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Influence of hardened layer structure and interface profile on character of plastic deformation development and fracture of surface hardened low carbon steels at the mesolevel
A.V. Romanenko and S.V. Panin1
Tomsk Polytechnic University, Tomsk, 634050, Russia 1 Institute of Strength Physics and Materials Science SB RAS, Tomsk, 634021, Russia
The investigation of influence of hardened layer structure and interface profile on plastic deformation development in boronized low carbon steels was carried out with the use of television-optical technique TOMSC and electron scanning microscope TESLA BS 300.
Substantial experimental and theoretical data describing deformation of surface hardened materials with plane profile of the interface between coating and matrix at the mesolevel were accumulated by now. By the group of Academician Victor Panin was shown, that a system of quasi-periodical transverse cracks form in the surface layer under active tension of such compositions. The distance between cracks depends on the hardened layer thickness [1,2]. In so doing, presence of high-strength nitrided layer on the surface promoted both significant yield and ultimate strength increase and plasticity decrease. Dispersing of stress mesoconcent-rators (meso SC), which can be achieved by formation of an interface with non-plane profile, would substantially change the cracking pattern, reduce the level of deformation localization and provide noticeable increase of operating characteristics of surface hardened structural steels [3]. Surface hardened layers formed by diffusion boronizing have a complex structure and consist of several phases with different mechanical characteristics located one after another. Depending on modes of the hardening process and substrate structure, the interface profile can be needled, toothed or plane [4]. It is evident that non-plane interface profile in boronized steel specimens is to provide redistribution of meso SC at
it, decreasing the degree of deformation localization in the substrate.
For boronizing were used low-carbon (~0.2 % C, 0.30.6 %Mn, >0.03 %Ni) and 15N3MA (0.15 % C, 0.20.3% Mo, ~3 % Ni) steels. The latter is used in the industry for heavy-loaded friction units (drilling bit pins, for example). Boronized layers on the studied specimens were formed by diffusion boronizing in powders. The boronized layer thickness was varied from 40 to 180 ^m by the process temperature. In the case of low carbon substrate the interface profile was needled (Fig. 1(a, b)). A pearlite interlayer formed between boride tooth and ferrite substrate to smooth the needled interface profile (Fig. 1(a)). Preliminary carbonization of low-carbon steel specimens was used to obtain toothed layers of 20-80 ^m (Fig. 1(c)) and plane layers of 260 |j,m (Fig. 1(d)) on pearlite sublayer.
Mechanical testing machine IMASH 20-78 was used for tension/compression tests. The rate of loading was 0.03-
0.05 mm/min. The loading diagrams were obtained with the use of Schenck-Sinus-100 hydraulic testing machine. The specimens for tension tests were bone-shaped with the gauge length 25x 1.5x4 mm3. The specimens for compression tests were made in the form of parallelepiped with the
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Fig. 1. The structures of boronized layers: needled formed on 15N3MA steel (a); needled formed on low-carbon steel (b); toothed (c); plane (d): 1 — FeB; 2 — Fe2B; 3 — pearlite; 4 — carboboride Fe3(C, B); 5 — substrate
© A.V Romanenko and S.V. Panin, 2004
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Fig. 2. Stress-strain curves of boronized low-carbon steel specimens (a) and preliminary carbonized low-carbon specimens (b) with different boronized layer thickness under tension
gauge length 3 X 1.5 X 1.5 mm3. Development of plastic deformation was studied on lateral faces in the interface area with the use of television-optical complex TOMSC, operation principle of which is based on the correlation analysis of optical images shot by double-exposure method [5]. The character of plastic deformation development at the meso-level was investigated by analysis of displacement vector fields. Longitudinal and transverse vector components were used for strain rate intensity calculation just as in [6]. Boronized layers cracking was studied on plane specimen faces with the use of a scanning electron microscope TESLA BS 300.
Low-carbon steel, tension
Development of plastic deformation at the mesoscale level in the specimens was determined by interface profile and boronized layer thickness. In Fig. 2 are shown stress-strain curves. The following peculiarities of the curves should be mentioned. First, increase not only ultimate strength, but also percent elongation of the specimens with thin (x 80 |j,m) boronized layers (Fig. 2(a), curves 1-3). For all the other specimens plasticity decreased with hardened layer thickness increase. Second, significant increase of mechanical characteristics of specimens with pearlite interlayer (toothed and plane interface, Fig. 2(b), curves 2-5). Third, rather high plasticity of the specimen with plane interface and boronized layer of 260 ^m, comparable with that of the specimen with toothed interface and boronized layer of 80 |j,m (Fig. 2(b), curves 5 and 4 correspondingly).
