Dimensional anisotropy during phase transformations in a chemically banded 5140 steel. Part I: experimental investigation
Introduction
Chemical segregation is an issue in steelmaking because it can cause microstructural banding and impart properties and performance that depend on banding orientation. A common type of microstructural banding is alternating layers of proeutectoid ferrite and pearlite, and the underlying source of this inhomogeneity has been associated with chemical segregation [1]. Chemical inhomogeneity develops during solidification when elements with a low partitioning ratio [2] are ejected into the interdendritic regions and this produces areas of high solute concentration. Subsequent deformation produces a segregation pattern that is aligned with the deformation direction. This inhomogeneity in substitutional alloying elements affects the local transformation behavior of the material. Solute rich areas stabilize austenite such that, during cooling, solute lean regions transform first followed by transformation of solute rich regions, and the reverse is true on heating except that austenite is the transformation product. It has been shown that manganese is prone to segregation [3] and has a significant effect on transformation temperatures [4].
A number of investigations have considered the link between banded microstructures and dimensional anisotropy. Siegmund et al. [5] reported that a non-transforming, austenite/ferrite, duplex stainless steel experienced dimensional anisotropy during thermal cycling between 20 and 900 °C. The work revealed that plastic deformation was required to accommodate dissimilar thermal expansion behavior for each phase. Because the structure was banded, microscopic accommodation strains were biased such that a macroscopic shape change was observed. Because the austenite has a larger CTE, during heating, the austenite was in compression and the ferrite in tension parallel to the banded microstructure. At high temperatures, the stress exceeded the yield strength of the ferrite and the phase deformed in tension. On cooling, the sign of the stresses reverses and the austenite was put in tension and the ferrite experienced a compressive stress. At low temperatures, the austenite is softer than the ferrite and the stress became large enough to induce plastic deformation in the austenite. This deformation was tensile and coupled with the tensile deformation of the ferrite on heating, a net elongation in the direction of the banded microstructure was observed for a thermal cycle. The key material properties responsible for this phenomenon are the dissimilar CTE and the relative yield strength of the phases which reverses over the temperature range. A finite element model was used to predict the experimental behavior. This phenomenon required that multiple phases exist over a large temperature range and such is not the case for the material of interest in the current research; however, it does illustrate a connection between microstructural morphology and material distortion.
Kennedy et al. [6] noted the occurrence of dimensional anisotropy for a variety of steels cooled at high and low cooling rates. Based on data collected for materials exposed to 100, 500 and 1000 cycles, several significant observations were made regarding the resulting shape changes:
- (1)
the length change per cycle is substantially consistent;
- (2)
a strong correlation exists between the shape change and the amount and direction of prior deformation;
- (3)
the magnitude of shape change decreases with increasing peak temperature; and
- (4)
a phase transformation is necessary in order for the phenomenon to occur.
The authors note the dependence upon prior deformation and suggest that the mechanism responsible for dimensional anisotropy may be related to texture and/or chemical segregation in the material.
In a series of papers [7], [8], [9], Goldberg observed transformation anisotropy in a 300-grade maraging steel undergoing martensitic phase transformation. The anisotropy was related to the prior deformation of the material and shape change was observed during both the martensitic transformation and the reverse transformation to austenite. It was proposed that chemical segregation results in variation of MS temperature and causes a “layer growth” phenomenon in which preferential formation of martensite occurs within definite layers of austenite. It was hypothesized that anisotropy was due to plastic deformation and/or oriented transformation of the austenite, as compressive stress caused by the preferential growth developed [9]. This mechanism was cited for the shape change during transformations on both heating and cooling.
Nagayama and co-workers [10] observed shape change during thermal cycling of Cr–Ni–Mo–Al–Ti maraging steel experiencing martensitic transformation. The net strain associated with a single temperature cycle was termed “ratchet strain”. While the source of the strain anisotropy was not investigated, it was assumed to be caused by crystallographic texture developed during forging. The researchers noted that the anisotropy was influenced by annealing temperature and annealing time.
