The blades consist of alternating sheets of clustered cementite particles and sheets of pearlite. The pearlite sheets can contain cementite particles. As the number of cementite particles in the pearlite sheets is reduced, the overall sheet sharpness is improved. When viewing the surface pattern of a Damascus blade from a comfortable distance, the boldness of the pattern perceived by the eye improves with sharper sheet formation for a given sheet spacing. Our experience examining both museum blades and blades made by Pendray have found that for a given sheet sharpness, the surface boldness is maximized with sheet spacings of around 40–100 microns. Our previous work3,4,–5 hypothesized that the microstructure is formed by a type of banding due to the microsegregation of V between the dendrites during the solidification of the ingots. One would therefore expect the spacing to be set by either the primary or the secondary dendrite spacing in the ingot from which a blade is forged combined with the amount the ingot is reduced on forging. If controlled by the primary spacing, the resulting cluster sheet spacing would be larger than if it were controlled by a combination of secondary and primary spacings. Our initial studies used a phosphorus addition of around 0.1 wt.% to match the composition measured on several well-patterned museum-quality blades. At this level of P, the ingots are hot short and very difficult to forge. We think this is why a 1722 report by Reaumur claimed that Parisian blacksmiths were unable to forge the small wootz cakes from India.1 The P forms a ternary eutectic called steadite, which becomes molten at the forging temperatures, thereby causing the embrittlement.
To overcome this problem we found that it was necessary to heat the ingots packed in an iron mill scale for around 10–12 h at 1100–1200°C. This treatment produces a rim of nearly pure iron on the ingot and is termed a rim heat treatment. The iron rim adequately contains the molten steadite within the ingot so that it can be forged without cracking. Apparently, the Parisian bladesmiths had not learned the trick of the rim heat treatment. (Pendray was able to obtain an ancient partially forged bar of steel verified to be a Damascus bar from the Anwar Armory in India. Additional forging of this bar produced a typical surface Damascus pattern, and metallographic sections of the bar verified that it had been given a rim heat treatment6). Eliminating the P addition removed the hot shortness problem. The blades still produced excellent cluster sheet formation but, unfortunately, without the rim heat treatment the cluster sheet spacing was found to be significantly smaller. On such blades forged to thicknesses of 3–5 mm, the spacings were running in the 20–30 micron range, and to easily see the surface pattern, it was necessary to use a magnifying glass. During the rim heat treatment, diffusion of V between the smaller distances would occur, which would be needed to homogenize the V compositions over secondary spacings. At the same time there would be some level of microsegregation of V between the primary spacings.
Experiments were recently carried out to produce blades made from ingots prepared as previously described3,4 and cooled at slow and fast cooling rates. The fast rate was obtained by turning off the propane gas to the furnace that heated the enclosed crucibles. The slow rate was obtained by a slow reduction of propane gas. Figure 1 presents the time–temperature history of the ingots made at the two cooling rates. The temperature was measured at the bottom outside edge of the crucible with a platinum thermocouple (type B). Experiments with thermocouples placed both below the bottom of the crucible and in a ceramic protection tube inside the crucible at the bottom of the melt showed that outside temperatures ran 33 ± 4°C higher and 18 ± 4°C lower than the inside melt temperature on heating versus cooling, respectively. Chemical analysis of ingot 8115 is presented in Table I. The iron carbon phase diagram7 gives a liquidus temperature of 1423°C for 1.57% C. Hence, the melt was heated to a maximum temperature of 1468°C and held above the liquidus of 1423°C (~ 1390 on heat up and ~ 1441 on cool down on the plot) for ~ 65 min. It is interesting to point out that when crucible steel was being produced in the late nineteenth century, to sufficiently kill the steel and avoid porosity in the ingots, they simply held the melt until it stopped bubbling.8,9 They had to hold the melt temperature above the freezing temperature for 30–40 min before gas evolution stopped. The authors do not specify what actual temperature was used. It must be that times of at least 30–40 min are required for the [O] + [Fe] ↔ CO equilibrium to be achieved adequately to avoid cavity porosity on their cooled ingots. Apparently the 30-min hold of ingot 8115 adequately killed the steel so that cavity porosity did not occur at the cooling rate of this ingot. The cooling rate given by the slope of the curve during subsequent dendrite growth was around 4.3°C/min.
Such was not the case at slower cooling rates. Several attempts were made to cool ingots at slower rates, but large cavity porosity was found in the ingots, which made it nearly impossible to forge them to blade shapes. It seemed most likely that this porosity was due to evolution of CO gas on cooling at the slow rates. Cavity porosity in the slow-cooled ingot 7315 was avoided by adding a borosilicate glass ampoule filled with 1.4 g of Al and 1.4 g of a chloride-based fluxing salt. The glass ampoule plus the fluxing agent prevented the Al from oxidizing before it could dissolve into the melt and thereby prevented CO evolution just as it does in aluminum-killed steels. Table I shows that the carbon composition of ingot 7315 is the same as that of ingot 8115; hence, it has the same liquidus temperature of 1423°C. It was heated to a maximum temperature of 1468°C, held above the liquidus for 108 min and subsequently cooled through the dendrite growth range at an average rate of around 1°C/min. There was no cavity porosity in this slow-cooled ingot.
Both ingots were given a rim heat treatment of 16 h at 1040°C. The ingots were 90 mm diameter by 43 mm height. Both ingots were forged to a blade shape with dimensions of 43 mm wide by a bit under 8 mm thick. A total of 36 forging cycles were employed with the first 7 carried out over a temperature range of ~ 1030°C to 700°C and the remainder over the range of ~ 925°C to 700°C followed by three thermal cycles between 950°C and 700°C. The blades were forged to a thickness of 7.75 ± 0.05 mm, and both displayed excellent cluster sheet alignment. As expected, the slower cooled ingots produced blades with a larger average cluster sheet spacing, 93 µm versus 71 µm for ingot 8115. These results present additional evidence that the formation of the aligned cluster sheet spacing is a result of the microsegregation of the carbide-forming element V between the dendrites during solidification of the ingot. Blade 8115 was forged further to a thickness similar to a knife blade or a sword, 2.8 mm, and the cluster sheet spacing dropped to 45 microns.
Figure 2 presents a micrograph of blade 7315 at 7.8 mm thickness, and the enlargement illustrates the nature of the clustered arrays of cementite particles in the cluster sheets as well as the pearlite sheets between the cluster sheets. The amount of cementite in the pearlite sheets has been reduced adequately to produce a bold surface pattern.