According to the classification in [1], cracks in hardened layers can be divided into primary and secondary. Primary cracks formed at the very beginning of plastic flow (yield plateau), and secondary — during following boronized layer fragments fragmentation. In so doing, primary cracks formed perpendicular to the direction of loading, while secondary cracking depended on interface profile and boronized layer thickness. Secondary cracks were oriented along the directions of maximum tangential stresses (xmax) in specimens with pearlite gradient interlayer (toothed and plane interface) and thin (x 80 ^m) needled boronized layers. In the case of thick (more than 80 |j,m) needled boronized layers, secondary cracks form parallel to the primary ones. Transverse cracks, which form in such boronized layers, lead to decrease of both specimen strength and plasticity (Fig. 2(a), curves 4 and 5).
Plastic flow of specimens with thin hardened layers began from the Luders band propagation. The latter defined the primary cracking period. With that, macrobands of localized plastic deformation formed along the T max directions (Fig. 3(b, c)). Intersecting, they formed mesovolumes in a form of triangular prisms. Spatial cracking period and mesovolumes size increased with boronized layer thickness. The further deformation of the specimens was characterized by self-consistent motion of primary mesovolumes. They moved both independently and also in groups to form larger mesovolumes, also in the form of triangular prisms. This effect was most pronounced in the specimens with boronized
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Fig. 3. Mesoband formation under primary cracking of boronized layer of 80 ^m: optical image of specimen lateral face (a); corresponding displacement vector field (b); strain rate intensity distribution (c). 8 ~ 1 %
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Fig. 4. Stress-strain curves of low-carbon steel specimens with needled (a) and toothed/plane (b) interface profile with different boronized layer thickness under compression
layers of 80 |j,m and less and characterizing by highest plasticity (Fig. 2(b), curves 2, 3).
Luders band propagation promoted uniform meso SC distribution. Self-consistence of a large number of meso SC along x max directions in the case of boronized layer thickness 80 |j,m promotes involving the whole specimen volume into shear deformation. It manifests itself in both pattern of boronized layer secondary cracking and realization of shears along Tmax directions in the substrate. As a result, the ultimate strength and the plasticity are increased (Fig. 2(a), curves 2, 3). Formation of through transverse cracks leads to abrupt lowering of the whole stress-strain curve (Fig. 2(a), curves 2, 3).
Low-carbon steel, compression
Development of plastic deformation in low-carbon steel specimens under compression also was defined by interface profile and boronized layer thickness (Fig. 4). The following peculiarities ofthe curves should be mentioned. First, significant increase of mechanical characteristics of the specimens with thin (x 80 |j,m) boronized layers (Fig. 4(a), curve
3, Fig. 4(b), curves 2-4). Second, decrease of loading stress with further hardened layer thickness increase (Fig. 4(a), curve 4, Fig. 4(b), curve 5). It should be noted that in the case of preliminary carbonized specimen, boronized layers even of significant thickness did not lead to yield strength decrease (Fig. 4(b), curve 5).
Deformation relief in non-hardened specimens and specimens with boronized layers of x80 ^m was uniform, without macrolocalizations. In the case of thick (>80 ^m)
hardened layer, a macrocrack formed in it to localize deformation extensively and to provide formation of macrobands of localized deformation.
So, boronized layers of <80 ^m promote significant specimen hardening under compression: the coefficients of surface hardening are KCTy = 2.5 for both specimen types, Ka = 1.5 for the specimens with needled layers and Ka = 1.5 for the specimens with toothed layers. As under tension, the thicker boronized layer lead to deformation localization under cracking and elimination of most part of the substrate from resistance to plastic deformation. However, it should be noted that pearlite interlayer allows to escape from yield strength decrease: in the case of boronized layer thickness 260 ^m surface hardening coefficients are KCT = 2.5 and Kn = 1.4.
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15N3MA steel, tension
Under loading of boronized 15N3MA steel specimens thin stochastically distributed cracks formed in hardened layers. It should be noted that boronized layers of any thickness led to decrease of both specimens strength and plasticity (stress-strain curves are given in Fig. 5(a)).
Under loading of 15N3MA steel specimens band of localized plastic deformation formed from the cracks in a bo-ronized layer into the substrate. These bands propagated non-symmetrically, mainly in the x max directions. As distinct from low-carbon steel specimens, the band did not propagate deep in the substrate. In the neck area, separated thin cracks united to form deep transverse cracks. In doing so, propagation of elongated bands of localized plastic
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Fig. 5. Stress-strain curves of the 15N3MA steel specimens with different boronized layer thickness under tension (a) and compression (b)
Fig. 6. Schemes of 15N3MA steel specimen shape changing under compression: 40 ^m (a); 100 |xm (b)
deformation in the xmax directions to form triangular prisms in the subsurface layer was observed. The main crack propagated along one of them.
It can be stated, that low plasticity of 15N3MA steel does not provide uniform involving of specimen value into shear deformation under stochastic cracking of boronized layer. It decreases meso SC role at the needled interface. Therefore, under loading of boronized 15N3MA steel plastic deformation localization and plasticity decrease take place. Boronizing of more soft low-carbon steel for the same depth promotes less deformation localization and plasticity increase.
