Hardness-controlled tool fabrication and application to cold forging of inner race with skewed ball grooves

Abstract

The aim of this study is to survey and verify the suitable quenching and tempering conditions of SKH51 high-speed tool steel material as a steel grade of AISI M2 for manufacturing an inner race with the six-skewed cross ball grooves used for a constant velocity joint. The longitudinally six-segmentalized asymmetric cold forging die fabricated by the SKH51 with a macro hardness (HRC) of over approximately 66.5 has often been broken and fractured during the cold forging operation, due to the excessive hardness of the tool material. The macro hardness (HRC) and the compressive strength were investigated for obtaining available and applicable properties of the tool material according to the quenching and tempering conditions, which can prevent fracture. Then, the hardness (HRC) was measured as approximately 64.6 for the quenched specimen at 1,150 °C, 62.4 for first tempered one at 550 °C, 61.8 for the second tempered material at 550 °C, and 61.7 for the third one at 540 °C. Using the quenched and tempered specimens, the compressive yield and ultimate strengths were also determined from a series of uniaxial compression tests as about 3,000 and 4,200 MPa, respectively. The hardness-controlled SKH51 material was considered to fabricate the longitudinally segmentalized cold forging die. With the reduced hardness and the measured compressive strength of the SKH51 tool material, the longitudinally six-segmentalized cold forging dies were refabricated and applied to the semi-closed die cold forging process for producing the inner race with the six-skewed cross ball grooves. It is shown that the hardness-controlled SKH51 high-speed tool steel could properly be applied to the inner race cold forging without any fractures, and the validity of the proposed heat treatment history is also confirmed.