Junro Kyono

Self-healing Effect of Boron Nitride Precipitation on Creep Cavitation in Austenitic Stainless Steel:

In this study, the precipitation of boron nitride (BN) on creep cavity surfaces during creep and its beneficial effect on the creep rupture properties are reported for a type 304 austenitic stainless steel. In the conventional 304 stainless steel, the trace of soluble S segregates strongly on the creep cavity surfaces and promotes its growth during creep. In the modified 304 stainless steel added with B, Ce, and Ti, the trace of soluble S is removed effectively by the formation of Ce and Ti sulfides, and the segregation of S on the creep cavity surface is replaced by the precipitation of the BN compound. As BN is very stable at high temperatures, its precipitation on the cavity surface is expected to suppress the creep cavity growth rate. It is suggested that the precipitation of BN on the creep cavity surface provides austenitic stainless steel with the function of self-healing for creep cavitation with an associated increase in the creep rupture strength and ductility.

Some chemical and microstructural factors influencing creep cavitation resistance of austenitic stainless steels

Creep cavitation in materials is greatly influenced by trace elements. To enhance creep cavitation resistance, the chemical composition of 304, 321, 347 austenitic stainless steels was modified with the addition of minute amounts of boron and cerium. The addition of boron and cerium to type 304 stainless steel led to an increase in its creep rupture life with an associated decrease in creep rupture ductility. The addition of boron and cerium to the titanium-containing 321 steel and niobium-containing 347 steel was found to increase their creep rupture life and ductility. Creep cavitation was highly suppressed in the 347 and 321 steels with the addition of boron and cerium. The chemistry of the grain boundary and creep cavity surface was analyzed by Auger electron spectroscopy. Extensive sulphur segregation was observed on the grain boundary and cavity surface of the steels without boron and cerium addition and even in the 304 steel containing boron and cerium. In the boron- and cerium-containing 347 and 321 steels, respectively, segregation of elemental boron and the BN compound on the cavity surface were observed. These segregations reduced cavity growth rate substantially in these steels and BN segregation was found to be more effective in reducing cavity growth rate than boron segregation. Cerium acts as a getter for soluble sulphur in the steels by precipitation of ceriumoxysulfide (Ce2O2S) to facilitate the segregation of boron on the cavity surface.

Self-healing of creep cavities by boron segregation and boron nitride precipitation autonomously developed during high temperature use

In heat resisting steels, creep cavities are formed at grain boundaries through long time use at high temperatures and stresses. These creep cavities grow along grain boundaries, lead to grain boundary cracks by linking up each other, and cause the low ductility and premature fracture. The long time creep rupture properties of heat resisting steels depend mainly on growth of creep cavities. As creep cavities are thought to grow by diffusive transport of matter from the cavity surface to the grain boundary, a surface control of creep cavities may be very effective for suppression of the creep cavity growth. In this study, the chemical composition of austenitic stainless steels have been modified with the addition of minute amount elements such as boron and cerium with the aim to suppress creep cavitation by segregation of boron and precipitation of boron nitride onto creep cavity surface. Chemistry of creep cavity surface was analyzed by Auger electron spectroscopy. Extensive sulfur segregation was observed on creep cavity surface of the steels without boron and cerium addition. In the modified steels, the segregation of elemental boron and the precipitation of boron nitride compound were observed on the creep cavity surfaces. The segregation and the precipitation were thought to suppress the surface diffusion of creep cavity, since both boron and boron nitride are very stable at high temperatures. Cerium acted as a getter for soluble sulfur in the steels by the precipitation of ceriumoxysulfide (Ce 2 0 2 S) to facilitate the segregation of boron and precipitation of boron nitride. The segregation of boron and the precipitation of boron nitride reduced creep cavity growth rate substantially, and improved markedly long time creep rupture strength coupled with long time ductility. It may be said that the segregation and the precipitation provide the steels with the self-healing function for cavitation, since they are autonomously developed during the high temperature use.

Preparation of new PTCR material by particle electrification processing

The purpose of the present work is to develop the materials with new functions by combining two kinds of particles electrified reciprocally. This paper reports a preparation method and the positive temperature coefficient of resistivity (PTCR) properties of complex particles consisting of semiconductive BaTiO3 granules and metallic indium powder particles. The conclusion obtained by the present experiment are as follows. (1) Vibrating cylindrical electrode can forced-electrify metal, semiconductor and insulator particles positively or negatively. (2) When the particles electrified reciprocally are mixed in the same region at the same time, complex particles can be created by the electrostatically attractive force working between the two kinds of particles. (3) Indium-semiconductive BaTiO3 complex particles made by this processing offer the new PTCR material which can be used in arbitrary shapes by filling and packing or drawing and painting.

