Mechanism of hot-shortness in leaded and tellurized free-machining steels
- PDF / 7,028,262 Bytes
- 16 Pages / 594 x 774 pts Page_size
- 110 Downloads / 168 Views
IT is well-known that the addition of controlled amounts of tellurium to steel significantly improves its machinability. A hot-shortness problem is encountered during hot-rolling of tellurized steels, however, which causes high yield losses and conditioning costs in the production of these steels. It is the purpose of this work to increase the current understanding of the basic mechanism of such hot cracking in tellurized steel grades. Previous studies have been undertaken on this subject. Tellurium-associated embrittlement has been reported by Brown in AISI 12L14 + Te steels, 1'2 and in other alloy steels by others in the literature, s,4 Only Heitmann and WrighP have postulated a possible mechanism of a preferential oxidation of iron relative to tellurium occurring during reheating (typically, between 1095 to 1315 ~C), leaving behind a surface layer enriched in tellurium as being responsible for the hot-shortness. It is argued that the solubility limit is soon reached due to this enrichment, leading to surface cracking. However, no conclusive experimental evidence exists to support this mechanism. This investigation has been aimed at a reexamination of the hot-shortness problem in the AISI 12L14 + Te grade to establish the underlying mechanism and to propose possible changes in practice to minimize the problem. In the first part of the investigation the chemical form in which tellurium exists in the steel microstructure is characterized, taking into account thermodynamic arguments and electron microprobe data. The existence of high temperature embrittlement and the temperature range of embrittlement is then established by hot tensile testing and associated fractography of the tensile fractures by scanning electron DEBANSHU BHATTACHARYA is Senior Research Engineer at Inland Steel Research Laboratories, East Chicago, IN. DENNIS T. QUINTO, formerly with the Inland Steel Research Laboratories, is now with the Philip McKenna Laboratory, Kennametal, Inc., Greensburg, PA. Manuscript submitted April 24, 1978. METALLURGICAL TRANSACTIONS A
microscopy (SEM). In the third part, the mechanistic aspects are investigated further with in-situ fracture surface analyses by Auger Electron Spectroscopy (AES) and subsequent SEM fractography. Finally, in order to test the embrittlement mechanism suggested by the experimental data, hot-rolling experiments on commercial 12L14 + Te ingot samples were performed in the laboratory mill to study the incidence of surface cracking as a function of hot-rolling temperature. EXPERIMENTAL METHODS The compositions of the steels used in this investigation are given in Table I. To determine whether tellurium occurs as a pure metal or an intermetallic phase in 12L14 + Te steel at temperatures typically used in hot-rolling, samples were Sen-packed and heat-treated to 1000 ~ for two hours in air and quenched to preserve the high-temperature inclusion compositions. These samples were then polished, etched for grain boundary appearance and analyzed by electron microprobe. A control sample of non-tellurize
Data Loading...
