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Acta Metall Sin  2019, Vol. 55 Issue (4): 496-510    DOI: 10.11900/0412.1961.2018.00191
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An Investigation of the Crystallography of Pearlites Nucleated on the Proeutectoid Cementite
Wensheng XU,Wenzheng ZHANG()
Key Laboratory of Advanced Materials MOE, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
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Abstract  

Pearlite is common microstructure in the carbon steel, which is widely applied in the railway steel and cold drawn steel where high wear resistance and strength are required. The pearlite colony is a circumscribed aggregate within which lamellae of cementite and ferrite phases have the same orientation. A cluster of wedge-shaped pearlite colonies will form the pearlite group nodules. The morphology of pearlite colonies will be influenced by the crystallography of pearlite. The common orientation relationship (OR) between pearlitic ferrite and pearlitic cementite is the Pitsch-Petch, Bagaryatsky, and Isaichev ORs. Combined with deep etching, SEM was used to investigate the morphology and crystallography of pearlite colonies and pearlite group nodules nucleated on the proeutectoid cementite in a Fe-1.29C-13.9Mn steel. The results showed that the initial morphology of the pearlite is irregular, but the pearlite possesses a parallel lamellar structure at the later stage of growth. Mutual ORs between phases of austenite, cementite, and ferrite in pearlite, proeutectoid grain boundary cementite, and Widmannstätten cementite were measured with the EBSD technique. Several reproducible ORs between cementite and ferrite lamellar have been observed, including the Pitsch-Petch, Bagaryatsky, and Isaichev ORs, without a particularly dominant OR. Since the two phases in the pearlite colonies have reproducible preferential OR, they are usually not independently nucleated, otherwise the independent nucleation of the cementite and ferrite inside the austenite has special crystallographic requirements for the mutual ORs between ferrite, cementite, and austenite. Thus, there will be a phase that nucleates first, which is called the "active nucleus". The active nucleus of pearlite has been carefully examined mainly according to the preferred OR between the pearlitic phases and existing phases. While the development of the pearlite crystallography is influenced by the active nucleus, no clear relationship was found between the ORs within the pearlite and active nucleus of the pearlite. The ORs between austenite and major pearlitic ferrite are near the K-S OR, but the ORs between austenite and the pearlitic cementite are various, depending on the preferred ORs between pearlitic ferrite and both austenite and pearlitic cementite. Widmannstätten cementite has never been seen to grow into pearlite. The measured data suggests that active nucleus of the pearlite colonies and pearlite group nodules nucleated on Widmannstätten cementite is ferrite. In some cases, grain boundary cementite was seen to grow as part of pearlite. Consequently, the grain boundary cementite is regarded as the active nucleus, though a preferred OR often coexists between pearlitic ferrite and either austenite or proeutectoid cementite. In other cases, the orientations of pearlitic cementite and grain-boundary cementite are discontinuous. For these cases, the ferrite is likely the active nucleus of pearlite. The orientation of pearlitic ferrite was seen to alter with the growth of pearlite, even causing the split of a single ferrite layer into two grain layers with a considerable misorientation. Significant distortion varying with the layers of pearlite was noticed in austenite near the pearlite growth front, indicating an evident strain field caused by the pearlite transformation. This requests a further investigation.

Key words:  pearlite      cementite      morphology      crystallography      orientation relationship      active nucleus     
Received:  15 May 2018     
ZTFLH:  TG113  
Fund: National Natural Science Foundation of China(No.51671111);National Key Research and development Program of China(No.2016YFB0701304)
Corresponding Authors:  Wenzheng ZHANG     E-mail:  zhangwz@tsinghua.edu.cn

Cite this article: 

Wensheng XU, Wenzheng ZHANG. An Investigation of the Crystallography of Pearlites Nucleated on the Proeutectoid Cementite. Acta Metall Sin, 2019, 55(4): 496-510.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00191     OR     https://www.ams.org.cn/EN/Y2019/V55/I4/496

