1. Introduction
High-strength eutectoid pearlitic steels are used as (i) constituent materials of rails [
1,
2] (
pearlitic rail steels) in mass after hot rolling; (ii) constituent materials of pre-stressed concrete structures, bridge cables and wire ropes in wire form as key elements in civil engineering [
3,
4,
5] (
cold-drawn pearlitic steel wires) after hot rolling and cold drawing; and (iii) reinforcement materials in vehicle tires [
6,
7] in the form of tiny wires after heavy cold drawing (
cold-drawn pearlitic steel wires).
While pearlitic rail steels do not undergo any mechanical treatment (only heat treatment), in cold-drawn pearlitic steels the cold drawing manufacturing procedure activates a strain hardening mechanism, thereby increasing both the yield strength and the ultimate tensile strength (UTS). The evolution of microstructure in pearlitic steel and its associated changes in strength and ductility during cold drawing were studied in the past by Embury and Fisher [
8], Langford [
9] and Ridley [
10], as well as by Lewandowski and Thompson [
11]. These important papers constitute, even nowadays, the key background of micromechanical analyses of pearlitic steels (either cold-drawn or not) and they conform the scientific bases of modern fracture mechanics approaches.
It is also worth mentioning the analyses by Nam and Bae [
12], Read et al. [
13], Toribio and Ovejero [
14,
15,
16,
17], Languillaume et al. [
18] or Nam et al. [
19] on microstructure changes in pearlite after drawing [
12,
13,
14,
15,
16,
17] or drawing-induced cementite dissolution [
18,
19]. The utmost importance of cold drawing effects on micromechanical features and fracture behavior is out of doubt.
The summary of such an utmost importance of the effects of cold drawing in pearlite is twofold: on one side, it produces orientation of both microstructural levels (pearlite colonies and lamellae) in the undoubtedly
hierarchical structure of pearlitic steels [
14,
15,
16,
17] and, on the other side, it can trigger local deterioration/degradation phenomena such as drawing-induced cementite dissolution [
18,
19].
A controversial topic [
20,
21,
22,
23] is the validity of the Hall-Petch equation to describe the better mechanical performance (increase in yield strength) with the decrease in the size of a characteristic microstructural unit: grain size, particle size, or, in the case of pearlitic steels, colony size or interlamellar spacing, the latter being usually considered as the key microstructural length representing the free distance for a dislocation to move before being blocked by the cementite barrier.
The phenomenon of blocking of free dislocation movement in the ferrite phase by the hardest cementite phase acting as a barrier provokes an increase in material strength. Although in
randomly-oriented pearlite (e.g., in a rail steel or a hot rolled pearlitic steel bar) the Hall-Petch equation can be valid to fit the strength
versus material characteristic size, in the case of
oriented pearlite (e.g., in a heavily cold-drawn steel) the relation between microstructure (represented by a characteristic length such as the pearlite interlamellar spacing) and strength does
not fit a Hall-Petch equation [
22] but an Embury-Fisher law [
23] in the case of drawn pearlite, to account for the evident microstructural orientation.
Many papers have dealt with the study of different hardening and strengthening mechanisms after cold-drawing of pearlitic microstructures [
24,
25,
26,
27,
28,
29,
30,
31]. With regard to fracture (
post-yield) behavior of pearlitic steels, one can find classical papers by Porter et al. [
32], Hyzak and Bernstein [
33], Park and Bernstein [
34], Alexander and Bernstein [
35] and Dollar et al. [
36], and more recent works [
37,
38,
39,
40,
41,
42] dealing with plasticity/yielding behavior, as well as fracture performance.
In the matter of environmentally assisted cracking [
43], in addition to pure stress corrosion cracking (SCC) by localized anodic dissolution (LAD), the deleterious phenomenon usually known as hydrogen embrittlement (HE) is harmful in cold-drawn pearlitic steel in the form of prestressing steel wires and cables in service [
44,
45,
46,
47,
48,
49,
50,
51,
52]. The analyses of such a phenomenon usually were performed in the past from the engineering point of view in the form of fracture mechanics approaches to avoid catastrophic accidents (an important practical aspect of HE and HAMD in high-strength pearlitic steel), so that there is a lack of purely-scientific material science approaches.
