Although it is important to eliminate blowholes or pinholes in iron and steel, the mechanism of growth of them has not been yet suffciently studied. As a first step to study this mechanism, vacuum-cutting apparatus was designed and chemical composition of gas in blowholes was studied. Blowholes caused by CO gas grow in rimmed steel but they do not grow in kilied steel, because they are deoxidized. This limit of blowhole formation can be decided by calculation from the equilibrium constant but experimentally was not obtained to this day. Therefore the relation between formation of blowhloles by CO gas and Si, Mn, Al deoxidation was explained experimentally and the results of experiments were compared with calculation values from the equilibrium constabt. Blowhole samples were melted in a 100kg basic high-frequency electric furnace. When the temperature of molten steel attained to 1600°C, deoxidation reagents (Si, Mn, Al) were added in different quantities. Some times after deoxidation oxygen gas was lanced into molten steel for 30 seconds and by reaction of C and O, CO gas evolved violently. 25kg of molten steel was immediately tapped to ladle and then poured to 16kg ingot. In succession, oxygen gas was lanced again to the rest molten steel in the furnace for 30 seconds and poured to a 16kg ingot. Accordingly from one melt of 100kg, four ingots were obtained and they were different in Si, Mn, Al and oxygen contents. These ingots were cut lengthwise and the distribution of blowholes were investigated. The results gained from this experiments were as follows. 1) Even if carbon content was as high as compared with usual rimmed steel. blowholes by CO gas were same as rimmed steel and primary and tecondary blowholes exists. 2) When deoxidized by Si, the limit of blowhole. formation was 0.10%Si for 0.33-0.40% C and 0.20% Si for 0.40-0.80% C. This limit was raised by carbon content and corresponds to a curve which was calculated by the equilibrium constant and the melting point. 3) When deoxidized by Al, the limit of blowhole formation was 0.01% Al. 4) When deoxidized by Mn, the blowhole formation was not prevented for weak deoxidation power of Mn.
The solidification rate of white cast iron and the effect of casting temperature, mould tem perature and casting size upon it were studied by means of the "pour-out method". By investigation of the roughness and microstructure in the solidification-front surface, the- mechanism of solidification was discussed. The results obtained were summarized as follows:- (1) In the solidification of white cast iron, the linear relation between the thickness of solidified layer and the square root of solidification time was satisfied only in the early stage of solidification and it deviated from the above relation as time proceeded. Then, the solidification curve was the form of S letter which had two points of inflexion, in other words, . white cast iron was solidified through three stages. (2) From the linear relation in the early stage, the following equation, what was called an "equation of rate of skin formation" could be derived, where, d: solidified layer (in inch) t: solidification time (in mn) k: solidification constant a: constant k or "rate of skin formation" decreased as the casting temperature and mould temperature became higher and the casting size became larger. (3) As solidification proceeded, the roughness in solidification-front surface became larger according to its solidification curve. And it becames larger with increase of the casting temperature, mould temperature and casting size. (4) The first point of inflexion in solidification curve was the beginning point of crystallization of a "bee-hive" like ledeburite and the second one was the end point of crystallization of dendritic cementite. With increase of the casting temperature, mould temperature and casting size, these points became later and their time interval became longer, and consequently the microstructure became rough. (5) From the results above mentioned, the schematic diagram of solidification of white cast iron was drawn and the reason for the appearance of two points of inflexion was explained.
General process of the induction surface hardening is as follows: a steel specimen of which the surface is austenitized with induction heating is used to be quenched in the water-jet, as soon as the heating has been stopped. If the quenching time is delayed after the induction heating, hardness of the specimen is not lowered than that of the instantanusly quenched specimen, this unsoftening condition of delay-time is amounted to several seconds for medium-carbon steel, and scores seconds for low alloy steels with medium-carbon content. This special quenching method above described is called as a "delay-quenching", and it takes effect to the stress relief on the induction surface-hardening. In the former report it was verified that a "stop-quenching", was a good application for the purpose of stress relief on the induction surface-hardening. However, it cannot be applied to the continuons induction hardening of a long steel rod, on the contrary, "delayquenching" can be applied to the continuous induction hardening.
