A Eutectoid Reaction for the Decomposition of Austenite into Pearlitic Lamellae of Ferrite and M23C6 Carbide in a Mn-Al Steel

Abstract

We discovered a eutectoid reaction in an Fe-13.4Mn-3.0Al-0.63C (wt pct) steel after solution heat treatment at 1373 K (1100°C) and holding at temperatures below 923 K (650°C). The steel is single austenite at temperatures from 1373 K to 923 K (1100°C to 650°C). A eutectoid reaction involves the replacement of the metastable austenite by a more stable mixture of ferrite and M 23 C 6 phases at temperatures below 923 K (650°C). The mixture of ferrite and M 23 C 6 is in the form of pearlitic lamellae. The morphology of the lamellae of the product phases is similar to that of pearlite in steels. Thus, we found a new pearlite from the eutectoid reaction of the Mn-Al steel featuring c fi a + M 23 C 6 . A Kurdjumov-Sachs (K-S) orientation relationship exists between the pearlitic ferrite (a) and M 23 C 6 (C6) grains, i.e., (110) a // (111) C6 and [111] a // [011] C6 . The upper temperature limit for the eutectoid reaction is between 923 K and 898 K (650°C and 625°C).

A general perspective of Fe–Mn–Al–C steels

“…Describing the phase diagrams of Fe-Mn-Al-C is not an easy task. Only in the last 8 years, new phases and phase transformations have been identified [41,44,48,50,62,96] in Fe-Mn-Al-C alloys. Thus, a full description of these phases is still challenging and maybe in some degree premature yet.…”

“…Recent discoveries in the Fe-Mn-Al-C equilibrium constituents have been reported. For instance, the eutectoid reactions; 𝛾 → 𝛼 + 𝑀 23 𝐶 6 [96], 𝛾 → 𝛼 + 𝜅𝑎𝑝𝑝𝑎 𝑐𝑎𝑟𝑏𝑖𝑑𝑒 + 𝑀 23 𝐶 6 [46] and 𝛾 → 𝛾 + 𝛼 + 𝜅𝑎𝑝𝑝𝑎 𝑐𝑎𝑟𝑏𝑖𝑑𝑒 + 𝑀 23 𝐶 6 [49] in the Fe-13.4Mn-3.0Al-0.63C, Fe-13.5-Mn-6.3-Al-0.78-C and Fe-17.9-Mn-7.1-Al-0.85-C steels, respectively. The reader is referred to these references to analyses these phase diagrams that have not been included in the present review.…”

“…Figure 5. TEM for the (a) Fe-13.4Mn-3.0Al-0.63C steel after isothermal holding at 600°C for 100 h (γ: austenite; α: ferrite; and C6: M23C6), reproduced from [96] with permission of Springer ® , (b) Fe-13.5-Mn-6.3-Al-0.78-C steel after isothermal holding at 650°C for 100 h (κ: κ-carbide and C: M23C6), reproduced from [46] with permission of Springer ® , (c, d) Fe-17.9-Mn-7.1-Al-0.85-C steel after isothermal holding at 600°C for 100 h (γ: austenite; α: ferrite; κ: κ-carbide and C6: M23C6), reproduced from [49] with permission of Springer ® and (e) Fe-Mn-Al alloy heated at 1573 K with argon protection for 30 min and air cooled, and (f) a TEM micrograph of (e), reproduced from [50] with permission of Springer ®…”

The analysis for morphological evolution and crystallography of degenerate pearlite in 100Mn13 steel

“…In fact, there are other non‐cementite carbides which constitute lamellar pearlite with ferrite in high carbon high manganese steel, such as M 23 C 6 carbide, kappa‐carbide etc. . However, it is not clear whether these non‐cementite carbides can also form degenerate pearlite, which needs a further study.…”

3D Growth of Intragranular Pearlite in an Aged 100Mn13 Steel

“…It has been reported that pearlite nucleated preferentially at the austenite grain boundary whether in Fe–Mn–C steel, [ 17–21 ] plain carbon steel, [ 22 ] or Mn–Al steel. [ 23–25 ] However, second‐phase particles precipitated at austenite grain boundaries were activated as intragranular nucleation sites of pearlite when some certain alloying element was added, and the intragranular pearlite had a specific OR with the particles, but had no OR with the austenite grain from which it formed. [ 26–28 ] Although pearlite formed on the proeutectoid M 23 C 6 was observed in 100Mn13 high‐carbon, high‐manganese steel, [ 29 ] it could directly nucleate in austenite grain, and the sites provided by the second‐phase particles has not been found in the steel.…”