The morphology and ageing behaviour of δ-ferrite in a modified 9Cr-1Mo steel

doi:10.1016/0022-3115(92)90377-W

Journal of Nuclear Materials

Volume 195, Issues 1–2, October 1992, Pages 198-204

https://doi.org/10.1016/0022-3115(92)90377-W Get rights and content

Abstract

Dual phase (martensite + δ-ferrite) microstructures were developed in a modified 9Cr-1Mo steel, by austenitising at 1523–1623 K, followed by water-quenching. These duplex structures were thermally aged at 973 K for ageing periods varying from 30 min to 21 h. Morphological aspects of δ-ferrite phase and its response to age-hardening were studied by optical, scanning electron and transmission electron microscopy, X-ray diffraction, electron probe microanalysis and microhardness testing.

It was observed that austenitizing at 1523 K produced fine, acicular δ-ferrite while the δ-ferrite formed by austenitising at higher temperatures (1573–1623 K) were massive, irregular-shaped and banded. Moreover the presence of 8-ferrite caused an abnormally strong (110) reflection, observed in X-ray diffraction patterns of martensite plus δ-ferrite structures. This behaviour is thought to be due to development of (110) texture in δ-ferrite phase. Thermal ageing at 973 K caused age-hardening of δ-ferrite with a peak hardness attained after 3.6 ks of ageing. Electron microscopic results suggest that the observed hardening was caused by the formation of Fe₂Mo Laves phase.

References (13)

Y. Hosoi et al.
J. Nucl. Mater.
(1986)
D.S. Gelles
ISIJ Int.
(1990)
S.J. Sanderson
V.K. Sikka et al.
C.M. Vitek et al.
Metall. Trans.
(1983)

There are more references available in the full text version of this article.

Cited by (13)

