The metallographic study has been carried out on the steel having the chemical composition 16%Cr-3%Si-4%Ni-4%Cu-0.44%N-Bal Fe which had been melted in magnesia crucible under ten atmospheric nitrogen by the induction heating and had been cast under the same pressure, and various factors in the steel to obtain a high strength PH type stainless steel have been clarified. The result is summarized as follows: (1) Thorough soaking at about 1100°C and repeated light deformations at the temperature in the initial stage of the forging are necessary in order to hot-work without forming cracks in the cast ingot. (2) The variation in nitrogen concentration occurs on the surface of the steel due to heating atmospheres; in the heating in vacuum and in argon—the denitrogenization, and in the heating in one atmospheric nitrogen—the nitrogen absorption. The weight increase due to the heating in air arises from both the oxidation and the nitrogen absorption on the surface of the steel. (3) The microstructure of the steel water-quenched at 1100°C after held for 10 min consists of austenite having a small amount of insoluble nitride particles. The austenite is still stable even if cooled to −180°C. The higher the solution-quenching temperature, the less the amount of the insoluble nitride becomes. (4) Precipitants, probably as Cr2N and Cu-rich solid solution, form in the austenite by the tempering at 500°∼550°C after the solution-quenching. If the tempering at 500°∼550°C is prolonged, a small amount of martensite is found to be formed at below −100°C by the subsequent subzero-cooling after the tempering. Both the precipitation and the ferrite formation from the austenite are found to proceed during the tempering at 700°C simultaneously. By the subsequent cooling from the tempering temperature, the remaining austenite transforms into martensite at temperatures considerably higher than room temperature. (5) On tempering of the solution-quenched steel, the strength increases, while the ductility decreases, the values being relatively low; σB=93 kg/mm2, σs=80 kg/mm2, δ=18.5%. To make the steel strong and tough, it is adequate to hot-cold work during the tempering at 700°C of the steel water-quenched at 1100°C, followed by the reheating at 450°C for 60 min to relieve the internal stress. When hot-cold rolled 33% in the above treatment, the final tensile-properties are found to be σB=129∼121 kg/mm2, σs=109∼116 kg/mm2, δ=22∼23%. Stronger magnetism and lower σs are obtained by the hot-cold working in the earlier stage of the tempering.
Investigations have been made on the vacuum melting of various ferroalloys, i.e. Fe-O,Fe-S,Fe-C,Fe-C-S, carbon saturated Fe-C-S and Fe-Si-S alloys. Experiments have been carried out at 1600°C, using a high frequency induction furnace and experimental results have been considered from kinetic and thermodynamic standpoints. The results of these investigations are summarized as follows: (1) In the case of Fe-O alloys, an alloy containing 0.2%O is deoxidized to 0.1%O and another alloy containing 0.06%O is not deoxidized. (2) In the case of Fe-S alloys,all the alloys containing from 0.07 to 0.51%S are desulfurized with increasing time. It is considered that desulfurization and deoxidation proceed independently of each other. If Ds⁄δs is assumed to be the order of 10−3 to 10−4, the rate of desulfurization can be approximately explained by the equation in which the diffusion rate of sulfur through a boundary layer at a surface is the rate-limiting factor. (3) In the case of carbon saturated Fe-C-S alloys, two kinds of processes are obtained. One of the processes is explained by the above-mentioned equation and the other process shows the rapid desulfurization in the early stage. (4) In the case of Fe-Si-S alloys, the rate of desulfurization can be explained by the equation which is the same type with Langmuir’s equation. (5) In the case of Fe-C and Fe-C-S alloys which have the carbon content of steel,the oxygen contents of the Fe-C-S alloys containing about 0.4%S are higher than those of the Fe-C alloys.
