A method for breaking a high carbon steel rod into billets was developed by the authors. The method consists of the three steps of forming a slight circumferential notch in the surface of the rod, developing a crack extending from the notch root toward the rod center by local heating and cooling of the surface area adjacent to the notch and breaking the rod along its notched portion by applying a mechanical load.
The heat treatment in the second step may be either single or double heating, the latter being applicable to the breaking of small-diameter rods (less than 50mm). In the second step of this method, it is supposed that the cracks initiate and propagate owing to the axial residual stress near the notch root during the local heat treatment.
In the present paper, after explaining the outline of the method, the hardness and residual stress distributions were measured with locally heat-treated unnotched specimens of high carbon chromium steel (SUJ2). The heat treatment conditions were selected so as to develope sufficient cracks in notched rods. And then the residual tensile stresses near the specimen surface caused by the single and double heating processes were discussed.
The results obtained are summarized as follows.
(1) In the single heating process, the residual tensile stress near the surface of the specimen occurred merely due to plastic shrinkage of the heated surface layer. The maximum value of tensile stress was about 40kgf/mm2.
(2) In the case of the double heating process, the surface layer of the specimen was hardened by the first heating and cooling, and tempered to the hardness of HV 400∼550 by the second treatment. The residual tensile stress near the surface of the specimen was about 100kgf/mm2, which was never obtainable by the single heating process.
