force for the diffusion of carbon atoms from cementite to austenite.

Abstract: Ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn-0.35N austenitic steels containing Ni and Cu was investigated by means of Charpy impact test and fractographic analysis. The commonly observed fracture mode of the specimens tested at -196 oC was transgranular cleavage-like brittle with flat facets occurring along {111} crystallographic planes, thereby leading to the occurrence of ductile-to-brittle transition. For all the steels investigated in the present study, the ductile-to-brittle transition temperature (DBTT) measured from Charpy impact tests was much higher by 90 to 135 oC than that predicted by empirical equation strongly depending on N content. The combined addition of Ni and Cu enabled the 18Cr-10Mn-0.35N steels to have the lowest DBTT, which could be explained by relatively high austenite stability and favorable effect of Cu as well as the absence of delta-ferrite.

Abstract: In order to investigate the susceptibility of steels to hydrogen embrittlement as a function of their microstructure X70 steel was chosen in different conditions: normalized transfer bar, as-received hot rolled strip and heat affected zone (HAZ). Notched and fatigue pre-cracked samples were subjected to electrochemical hydrogen charging to achieve 2 ppm hydrogen content. Three point bend tests were conducted on as-received and hydrogen charged samples. The results showed that HAZ samples are more susceptible to hydrogen embrittlement than the others. This was supported by fracture surface observations.