ɛ AND α MARTENSITE FORMATION AND REVERSION IN AUSTENITIC STAINLESS STEELS

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1 ɛ AND α MARTENSITE FORMATION AND REVERSION IN AUSTENITIC STAINLESS STEELS K. Guy, E. Butler, D. West To cite this version: K. Guy, E. Butler, D. West. ɛ AND α MARTENSITE FORMATION AND REVERSION IN AUSTENITIC STAINLESS STEELS. Journal de Physique Colloques, 1982, 43 (C4), pp.c4-575-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1982 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C4, szipple'ment au no 12, Tome 43, de'cembre 1982 page C4-575 E AND a' MARTENSITE FORMATION AND REVERSION IN AUSTENITIC STAINLESS STEELS K. Guy, E.P. ~utler* and D.R.F. west* Engelhard Industries Ltd, Chessington, England "~m~erial CoZZege of Science & TechnoZogy, London, England (Accepted 9 August 1982) Abstract - The structures of a' and E martensite in 18/8 and 18/12 steels, observed using electron microscopy techniques are reported. Structural changes associated with the reverse transformations of these phases to austenite, y, on heating, together with compositional variations are presented and mechanisms of both the direct and reverse transformations are discussd. Introduction - In Fe-Ni-Cr austenitic stainless steels, two martensitic phases form after cooling below MS or deformation below Md. E martensite has an hcp structure and forms as plates, while bcc a' martensite forms as laths. Mechanisms for the y + E transformation have been proposed (1-4) which either predict the transformation to occur by a regular (2,3) or a random (4) faulting process on alternate {Ill} planes. The reverse transformation of E + y has also been reported (5) toyoccur by similar dislocation motion at the plate "ends", causing them to shrink. The a' martensite laths form in bands and contain a heavily dislocated substructure. Various habit planes (1,6,7) and orientation relationships (7-1 1) have been reported. In general the orientation is given as being uniquely either Nishiyama-Wasserman (N-W) or Kurdjumov-Sachs (K-S) (7,8), with some authors reporting both to occur simultaneously (9-11). The mechanism of the reverse transformation of a' + y is unclear. Both athermal (12) and diffusional (12,13) processes have been reported, together with the transformation proceeding in distinct stages (14). Various microstructural features have been observed with in reversed y, including stacking faults, twins and dislocation arrays (15,16). The present paper reports microstructural observations made on an 18/8 steel cooled to -196 C (producing 20%a1 and 10%~) and an 18/12 steel rolled at -196OC to give a 20% reduction in thickness (producing 20%a1 and 30%~). Changes produced on heating up to 900 C are also reported. Experimental Procedure as follows: - The compositions (wt%) of the two steels studied are Table I Composition of Steels (wt%) Fe Cr Ni C N M n S i Mo Ti V Co P S 1 18/8 Bal < < /12 Bal ( < Rolling the 18/12 steel was carried out by immersing strips, solution treated at 1050 C, in liquid nitrogen before deformation. Each rolling pass was limited to a 5% reduction in thickness. The a' content was determined using a magnetic balance and the E by X-ray diffraction techniques. Thin foils were prepared from 3mm diameter discs. Microanalysis was carried out using EDX analytical equipment on a JEOL 120 CX STEM. In situ heating experiments were conducted within an AEI EM7, lmev electron microscope, operated at 500 kv. Reversion of bulk samples was perfolm?d oy salt bath heating. Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE Results 1. Observations of martensitic structures. - Fig. 1 shows the a'-laths martensite in the 18/8 steel, having a 12251, habit plane and lying in bands, bounded by plates of E. Diffraction evidence shows streaked hcp E reflections, due to the thin plate morphology, with maxima at positions consistent with the previously observed (1) orientation relationship of (111$/~0_001),, [&I]// [1~101,. Two bcc patterns are evident, however, with one, L1111 consiztent with the K-S relationship (111) // (01 I),', [701Iy// [711],', and the other [I001 indicating the N-W relalionship ( 1 1 ) (Oll)a, [ I / Dark field imaging revealed individual lath orientations as labellex in Fig. 1 ; a' contains a high dislocation density, and in Fig. 1 three distinct slip-traces are evident within the K-S laths, consistent with (Oll),~, (101),1? and (110),1 slip planes. In the deformed 18/12 steel (Fig. 2), apart from the higher E content and a more heavily worked structure in much of the retained y the a' structure is very similar to the cooled 18/8 material. The E is clearly faulted and at the intersection of many y slip bands is in an early stage of formation. 2. In-situ heating. - No structural changes occurred on heating these martensitic structures up to 250 C. Between C, below the a'.% of either steel, (540 C for the 18/8 and 470 C for the 18/12) stacking faults within the retained Y shrink (Fig. 3), as deduced from changes in the fringe contrast (eg Fig. 3b). Similar effects were found for both the E within the retained austenite and that associated with a' bands,(fig. 4). 3. TEM observations on material heated in hulk form. - Samples of both steels were heate or an 30 min between C. On heating between the A and 600 C, the r rev:reed to adtwinned substruct:re. Above 600 C, a subqrain struchre was produced. On holding at 600 C for 2 mins a twinned structure formed which was replaced by subgrains, after 30 mins, (Fig. 5). Samples were also held for times of up to 10,000 min at 550 C. Fig. 6 demonstrates various aspects of the twinned structure. Most of this reverted structure has the same orientation as the retained y (labelled y,,). However, in the centre of the band is reversed austenite y2, rnisorientated to the rest by a 90" rotation about a <llo>y axis. Some a' martensite is also still present. Also, with the samples held at 550 C, steps (arrowed) were observed along a'/ a' interfaces during the early states of transformation and along al/reversed y boundaries after longer times, (Fig. 7). 4. Microanalysis. - In the 18/8 steel held at 550 C (Table 11) a significant decrease in Ni content is produced in the remaining bcc structure and an increase in Ni and decrease in Cr in the twinned, reversed y. TABLE I1 COMPOSITIONAL DATA OF THE 18/8 STEEL COOLED TO -196 C AND AGED AT 550 C Time (mins) Retained y Remaining a' Twinned y %Cr %Ni %Cr %Ni %Cr %Ni Discussion 1. E-martensite. - The fringed nature of the E-martensite, (Fig. 2), demonstrates that this is not a perfect hcp structure, but can be considered to be heavily faulted, or equally to be a network of stacking faults within the y phase. It has been

