Abstract

Forging was used by smiths to make products such as kitchenware, hand tools, edged weapons and jewelry. Kabiye smiths in Togo (West Africa) used hammer and anvil to produce traditional objects onto desired shape and size. Long time ago, only Bassar steel was used as a raw material. Nowadays, the steel parts resulting from metal recovery were widely used as raw materials due to their low cost and easy availability. But the final users prefer forged objects produced with Bassar steel. However, microstructure evolution of Bassar steel during traditional Kabiye forging process and its influence on mechanical properties of forged products remains an outstanding issue. The present study investigated the microstructural evolution of the Bassar Steel during the traditional Kabiye forging process at laboratory scale. Some pieces of Bassar steels ingots were subjected to a series of annealing followed by hot rolling in the laboratory. To follow microstructure evolution, samples were taken at different stages: after cold rolling, the first annealing-rolling, the first annealing-rolling-annealing, the fifth annealing-rolling and finally the fifth annealing-rolling-annealing. The microstructure and mechanical properties of the last annealing-rolling-annealing sample were then compared to the traditional forged one. The results showed that the repeated cycles of annealing-rolling operations progressively homogenize the microstructure of the sample. The brittle cementite phase present in the BS pearlite phase is the key element that allows the forging operation to be effective. The last annealing-rolling-annealing yielded equiaxial grain structure similar to the traditional forged one. However, the size and distributions of inclusions were different from one sample to another. Both samples had similar mechanical properties except for the ductility. The laboratory annealed-rolled sample showed very low ductility compared to traditional forged one.

1. Introduction

The Bassar iron smelting process and the traditional Kabiye forging technique may be one of the most important aspects of the ancient West African iron and steel making industry. Recently, few progresses have been made to understand the nature of the Bassar Steel (BS) and the Kabiye forging process using Bassar Steel as a raw material ^{1, 2, 3}. The Bassar steel process is an ingenious and controlled know-how for the elaboration of an inhomogeneous Wiedmannstätten structure steel containing non-metallic inclusions and bubbles ^{1, 2}. The iron smelting furnace ^{1, 4} is specifically designed for smelting hematite iron ore, which is abundantly available in Bassar area. Recently, some investigations have been undertaken on understanding the Kabiye forging process and the metallurgical properties of the final forged material ². Two steps were used by Kabiye smiths, mechanical and thermomechanical treatments to forge the Bassar steel. The initial BS ingots were first cold hammered in order to remove slag and reduce bubbles. Some small pieces were then cut off and subjected to repeated series of cold forging and folding before being transformed into relatively thick plate. The plates were then successively annealed and hammered at high temperature to make finished products with a specific form. This Kabye forging process is similar to the traditional Japanese sword making ^{5, 6, 7}.

Agricultural tools and weapons elaborated by forging BS were generally preferred by farmers despite their relatively higher cost ³. According to them, these tools had better wear and corrosion resistance than the other forged tools made from imported steel.

Despite the reputation of the tools manufactured by forging Bassar Steel (BS) very few scientific studies have been conducted on the microstructural evolution from the initial Bassar Steel ingots to the final forged product structure and how the last heat treatment affect the mechanical properties of the final product remains unknown.

Therefore, the aim of this work was to investigate the microstructure evolution of Bassar Steel during it Kabiye smiths forging process by using modern thermomechanical treatments and characterization techniques at laboratory scale.

2. Experimental Procedure

To understand the microstructural transformations involved in the traditional Kabiye forging process, some pieces of Bassar Steel ingots were subjected to a series of cold rolling and annealing followed by hot rolling in the laboratory. In Kabiye traditional forging process, BS ingots pieces were cold worked to remove porosity and impurities resulting from the solidification during smelting process ^{1, 3}. After cold working these pieces were subjected to a series of annealing at 800 °C followed by hot forging.

