Microstructural Modifications in AISI 1080 Eutectoid Steel Under Combined Cyclic Heat Treatment

Abstract

In this research work annealed AISI 1080 steel bars are subjected to a typical combined cyclic heat treatment involving two cycles. The first cycle consist of inserting the specimen in an electric resistance furnace at a temperature of 775 8C (in fully austenitic region) and holding for a short duration (6 min), followed by forced air cooling to the room temperature. In second cycle holding temperature and time being same, the specimen is subjected to oil quenching. Such a typical combined thermal cycling results in the evolution of a novel microstructure that comprises of submicroscopic cementite spheroids and fine microscopic cementite clusters in a matrix of martensite. Accordingly, very high hardness (894 HV) is achieved which envisaged a new heat treatment route of the development of plain carbon tool steel.

Effect of Combined Cyclic Heat Treatment on AISI 1080 Steel: Part II-Mechanical Property Evaluation

“…The matted fine pearlitic structure in MPSC region is originated through interlocking of prior fragmented cementite lamella growing through divorced eutectoid transformation and cementite lamella appearing through normal eutectoid transformation (Figure b). Further detailed discussion is available in author's previous research publication . As envisaged in Table , the preliminary investigation corroborates the presence of cementite clusters and submicroscopic cementite spheroids in martensite matrix for specimen subjected to two cycles of combined heat treatment.…”

“…Further detailed discussion is available in author's previous research publication . As envisaged in Table , the preliminary investigation corroborates the presence of cementite clusters and submicroscopic cementite spheroids in martensite matrix for specimen subjected to two cycles of combined heat treatment. However, in the present investigation, further detailed FESEM study reveals the dispersion of nano‐sized cementite spheroids in martensite matrix as well (Figure a–b).…”

“…The microstructure of as‐received annealed specimen, conventional hardened and tempered specimen and specimens subjected to cyclic heat treatment are summarized in Table . Detailed information regarding microstructure evolution in these specimens is available in previously published research paper of the present authors …”

“…In first heat treatment cycle, due to short duration (6 min) of holding, the microstructure remains heterogeneous at 775 °C. Dissolution of cementite lamella in austenite is almost complete in some region which forms carbon enriched austenite (CEA); while in other regions dissolution remains partial and marginal, which are termed as partially dissolved pearlite (PDP) and marginally dissolved pearlite (MDP), respectively . Upon forced air cooling, TCF (Thickened cementite in ferrite matrix), MPSC (Matted fine pearlite containing submicroscopic cementite spheroids), and FLP (Fine lamellar pearlite) regions originate from MDP, PDP, and CEA regions, respectively.…”

“…The different heat treatment conditions and corresponding specimen codes are summarized in Table , and schematically presented in Figure . All heat treated steel bars were studied with optical metallography, SEM metallography, transmission electron microscopy, hardness and microhardness tests; the detailed description of which is available elsewhere . Further in depth analysis of microstructure was carried out with field emission scanning electron microscope (FESEM, Model: SIGMA HD, Zeiss, Germany).…”