Such a phenomenon has received many names in the scientific literature such as hydrogen degradation (HD), hydrogen-assisted fracture (HAF) or hydrogen-assisted cracking (HAC) when a crack is present in the material. The first term (HD) is especially important because it includes any deleterious effect caused by hydrogen on a specific material, either typical (pure) embrittlement in the form of lack of ductility or decrease in fracture toughness, or more complicated phenomena involving a sort of interplay between locally ductile processes (localized plasticity or hydrogen-dislocation interactions) and global material degradation.
In this paper, the
hydrogen-assisted microdamage (HAMD) in the form of the so-called
tearing topography surface (TTS), a term coined by Thompson and Chesnutt [
53] and Costa and Thompson [
54] but just described as a new, non-conventional, fractographic mode but, in principle, not associated with a fracture phenomenon of specific nature (either brittle or ductile) or linked to a particular material (although all provided examples deal with metals and alloys).
At the
macro-,
micro- and
nano-levels, the TTS fractographic mode has been associated [
55,
56,
57,
58,
59] with HE/HD processes and HAMD in pearlitic steel, such a relationship thoroughly analyzed by Toribio (the author of the present paper) and co-workers. The present work goes further in the brief analysis on HE and HAMD in the presence of notches (
hydrogen-assisted notch-induced fracture and
notch tensile strength) published in a short paper included in conference proceedings [
60].
4. Fractographic Analysis at the Macro-, Micro- and Nano-Scales
In the tests performed
in air [
63], scanning electron microscope (SEM) fractographic analysis showed that the fracture process always initiates in a
fibrous or
dimpled region by a mechanism of
micro-void coalescence (MVC) and later propagates in an unstable manner by a
cleavage-like (C) micro-mechanism in geometries A, B and C, or in stable manner by MVC followed by shear lip (L) in the case of geometry D.
To have a complete picture of the fracture process at the micro-mechanical level of pearlitic steel in the presence of notches of very different geometries (with very distinct constraint/triaxiality levels),
micro-fracture maps (MFMs) were assembled covering the whole fracture surface of all geometries and containing information on the micromechanisms of fracture at each portion of the cross-sectional area (
Figure 2).
The macroscopic fracture mode is peripheral and plane in the first case (geometries A, B and C) and central with
cup and cone shape in the second case (geometry D). The cleavage-like topography associated with unstable final fracture exhibits the typical river patterns indicating the direction and sense of fracture propagation and therefore can explain the different stages of fracture (see
Figure 2).
In the case of the first set of geometries exhibiting the most brittle fracture mode (geometries A, B, and C containing the main area of cleavage/brittle fracture in the cross-sectional area, cf.
Figure 2), the cleavage micro-fracture mode is
oriented cleavage in the case of samples A and C (
single fracture initiation point) and
randomly-oriented cleavage sample B (
multiple or extended fracture initiation locations).
A characteristic value of triaxiality seems to exist below which fracture is by MVC and above which it is cleavage-like. This is consistent with the existence of a critical size of micro-void that is a decreasing function of the triaxiality.
In the HE tests, HAMD started at the notch tip, and fractographic analysis by SEM showed a characteristic microscopic fracture mode (non-conventional) with a kind of ductile tearing appearance: the so-called
tearing topography surface or TTS (
Figure 3), cf. [
53,
54,
55,
56,
57,
58,
59]. Such a TTS region may be analyzed at the
macro-,
micro- and
nano-levels. To this end,
Figure 3 also includes the fully-pearlitic microstructure of the steel, showing the importance of the
nano-scale, considering the level of the pearlite interlamellar spacing (0.3 μm), i.e., a
sub-micron scale for the softer (and more ductile) ferrite constrained between harder (and more brittle) cementite, thus allowing a consideration of the fully-pearlitic steel under study as a real
nano-composite material.