Orientation relationshipMathematical equation
Isaichev[13]1ˉ03C//(110)F; 010C//11ˉ1ˉF; [311]C0.91°//[11ˉ1]F
Bagaryatsky[12][100]C//[11ˉ0]F; 010C//11ˉ1ˉF; [001]C//[112ˉ]F
Pitsch-Petch[14,15][100]C2.6°//[131ˉ]F; 010C2.6°//113F;[001]C//[52ˉ1ˉ]F
New-2[16,17]1ˉ03C//(1ˉ01)F; 010C8.5°//131F; [311]C//[11ˉ1]F
New-3[16,17]022ˉC//(1ˉ01)F;[311]C//[11ˉ1]F; [100]C2.4°//[13ˉ1]F
New-4[16,17]210C//(1ˉ01)F; [001]C//[131]F; [1ˉ21]C5.9°//[101]F
Table 1  Several common orientation relationships between cementite and ferrite layers inside pearlite colony[12,13,14,15,16,17]
Fig.1  SEM images of early pearlite colonies and pearlite group nodules in Fe-1.29C-13.9Mn steel after heat treatment at 650 ℃ for 15 h(a) intact pearlite colony nucleated on Widmannstätten cementite plate(b) pearlite group nodules nucleated on grain-boundary cementite (The morphology of pearlite in colonies I and III is irregular, but the morphology of pearlite in colony II is the common lamellar morphology)
Fig.2  Cross section SEM images of large pearlite group nodules in Fe-1.29C-13.9Mn steel after heat treatment at 650 ℃ for 72 h (The growth orientations of the pearlite in colonies I~V in Figs.2c and e are different)(a) pearlite group nodules nucleated on the Widmannstätten cementite plates(b) pearlite group nodules nucleated on the grain-boundary cementite(c) pearlite group nodules nucleated on the cementite plate (deep-etched)(d) 56° tilt of Fig.2c(e) pearlite group nodules nucleated on grain boundary (deep-etched)(f) 45° tilt of Fig.2e
Fig.3  EBSD results of an early pearlite group nodule in Fe-1.29C-13.9Mn steel after heat treatment at 650 ℃ for 8 h(a) SEM image of the pearlite group nodules and proeutectoid cementite
(b) phase color of the pearlite group nodules and proeutectoid cementite
(c) inverse pole figure (IPF) map of the phases in the pearlite group nodules, austenite matrix and the proeutectoid cementite (The pearlite group nodule is composed of three pearlite colonies: colony 1, colony 2 and colony 3)
ColonyF/CF/AC/AF/WC or F/GCC/WC or C/GC
1Pitsch-Petch[14,15]UnreportedUnreportedUnreportedDiscontinuous
2Pitsch-Petch[14,15]Near K-S (I)Switched F-E[32]UnreportedDiscontinuous
3Isaichev[13]K-S[26]UnreportedNew-3[16]Discontinuous
Table 2  Orientation relationships between pearlitic ferrite (F), pearlitic cementite (C), grain-boundary cementite (GC), Widmannstätten cementite (WC) and austenite matrix (A) in Fig.3
Fig.4  EBSD results of a single pearlite colony nucleated on Widmannstätten cementite plate in Fe-1.29C-13.9Mn steel after heat treatment at 650 ℃ for 72 h (The dashed lines in Figs.4a, b and d show the split of single ferrite layer, which are clear shown in the corresponding rectangulars)(a) phase figure (b) IPF map (c) image quality map (d) ferrite part of Fig.4b with part of ferrite colored (The scatter of blue part is within 12°)
Fig.5  Pole figures of F, C, WC and A in Fig.4 (TD—transverse direction, RD—rolling direction, TD and RD are the axis of sample coordinate system in the equipment) (a) (100)C (b) (010)C (c) (001)C (d) {011}C (e) {210}C (f) {131}F (g) {101}F (h) {111}F (i) {111}A (j) {101}A (k) pole figure of {100}A and {100}F under the orientation relationships with the Bain correspondence (K-S, N-W and G-T) and the experimental results