A trial to reproduce microstructural evolution during traditional forging process, modern thermomechanical treatments were applied to BS pieces ¹ at laboratory scale These pieces were therefore cold rolled and hot rolled at laboratory. After the cool rolling step, these pieces were annealed at 800 °C for 30 minutes and then hot rolled for 2 minutes. Five cyclic of this annealing-rolling treatment were applied and final annealing was done before being air cooled.

To follow microstructural evolution during thermomechanical treatments at laboratory scale, samples were subjected to microstructure observation after these steps: cold rolling, first and fifth annealing-rolling, and final annealing.

For microstructure comparison, samples cut from a weapon (spear) forged from BS through the application of the traditional Kabiye’s forging process were also subjected to microstructure observation. Before microstructure observation, small specimens were mounted and treated following a standard metallographic procedure. These specimens were then etched for 5 seconds using 3% volumetric nitric acid solution in balance with methanol. The microstructure of samples was assessed using an Olympus PME3 optical microscope and scanning electron microscope using Hitachi S-2500 type.

Micro hardness was done with MHT-200 Series Micro Vickers Hardness Tester, applying a load charge of 300 g during 5s. For tensile test, only two samples were used: one cut along the axis of the traditional weapon (cross section) and the second from BS ingot obtained after final annealing and air-cooling step at the laboratory scale. Tensile test was conducted at room temperature using an INSTRON 5966 machine according to ASTM E8-04 specification ⁸. Mechanical properties of the two samples were compared after tensile tests.

3. Results and Discussion

Figure 1 presents the optical micrograph of the Bassar Steel (BS) ingots sample used as raw material before forging process. The BS shows a biphasic microstructure made of ferritic grains (bright) in a perlite matrix (grey area). This microstructure is well-known as Wiedmannstätten structure ^{1, 9} Some dark points were also observed, probably non-metallic inclusions or porosities.

High magnification of gray area (Figure 2) using scanning electron microscope confirmed that it was a perlite component. This micrograph shows a two-phased, lamellar structure composed of alternating layers of ferrite and cementite.

The ferrite (bright contrast) and cementite have similar thickness of about 1 µm.

Figure 3 is the optical micrograph obtained for the BS sample after cold rolling without annealing operation. The microstructure is the Wiedmannstätten structure ^{1, 9}, composed of ferrite plates in light contrast in the dark pearlite matrix. However, the ferrite needles were exploded under the cold rolling effect.

The pearlite matrix of BS was lamella (Figure 2). Under the effect of rolling, the grains were distorted, elongated in the rolling direction, the imperfections increased, fragile cementite lamella break leading to generalized fragmentation of the pearlite matrix. The cracks born in the matrix propagate in some areas of the ferrite grains (Figure 3). When a metal is cold worked, it becomes harder and stronger, whereas, the annealing of cold worked metal causes relaxation and softening.

Figure 4 presents the optical micrograph of the BS sample after the first annealing-rolling cycle. The microstructure was quite homogeneous, with dislocations that are clustered along the grain boundaries. On these parts the rolling direction was not clearly visible showing that the restoration has occurred. However, other parts of the surface exhibits large dark inclusion surrounded by extensive dislocation pile-up and large dendritic ferrite grain.

Figure 5 presents the optical micrograph of sample after the first annealing-rolling cycle followed by an annealing at 800°C. The microstructure was completely granular with equiaxial large and small grains. Smaller grains were located at the triple grains joints boundaries. During hot-rolling, the microstructural changes that occurred were elongation of grains in rolling direction, nucleation of new grains at the grain boundaries and growth of nucleated grains to form fully recrystallized microstructure. Large grain growth was abnormal and small ones were probably due to the primary grain’s residue. Some large inclusions located at the grain boundaries were also observed.