4 proposed (4,17) that c forins prior to the a' by the random overlapping of y stacking faults, generated either by a decrease in stacking fault energy on cooling or by dislocation cross-slip mechanisms at the intersection of y slip planes, (Fig. 2). On heating above %25O0C, the E -t y transformation then appears to proceed in an opposite manner. Thus, considering the phase to be a "bundle" of y stacking faults, these faults shrink (Figs. 3 & 4), decreasing in length and number throughout the E phase until an fcc structure remains, containing dislocations and a few "remnant" stacking faults (5). 2. a'-martensite. - It has been reported (9,10,11) that the two orientation relationships (K-S and N-W) can exist simultaneously in certain alloy systems. The diffraction evidence in Fig. 1 demonstrates that this is so for the 1818 steel. Both relationships were also found in the 18/12 steel. It has been suggested (10) that the relationship changes during the course of the transformation, with N-W predominant in the early stages, a concept consistent with tie featurgof Fig. 1. However, this is not wholly typical of the amount of each orientation found and large a' laths were observed in the N-W orientation. The two relationships differ by a 5'16' rotation about a <I lo>,' axis. Electron diffraction is accurate to within.l.zo (1 I), so it is, therefore, not possible to distinguish as to whether more than one precise orientation relationship exists as proposed by Rao (11) or whether a range of orientations occurs, clustering close to N-W and K-S as demonstrated by Sari kaya (9) using microdiffraction techniques. The dislocation defect structure of a' may arise by inheritance of faults from the faulted c. Considering the orientation relationships between the three phases y, ~&a', slip on one {IIO},' plane could be inherited from (0001) faults. Further slip on this plane and the other two, could then only be a result of the transformation shape strain. However, without knowing the precise slip systems and amount of shear, it is difficult to determine their precise role. The reverse transformation a' + y can occur isothermally and athermal ly (12-14). Fig. 7 and Table I1 provide supporting evidence for a diffusional transformation mode occurring close to the AS, involving movement of stepped (18, 19) al/y interfaces and compositional changes. A further proposal (16) has been that a subgrain reversed y structure is formed by the athermal transformation of neighbouring a' laths having small orientational variations, the twinned structure being produced by the recovery of stacking faults. However, in this investigation, stacking faults are only associated with the E + y transformation and as the a' + y transformation proceeds at 600 C a twinned structure forms prior to subgrains. It is therefore proposed that the twinned structure is associated with the a' -t y transformation, ie a' + twinned y + y. The subgrains are then formed by the recovery, at temperatures at and above 600 C, of this twinned substructure. In general the reversed y has the same orientation as the retained Y, though in areas close to the centre of the lath bands y of a different orientation is produced (Fig. 7). y of the same orientation is formed by the movement of the original aily boundaries, shrinking the laths. y of a different orientation is associated with y forming within the a' lath bands by a nucleation and growth process. Further observations involving the a' + y transformation have been reported el sewhere by the present authors (20,21). Conclusions 1. Plates of E-martensite are formed, with a faulted structure, in association with the a'-martensite, orientated with the close packed planes and directions of both E and y para1 lel. 2. a'-martensite, {225} laths form with an internal dislocation substructure, predominantly on three {TIO~,~ slip planes. These do not have a single unique orientation relationship. 3. The reverse transformation of E -+ y is the direct opposite of its formation, occurring by an unfaulting mechanism.

