I'm back... If my grant goes through it should answer some of these questions. Here is an excerpt from my PhD, but we know a bit more now than then.. Particularly note Ebner and Maurer (1982) study, I think ductility might be the key, particulary when on horseback.
'' The quality of different swords was first noted by al-Kindi. He used the terms, translated as “antique” for good, “modern” for not good, and “not antique but not modern” for medium quality. Al-Kindi said that the terms did not reflect age but quality. There is no consensus of opinion on the quality of crucible steel or Damascus steel either in antiquity or by modern researchers, “Some say the blades were flexible and tough; others conceded that they were stiff and even brittle but extraordinarily sharp...”(Bronson, 1986, 13). The appearance and behaviour of a metal is the result of the microstructure. Before modern times, when elaborate scientific equipment became available, the quality of a blade was judged on external factors rather than microstructure. However, steels made by different methods, with different microstructures, could have similar behaviour properties or hidden defects.
Anosov wrote that Damascus swords were assessed by four “tests”:
1) “Ring: – the clearer the tone, the better is the quality of steel,
2) Sharpness of the cutting edge: - while testing the edge, damask steel must cut a fine silk handkerchief in one stroke,
3) Strength of the blade: - on cutting an iron bar, damask steel should not acquire notches,
4) Elasticity: - on bending, damask steel should not break and should not become permanently deformed” (Bogachev, 1952, 40).
Al-Beruni also refers to these same characteristics. He refers to qala` swords which have clangour, whereas non-qala’ swords “possess an irritating sound” (Said, 1986, 213). Whichever type of swords these were, the passage does suggest that sound was an important feature considered when assessing the quality of the sword. More recently Massalski stated that a sabre should possess a good sound (Allan and Gilmour, 2000, 539). Indeed, the composition of the sword would affect its sound. According to Rostoker and Bronson (1990, 151) iron and steel are used to made musical wire because they have better properties than other metals, such as capacity for tension and good resistance to fatigue fracture. No specific studies have addressed the sounds different types of blades make. Factors that would affect the sound include the shape of the blade and any faults. For example, an internal crack or atomic-scale changes will have a dampening effect (Gordon, pers. com.). Thus, a clear long ring would suggest a quality blade.
The relationship between the sharpness of the blades and the pattern was noted by a number of scholars. Sharpness is primarily due to the presence of cementite in steel, which is hard yet brittle, thus it will cut well but will shatter if struck. Contrarily, iron areas composed of soft ferrite will not hold a sharp edge. Already, al-Beruni stated that the sharpness of farand (the pattern) comes from its hardness, but that it is brittle (Said, 1989, 217). Too many “threads” (i.e. aligned cementite in hypereutectoid blades) would produce a sharp yet brittle edge. Above it was discussed that prominent threads would be formed in slowly cooled ingots, which were extensively forged at low temperatures producing the coarser and clearer pattern.
The ductility of Damascus blades was one feature that distinguished it from other types of steels. Damascus steel blades typically contain spheroidal/globular cementite in a ferrite/pearlite matrix. Metallurgical experiments conducted by Ebner and Maurer (1982) on steel concluded that toughness and ductility coincide with a spheroidization of carbides. They also noted that additional tempering decreases the strength whereas toughness and ductility vary only slightly (Ebner and Maurer, 1982). Thus, the microstructure of hypereutectoid Damascus steel is optimum for ductility.
Given the variety of crucible steel, some with a high cementite content and others with a high ferrite content, in addition to the variety of forging methods, the range of microstructures, and the presence of phosphorous and other minor or trace elements, it is not surprising that there is no consensus of opinion. The presence of small amounts of phosphorus would have affected the forging and performance of the blade, particularly the elasticity. The effects of less than 1% P in the steel would have greatly influenced the performance of the blade. It appears that there were different types of crucible steel available, such as those that were made of hypoeutectic or hypereutectic steel, with or without a pattern and that each possessed different qualities because of their microstructure, the presence of minor and trace elements, and their subsequent heat treatments.
Not only would phosphorus have made the ingot “hot short” (see above), it would have made the finished product “cold short” (brittle when cold) and this property was noticed in the past. In fourteenth century Moorish Spain, Aly ben ’Abderrahman Ibn Hodeil observed that “… the Hindy sabre often breaks when the weather is cold and shows itself better when the weather is warm” (Bronson, 1986 from Mercier, 1924, 231). This is probably due to the presence of phosphorus in the steel. Hindi sabres derived from Sri Lanka (see above), and indeed Wayman and Juleff (1999, 36) identified steadite, the iron-phosphorous compound, in a crucible ingot from there, suggesting that blades produced in Sri Lanka contained phosphorus. Blades that contain phosphorus in percentages over c. 0.3% can be “cold short” and those that work well and be malleable in the summer can shatter during a cold spell (Rostoker and Bronson, 1990, 22; Percy, 1864, 64).
In addition to being decorative, the Damascus pattern was a hallmark of a potentially very high quality blade. Crucible steel blades that did not have a pattern could have been just as good quality as those with a pattern, yet, those with a pattern may not have been as good as some without. However, it may not have been possible to distinguish crucible steel blades without a pattern to blades made from non-crucible steel. While blades made of other types of steel could have been equally as sharp and strong, they would not have remained as ductile because they did not have the microstructure of spheroidal cementite in a ferrite/pearlite matrix. Ductility would have been a highly important feature, particularly in combat, because a bent or shattered blade could cost the user his life. A man would purchase the best quality blade available, for himself or possibly his son who had come of age, because not only was the blade a symbol of masculinity and prestige, but it would be his defence in a confrontation, hence his reputation, status, and life depended on the chosen blade. By using the above-mentioned tests and by observing the type of pattern, a blade would be chosen. The name of a particular type of decorative pattern was often associated with a specific location, workshop or smith, who would have had a reputation for making blades of a specific quality."
I think that just gives more fuel to the argument rather than an answer.