Don't take my word for it but from my understanding the mold for the cast iron would be partially destroyed (unlike bronze piece molding), so it would be much harder to mass produce it. You technically could, but it was be very hard to mass produce triggers with interchangeable parts when the mold keeps being destroyed or damaged after each casting. Again, I'm not sure about this.
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From Yang Hong in Weapons of Ancient China on Han dynasty steel making (pg 185-186):
The pig iron from smelting furnaces or the scrap iron of used iron wares was used as the raw material for melting. Cupola furnaces were discovered at Wafangzhuang of City Nanyang, Tieshenggou of Gongxian, Guxingzhen of City Zhengzhou and Wangchenggan of Lushangxian. The adoption of the cupola furnace greatly improved the quality of the liquid molten iron, and therefore the quality of the articles cast. It is estimated that the average inner diameter of the cupola furnaces discovered at Wafangzhuong of City Nanyang was about 1.5 metres and their height about 3-4 metres. The operation of a furnace of this size is believed to have been semi-continuous. At the same time, the art of casting with clay, layered, and metal moulds was also perfected to a high degree.
The method of steel making and quenching techniques were also developed, and these were closely related with the manufacture of weapons. The low carbon steel made from pig iron during the Warring States Period developed into low-grade bai liangang (steel forged a 100 times) during the Western Han Dynasty. This was a significant breakthrough in steel making. Broad swords and double-edged swords made of this kind of low grade tempered steel were already in use by the end of the second century B.C. This has been proved by metallographic examination of the double-edged swords which were carried on the belt, other steel double-edged swords, and knives for cutting paper which are inlaid with gold. All of these were unearthed from the tomb of Emperor Liu Sheng at Mancheng, Hebei. The raw materials from which the steel of these swords was made was no different from that of the relics of the last years of the Warring States Period. Both are low carbon steel made from pig iron mixed with carbon, but the quality of the former is much better. Specifically, both the size and the number of impurities are reduced, the carbon content is also lower, and the various layers have a more homogeneous structure. The low-carbon layer of the double-edged steel swords unearthed from the secondary capital of state Yan is about 0.2 mm thick, and that of the double-edged steel swords unearthed from the tomb of Emperor Liu Sheng, about 0.05 – 1 mm. This reduction in the thickness of the layer is due to repeated forging, a process leading to bai lian gang (steel forged a hundred times).
The above findings show that the art of repeated heating and forging, adding solid-state carbon and hammering after repeated folding was known at that time and that a low-grade tempered steel was successfully produced. At the same time, following the development of the technique of annealing, the technology of making steel by decarbonizing cast iron through annealing was perfected. On the basis of this, therefore, a new technology using as open hearth process to decarbonize pig iron and turn it into low carbon steel while still in a semi-molten state developed. The steel made in this way is the so called “stir-fried steel” (chao gang). In the excavation of the site at Tieshenggou of Gongxian, a jar-shaped coverter furnace was discovered. According to the opinions of experts which are based on a study of the furnace, when air is blown into the furnace from the top, the shape of the furnace would ensure the oxidizing condition within it and accelerate the decarbonization therein. The capacity of this jar-shaped steel furnace is small, and it is easy to maintain the temperature in it due to its shape. Moreover, since it is built underground, the dissipation of heat is negligible and thus it is easy to raise the temperature in the furnace. The opening in the lower part is large, so it is easy to charge, to stir, and to let out the content of the furnace. The structure of the furnace is simple and it therefore is easy to build. These advantages facilitated its popularity at the time. Several such furnaces were found on the site of iron smelting at Wafangzhuang of City Nanyang. They are quite similar to the one at Tieshenggou of Gongxian, and some still have lumps of iron at the bottom. The advent of this new technique of decarbonization gave new impetus to the development of highly tempered steel (bai lian gang) and removed the barrier resulting from low steel production and inefficiency of pig iron blending with carbon. Thus it raised the production and quality of the steel and made it possible for highly tempered decarbonized steel (bai lian gang) to come into its own. Recently two steel weapons with inscriptions were discovered in Eastern Han tombs in Shandong and Jiangsu. One of them, a ringheaded steel sword was made in the year of the reign of the Eastern Han Dynasty (112 A.D.), was unearthed from Changshan or Shandong in 1974. An examination showed that the sword was made of chao gang (Fig. 304). The structure of the steel of the sword magnified 10,000 times under a microscope shows the sword has 30 layers. This agrees with the inscription on the sword which says the sword was forged 30 times, that is the steel was heated, folded and hammered 30 times.
The other weapons is a steel double-edged sword (jian) made in the second year of the reign of Jian Chu of Eastern Han (78 A.D.) and unearthed in Xuzhou in 1978 (Fig. 300). Its date of manufacture is a quarter of a century earlier than that of the iron broad sword unearthed at Cang Shan. It inscription says it was made by being forged 50 times under the instructions of the governor of the prefecture of Shu. Miscroscopic examination of the metal structure revealed that it was forged from high-carbon “stir-fried” steel (chao gang). These two specimens are concrete evidences that the technology of bai lian gang steel, i.e. highly tempered and refined steel, was completely mastered during the Eastern Han Dynasty. They also demonstrate that the technology was applied to the manufacturing of weapons.
The advancement of the technique of quenching is very important to the manufacturing of weapons. The steel broad sword and double-edged swords unearthed from the tomb of emperor Liu Sheng at mancheng, show that the technique of partial quenching, that is, quenching the cutting edge to increase its hardness while leaving the toughness of the other parts of the weapon such as the back, central ridge untouched, was already mastered at the time. This technique increased the quality and effectiveness of weapons greatly. At the same time, methods of decarbonization and annealing to increase the malleability of the steel and to make it more pliable for processing, was also used.
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And now for some tables:
Above two tables came from: Sarah Taylor, Early Chinese Iron Technology: Some Social And Historical Implications
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Anyway, I would appreciate it if we focus on crossbows. I might make a separate thread about Chinese metallurgy in the future.