Fig.6  EBSD results of pearlite group nodules nucleated on WC (a, b) and GC (c, d) in Fe-1.29C-13.9Mn steel after heat treatment at 650 ℃ for 72 h(a, c) phase maps with different colors(b, d) IPF color maps of the pearlite group nodules (I~VI are pearlite colonies with different orientation inside the pearlite group nodules)
ColonyF/CF/AC/AF/WC or F/GCC/WC or C/GC
IBagaryatsky[12]UnreportedUnreportedUnreportedDiscontinuous
IIPitsch-Petch[14,15]Near K-S[26]UnreportedUnreportedNear (10°)
IIINew-2[16]Near K-S[26]UnreportedUnreportedDiscontinuous
IVPitsch-Petch[14,15]Near K-S[26]UnreportedBagaryatsky[12]Discontinuous
VPitsch-Petch[14,15]Near K-S[26]UnreportedNew-3[16]Continuous
VIPitsch-Petch[14,15]Near K-S[26]F-E[30]Pitsch-Petch[14,15]Continuous
Table 3  Orientation relationships between F, C, GC, WC and A in Fig.6
Fig.7  Pole figures of pearlitic ferrite and austenite matrix in Fig.6 (ND—normal direction)(a, b) pole figures {101}F and {111}F of pearlitic ferrite in Fig.6d (The black and blue parts are corresponding to the pearlitic ferrite in colonies IV and V, respectively) (c, d) pole figures {111}A and {101}A of the austenite in left grain
No.F/CF/WC or F/GCF/AC/WC or C/GCC/AWC/A or GC/A
1Pitsch-PetchUnreportedUnreportedDiscontinuousUnreportedF-E
2New-4UnreportedUnreportedDiscontinuousUnreportedF-E
3New-3/New-4Pitsch-PetchNear K-SDiscontinuousUnreportedF-E
4Pitsch-PetchUnreportedNear K-S10°UnreportedUnreported
5Pitsch-PetchUnreportedK-SDiscontinuousSwitched F-EPitsch
6IsaichevNew-3K-SDiscontinuousUnreportedF-E
7New-3-Near K-S-Unreported-
8BagaryatskyUnreportedUnreported10°UnreportedUnreported
8UnreportedIsaichevK-S10°UnreportedUnreported
8New-3New-3UnreportedDiscontinuousUnreportedUnreported
8BagaryatskyIsaichevK-SDiscontinuousUnreportedUnreported
9BagaryatskyUnreportedNear K-SDiscontinuousUnreportedPitsch
9Pitsch-PetchUnreportedNear K-SDiscontinuousUnreportedPitsch
9Pitsch-PetchUnreportedNear K-SDiscontinuousSwitched F-EPitsch
10Pitsch-PetchPitsch-PetchK-S (I)ContinuousUnreportedUnreported
11Pitsch-PetchPitsch-PetchK-S (I)ContinuousF-E (I)F-E (I)
12Pitsch-PetchBagaryatskyNear K-SDiscontinuousUnreportedUnreported
12New-3Pitsch-PetchNear K-S/K-S (I)UnreportedNew OR[39]
12Pitsch-PetchPitsch-PetchNear K-S (I)ContinuousNew OR[39]New OR[39]
13Bagaryatsky-K-S (I)-Pitsch (I)-
13BagaryatskyBagaryatskyNear K-S (I)ContinuousPitsch (I)Pitsch (I)
13Pitsch-PetchPitsch-PetchNear K-SContinuousPitsch (I)Pitsch (I)
13New-3New-3Near K-S (I)ContinuousF-E (I)F-E (I)
13New-3New-3UnreportedContinuousPitsch (I)Pitsch (I)
14UnreportedUnreportedK-SContinuousUnreportedUnreported
14Pitsch-PetchPitsch-PetchUnreportedDiscontinuousUnreportedF-E (I)
14Pitsch-PetchPitsch-PetchNear K-SDiscontinuousUnreportedF-E (I)
Table 4  EBSD results of different orientation relationships between C and F, proeutectoid cementite, and A for the pearlite colonies nucleated on proeutectoid cementite
Fig.8  
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