An investigation was carried out on the effect of stress, chloride concentration, temperature and oxygen on the stress corrosion cracking behaviour of 18-8 and 18-13 Mo steels. Tests were made in autoclaves with the chloride ion concentration of 0∼300,000 ppm at 130°∼250°C. The susceptibility for stress corrosion cracking was found to increase with the increase of chloride concentration and/or temperature. At temperatures blow 150°C, no stress corrosion cracking was observed except in boiling 42%MgCl2 solution. Above 180°C, susceptibility was much greater in vapor phase than in liquid phase. In the atmosphere of air 18-8 steel cracked at 180°C with 30,000 ppm chloride concentration in liquid phase and 30 ppm in vapor phase, wherease at 250°C it cracked with 10 ppm chloride concentration in vapor phase. Addition of oxygen in autoclaves promoted the tendency to stress corrosion cracking; at 180°C 18-8 steel cracked with 300 ppm chloride ion concentration in liquid phase and 10 ppm in vapor phase. The 18-13 Mo steel was observed to be more susceptible than 18-8 steel in vapor phase and less susceptible in liquid phase in dilute chloride solution, whereas in boiling 42%MgCl2 solution both steels were almost equally susceptible. These results suggested the pressence of definite threshold stresses for stress corrosion cracking which depended on chemical composition and corrosive environments such as temperature, chloride and oxygen concentrations. It was also observed that in dilute chloride solutions all cracks were originated at pits formed in both vapor and liquid phase and that complete exclusion of oxygen eliminated the occurrence of stress corrosion cracking as well as pitting corrosion.
The breaking behaviours of a common mild steel in the standard Charpy test and the interrupted impact bending test (impact velocity 1m/sec) have been studied by observing load-time curves, strain distributions and fracture appearances. The results obtained in this study are summerized as follows: (1) The 15 ft-1b transition temperature (Tr15) is raised linearly with an increase in grain size of specimen. Although the total absorbed energy is lowered with an increase in grain size, the crack initiation load becomes higher in coarse grained specimen than in fine one. The critical crack size for initiation and propagation is observed of the order of the grain size. The breaking behaviours depend on the length and the numbers of cracks which are developed in the initial stage of blow. (2) The effects of the prestraining on the transition temperature and on the crack initiation load are also similarly observed as above-mentioned for the coarse grained specimens. In prestrained specimens, however, the breaking behaviours depend on the conditions of hardening due to the strain aging. (3) The cracks developed below the transition temperature appear about 5.5 mm or more in length by the interrupted impact tests which are applied the definite impact arrested length at each testing temperature, such as 1.0 mm, 0.6 mm and 0.2 mm at −60°, −80° and −196°C respectively. The cracks do not appear below the transition temperature less than 5.5 mm in length in these interrupted tests.
In low carbon -2 1/4 Cr-1Mo multilayer welded deposit for joining main steam pipes, ferrite grains are sometimes coarsened during stress relief annealing process after welding. Although the pipe-metal temperature in operation is far lower than the annealing temperature of them, the coarsening of ferrite grains may be accelerated due to the creep strain on internal pressure. In order to clarify the effect of creep strain on the coarsening of ferrite grains, conditions of coarsening in the deposit during heating tests or creep tests at 700°C and 740°C were investigated. As the results, the following items have been proved: (1) The higher the heating temperature is, the larger the ferrite coasening rate becomes. (2) The coarsening rate is not affected by the strain rate 10−7/hr, so far as the amount of coarsened ferrite does not exceed about 35% of the whole volume of deposit, but it is affected when coarsened ferrite exists beyond that amount. As the reason of the fact, it is assumes that the coarsening associated with recrystallization takes place at latter stage of the coarsening. (3) The coarsening rate is markedly diminished under the larger creep rate of 10−4/hr, because the larger strain destroys casting structure of the welded deposit. (4) The tangential strain rate of a pipe in operation due to the internal pressure is limited to the smaller value than 10−7/hr, and the time when the amount of coarsened ferrite reaches 35% under the strain rate of 10−7/hr at the operating temperature 568°C is estimated as a hundred years or more.
