原田 泰典

The workability of cast pure chromium produced by an induction-slag-melting process was investigated through the study of rolling and isothermal upsetting. The cast material had large crystal grains elongated along the cylindrical ingot axis or the solidification direction. The effect of rolling, which was performed to change the coarse cast structure to a finer one, on the ductile-to-brittle transition temperature (DBTT) was also examined. The present results are summarized as follows.<BR>DBTT of the as-cast specimen tested in tension at a strain rate of 2.4×10<SUP>−2</SUP> s<SUP>−1</SUP> was about 500 K. A minimum ductility and a maximum flow stress appeared at about 973 K owing to dynamic strain aging.<BR>The forming limit in upsetting, percent reduction of height at beginning of cracking on the specimen wall, increased to 80% at 773 K, and no crack appeared at temperatures above 873 K.<BR>At all temperatures, there occurred cracks in the as-cast specimen deformed by a conventional rolling. Cracks occurred slightly in the specimen even in a sandwich-rolling by which a sintered chromium was successfully deformed. Cracking in the as-cast specimem was mainly due the coarse structure with large crystal grains. Once the structure was changed to a finer one by the upsetting, the specimen was easily rolled to a thin sheet without crack occurrence.<BR>DBTT of the specimen rolled to 80% and followed by annealing at 1273 K was higher by about 200 K than that of the as-cast specimen, while annealing at 1473 K yielded a slightly lower value than that of the ascast material.

Tensile test for sintered and cast chromiums was carried out in vacuum at temperatures up to 1273 K to exartme the dynamic strain aging behavior. Three strain rates 2.4×10<SUP>−2</SUP>, 2.4×10<SUP>−1</SUP> and 2.4 s<SUP>−1</SUP> were applied.<BR>With increasing temperature, tensile strength decreased gradually at first and showed a minimum at a certain temperature. Above this temperature, the strength continued to increase to a maximum value. Serrated flow appeared between the temperatures of the minimum and maximum tensile strengths. A minimum tensile ductility evaluated by the fracture elongation was observed around the temperature at which the tensile strength became a maximum.<BR>Relation between the strain rate \dotε and the temperature <I>T</I> of the maximum tensile strength could be well described by Arrhenius’ equation, \dotε=\dotε<SUB>0</SUB>exp(−<I>Q</I>⁄<I>RT</I>), where \dotε<SUB>0</SUB> is a constant, <I>Q</I> is the activation energy for dynamic strain aging, and <I>R</I> is a gas constant. Activation energy of 101.3 kJ/mol was obtained, which was in good agreement with the activation energy for diffusion of the nitrogen atom in chromium.<BR>It was found previously that chromium lost its rolling workability at temperatures above 1173 K owing to the large tensile stress in the rolling direction of the workpiece-surface in contact with rolls. The decreased rolling workability of chromium was related to the dynamic strain aging which brought about less ductility.

The enhancement in rolling workability of a sintered powder chromium (Cr 1, 99.9 mass%) by CIP and HIP processes was made at elevated temperatures by means of a sandwich rolling, in which the workpiece sandwiched in between two mild steel sheets was rolled. The behavior of ductile-to-brittle transition and recrystallization was also examined in the rolled specimens of 99.99 mass% purity sintered powder (Cr 2) and the above specimen Cr 1, respectively. The results are summarized as follows.<BR>The sandwich rolling was very efficient to prevent the crack generation during rolling of the workpiece at elevated temperatures, especially at the temperatures ranging from 1173 to 1373 K, at which the workability was extremely poor in a conventional rolling.<BR>The ductile-to-brittle transition temperature (DBTT) was lowered by a high rolling reduction given to the specimen. Namely, the DBTT of the specimen rolled to 85% at 973 K and annealed at 1473 K was 510 K which was about 200 K lower than that of the specimens as sintered and/or rolled to 50% after annealing at the same temperature.<BR>Room temperature hardness of the specimen rolled to a given reduction at 973 K, the temperature at which the dynamic strain ageing occurred, was considerably larger than the hardness measured in the specimen rolled to the same amount at 773 K at which the strain ageing did not occur. The recrystallization temperature of the present sintered powder was similar to that of a cast material. The recrystallization during the sandwich rolling at the temperatures beyond the recrystallization temperature occurred evenly on the surface and in the inside of the workpiece, while it occurred only in the inside during the conventional rolling.

The rolling workability of sintered chromium, which is very brittle at room temperature was examined as a function of temperature. The present sintered chromium of 99.9% purity was produced by CIP (cold isostatic press) and HIP (hot isostatic press).<BR>The specimen could be successfully rolled in a limited temperature range from 673 to 1073 K. This formability characteristic is different from the experimental results found in the upset and the tensile tests in which no upper limit of temperature was found for workability and ductility. The deterioration in rolling at temperatures above 1173 K was caused by rapid cooling of the specimen by the rolls. The temperature of the specimen during rolling decreased greatly in the roll-contact region, while the inner part of the workpiece retained its starting temperature. Hence, the large tensile stress in the rolling direction occurred near the roll-contace material because of a large difference in the flow stress between the roll-contact region and the inner part of the workpiece. The tensile stress was responsible for cracking on the surface of the specimen heated at temperatures above 1173 K.<BR>The steel/chromium/steel sandwich plate could be successfully rolled to a large reduction even at temperatures above 1173 K without occurrence of the surface cracks because the steel skins prevented the temperature decrease of the specimen during rolling.

Tensile properties of sintered powder chromium (99.9 mass% purity) produced by a cold isostatic press (CIP) and a hot isostatic press (HIP) were preliminarily examined to find conditions favorable to the plastic working of the sintered chromium. Specimens A and B<SUB>1</SUB> taken from a sintered ingot A and a half of an ingot B, respectively, were annealed at 1823 K higher than 1473 K encountered in the present HIP. Specimen B<SUB>2</SUB> from the remainder of the ingot B was not heat treated.<BR>Transition from ductile to brittle fracture occurred very sharply, i.e., in a small range of temperature, and at the higher temperature in a testing at the higher speed of deformation. Ductile-to-brittle transition temperature of the specimen B<SUB>1</SUB> was lower than that of the specimen B<SUB>2</SUB> suggesting that the annealing at 1823 K is effective in lowering the transition temperature. Small plastic tensile prestrain given to a specimen at a temperature slightly beyond its transition temperature considerably reduced the brittleness of the specimen. This prestraining effect is supposed to come from free dislocations generated in the prestrained specimen. At temperatures between 700 K to 1023 K, flow stress was never changed even when the strain rate was increased from 1.7×10<SUP>−4</SUP> s<SUP>−1</SUP> to 1.7×10<SUP>−1</SUP> s<SUP>−1</SUP>, and the decrease of the stress with increasing temperature was small.