Prediction and analysis of magnetically impelled arc butt welded dissimilar metal
2019, Materials Today: Proceedings
Citation Excerpt :
Additionally, in the nuclear reactor systems the above mentioned steels have considered as the alternate for the austenitic steel, due to its high amount of resistance against the embrittling due to irradiation and ducting. Currently some alloy steels are altered for the applications such as modified 9% Chromium – 1% Molybdenum steel (T91) [7]. In concern with improving the working properties tempered and normalized T91 have been utilized in the thermal power plant [8].
In the emerging welding techniques, Magnetically Impelled Arc Butt Welding (MIAB) is a speedy. Consistent process among the arc welding processes. MIAB welding employs magnets for producing a magnetic field, heat energy from an electrical source for generating melting or fusion between two parent metals followed by forging process to complete the welding action. This paper presents the mechanical joining of two dissimilar metal which made up of T11 and other T91 boiler graded tube materials with thickness varying between 3.5–5.5 mm, 200 mm as the length of the tube and 44.5 mm as the outer diameter using Magnetically Impelled Arc Butt welding process. The process parameters are varying input current between 900 and 1100 A and time of joining 15–14 s. Post welding an investigation on mechanical characteristics based on hardness test and microstructural characterization of the Heat affected zone (HAZ) and Weld Metal, along with the Base Metal have been performed. Based on the obtained results, the welded part shows good strength and homogeneous microstructure.
Creep rupture behavior of welded Grade 91 steel
2016, Materials Science and Engineering: A
Creep rupture behavior of fusion welded Grade 91 steel was studied in the temperature range of 600 – 700 °C and at stresses of 50–200 MPa. The creep data were analyzed in terms of the Monkman-Grant relation and Larson-Miller parameter. The creep damage tolerance factor was used to identify the origin of creep damage. The creep damage was identified as the void growth in combination with microstructural degradation. The fracture surface morphology of the ruptured specimens was studied by scanning electron microscopy and deformed microstructure examined by transmission electron microscopy, to further elucidate the rupture mechanisms.
A roadmap for tailoring the strength and ductility of ferritic/martensitic T91 steel via thermo-mechanical treatment
2016, Acta Materialia
Citation Excerpt :
TMT of T91 steels further enhances their mechanical strength with moderate loss of tensile ductility as shown in Fig. 15b. In the current study, δ-ferrite was absent in the selected austenization range of 800–1200 °C in T91 steel, consistent with earlier studies that show δ-ferrite formed during normalization above 1250 °C for 5 min or longer [46,47]. δ-ferrite is detrimental to the creep strength of T91 [48].
Ferritic/martensitic (F/M) steels with high strength and excellent ductility are important candidate materials for the life extension of the current nuclear reactors and the design of next generation nuclear reactors. Recent studies show that equal channel angular extrusion (ECAE) was able to improve mechanical strength of ferritic T91 steels moderately. Here, we examine several strategies to further enhance the mechanical strength of T91 while maintaining its ductility. Certain thermo-mechanical treatment (TMT) processes enabled by combinations of ECAE, water quench, and tempering may lead to “ductile martensite” with exceptionally high strength in T91 steel. The evolution of microstructures and mechanical properties of T91 steel were investigated in detail, and transition carbides were identified in water quenched T91 steel. This study provides guidelines for tailoring the microstructure and mechanical properties of T91 steel via ECAE enabled TMT for an improved combination of strength and ductility.
Microstructural evolution of delta ferrite in SAVE12 steel under heat treatment and short-term creep
2012, Materials Characterization
Citation Excerpt :
Therefore, delta ferrite can be formed during solidification process, hot working, and welding process. The presence of delta ferrite in steels reduces the attainable strength, fatigue endurance limit and impact resistance [8–11]. For austenitic stainless steels [12–14] and martensitic stainless steels [15,16], plenty of investigations about delta ferrite have been carried out.
This research focused on the formation and microstructural evolution of delta ferrite phase in SAVE12 steel. The formation of delta ferrite was due to the high content of ferrite forming alloy elements such as Cr, W, and Ta. This was interpreted through either JMatPro-4.1 computer program or Cr_eq calculations. Delta ferrite was found in bamboo-like shape and contained large amount of MX phase. It was surrounded by Laves phases before creep or aging treatment. Annealing treatments were performed under temperatures from 1050 °C to 1100 °C and various time periods to study its dissolution kinetics. The result showed that most of the delta ferrite can be dissolved by annealing in single phase austenitic region. Dissolution process of delta ferrite may largely depend on dissolution kinetic factors, rather than on thermodynamic factors. Precipitation behavior during short-term (1100 h) creep was investigated at temperature of 600 °C under a stress of 180 MPa. The results demonstrated that delta ferrite became preferential nucleation sites for Laves phase at the early stage of creep. Laves phase on the boundary around delta ferrite showed relatively slower growth and coarsening rate than that inside delta ferrite.
Influence of microstructural inhomogeneities on the fracture toughness of modified 9Cr-1Mo steel at 298-823 K
2012, Journal of Nuclear Materials
Citation Excerpt :
Austenitising temperature and cooling rate are known to influence the microstructure of this class of steels [19,21]. Water quenching from a high austenitising temperature (>1573 K) results in presence of delta ferrite in the microstructure [20,21]. At lower [20] austenitising temperature (1323 K) furnace cooling results in formation of proeutectoid-ferrite regions within the martensite.
Quasistatic fracture behaviour of two heats of modified 9Cr–1Mo steel for steam generator applications have been assessed at 298, 653 and 823 K. J–R curves were established and the elastic–plastic fracture toughness values at 0.2 mm crack extension (J_0.2) were determined. The fracture mechanisms were entirely different for the two steels at 298 K, with brittle fracture controlled by cleavage crack initiation in one and ductile fracture in the other by void nucleation and growth. At 653 and 823 K, fracture in both materials was by ductile crack growth. The difference in behaviour between the two steels at 298 K was attributed to the differences in microstructure, distribution and density of inclusions as well as phosphorous contents.
A study on martensitic phase transformation in 9Cr-1W-0.23V-0.063Ta-0.56Mn- 0.09C-0.02N (wt.%) reduced activation steel using differential scanning calorimetry
2010, Journal of Nuclear Materials
Citation Excerpt :
The formation of hard and brittle martensite as a result of the solutionising and quenching heat treatment and its subsequent tempering to achieve the desired balance between toughness and strength constitute the core physical metallurgy of modern power plant steels [1–10]. Not surprisingly therefore, numerous studies, both on experimental [11–48] and modelling – simulation fronts [49–84] have been conducted till date to understand the phase and microstructural stability of these alloys upon high temperature service-exposure and the attending changes in physical and mechanical properties in terms of simple intuitively understood metallurgical concepts. Thanks to the immense and coordinated worldwide research, the present day high chromium power plant steels have scientifically tailored chemical compositions, melting practices and thermomechanical processing schedules that make possible the attainment of fairly stable microstructure with desired physical and mechanical properties [1,2].
The salient characteristics of martensite formation kinetics in 9Cr–1W–0.23V–0.063Ta–0.09C–0.02N (wt.%) reduced activation steel have been investigated using differential scanning calorimetry and supplemented by microscopy characterisation. Accurate determination of the martensite start (M_S) and finish (M_f) temperatures as a function of cooling rate (1–99 K min⁻¹) and holding time (1–120 min) in the γ-austenite phase field have been made. The critical cooling rate to form nearly 100% martensite is found to be of the order of 5–6 K min⁻¹. The measured latent heat for the martensite transformation is found to be in the range 62–75 J g⁻¹. In the case of austenitisation at 1323 K, it is found that the grain size of the parent austenite exhibited significant growth and the observed M_S temperature too exhibited a systematic increase with austenite grain size. However, for those samples that are solution treated at a slightly lower temperature of 1253 K, the M_S is found to be initially insensitive to the increase in holding time for up to about 30 min. This trend is followed later on by the usual increasing character of M_S for further rise in the holding periods, up to about 120 min. This unusual initial behaviour is also reflected by the corresponding variation in the hardness values of the quenched in martensite. Moreover, the growth of parent austenite grains has also been restricted during the initial phase of solution annealing at 1253 K. This is attributed to the presence of undissolved carbide particles, which exert a pinning influence on the movement of austenite grain boundaries. The observed athermal martensite formation kinetics is described well by the Koistinen–Marburger relation.

View all citing articles on Scopus

View full text

The morphology and ageing behaviour of δ-ferrite in a modified 9Cr-1Mo steel

Abstract

J. Nucl. Mater.

ISIJ Int.

Metall. Trans.