Application of Forced Electrification for Preparation of Complex Particles

The purposes of the present work are to describe the importance of the application of forced electrification on complex particle preparation and to develop new materials with multiple functions by combining two kinds of particles electrified reciprocally. This paper reports a preparation method and the positive temperature coefficient of resistivity (PTCR) characteristics of composites consisting of semiconductive BaTiO3 particles and metallic indium powder. The forced electrification technique was applied to a complex particle preparation process and confirmed to be effective as follows: 1. A vibrating cylindrical electrode can force-electrify metal, semiconductor and insulator particles with positive or negative charges. 2. When two kinds of particles electrified reciprocally are mixed in the same region at the same time, complex particles can be produced by the electrostatically attractive force working between them. 3. Indium-semiconductive BaTiO3 complex particles made with this processing offer a new PTCR material which can be used in arbitrary shapes by simple packing. This behavior cannot be accomplished by the conventionally sintered PTCR materials.

Self-healing of damage in heat resisting steels

Material damage is too fine to be detected by non-destructive tests and difficult to be repaired during use. Therefore self-healing of damage is most desirable to improve the reliability of materials and structures. In heat resisting steels, creep cavities nucleate at grain boundaries by long time use at high temperatures. These creep cavities grow along grain boundaries, from grain boundary cracks by linking up each other and cause the premature and low ductility fracture. Therefore long time creep rupture life and ductility chiefly depend upon the behavior of nucleation and growth of creep cavities. If the growth of creep cavities could be suppressed, the creep rupture properties of creep rupture life and ductility should be improved due to prevention of the premature fracture. Ordinary austenitic stainless steels contain sulfur as impurity and the sulfure segregates preferentially to creep cavity surface because of its high surface actiivty. It is possible to remove sulfur almost completely by doping cerium and adding titanium to the austenitic stainless steels. By adding boron and nitrogen, boron nitride precipitates on creep cavity surface. It was thought that the boron nitride on creep cavity surface suppresses creep cavity grwoth and improve creep rupture life and ductility by its healing effect on cavities.

Self-healing of creep damage in heat resisting steels

In heat resisting steels, micro holes, called creep cavities, are formed at grain boundaries by long term use at high temperatures. These creep cavities grow along grain boundaries, form grain boundary cracks by linking up each other anc cause low ductility and premature fracture as shown in Fig. 1. Therefore long term creep rupture strength and ductilities chiefly depend upon the behavior of nucleation and growth of creep cavities. If the growth of creep cavities could be suppressed, creep rupture strength and ductilities should be improved remarkably. Present work is intended to propose a self-healing process for the cavitation, and improve the creep rupture properties by the self-healing. It is thought that chemical compound of BN precipitates at inside surface of creep cavity by addition of B and N to heat resisting steels. As the BN is very stable at high temperatures, the precipitation of BN at creep cavity surface is expected to suppress the creep cavity growth and bring about the healing effect on the cavitation.

I-V characteristics of the contact interface in a semiconductive BaTiO3-In composite particle

Abstract The authors previously fabricated semiconductive BaTiO 3 -In composite particles. The aggregate could be used as a new positive temperature coefficients (PTC) material, which could be used in arbitrary shapes, differing from the conventional rigid PTC materials. In the composite particle, the interface between the semiconductive BaTiO 3 particles and indium particles plays an important role. In this work I-V characteristics of the interface are investigated in detail. The conclusions obtained in this research are as follows. (1) The existence of indium particles at the interface between two semiconductive BaTiO 3 particles lowered the electric resistance markedly. This effect was ascribed to the good plasticity and a low value of the work function of indium. (2) In-Ga eutectic liquid alloy and physically vapor-deposited indium film satisfied Ohms law and did not form a Schottky barrier at the interface with the semiconductive BaTiO 3 material. (3) Physically vapor-deposited gold film formed a high electric resistance at the interface with the semiconductive BaTiO 3 material. This high resistance might be caused by a Schottky barrier.