An investigation was made of the effect of cold working on the susceptibility of seven austenitic stainless steels belong to AISI Type 304(or 304 L) to stress-corrosion cracking in high temperature NaCl solution (300°C, 500 ppm as Cl−) and in boiling 42%MgCl2 solution. The stress-corrosion was compared with the general corrosion in high temperature pure water and high temperature NaCl solution. These results are summarized as follows: (1) The carbon contents and heat treatments before cold working do not affect appreciably the susceptibility of stainless steel to the stress-corrosion cracking in high temperature NaCl solution. (2) The degree of cold working does not seem to have a direct relation to the susceptibility of stainless steel to the stress-corrosion cracking in high temperature NaCl solution and in boiling 42%MgCl2 solution. However, the ferrite transformed from the austenite by cold working decreases significantly the susceptibility of stainless steel to the stress-corrosion cracking in both solutions. (3) The general corrosion rate of uncracked specimens is higher than that of cracked ones.
An investigation was conducted on both mechanical, electrical properties at room temperature and tensile creep-rupture properties at 800°C as well as microstructures of Nichrome, which were prepared by cold drawing of forged and rolled ingots melted in an induction furnace under air of 100 mmHg using various sorts chromium as raw material. The results were as follows. (1) The difference of both mechanical and electrical properties at room temperature among Nichrome-specimens due to the sort of the raw chromium was almost not able to be detected. From the result of the tensile creep-rupture test at 800°C, it may be said that the properties of Nichrome which was melted with the home-made electrolytic chromium and then deoxidized by the Ni-C alloy were decidedly not inferior to those with the imported chromium. (2) The tensile creep-rupture life at 800°C of specimens was found to be improved considerably by the preheating for a long time after the cold rolling. Particularly, specimens preheated at 900°C·5 hr followed by air cooling had a long rupture-life of 3∼4 times of that of specimens preheated at 800°C·5 hr, although the rupture elongation was smaller in the former than in the latter. (3) As evidences of the anomaly of Nichrome at about 540°C, which has been said to be a short range ordering, the increase of the electrical resistance, the increase in tensile strength and the decrease in elongation were found with decreasing the cooling rate.
Inert gas fusion-capillary trap condensation method was applied for the determination of oxygen in iron, steel and ferrous alloys. The flow rate of argon used for the carrier gas was controlled to 250 mL/min in each case and degassing was carried out at 2400°C for about 30 min, until the blank value became less than 5 γ/min as oxygen. In the case of iron or steel,the reaction temperature for the extraction of gas from sample was controlled to 1700°C. For the analysis of Fe-Cr alloy, the extraction temperature was controlled to 1950±50°C, without using any metal baths. The determination of oxygen in Fe-Mn alloy, which has never been done by vacuum fusion method, was succeeded by the proposed method by using tin as bath and controlling the reaction temperature at 1650°C. In this case, however, fresh tin must be added every time when each sample was charged for the analysis. The results of oxygen analysis by using this method gave a very good reproducibility.
Magnetic properties of ferromagnetic Co-Al alloys containing 4.32∼24.90% of Al have been measured. It has been found that the alloys containing more than about 10% of Al shows high coercive forces when tempered at 500°∼550°C after water-quenching from 1380°C (non-magnetic β′ phase): The 15.02%Al alloy showing the highest coercive force have a residual magnetic flux density of 4,200 G, a coercive of 1,200 Oe and a maximum energy product of 1.71×106 G·Oe. Thus the present investigators have named these alloys “Malcolloy” which is a abbreviation of Magnetic Al-Co alloy. These alloys having high coercive forces consist of many ferromagnetic elongated particles about 300∼1000Å in mean diameter dispersed in the matrix of non-magnetic β′ phase and consequently it may be concluded that the high coercivity of these alloys is probably caused by the existence of small these particles, each of them being composed of a single magnetic domain.
Die Autoren versuchten die Beständigkeiten der silizierenden Stahlproben, welche porenfreie und porige Ferrosiliziumschicht einzeln trugen, gegen Schwefelsäure, Salzsäure und Salpetersäure bei Raumtemperatur. Die porenfreie silizierende Stahlprobe war in Schwefelsäure und Salzsäure unabhängig von ihren Konzentrationen sehr beständig. Anderseits wurde die porige silizierende Stahlprobe in Schwefelsäure und Salzsäure schneller als die unsilizierende Stahlprobe angegriffen. Das kommt wahlscheinlich daher, dass die durch die Poren eindringende Säure die etwa 8%Si enthaltende Schicht, welche mit dem Eisenkern in Berührung gekommen ist, angriff.