Figure 6 presents the optical micrograph of the BS sample after five annealing-rolling cycles. The microstructure was almost homogenous and biphasic consisting of a network of light phase in a matrix of a dark one. The light phase was probably the initial ferrite phase. The grinding process started in Figure 3 has reached its completion in Figure 5, after a series of rolling the ferrite needles were completely cut. The pearlite matrix, which is more fragile because of the cementite, mixes almost homogeneously, leading to the microstructure of Figure 6. The lamellar cementite therefore plays an important role in the process of microstructure transformation of BS during its shaping in the forge. The original coarse dendrites and columnar grains become the equiaxial recrystallized structure with fine grains and uniform size due to the deformation and recrystallization of the metal.

Figure 7 shows the microstructure of the sample after the fifth rolling and 800°C annealing. The microstructure consists of equiaxial grains mainly large. The small ones appear at the triple or quadruple join of the large ones. The presence of small grains could suggest that the large grains were the result of the growth by coalescence of grains having similar crystallographic orientation. Some large non-metallic inclusions were clearly observed.

For comparison, the micrograph of sample cut from an old weapon that had been forged by Kabiye blacksmiths using BS ingots was showed in Figure 8. The microstructure was equiaxial light ferrite grains with many dark precipitates of non-metallic inclusions. These precipitates were located mostly at the grain’s boundaries and the less large ones were formed inside some grains. Such inclusions indicate the bloomery nature of the process leading to the initial Bassar Steel and the forging technique applied to develop the finished products. The inclusions observed in this sample were finer and regularly distributed over the entire surface, whereas those observed in the sample resulting from the thermomechanical treatment at the laboratory scale (Figure 7) present large inclusions having the average grain size and unevenly distributed over the entire surface.

Figure 9 show the stress-strain curve of traditional forged and laboratory annealed-rolled BS samples. Mechanical parameters such as Young's modulus (E), yield strength (σ_e), ultimate strength (σ_r), total strain at break (ε_r) of traditional forged and laboratory treated Bassar Steels were reported in Table 1. The mechanical properties were almost close for booth studied materials except for the total strain at break which was very low for the laboratory annealed-rolled-annealed sample. This difference in deformation behavior could be explained by the inclusions size and distribution inside each sample. The difference on inclusions distribution and size was due to the process used for each sample thermomechanical treatment. In terms of degree of transformation, it’s generally known that the degree of deformation of the forging is much greater than that of rolling, which means that the effect of breaking inclusions through forging is better than that of rolling. During traditional ways of steel forging, the hammering is repeated several times with the master exposing out all directions. This step of the process helps to remove and homogenies as many as possible impurities before the next stage. Forging process gives small inclusions and uniformly repartition inside the sample. The difference in axial and radial mechanical properties of forging is smaller than that of rolled products. Many authors ^{6, 7, 8, 9} underlined the effect of inclusions size on mechanical properties Inclusions that occupy a significant portion of the material cross section during hot or cold work or that are in regions where processing deformation is high may cause fracture during processing. Yakura et al ¹³ showed that reducing the inclusion size can improve not only the fatigue limit due to fractures initiated from surface inclusions, but also the fatigue limit due to fractures initiated from internal inclusions. André Luiz et al ¹¹ showed that inclusion distribution in steel products greatly influences how they affect steel properties. A cluster of relatively large alumina inclusions has caused the defects during foils cold rolling.

4. Conclusion

The present study has tried to follow the microstructural evolution of Bassar Steel (BS) during it forging process. The laboratory annealing-rolling treatment has reproduced a final microstructure very similar to that of the traditional forged BS. Many mechanical parameters were similar for booth materials. However, a very low total strain was recorded for the laboratory annealed-rolled sample. The fragile nature of cementite allows it to play a key role in the process of homogenization of the BS microstructure during its shaping in the traditional forge The easy fragmentation of lamellar cementite and the ductile ferrite needles whose surface area is being enlarged upon forging were found to accelerate the carbon diffusion to achieve the final microstructure of the forged BS.

Acknowledgements

This project was supported by the French government through the “Service de Coopération et d’Actions Culturelles” of the French Embassy in Lomé, Togo.

References