At the
macro-scale, the TTS zone or domain is a portion of area (region) in the MFM including all fractographic modes in the fracture surface. The MFMs corresponding to the HE tests are given in
Figure 4, showing that the TTS region is always peripherical, but in some cases it has thumbnail shape (shallow geometries A and C, either shallow sharp A or shallow blunt C) and in other cases it has a ring shape (deep geometries B and D, either deep sharp B or deep blunt D).
A question should be raised here regarding the macro- and micro-levels of analysis. The concept of MFM corresponds to the macro-scale, since such a map covers the whole macro-fracture surface (circular cross-sectional area of the notched sample in this case). Therefore, it is a macroscopic map containing microscopic modes of fracture, i.e., a map of micro-fracture or micro-fracture map (MFM).
After this origin by TTS (environmentally-assisted sub-critical cracking), a fracture progresses in an unstable manner by a cleavage-like (C) topography. In shallow geometries A and C with local thumbnail TTS shape, cleavage develops in a divergent progression and lines form the initiation by TTS, whereas in deep geometries B and D with external circumferential shape by TTS, cleavage progresses towards the inner point of the sample in a random manner (following diverse directions).
A plausible explanation of the type of MFM in each case of
Figure 4 would be the following: in sharply-notched geometries A and B, the intense stress concentration produced by the sharp notch creates a maximum of hydrostatic stress near the notch tip, thereby “pumping” hydrogen to locations in the near-tip zone and thus creating the
worst area (the
most damaged region) as a local TTS zone placed just at the specimen surface in the form of either a surface crack (sample A) or a surface external ring (sample B).
The same happens in shallow blunt notch C in which the maximum hydrostatic stress is also near the notch tip and thus a thumbnail TTS sub-critical crack is created at the notch tip. With regard to the deep blunt notch D, it exhibits a surface external ring by TTS. In this case, the maximum hydrostatic stress is achieved at the sample axis (place towards which hydrogen diffuses along radial directions), in such a manner that the axial (radial) symmetry is preserved in this case.
Thus, the two shallow geometries (A and C) promote a loss of axial symmetry in the HAMD area with external (surface) thumbnail TTS region whereas the two deep geometries (B and D) preserve the axial symmetry in the HAMD area with external (surface) ring-shaped TTS region.
At the
micro-scale, the TTS zone contains features resembling micro-damage, micro-cracking or micro-tearing (
Figure 3), since it does not have a purely brittle appearance (at the micro-level), in spite of the fact that it is associated with embrittlement by hydrogen (hydrogen embrittlement/HE) in pearlitic steels at the macro-level, so that it is more adequate to speak about hydrogen degradation (HD) in pearlitic steels. At the micro-scale, the TTS zone can be qualified as hydrogen-assisted microdamage (HAMD). The TTS fractography appearing in
Figure 3 does not show any evidence of orientation or alignment in privileged direction, i.e., it is randomly oriented. This is the most frequent situation regarding the TTS area (
non-oriented or
randomly oriented TTS).
At the nano-scale, the TTS region also contains some evidence of tear ridges at the sub-micron level, i.e., at the nano-level; it is the level of the pearlite interlamellar spacing of the considered pearlitic steel (about 0.3 μm, i.e., 300 nm). A plausible explanation of this sort of hydrogen-assisted sub-micron nano-damage inside the pearlitic microstructure could be any local plasticity/yielding phenomenon related to movement of dislocations along a glide plane in the ferrite phase, with possible breaking of the cementite lamellae by a nano-shear mode or by nano-tearing mode.
5. Evolution of Hydrogen-Assisted Microdamage (HAMD)
This section of the paper includes a thorough study on the microscopic features of the HAMD area in the different notched specimens (with distinct triaxiality/constraint level) and its time evolution.
