Study of the Precipitation of X-carbide in Tempered Medium and Low Carbon Steels(Metallurgy)

Observations by transmission electron microscopy have been made on quenched and tempered carbon steels containing 0.017%, 0.15% and 0.48% carbon to investigate the precipitation behaviours of X-carbide in lower carbon steels with regard to those already clarified for high carbon steels by the present authors. The main results obtained are summarized as follows : (1) There exists X-carbide in the tempering process of low (0.017%, 0.15%C) and medium (0.48%C) carbon steels. (2) The habit plane of the X-carbide in the medium carbon steel is {112}_a, suggesting that the carbide precipitates preferentially on martensite twin boundaries. (3) The orientation relationship of the X-carbide with ferrite matrix of low and medium carbon steel is approximately expressed as (100)_x//(121)_a, (010)_x//(101)_a, and [001]_x//[111]_a

Recovery of Lattice Defects in Cementite in Cold-Rolled Carbon Steels(Metallurgy)

Transmission electron microscopic observations were made on the recovery process of lattice defects in cementite in high carbon steels annealed after 92% cold rolling. Thermomagnetic and X-ray analyses were also performed as additional examinations. No observable change in the defect structure of cementite occurs at temperatures below about 400℃. Annealing at higher temperatures results in the disappearance of the moir6 pattern, a considerable decrease of dislcoation density and the formation of well-developed subboundaries. Above about 600℃, these defects disappear gradually with the progress of spheroidization. These results suggest that the recovery of lattice defects in cementite is caused by polygonization accompanied by climbing or cross slipping of dislocations

The Precipitation of χ-carbide in the Tempering Process of High Carbon Steels

To make clear the precipitation behaviour of iron-carbides, especially to show if χ-carbide precipitates or not, in the tempering process of steels, observations were made by transmission electron microscopy on some quenched and tempered high carbon steels containing 0.86, 1.11, and 1.34 wt% carbon. Thermomagnetic and thermal dilatational measurements were also made for comparison\u27s sake. The main results obtained are summarized as follows : (1) In the high carbon steels tempered under appropriate conditions, χ-carbide was observed by transmission electron microscopy. (2) The habit plane of χ-carbide was determined to be {112}_α, suggesting that the carbide may precipitate preferentially on twin boundary in martensite. (3) In electron micrographs, the morphology and the growth direction of χ-carbide were different from those of ε-carbide, but were very similar to those of θ-carbide which precipitates on {112}_α. From these observations, it was supposed that the transitions of ε→χ andχ→θ were due to a separate nucleation and an in situ transformation, respectively. θ-carbide precipitated on {110}_α was also found, suggesting the precipitation to take place by a separate nucleation directly from ε-carbide. (4) Using indices of main crystal planes and a direction in χ-carbide, i.e., (100)_x, (010)_x, and [001]_x, the orientation relationship of the carbide with ferrite matrix was expressed as (100)_x//(121)_α, (010)_x//(101)_α, and [001]_x//[111]_α. (5) The precipitation of χ-carbide was also detected by thermomagnetic and thermal dilatational measurements on the high carbon steels tempered under the most favourable condition to obtain χ-carbide

Microstructures of Deformation and Fracture of Cementite in Pearlitic Carbon Steels Strained at Various Temperatures(Metallurgy)

In order to make clear the effect of temperature on deformation and fracture of cementite in steels, observations by transmission electron microscopy were made on cementite in carbon steels strained in tension at various temperatures ranging from -196 to 700℃. Hardly any plastic deformation of cementite was detected at -196 and -78℃. It was confirmed that above room temperature cementite in steel can deform due to dislocation slip and that the deformation becomes easier as the temperature increases. Slip in cementite at room temperature and at 300℃ seems to be confined to only (100) or (001). Dislocations observed at room temperature and at 300℃ were mostly isolated and straight. Above 400℃ (100), (010), (001), and some {110} planes are all operative slip planes. Dislocation loops, dipoles, cusps, and networks were frequently found. These observations indicate that double slipping and the interaction of dislocations can occur and that the deformability of cementite above 400℃ is very large. An appreciable degree of dynamic recovery was detected above 500℃. The fracture of cementite at -196 and -78℃ occurred in a cleavage manner along some crystallographic planes such as (110), (100) or (210) . Above room temperature fracture occurred along an activated slip plane and was preceded by some amount of slip on that plane. Above 400℃ cementite fracture, caused by joining of voids, formed along an activated slip plane was frequently observed