Microstructural Changes in Low-carbon Steel Occurring by Heating in Mixtures of Iron, Graphite and Alumina Powders

Abstract

Carbon and nitrogen are easily diffused into stainless steel and titanium, when these are embedded in mixtures of iron, graphite and alumina powders and held at high temperatures in a nitrogen flow. To promote an understanding of this new diffusion technique, which we call "iron-powder pack treatment", we focused attention on the microstructures of low-carbon steel subjected to such a treatment. The diffusion of carbon into the steel was understood by evaluating area fraction of pearlite emerging in ferrite. A pearlite area in the steel, which was heat-treated at 1 273 K for 3.6 ks using a mixture of iron and alumina powders, was larger than before heating, because the steel was carburized by carbon in the iron powder. On the other hand, an increase in the pearlite area was hardly observed in the case of a mixture of graphite and alumina powders. By replacing a part of graphite in this mixture with the iron powder, however, pearlite was most remarkably produced in the steel. Instead of the iron powder, the use of nickel powder did not exert a significant impact on the diffusion of carbon into the steel. These indicate that the iron powder acts as an important agent for enhancing the migration of carbon from a powder mixture to lowcarbon steel. The relationship between a diffusion phenomenon of carbon and the generation of carbon monoxide gas in heat treatment is also discussed.

Novel Solid Carburizing Method for Steel Using a Mixture of Iron and Graphite Powders

“…This ratio was determined by referring to our previous study. 14) The crucible containing a steel specimen and approximately 8 mL of the mixture was placed in an electric furnace and heated in the temperature range of 973-1 373 K for 3.6 ks in the atmosphere. This was allowed to cool in the furnace to room temperature.…”

“…It has been confirmed that the iron powder acts as an important agent for enhancing the diffusion of carbon from the powder mixture to the specimen. 14) IPP treatment is carried out in a nitrogen flow to avoid oxidation of a specimen and a powder mixture at high temperatures. On the other hand, when a heating atmosphere was changed to air, an increase in the area fraction of pearlite was observed in a heat-treated low-carbon steel specimen.…”

Surface Modification of Ferritic Stainless Steel by Heating in Iron, Graphite and Alumina Powders

“…7) This phenomenon means that the chromium oxide film on the surface of the SUS430 steel decomposes during IPP treatment. Since carbon monoxide (CO) gas, which is generated in the heating step of IPP treatment, is exhausted via the nitrogen flow from the electric furnace, 7,10) the oxygen partial pressure should be low around the SUS430 steel. Therefore, it is thought that this environment contributes to the decomposition of the oxide film on the steel surface.…”

“…It is known that this operation leads to the generation of carbon monoxide (CO) gas in the heating step, whereas the CO concentration detected in the holding step is very low. 7,10) The CO gas is exhausted via the nitrogen flow from the electric furnace during heat treatment. Therefore, it is thought that the consumption of oxygen in the furnace contributes to the lowering of the oxygen partial pressure around the specimen.…”

“…In addition, it has been experimentally confirmed that the iron powder acts as an important agent for enhancing the migration of carbon from the powder mixture to the specimen. 10) In IPP treatment, a powdery mixture of iron and carbon becomes solid by heating at high temperatures which should be avoided for sample recovery after heat treatment. Therefore, alumina powder was added to the iron and carbon powders as an anti-sintering agent.…”