Abstract: The development of high strength materials, coupled with good formability, has always been the aim of material scientists. The effect of degree of deformation on the mechanical properties of high temperature themomechanically treated steel (HTMT) has been investigated in this study. It has shown that an increase in percent deformation results in improvement in mechanical properties. In order to have maximum strengthening, heavy deformation and low finishing temperature should be chosen.
INTRODUCTION
Thermomechanical treatment involves the deformation of austenite at a temperature above the recrystallisation temperature. The austenite formed is quenched immediately after mechanical working to avoid development of recrystallisation process. The strengthening achieved arises from austenite grain size refinement, but optimum properties are often obtained if austenite is prevented from recrystallisation (Hurley et al., 2000; Askeland, 1994; Kamma, 1990).The application of low stress and strain rates, presents rapid recovery rate and the materials do not dynamically recrystallise.Thus, a dislocation density and configuration sufficient to nucleate and drive the recrystallisation process is not developed in them. The degree of deformation in HTMT must not be excessively high; otherwise recrystallisation might reduce the strengthening effect (Novikov, 1978).
The grain size dependence of flow stress in metals is evident from the well known Hall-Petch relationship (Prasad, 1975). In fact, nowadays it has become fashionable to influence and control the mechanical properties of materials by systematic control of the grain structure and grain size (Hurley and Hodgson, 2001; Embury, 1975; Dahl and Rees, 1974). Thus, the ever growing interest of physical and mechanical metallurgists in grain-size controlling research efforts is understandable. The conventional method of grain refinement and improvement of mechanical properties is through the application of alloying elements. A rather unorthodox, but technologically viable method is the so-called thermo mechanical process employing heat treatment and mechanical deformation in a repetitive cycle-a familiar procedure in conventional rolling operations. In terms of economy, effectiveness and procedural reproducibility, the thermo mechanical treatment appears to be the best method of grain size control (Kamma, 1990; Bolton, 1988).
This study therefore seeks to investigate and establish the combined effect of HTMT and degree of deformation in order to achieve optimum results.
MATERIALS AND METHODS
The sample was a low carbon steel obtained from Universal Steel Ikeja Lagos Nigeria with following composition (wt%) C-0.368, Si-0.226, S-0.033, P-0.039%, Mn-0.688, Ni-0.177, Cr-0.119, Mo-0.021, V-0.002, Cu-0.227%, W-0.003%, As-0.014%, Sn-0.003%, Fe-98.055%. The samples were in 16 mm, 12 mm and 10 mm diameter. The sizes were obtained by repetitive (cyclical) process of rolling and recrystallisation (HTMT) carried out a number of times resulting in varying degree of deformation. At the end of the HTMT, the samples were examined for microstructure. Tensile (strength), impact and hardness samples were prepared from different sizes.
The study was carried out in February, 2000 in the Materials Laboratories of the Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria; Obafemi Awolowo University, Ile-Ife, Nigeria and in the Department of Mechanical Engineering, Federal Polytechnic Ado-Ekiti, Nigeria.
Calculation of the degree of deformation of the samples: Initial dimension of the billet = 100x10x10 cm Initial cross sectional area of the billet = 10x10 = 100 cm2 = 10000 mm2
| • | For 10 mm diameter sample, cross sectional area = πr2 = 3.142x52 = 78.55 mm2 |
| • | For 12 mm diameter sample, cross sectional area =πr2 = 3.142x62 = 113.11 mm2 |
| • | For 16 mm diameter sample, cross sectional area = πr2 = 3.142x82 = 201 .09 mm2 |
CALCULATION OF THE AVERAGE GRAIN SIZE OF DEFORMED SAMPLE
The determination of the average grain size (D) according to Kamma (1990) was by the well known ‘intercept method’, based on calculations of the average cross-sectional area (Fk) of a circle(diameter, dmm) drawn on a micrograph of known magnification (V), whereby
| (1) |
Average cross sectional area of grain (Fm)
| (2) |
| (3) |
| z | = | number of grain lying completely within the circle. |
| n | = | number of grains intercepted by the circle |
| 0.67 | = | Oertel factors |
RESULTS AND DISCUSSION
The result of the mechanical tests carried out is presented in Table 1. The corresponding stress-strain curves are shown in Fig. (1-3) for the 10 mm diameter, 12 mm diameter and 16 mm diameter samples respectively.
