Kenji Abiko

Site competition between sulfur and carbon at grain boundaries and their effects on the grain boundary cohesion in iron

Grain boundary segregation in iron-sulfur-carbon alloys containing up to 100 wt ppm sulfur and up to 90 wt ppm carbon has been investigated with Auger electron spectroscopy (AES). The results show the site compctition on grain boundaries between the segregation of sulfur and carbon. The segregation energy of sulfur is estimated to be 75 kJ/mol. Impact tests of these alloys were carried out. Iron-sulfur alloys with less than 20 wt ppm carbon fractured by the intergranular mode with high ductile-brittle transition temperatures (DBTT’s). Addition of up to 90 wt ppm carbon to the binary alloys prevented the intergranular fracture caused by the grain boundary segregation of sulfur, and decreased the DBTT. Carbon, when segregated to grain boundaries, drives sulfur away from the boundaries and also increases the grain boundary cohesion. The DBTT values of the iron-sulfur-carbon alloys are analyzed in terms of the degree of grain boundary segregation of sulfur and carbon. It is shown that sulfur decreases the grain boundary cohesion of iron more severely than phosphorus if compared at the same degree of grain boundary segregation.

Formation of σ Phase in Highly Purified Fe–Cr Alloys

The formation behavior of the σ phase in Fe–Cr alloys was investigated using highly purified Fe–Cr alloys containing between 40 and 50 mass% chromium. The amount of σ phase was determined by optical microscopy. In annealed highly purified Fe–40 and 45 mass% Cr alloys, the σ phase nucleates at the surface of the specimen and grows inward, and the formation of this phase is finished completely within 360 ks. In annealed highly purified Fe–50 mass% Cr alloy the nucleation rate and the growth rate of the σ phase were extremely small. The formation of the σ phase was suppressed by purification, by the reduction of nitrogen, and by the presence of a small amount of carbon.

Preparation of 10 kg Ingot of Ultra-Pure Iron

A new induction melting furnace with a cold-copper crucible has been designed and constructed using ultra-high vacuum technology. The main chamber can be evacuated to 6.7 × 10—8 Pa as the base pressure. The furnace can melt high-purity iron of 10 kg, at most, within 300 s. The total pressure becomes less than 4 × 10—6 Pa during melting of iron under the best condition. As starting materials for melting, high-purity electrolytic iron containing impurities of 25.2 massppm which is determined after the analysis of 18 elements was prepared. The content of gaseous impurities such as carbon, nitrogen, oxygen, sulfur and hydrogen is 19.6 massppm in total. It is concluded that ultra-high vacuum melting of iron is quite useful to increase the purity of iron. The total content of impurities in iron is reduced from 25.2 to 12.6 massppm by melting under the atmosphere of 7.5 × 10—6 Pa. The amount of gaseous impurities can be reduced from 19.6 to 10.1 massppm in total. Especially, the amount of oxygen is reduced from 14.1 to 7.5 massppm. These amounts of gaseous impurities are almost the limit of detection by conventional methods of analysis.

Effect of boron on the grain boundary segregation of phosphorus and intergranular fracture in high-purity Fe-0.2 Pct P-B alloys

The effect of boron on the grain boundary segregation of phosphorus in a high-purity Fe-0.2 pct P alloy has been investigated by Auger electron spectroscopy (AES). The segregation of phosphorus decreases markedly with the segregation of boron; phosphorus atoms are replaced by boron atoms at grain boundaries. The free energy of segregation of boron at 1073 K is determined to be 100 kJ/mol. The effect of boron on the phosphorus-induced intergranular fracture (IGF) has been examined with impact testing, and the fractography has been studied with scanning electron microscopy (SEM). Addition of 12.5 wt · ppm boron completely prevents the IGF induced by the segregation of phosphorus and decreases the ductile-brittle transition temperature (DBTT) by about 170 K when quenched from 1073 K. The suppression of the IGF due to the addition of boron is caused by two mechanisms. One is the increased grain boundary cohesion of iron caused by the segregated boron as its inherent effect. The other is the decrease in the segregation of phosphorus caused by the segregation of boron. The former has been shown to be more effective than the latter in suppressing the IGF.