The cause of the hard tempered martepsite brittleness which has come to be known as "500°F embrittlement" has not been explained until today. The phenomenon was much relatd to the deoxidation, nitrogen-fixation, and austenite grain size of steel. The tempering characteristics of the steel plays a major role in the development of embrittlement. This investigation consists of a stuay of the effect of Al, Ti and B addition on the impact-resistance and an electron micrgscopic study of the manganese-chromium case hardening steel when tempered between 120° and 420°C. The results obtained are as follows: (1) The Al-Ti (0.05-0.10%) addition markedly improves the impact values during tempering at these temperatures, especially with Ti addition the benefibal effect in the ternperature range of embrittlement is distinct. (2) The tempering embrittlement temperqture is not changed with the Al or Al-Ti addition and it occures at 300°C to 360°C; however in the case of Al-Ti-B addition it is the temperature range at 300°C to 420°C (3) The impact minimum occurs in 20 to 30 minutes at the tempering embrittlement temperatures and the value is decreased with keeping time at 300°C, it is almost constant up to 10h at 360°C. (4) The transition temperature is lowered by the Al, Ti and Al-Ti-B additiop; with the Al (0.04%), Ti(0.10%) and Al-Ti (0.06%)-B addition it falls down to 0°C-10°C. (5) It is obeerved that the embrittlement arises from a precipitation of elongated thin cementite network along ferrite grain boundaries, with the B addition the formation is obserVed at the higher tempering temperature.
To investigate the influence of vanadium, molybdenum, tungsten and silicon on the properties of the hot-working tool steel containing 0.35% carbon and 5% chromium, the authors measured the critical point, S-curves for the transformation of austenite, as quenched and as tempered hardness, retained austenite, dimensional changes, and mechanjcal properties at elevated temperature. The results obtained were as follows: (1) The critical point was raised with the vanadium, silicon and molybdenum content, respectively. The Ms-point was raised with the vanadium content, butlowered with the molybdenum additon. (2) As the vanadium content increased, the temperature of the tip ofthe pearlite knee occured at a higher temperature and the pearlitic reaction was accelerated, moving the pearlite knee to the left, and displacing the beginning line for thebainite reaction to the left. And as the tungsten content increased, the beginning line for the pearlitic reaction was moved to the left and for the bainite reaction moved to the right. (3) Full hardness was obtained by air-cooling from 1020°C to 1050°C, and as the vanadium content increased, higher austenitizing temperature was necessary for the obtaining the full hardness. (4) The quantity of retained austenite after air-hardening from 1020°C measured by the magnetic method increased with vanadium addition, and the tungsten and silicon had little effect on the retention of austenite. This retained austenite decomposed at a temperature between 550 to 650°C and resulted in the volume expansion. (5) During tempering, the hardness decreased slightly up to about 300°C and then increased. Maximum secondary hardness was exhibited at a temperature between 500 to 550°C. As the vanadium, molybdenum and silicon content increased, respectively, the room temperature hardness after tempering above 500°C increased. (6) The volume ingrement resulting from air-hardening was less than oil-quenching from the ordinary temperature. As the vanadium content increased, the amounts of iength change after air-hardening from 1020°C decreased. (7) As the vanadium and molybdenum content incresaed, the tensile strength at elevated temperature increased and impact strength slightly decreased. And both the tensile strength and impact strength increased with the silicon addition.
The material corresponding to a die steel No. 5 as spring material in high temperature was heat-treated in the constant hot bath which was considered as the best one being based on the former experiments. Then, impact hardness test in high tempetature, hardness test in room temperature, impact test, fatigue test by the Upton Lewis testing machine and microscopic test were carried out and also, impact hardness test of ordinary quench-tempered Si-Mn steel at high temperature was carried out. These results were summarized as follows. (1) On the material corresponding to a die steel No. 5, the treatment in 600°C constant hot bath quenched from 1100°C was ieconfirmed as the best heat-treatment. (2) In the impact hardness test of ordinary quench-tempered Si-Mn steel, the almost linear descending inclination of impact hardness was recognized with ascent of testing temperature.
The authors studied the effects of calcium hydride addition to grey cast iron and the following results were obtained: 1) Spheroidal graphite cast iron could be obtained by addition of calucium hydride (CaH2) in excess of 4% to a suitable molten iron (Swedish charcoal pig iron) covered with the flux of fluorspar. 2) When CaH2 and fluspar were added without any other additions, it was difficul to give all spheroidal graphite structure, and always quasi-flake graphite was distributed in it. 3) More spongy blow holes occurred with higher addition of CaH2 and fluospar, but when the spheroidal graphite structures occurred, most of the blow holes disappeared. 4) For suitable iron (Swedish charcoal pig iron), the addition of both 2% CaH2 and Mgalloy was not so effective to decrease the amount of Mg which was necessary to make spheroidal graphite. 5) For the pig iron seemed to be oxidized, when blow holes appeared by addition of CaH2, spheroidalization of graphite might occur by the same amount of additional Mg as to Swedish charcoal pig iron. 6) The effect of desulphurization was observed by addition of the CaH2 spread over the molten iron.