The micrographs (Fig. 5-8) show the dark patches as the pearlite phase and the white patches as the ferrite phase (proeutectoid) because the carbon content of the alloyed steel falls within the hypo eutectoid range (Lakhtin, 1990).
From Table 1, it can be seen that the 10 mm diameter sample has the highest ultimate tensile strength (UTS), followed by 12 mm diameter and 16 mm diameter respectively.
| Fig. 1: | Stress-strain curve for HTMT 99.2% deformation |
| Table 1: | |
| A | Tensile test |
| (i) | 10 mm diameter as-rolled samples |
| Fig. 2: | Stress-strain curve for HTMT 98.78% deformation |
| Fig. 3: | Stress-strain curve for HTMT 97.99% deformation |
| Fig. 4: | Stress-strain curve for as-cast sample |
The grain size decreases as the degree of deformation increases during the high temperature thermo mechanical treatment (Bolton, 1988). Thus, the 10 mm diameter sample, with the finest grains, explains strengthening by ferrite grain refinement. This is confirmed by the Hall-Petch equation:
| Fig. 5: | 10 mm diameter as-rolled (HTMT) sample showing pearlite (dark
patches) and ferrite (white patches) Magnificationx100 Etchant -Nital |
| Fig. 6: | 12 mm diameter as-rolled (HTMT) sample showing pearlite (dark
patches) and ferrite (white patches) Magnificationx100 Etchant -Nital |
| Fig. 7: | 16 mm diameter as-rolled (HTMT) sample showing pearlite (dark
patches) and ferrite (white patches). Magnificationx100 Etchant -Nital |
Where σy = yield strength, σi= friction stress, ky = a constant, d = ferrite grain size.
| Fig. 8: | As-cast sample showing pearlite (dark patches) and ferrite
(white patches) Magnificationx100, Etchant -Nital |
It shows that the strength increases as the grain size decreases (Llewellyn, 1992). Microstructurally, Fig. 5, shows the best dispersed structure and the least presence of inclusions, blowholes and internal coring which contributes to the highest strength value. The dispersion fineness and uniformity trend in the micrographs are in the order of Fig. 5 >Fig. 6 >Fig. 7 >Fig. 8.
In terms of ductility, Table 1 columns for percent elongation and percent reduction in area show that the 16 mm diameter sample is the most ductile. Its relatively coarse grain (Fig. 7) enhanced ductility because more slippage can take place during plastic deformation before fracture (Lakhtin, 1990). Also, as the degree of deformation increases, the more difficult deformation or slippage becomes. Hence, the fewer the grain boundaries present, the better the ductility (Bolton, 1988).
In terms of hardness, Table 1 shows that the 10 mm diameter sample is the hardest. The high temperature thermo mechanical treatment confirms that as the degree of deformation increases, the strength and hardness increase concurrently (Bolton, 1988). From Fig. 5, the 10 mm diameter sample has the finest grains (many grain boundaries), poor ductility, giving rise to more resistance to deformation due to the repeated strain hardening during the working (HTMT). In the other plates, namely Fig. 6 and 7, respectively there is relative increase in grain size in that order over that in Fig. 8 where there is no thermomechanical working. As such the grain size in Fig. 4 remains the same.
The toughness property is highly sensitive to change in grain size. Therefore, the smaller the grains, the tougher the sample becomes (Novikov, 1978). Hence, from Table 1, the 10 mm diameter sample is the toughest. The Petch equation linking toughness to grain size is given by:
| where β and C are constants | ||
| T | = | impact energy (toughness) |
| d | = | ferrite grain size |
CONCLUSIONS
This study has now established the fact that the coarser the grains, the better the ductility because more extensive slippage can take place during plastic deformation before fracture (Lakhtin, 1990). As the degree of deformation is increased, the grain boundaries are increased and thereby making further slippage or deformation more difficult (Bolton, 1988). Hence, the 16 mm sample (relatively coarse grains) has better hardenability and machinability due to fewer grains boundaries present (Courtney, 1990). Since the toughness property is highly sensitive to change in grain size, hence, the smaller the grains, the tougher the sample becomes (Novikov, 1978).
Therefore, for applications (HTMT) that require adequate mechanical properties, a large degree of deformation should be employed.