Estimation of the thickness or composition of a covering layer on a solid by XPS or AES

Abstract A method is proposed for semi-quantitative determination of the thickness or composition of a covering layer on a solid by XPS or by micro-AES using neither a series of standard samples nor argon-ion etching. In the case of XPS, the thickness or relative composition of the covering layer is calculated by means of the intensity ratio between one element and another in a specimen and using physical parameters such as inelastic mean free paths of electrons and photoionization cross-sections. In the case of AES, the physical parameters for the quantification can be eliminated by using the ratio of the relative intensity for two elements in the sample to that in a suitable, comparable reference.

Effects of Neutron Irradiation on Tensile Properties in High-Purity Fe–Cr Alloys

The tensile properties of high- and low-purity Fe–9, –18 and –30Cr alloys irradiated by neutrons up to a dose of 5 × 1024 n/m2 (E >1 MeV) at 613, 673, or 763 K have been examined. The yield strength and the ultimate strength are increased and the elongation is decreased by irradiation. The enhancement of these strengths due to the irradiation has a tendency to increase with chromium and impurity content. Large stress drops are often observed, especially at 763 K, in stress–strain curves of high-purity and high-chromium-content alloys except for Fe–9Cr alloys. Irradiation-induced precipitates, with 2% larger interplanar spacings than the α′-phase, on dislocation loops are more easily formed in the specimens of higher chromium content and higher purity. The precipitates are formed even in the irradiated Fe–9Cr alloy of high purity. The stress drop behaviour during the tensile tests is predominant in the specimens of higher chromium content and higher purity.

Recent progress and future R&D for high-chromium iron-base and chromium-base alloys

Attractive characteristics of high-chromium iron-base alloys and chromium-base alloys have been demonstrated compared with those of conventional ferritic and austentic stainless steels. Recent progress especially on purification techniques indicates the possibility for these alloys to be developed into engineering materials. It is argued that the purification may overcome the brittleness of drawback of these alloys.

High Temperature Deformation Mechanism of a High-Purity Fe–50 mass% Cr Alloy

The deformation mechanism of a high-purity Fe–50 mass% Cr alloy and a floating-zone refined Fe–50 mass% Cr alloy was investigated by tensile testing between 873 and 1073 K and microstructural observation. It is concluded from the present experimental research that: 1. A stress-drop appears after yielding in both alloys between 873 and 1073 K. The stress-drop is the result of grain boundary sliding and is related to the formation of dislocation sources at the beginning of deformation. 2. Intergranular cracking occurs in the high-purity Fe–50 mass% Cr alloy and also in the fine-grained zone-refined alloy and the cracking is strongly influenced by grain size. The mechanism is not grain boundary decohesion caused by segregated sulfur. 3. The stress-drop is also observed in the Fe–50 mass% Cr–8 mass% W alloy, but grain boundary cracks are seldom observed in this alloy. 4. The deformation behavior after the stress-drop is determined by a competition between increasing dislocation density and dislocation annihilation at high temperatures. No twinning is seen in this temperature range, not even in the W-doped alloy.

Aging Properties of Ultra-High-Purity Fe–High-Cr Alloys

The paper investigates the secondary phase precipitation behavior as well as changes in mechanical properties accompanying the aging treatment of two types of high-Cr alloys (Fe–50Cr and Fe–30Cr–4W) having different purity and additional impurities. The results show that ultra-high purification can prevent the σ-phase formation even after aging. Carbon and nitrogen are hardly dissolved in the base phase and they do not bring about the formation of the σ phase. The differences in the formation mechanisms of σ and Laves phases, representative harmful phases of Fe–high-Cr alloys, are discussed.

Reduction of intergranular fracture in FeP alloys by the addition of nickel

Abstract Nickel in solid solution reduces the susceptibility of FeP alloys to the intergranular fracture caused by the grain boundary segregation of phosphorus under certain conditions. Among factors affecting the intergranular fracture, the degree of phosphorus segregation and the grain size are independent of the nickel concentration. Two other factors are the solution softening by nickel and the interaction between nickel and phosphorus at grain boundaries. By quenching from the γ phase region, dislocations are introduced and the solution softening due to nickel is masked; the yield stress becomes independent of the nickel concentration. In this case a reduction in the intergranular fracture due to nickel is not observed. The solution softening is responsible for the reduction in the intergranular fracture due to nickel.