An Investigation of the Hot-torsion Test Used in Studies of Hot Workability

Abstract

The use of as-cast and wrought metals for the manufacture of various shapes by forging requires a knowledge of the ductility of metals and alloys at high temperatures. reveals that each type of test used for hot workability measurement has some characteristic that usually gives different results than those obtained by using one of the other tests. For this reason, to enable a better assessment of the hot workability of materials to be made, a correction formula is proposed. Hot workability measurement by means of the torsion test is of considerable interest and the main factors which characterise this type of testing are discussed in detail, It was found that the axial force which appears during twisting is closely connected with deformation behaviour and is not significantly affected by fibre structure or crystal lattice. Compressive force is suggested to be due to grain deformation and tensile force to grain boundary sliding. These phenomena can be connected by a formula which allows axial force in the tension test to be used for studying a material's behaviour from the point of view of grain boundary sliding in a very easy way. Phase transformations and recrystallisation may also be studied by measuring the variation of axial force with time at various temperatures. It was also found that the generally accepted plasticity equations cannot be used in a simple way for studying the axial stress distribution across the specimen cross section, on account of the factors which give rise to the force. A formula is proposed for taking account of the influence of axial force on the hot ductility measurement by torsion test, thus allowing a better comparison between the ductilities of various metals and alloys to be made. Studies of the manner of fracture revealed that cracks which first form within the specimen at elevated temperature in the steels used originated at inclusions along the fibre structure. Although axial tensile stresses markedly influence the appearance of these cracks they are not the main cause. Factors connected with specimen dimensions which may effect the ductility were also studied, and revealed that specimen diameter seems to affect the ductility more than its length. Experiments on as-cast mild steel showed thot different results are obtained on specimens taken from different parts of the ingot and that wrought material has generally better ductility than cast, Nodular cast iron has some ductility and can be deformed in the temperature range of 850 = 1000ºC in the cast state, and720 = 1020ºC after annealing, Ductility was slightly less than half that of specimens cut from mild steel ingots.