Figure 5 shows the microscopic fracture mode in specimen A10, i.e., that of minimum notch depth and minimum notch radius that has undergone the fastest HE experiment (HE test of about 10 min up to final fracture under a hydrogen environment). The fractograph in
Figure 5 shows subcritical fracture initiation and slow initial cracking/damage/fracture propagation produced by unconventional TTS (assisted by hydrogen) and critical (unstable) cracking/fracture quick propagation taking place in the form of a typical cleavage-like micro-fracture mode that is purely mechanical and not assisted by hydrogen. In the slowest test A1 (test duration of about 13.5 h up to final fracture), the micro-fracture map is similar (TTS plus cleavage), but the depth of the HAMD area is higher.
The cleavage-like (C) fractographic mode exhibits the typical river patterns signaling the direction of fracture propagation. Such river patterns are globally divergent from the area associated with fracture sub-critical initiation by TTS (from bottom to top) and at the same time they are locally convergent (in the same way as tributaries approaching the main river flow) in specific areas of the fracture topography (following the sense of fracture propagation direction from the bottom of the fractograph to the top of it). While the TTS fractographic mode represents a sub-critical (hydrogen-assisted) fracture mode associated with slow crack growth, the cleavage-like propagation is quick and unstable while approaching final catastrophic fracture.
Figure 6 presents three fracture areas inside the sample B6, i.e., that of maximum notch depth and minimum notch radius and the fastest HE experiment with a relatively short test duration (time to final fracture of 12 min). At the notch tip (initiation of HAMD) a sort of
orientation of the TTS topography (
oriented TTS) may be observed, with an orientation along a radial axis representing the micro-damage (or nano-damage) propagation direction. Such an orientation inside the TTS zone is less defined in areas placed far from the notch tip where the fractographic mode becomes
randomly-oriented TTS (
non-oriented TTS in
Figure 6c).
After the two TTS regions (
oriented and
randomly oriented) that are clearly assisted by hydrogen, the microscopic fracture mode evolves in such a manner that it becomes more similar to the
micro-void coalescence (MVC) topography (probably also produced with hydrogen assistance) and thus it will be denoted as
quasi-MVC (QMVC) micro-fracture mode throughout this article, cf.
Figure 6a,c, and finally catastrophic fast failure takes place by cleavage (C) with no need of hydrogen assistance (it is quick/brittle mechanical final fracture), as shown at the top of
Figure 6a.
Figure 6b offers an enlarged and detailed view of the TTS fractographic mode. In the slowest test B21 (with 73 h of test duration up to final fracture), the micro-fracture map (MFM) is similar (TTS plus QMVC plus cleavage), but the depth of the HAMD area is higher in this case.
Figure 7 describes the evolution of HAMD in specimen C6, i.e., that of minimum notch depth and maximum notch radius (with a duration of 9 min in the HE test; i.e., the fastest experiment for this geometry). The sequence of fracture events is as follows: (i) slow (sub-critical) initiation of hydrogen-assisted fracture/cracking by
oriented TTS; (ii) further slow progress by
randomly oriented TTS (
non-oriented TTS) that is also assisted by the hydrogen environment and sub-critical; (iii) final slow fracture development by a
poorly-defined quasi-MVC (probably with certain hydrogen assistance); (iv) final quick fracture (catastrophic/unstable) by cleavage (C) with no need of environmental (hydrogen) assistance. In this geometry C, it is important to emphasize that, in the slowest HE test C7 (21 h of test duration up to final fracture), the micro-fracture map (MFM) is different (TTS plus cleavage), i.e., the QMVC area
disappears in this case, apart from the fact that the depth of the HAMD area is higher.
Figure 8 corresponds to specimen D5, i.e., that of maximum notch depth and maximum notch radius with an intermediate HE test duration of 2.5 h up to final fracture. The same trends of evolution of HAMD are observed, i.e., the following sequence (
Figure 8a):
The transition from TTS to quasi-MVC (specimen D5) is given in
Figure 8b and the transition from quasi-MVC to cleavage (also in specimen D5) is shown in
Figure 8c. With the same bluntly-notched geometry D and the slowest test D24 (with a long test duration of 90 h up to final fracture) the micro-fracture map (MFM) is similar (TTS plus QMVC plus cleavage) but the depth of the HAMD area is higher in this case.
