Reversion Of Strain-induced Martensite Research Articles

Two-step annealing treatment for grain refinement of cold-worked AISI 316L stainless steel

Abstract Two-step annealing after cold rolling is proposed for the optimization of the annealing process of AISI 316L austenitic stainless steel toward more intense grain refinement. It was revealed that the first step of the annealing process at lower temperatures (i. e. 750 0C) can be used for the reversion of strain-induced martensite to austenite without uninhibited grain growth and recrystallization of the retained austenite. Afterwards, the primary recrystallization of the retained austenite takes place in the second step of the annealing process at higher temperatures (in the range of 850 to 950 0C). Carefully controlling the second annealing step resulted in a finer grain size and better mechanical response compared to those of a conventional one-step annealing approach.

Thermal Mechanisms of Grain Refinement in Steels: A Review

Thermal mechanisms of microstructural refinement in steels were reviewed. These include thermal cycling, martensite process, and static recrystallization (SRX), in which the dominant stage of microstructural refinement is governed by an annealing treatment of a deformed or an undeformed material. Recent progress in grain refinement by thermal cycling for the body-centered cubic, face-centered cubic, and dual phase (DP) steels was introduced. The application of the cold rolling and subsequent annealing of a martensite starting structure (martensite process) for grain refinement of low-carbon and DP steels was reviewed. The formation and reversion of strain-induced martensite in metastable austenitic stainless steels and their effects on the microstructural evolutions were critically discussed. Moreover, the repetition of the martensite process and its limitations were explained. Important findings on the SRX of ferrite and austenite for grain refinement as well as the recrystallization kinetics were presented. Finally, the concepts of controlled rolling for grain refinement and the interaction of austenite recrystallization and strain-induced precipitation in microalloyed steels during thermomechanical processing were also reviewed. This short overview presents the opportunities that the conventional and innovative processing routes can offer for grain refinement of steels.

Microstructural evolution and mechanical behavior of phase reversion-induced bimodal austenitic steels

The present study aimed at achieving excellent mechanical properties in austenitic stainless steel by the design of bimodal (BM) microstructure fabricated through cold rolling and reverse annealing. The evolution of the BM microstructure during reverse annealing was investigated in detail. Moreover, the strengthening mechanism and the work-hardening behavior of BM steels were also analyzed. It was found that when the annealing temperature was below 634 °C, the reversion of strain-induced martensite (SIM; approximately 80%) into austenite (α' → γ) was mainly controlled by the diffusion mechanism. However, when the annealing temperature exceeded 634 °C, the reversion of SIM was gradually controlled by the shearing mechanism due to an increase in the driving force for shearing reversion. When annealing was carried out at 800 °C for 1–1000 s, the BM microstructure was successfully obtained because of the differences in nucleation rate and stored energy between the reversed austenite zone (approximately 80%) and the deformed austenite zone (approximately 20%) during recrystallization. The yield strengths (YS; 658–775 MPa) and the tensile strength (TS; 1157–1183 MPa) of the as-prepared BM steels were significantly improved as compared to those of the solid-solution-treated (coarse-grained; CG) steel (YS = 332 MPa and TS = 1022 MPa). Furthermore, the effects of twinning-induced plasticity (TWIP) and multi-stage transformation-induced plasticity (TRIP) (caused by the BM microstructure) contributed to strong work-hardening ability and high ductility. The uniform elongation and the total elongation of the as-obtained BM steels were measured as ~45% and ~55.5%, respectively.

Comparative study on the forming and reversion of strain-induced martensite in two duplex stainless steels: Developing a model for VSM analysis of powders or fine chips

Abstract An original feature of this work is the proposal of two equations to fit the volume fraction of ferromagnetic (α + α′) phases that can be applied in the measurement of magnetic saturation of comminuted duplex and superduplex stainless steels (for example, powders or filed chips). Duplex stainless steels contain similar volume fractions of austenite (γ) and ferrite (α) in their microstructure. Two steels exemplify this class, namely the most widely used duplex UNS S31803 and superduplex UNS S32520 stainless steels. The phenomena of work hardening, formation, and reversion of strain-induced martensite (α′) in austenite were compared in both stainless steels. Samples were work-hardened and annealed under identical conditions, and their behavior was evaluated mainly through X-ray diffraction and magnetic measurements. Notably, the volume fraction of strain-induced α′ in duplex stainless steel was as high as 32%, which indicated that this steel had a greater tendency to form α′ than superduplex stainless steel, for which the corresponding value equaled 15%. Annealing at 650 °C for 2 h promoted the reversion of strain-induced α′ into γ, decreasing the volume fraction of the former phase from 32 to 2% (duplex) and from 15 to 6% (superduplex).

Improvement in Properties of 301LN Austenitic Stainless Steel for Metro Coach Manufacture

Metastable austenitic stainless steel type 301LN is widely used for fabrication of structural components of Metro Coaches. The steel exhibits both high strength and enhanced plasticity due to strain hardening as well as formation of strain-induced martensite (α) during cold deformation (TRIP effect). The current market requirement, as projected by many of the ongoing Metro Rail Projects in India, calls for manufacture of this steel with ultimate tensile strength (UTS) in excess of 1000 MPa and yield (YS/UTS) ratio of less than 0.8, as this would facilitate substantial reductions in tare weight and crash-worthiness of metro coaches. The typical property requirements in high temper (HT) as per one typical Metro Coach specification are: Yield Strength (YS) ~ 751-921 MPa, Ultimate Tensile Strength (UTS) ~ 1001-1151 MPa, Elongation ~ 22% min, Hardness ~ 36 HRC max and YS/UTS ratio <0.8. previous="" plant="" experience="" has="" shown="" that="" the="" maximum="" attainable="" uts="" in="" this="" grade="" through="" cold="" rolling="" is="" only="" to="" tune="" of="" 970="" mpa="" and="" any="" excess="" deformation="" severely="" impairs="" both="" ductility="" 22="" hardness=""> 36 HRC) beyond acceptable limits. In the present work, an innovative thermomechanical processing (TMP) methodology has been evolved for the attainment of this seemingly unlikely combination of properties through experimental cold rolling and short annealing simulations in Gleeble 3500 C thermomechanical simulator. The novel process methodology entails imparting heavy cold reductions (CR) of 45-50% in Sendzimir Mill followed by brief/ short reversion annealing treatments (80-160 s) by means of single furnace operation at 750 °C at standard line operating speeds for 300 series in Annealing Pickling Line-1 (AP Line-1) of Salem Steel Plant (SSP) in India. The improvement in properties (strength-ductility combination) has been attributed to grain refinement through formation of submicron grained austenitic (γ) microstructure by controlled reversion of strain-induced martensite (α) during the short annealing treatment. The process is distinct from conventional long annealing treatments (300-360 s), which are employed to soften the steel after cold rolling by means of recovery and recrystallization processes.