Mechanical Properties of Xviii Century Persian Bulat Steel Compared with Modern Tool Steels

It is shown that the Persian Kurdistan bulat steel blades (end of the XVIII century) correspond with respect to chemical composition to contemporary high-carbon steel (1.48 and 1.67% C), with a high content of phosphorus impurity (0.192 and 0.236% P). However, contemporary industry carbon tool steels are not used with this carbon and phosphorus content. It is revealed that the main distinguishing feature of ancient bulat steels from modern tool steels is layered heterogeneity with respect to composition, structure, and hardness distribution. Layered carbide heterogeneity develops in the form of patterns after polishing and etching in 3% alcoholic nitric acid solution. It is found that the carbide layers located in a troostite matrix consist of oblong shaped cementite with a 1/3 ratio of axes. The microhardness of carbide layers is determined in the range of values from 820 to 1020 HV. In troostite layers the range of microhardness values is from 390 to 560 HV. The layered distribution of microhardness in the edge of a bulat blade is a micro-saw with teeth from 50 to 100 microns. Tests for cutting edge resistance on cutting felt in a unit with a reciprocating mechanism are carried out. It is revealed that with low cutting forces (up to 40 N) bulat steel withstands a greater number of cuts than carbon tool steel with a uniform structure. With an increase cutting edge force from 6 to 12 kg bulat steel type Ds15P demonstrated 30% fewer cuts than by carbon tool steel type Y15А. It has been established that within the layered structure of bulat steel fatigue crack propagation from the instant of its occurrence to complete destruction occurs in a greater number of cycles than in the homogen eous structure of carbon tool steel. Old blades of bulat steel type Ds15P according to fatigue life indices show twice as long life compared to modern carbon tool steel type Y15А. Loss of cutting capacity is compensated by an increase in blade “viability” with repeated fatigue loads. Thermal tests are conducted for the impact strength of genuine bulat steel type Ds17P. It is shown that in the temperature range from 0 to +40°C impact strength increases by a factor of four. It is established that chemical and structural layering of bulat steel minimizes the negative impact of phosphorus impurities, and impact energy at failure increases by 35%.