To complete the information about the TTS features,
Figure 9 shows the initiation of HAMD by TTS in the annular external ring of sample D10, i.e., that of maximum notch depth and maximum notch radius with the minimum test duration of 11 min. Hydrogen damage at the micro-level can be observed in the external circumferential ring, in spite of the short duration of the test.
In the matter of the triaxiality effects on HAMD of the pearlitic steel considered in the present paper and its dependence and evolution with the embrittlement time, the fractographic features at the
macro-,
micro- and
nano-scales in all notched geometries should be analyzed considering two limit experiments: (i)
quasi-instantaneous tests (short enough to avoid deep hydrogen penetration distances from the notch tip) and, on the other hand, (ii)
quasi-static tests (long enough to allow the stationary condition for the hydrogen diffusion problem to be reached, thus permitting deep hydrogen penetration distances from the notch tip). The embrittlement time for the quasi-instantaneous tests was about 10 min, whereas for quasi-static experiments, such a time (test duration) ranged from 10 to 100 h (see
Table 3).
Table 4 shows the progression of HAMD in the considered pearlitic steel for distinct triaxiality levels in the specimen geometry. The transition region between the TTS and the cleavage areas in the form of quasi-MVC fracture zone appears
only in certain cases: firstly, in geometries containing deep notches, either a sharp deep notch (geometry B) or a blunt deep notch (geometry D), in both cases of short and long HE tests; secondly, the quasi-MVC topography also appears in geometry C containing a blunt shallow notch (but the quasi-MVC are appears
only in the case of short HE tests).
In other words, the quasi-MVC zone disappears (and all the HAMD region is pure TTS) for the geometry A (with a sharp shallow notch) and for long tests on geometry C (blunt shallow notch), and it becomes less defined for long tests of geometry B (sharp deep notch) and for short tests of geometry C (blunt shallow notch). In the next section, this effect is analyzed on the basis of the hydrostatic stress distribution in the different notched specimens used in the experiments.
8. Conclusions
The evolution of HAMD was analyzed, firstly showing an oriented TTS mode that turns to non-oriented (or randomly-oriented) TTS and finally to a quasi-MVC topography before final fracture by cleavage (purely mechanical) takes place.
The TTS represents a sort of HELP mode in which local micro- and nano-damage takes place in the form of hydrogen-plasticity interactions and dislocation transport of hydrogen developing at a very fine scale of analysis.
Although the main mechanism of hydrogen transport in pearlite is diffusion assisted by stress and strain fields at the macro-scale, hydrogen transport by dislocational movement (dragging) could be operative at the micro- and nano-scale.
At the micro-level, the formation of TTS could involve an interplay also between HELP and HEDE in the form of pearlite tearing (PT) as a mode of HELP or, on the other hand, in the form of pearlite delamination (PD) as a mode of HEDE.
In the case of a local phenomenon of HELP by PT to create TTS, the Miller-Smith mechanism of shear cracking of pearlite (SCP) could be operative in this case, with breaking of brittle cementite lamellae by local shear of ductile ferrite bands.
The locally-ductile micro-mechanism of PT or SCP (a mode of HELP) produces a trans-lamellar cracking path (TLCP), whereas the locally-brittle micro-mechanism of PD (a mode of HEDE) produces an inter-lamellar cracking path (ILCP).
The TTS fractographic mode represents a subcritical hydrogen-assisted fracture mode associated with the stage II or plateau in the CGK curve da/dt-K in which the CGR is independent of the SIF K.
The CGR during TTS crack propagation was measured in pre-cracked samples of pearlitic steel under constant strain tests (CSTs), with a value of CGR v = 10−7 m/s for hydrogen-assisted sub-critical cracking by TTS.
The TTS appears in strongly-hydrogenated areas, whereas the pseudo-MVC areas (transition regions) correspond to weakly-hydrogenated zones, as demonstrated by comparing the HPP to the maximum hydrostatic stress point and the HE test duration.
The quasi-MVC topography could be considered as a candidate to TTS that becomes TTS itself when a sufficient amount of hydrogen penetrates this zone of sub-critical hydrogen-assisted cracking/fracture.