15N3MA steel, compression
Under compression of specimens with thin boronized layers increase of their strength was took place (Fig. 5(b), curve 2): surface hardening coefficients are KCT y = 1.5 and K a = 1.2. Like low-carbon specimens with needled interface, ultimate strength of the specimen with boronized layer of 100 |j,m was significantly decreased (Fig. 5(b), curve 3).
Folds of extruded material, oriented normal to the direction of applied loading, are characteristic elements of deformation relief (except grain boundary relief) under compression ofnon-hardened 15N3MA steel specimens. Thin boro-nized layers changed the fold orientation: in the case of boronized layer thickness 40 ^m they were oriented at 7075° to the loading axis, and in the case of boronized layer thickness 60 |j,m — 50-60°. As in the case of low-carbon steel, cracking of boronized layer of 100 ^m provided significant deformation localization and formation of macrobands of localized plastic deformation, propagating in the xmax directions that caused loading stresses decrease (Fig. 5(b)).
Changing of 15N3MA steel specimens shape is worthy of notice. In the case of thin hardened layer (about 4060 |J,m), a specimen got barrel-like shape as a result of compression (Fig. 6(a)). In the case of thick boronized layer, however, initially parallelepiped specimen got prismatic shape (Fig. 6(b)). Analysis of SEM-images of boronized layers of such specimens has shown that a macrocracks forms in it at the initial stage of plastic deformation. An area with decreasing flow stress values in the stress-strain curves corresponds to it. In low-carbon steel specimens, both carbonized and not, the pearlite sublayer smoothed the difference in mechanical characteristics of hardened layer and substrate and did not allow the cracks to penetrate into the substrate and changing of specimen shape took place according to the scheme given in Fig. 6(a).
So, thin boronized layers, formed on 15N3MA steel, exert positive influence under compression, promoting mechanical characteristics increase and high specimens plasticity. Boronized layers of larger thickness lead to significant deformation localization, non-uniform specimen shape change and mechanical characteristics decrease.
Conclusion
1. Meso SC, distributed at needled interface in boronized low-carbon steel with hardened layer of 80 |j,m provide uniform shear deformation in the bulk material and boronized layer cracking in the conjugated directions of maximum tangential stresses that promotes increase of both strength and plasticity of the specimens. Further increase of boronized layer thickness leads to decrease of all mechanical characteristics.
2. Boronized layers with toothed interface, formed on preliminary carbonized low-carbon steel promote increase of mechanical characteristics not depending on thickness and loading conditions.
3. Presence of boronized layer with needled interface profile on the surface of 15N3MA steel specimens leads to decrease of their strength and plasticity characteristics under tension. Under compression, boronized layers of not more than 80 |j,m promote significant increase of strength characteristics of the surface hardened specimens.
4. Formation of toothed/needled “hardened layer - substrate” interface profile, as well as gradient transitional layer can be recommended for low-carbon steels surface hardening.
5. The technique of visualization of meso SC relaxation pattern with the use of television-optical complex TOMSC, which was put forward in the work, can be effectively used for surface hardening modes optimization.
References
[1] V.E. Panin, Synergetic principles of physical mesomechanics, Phys. Mesomech., 3, No. 6 (2000) 5.
[2] V.E. Panin, V.M. Fomin, and V.M. Titov, Physical principles of mesomechanics of surface layers and internal interfaces in a solid under deformation, Phys. Mesomech., 6, No. 2 (2003).
[3] R.R. Balokhonov, S.V. Panin, V.A. Romanova, P.V. Makarov, Simulation of Stress Concentration and Localized Plastic Flow in Coated Materials on the Mesolevel, in Proceedings of the International Conference on New Challenges in Mesomechanics, Aalborg University, Denmark, V. 2 (2002) 587.
[4] O. Sizova, A. Kolubaev, and G. Trusova, Einflu|3 der Struktur von Borid-Schutzschichten auf Reibung und Gleitverschlei|3, Metall, No. 12 (1997) 713.
[5] V.I. Syryamkin, V.E. Panin, E.E. Deryugin, A.V Parfenov, G.V. Ne-rush, and S.V. Panin, Optical-Television Techniques for Research and Diagnostics of Materials at Mesolevel, in Physical Mesomechanics and Computer-Aided Design of Materials, Ed. by V.E. Panin, V. 1, Nauka, Novosibirsk (1995) 176.
[6] L.S. Derevyagina, V.E. Panin, I.L. Strelkova, and A.I. Mirkhaidarova, Self-organization of zones with high plasticity in the region of geometric stress concentrators and fracture behavior of copper in tension, Phys. Mesomech., 6, No. 5 (2003) 11.