Recent work [11], [12] has identified the dependence of dimensional anisotropy on chemical segregation. Kop et al. focused on the near equilibrium ferrite formation associated with slow cooling in a 0.147 wt% C, 0.926 wt% Mn steel. The hot-rolled plate had a planar segregation pattern and orientation dependence was observed in dilatometry data. Farooque et al. showed orientation dependence in hot-extruded rod of 18 wt% Ni maraging steel. The work indicated that prior deformation and annealing temperature affect the amount of length change per cycle. The steel expanded perpendicular to and contracted parallel to the extrusion direction. Plastic accommodation of inhomogeneous transformation was proposed as the source of anisotropy in both efforts. Although clear evidences of chemical segregation were presented, neither group of authors provided a quantitative relationship between the chemical segregation and dimensional anisotropy.
One of the obstacles in elucidating the link between chemical segregation and dimensional anisotropy is that the banding itself is difficult to control and vary. However, recent investigations [13], [14], [15] have shown that it is possible to fabricate steels with precisely engineered manganese bands. Because the banding pattern in such roll bonded steel is much more controllable than the banding produced in commercial processes, the synthetically banded materials admit quantitative relationships between chemical segregation and material response to temperature and mechanical loading. One means of accomplishing this is through dilatometry – an experimental technique by which dimensional change of a material is measured for a specified temperature history. Because density changes occur during solid state transformations in steel, the technique can be used to characterize phase transformation kinetics [16], [17]. Dilatometry data have, in fact, been used to investigate dimensional changes associated with thermal cycling [5], [6], [7], [8], [9], [10], [11], [12].
In this work, dilatometry was used to investigate transformation anisotropy in a 5140 steel with a fabricated chemical segregation pattern. Both chemically banded and homogeneous specimens, machined into cylinders, were subjected to multiple thermal cycles. The axial length was recorded throughout the cycling. The banded specimens were machined with axis orientations both parallel and transverse to the planar bands. Axial strain measurements for the two orientations were used to quantify the difference in dimensional change in directions perpendicular and parallel to the bands.
Section snippets
Material
The chemical analysis of the SAE 5140 and modified, high manganese 5140, hereafter identified as 5140M, is given in Table 1.
Two experimental heats, provided by the Timken Company, were vacuum induction melted and cast into 50 kg ingots. The ingots were hot-rolled into 500 mm long by 165 mm wide by 16 mm high plates and given a vacuum homogenization at 1300 °C for 12 h. The plates were ground to a thickness of 12 mm to produce a clean, smooth surface.
Specimen fabrication
The method for producing a steel with a
Homogeneous 5140 and 5140M steels
A set of dilatometry data representing the base 5140M steel, transverse orientation is shown in Fig. 3. The figures plot axial expansion versus temperature for 16 thermal cycles. The arrow in the figure indicates the direction of length change per cycle.
The figure indicates a negative axial strain or contraction. Such behavior was recorded for each homogeneous specimen, independent of chemistry or orientation with respect to the rolling direction. Sixteen thermal cycles plotted in Fig. 3
Plastic deformation in base steels
The length change measured in the homogeneous 5140 and 5140M specimens was found to be independent of orientation with respect to rolling direction and manganese content. These observations suggest that the deformation is most likely related to the experimental setup and possibly specimen geometry. Calibration runs using a non-transforming 316L stainless steel did not produce any length change. This suggests that phase transformation plays a role in the observed length change in the base
Conclusions
- 1.
Transformation anisotropy was measured in a 5140 steel fabricated with a planar manganese segregation pattern. A consistent length change associated with thermal cycling between the high temperature austenite phase and a room temperature martensite showed a strong dependence upon the specimen orientation with respect to the segregation pattern.
- 2.
Dilatometry traces indicate that plastic deformation occurs during both the austenite transformation on heat up and martensitic transformation during
Acknowledgements
The authors acknowledge the Advanced Steel Processing & Products Research Center (ASPPRC), an NSF/University Cooperative Research Center, at the Colorado School of Mines and its sponsor members, for their support of this work. Also, Prof. David K. Matlock of the ASPPRC is acknowledged for his support and valuable insight. Dilatometry experiments were performed at Los Alamos National Laboratory under the auspices of Dr. Dan J. Thoma and technical support of Mr. Lawrence B. Dauelsberg. This work
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