5 C4-578 JOURNAL DE PHYSIQUE 4. The transformation a' -t y proceeds both athermally and by diffusional processes producing y of the same orientation as the retained phase, having a twinned sub-structure which subsequently recovers to a subgrain substructure. Austenite of a different orientation, however, can also be produced within the a' lath bands. Acknowledgements - are made to Professor D.W. Pashley FRS for the provision of research facilities, and to SERC and to AERE Harwell for support. References 1. Venables, J.A., Phil. Mag., 7 (1962) Bollman, W., Acta. Met., 9 (T961) Seeger, A., Z. Metall., 4F (1953) Fujita, H. and Veda, S.,Tcta. Met., (1972) Enami, K., Nenno, S. and Minato, J., Trans. Jap. Inst. Met., 18 (1977) Lagneborg, R., Acta. Met., 12 (1964) Kelly, P.M., Acta. Met., ) Suzuki, T., Kojima, H., Suzuki, K., Hashimoto, T. and Ichihara, M., Acta. Met., 25 (1977) ~Fikaya, M., Proc. 39th Annual EMSA Meeting, (1981) Manganon, P.L. and Thomas, G., Met. Trans., 1 (1970) Narasimha-Rao, B.V., Met. Trans., 10 (1979) K Kessler, H. and Pitsch, W., Acta. Kt., 15 (1967) Jana, S. and Wayman, C.M., Trans. AIME, z 9 (1967) Coleman, T.H. and West, D.R.F., Metal Science, 9 (1975) Breedis, J.F., Trans. AIME, 236 (1966) Smith, H. and West, D.R.F., J. Mat. Sci., 8 (1973) Brooks, J.W., Loretto, M.H. and Smallman, K.E., Acta. Met., 27 (1979) Kinsman, ICR., Eichen, E. and Aaronson, H.I., Met. Trans., 6A(1975) Rigsbee, J.M. and Aaronson, H.I., Acta. Met., 27 (1979) 351, 20. Guy, K.B., Ph.D thesis, University of London, Guy, K.B., Butler, E.P. and West. D.R.F., Metal Science (to be published). Fig. 1-18/8, cooled to -196"C, showing E martensite and bands of a' martensite in retained y. Both N-W and K-S relationships are represented, as deduced from the diffraction pattern analysis.

6 Fig. 2-18/12, rolled by 20% at -196"C, showing a' obeying different variants of both N-W and K-S orientation relationships. Fig. 3-18/8, cooled to -196 C. (a) RT structure, (b) after heating to 250 C for 2 mins, (c) after heating to 450 C for 5 mins. Fig. 4-18/12, rolled by 20% at -196 C. (a) RT structure, (b) after heating to 425 C for 7 mins, (c) after heating to 475 C for 2 mins.

7 C4-580 JOURNAL DE PHYSIQUE Fig. 5-18/12, rolled by 20% at -196OC then aged at 600 C for 30 mins, showing y twins and subgrains within the reverted regions. Fig. 6-18/8, cooled to -196 C then aged at 550 C for 10,000 mins, showing twinned reverted y. Fig. 7-18/8, cooled to -196 C then aged at 550 C for 10,000 mins, showing steps, as arrowed on a'/ y boundaries.