Yoshinori Shiota

Structure and mechanical behavior of heavily drawn pearlite and martensite in a high carbon steel

Neutron diffraction measurements have revealed that cementite peaks disappear in a pearlite steel with drawing and that the residual intergranular stresses are generated. The diffraction profiles in a heavily drawn specimen suggest the tetoragonality with a small c/a in the ferrite matrix. Although cementite was hardly observed in the heavily drawn specimen, its c/a value determined by neutron diffraction and mechanical behavior are quite different from those of as-quenched martensite. The changes in hardness and c/a with annealing or tempering were also different between heavily drawn pearlite and marteniste. Hence, most of carbon atoms do not exist inside the ferrite lattice in the drawn pearlite and multi-scaled heterogeneous plastic deformation in pearlite seems to affect the asymmetry in the diffraction profile. Fracture behavior and hardness change with tempering is different in the two microstructures.

Inverse pole figure mapping of bulk crystalline grains in a polycrystalline steel plate by pulsed neutron Bragg-dip transmission imaging

A new mapping procedure for polycrystals using neutron Bragg-dip transmission is presented. This is expected to be useful as a new materials characterization tool which can simultaneously map the crystallographic direction of grains parallel to the incident beam. The method potentially has a higher spatial resolution than neutron diffraction imaging. As a demonstration, a Bragg-dip neutron transmission experiment was conducted at J-PARC on beamline MLF BL10 NOBORU. A large-grained Si–steel plate was used. Since this specimen included multiple grains along the neutron beam transmission path, it was a challenging task for existing methods to analyse the direction of the crystal lattice of each grain. A new data-analysis method for Bragg-dip transmission measurements was developed based on database matching. As a result, the number of grains and their crystallographic direction along the neutron transmission path have been determined.

Neutron Diffraction Profile Analysis to Determine Dislocation Density and Grain Size for Drawn Steel Wires

Dislocation density and crystallite size of steel wires with various carbon concentrations and drawing strains were determined by profile analyses for neutron diffraction profiles. The density is found to increase while the size decreases with increasing of carbon concentration and/or drawing strain. Both of the Bailey-Hirsch relation and Hall-Petch relation hold for the present results to suggest that these two are not independent., i.e., indicating an identical strengthening mechanism from a different point of view.

Microstructure and Residual Strain Distribution in Cast Duplex Stainless Steel Studied by Neutron Imaging

The neutron imaging and diffraction instruments at Materials and Life Science Experimental Facility (MLF) at J-PARC are expected to play an important role in the microstructure characteristic evaluation of steel materials further for industry application. Neutron transmission spectrum measured at a neutron imaging detector coupled with time-of-flight (TOF) method at a pulsed source, can quantitatively and non-destructively visualize the spatial distributions of the wider area by 2D mapping of textures and the microstructures information inside a relatively thicker material than the traditional electron, X-ray and neutron experiments. In this study, neutron imaging experiment was performed using NOBORU, BL10 of MLF at J-PARC. Four kinds of cast duplex stainless steel with ferrite and austenite microstructure were studied here, which were produced by different casting method at different temperature. Firstly, a two-dimensional scintillation detector using wavelength-shifting (WLS) fibers [1] with pixel size of 0.52mm × 0.52mm and illuminated area 55mm × 55mm was used for data collection. Then, measurement by Micro Pixel Chamber (μPIC)-based neutron imaging detector [2] having higher spatial resolution about 0.2mm was conducted. Data analysis code RITS (Rietveld Imaging of Transmission Spectra) [3] will be used for microstructure including crystalline phase, lattice strain, crystallite size, texture evaluation. This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Development of Energy-Selective Neutron Imaging

A neutron transmission image gives different features depending on neutron energy. Therefore, energy-selective imaging has become increasingly popular for obtaining comprehensive information. Neutron imaging using a pulsed neutron source can offer energy-selective imaging much more easily than a reactor neutron source, since it can use the time-of-flight method for energy analysis. Furthermore, by analyzing a transmission spectrum we can obtain physical quantities such as crystallographic information, elemental information, temperature, magnetic field, and so on. Such a spectrum analysis method, namely spectroscopic imaging performed at a pulsed neutron source, is a useful method for the material analysis and inspection of industrial products. Here, the principle of the method is explained and recent results are presented for a quenched iron rod, a hydrogen storage material, and elemental distributions in stainless steel.

Radiographic and Tomographic Neutron Bragg Imaging for Quantitative Visualization of Wide Area Crystalline Structural Information

Recent status of the technical development of the Bragg-edge neutron transmission imaging and its application to material science is presented. The neutron Bragg imaging has the advantages in measuring large area with reasonable spatial resolution, and it is a non-destructive method capable of looking inside a bulk material. Therefore, various information that are quite different from EBSD, synchrotron microtomography and X-ray/neutron scattering can be obtained by this method. We carried out quantitative imaging to obtain crystalline microstructural information in ultralow-carbon steels that received the high pressure torsion (HPT). The real-space distributions of texture and grain/crystallite size of HPTed steels of four torsion numbers were quantitatively visualized at once. As a result, we could deduce unique distributions of microstructural information depending on each torsion number, and correlated them with real-space distributions of the Vickers hardness. We also successfully developed a versatile strain tomography technique that can obtain tensor values for strain although traditional CT techniques can deal with only scalar values. The new CT algorithm, the tensor CT method, is based on our original algorithm called FBP-EM. The strain tensor tomography using FBP-EM was successfully applied for the experimental measured result obtained with the VAMAS neutron strain analysis international standard sample.

Dissolution of Cementite Plates by Drawing, Re-Precipitation with Annealing and Corresponding Changes in Tensile Behavior in a Pearlite Steel

The microstructural change with drawing and subsequent annealing for a patented pearlite steel was investigated by means of neutron diffraction. The dissolution of cementite plates with drawing and re-precipitation of spherical cementite particles with annealing after sever drawing were observed. In situ neutron diffraction during tensile loading was performed and it is revealed that the strengthening mechanism of the specimen without cementite differs from that for a ferrite-cementite steel where the load transfer is a main mechanism. The possible strengthening mechanism for the heavily drawn specimen is proposed.

Neutron Diffraction Measurement of Thermal Residual Stresses Caused by Solution Treatment for a SUS 304 Stainless Steel Mechanical Part

Neutron diffraction technique was employed to measure stress distribution in an interior region of a SUS304 rotating mechanical component with a fringe after solution treatment and after subsequently conducted stress-relief annealing. A coupon with 3x3x3 mm was prepared by spark cutting to determine stress-free spacing d0. The elastic strain was determined by using the measured (111) spacing d and d0 by (d - d0)/d0. Then the obtained strains were input into the general Hooke’s law to calculate stresses. The residual hoop stress near the surface is compressive which is lowered in the inner part. The residual stresses near the center of the component are hydrostatic tensile stresses. These residual stresses were not relaxed by the annealing treatment recommended in a handbook. Hence, more appropriate stress relief treatment has to be developed. In case of a cast specimen, coarse grains with a strong texture made it difficult to determine residual stresses precisely.