Superplastic steel

Ninety Percent of Steel Is Carbon Steel Superplastic Steel Is New Again... 

Wootz steel later became known as Damascus steel because sword blades made from it had wavy surface patterns like Damask fabric. Damascus steel became famous because these swords kept their sharpness and strength after many battles. The knowledge of how to make Damascus steel was lost in the 1800s, but recently the process was redeveloped under the name superplastic steel. Collector hunting knives worth thou sands of dollars are being made from superplastic steel. 

A new steel that can be formed into complex shapes called superplastic or ultrahigh-carbon steel (UHCS) has been developed. Damascus steels can be stretched up to 100 times their length without breaking. The limit for most steels is less than 1 1/2 times. At high temperatures, UHCS puUs like taffy. There is almost no waste from their use. 

SWORDS OF DAMASCUS LEAVE LEGACY OF SUPERPLASTIC STEEL... 

Unlike most steels, which fail after being stretched to 2 times their original size, superplastic steel at elevated temperatures can be stretched to 11 times its size without cracking or pulling apart. As a result, heated superplastic steel can be formed into complex shapes. It can even flow like molasses and be poured into a mold. This property eliminates the need for machining, which typically results in about 50% scrap. Superplastic steel is also similar to stainless steel in its resistance to corrosion, but is made with less scarce and expensive materials than the nickel and chromium in stainless steel. 

The superior properties of superplastic steel are attributed to a much finer grain structure than that in ordinary carbon steels, which are quite brittle. Once its manufacture was understood, a group of industrial firms banded together to develop its use. As... 

Superplastic, ultrahigh carbon steel can be stretched to 11 times its size at 900 C. The original size of the bottom steel piece was 1 inch, but it was pulled to a length of 14 inches. Conventional steels collapse when they are extended to two times their size. 

Phosphorus tends to segregate in poor-quality steels. In rolled steel with too much P, polished and etched sections show distinct bands of metal phosphide ( phosphorus banding ). Very fine grain structure and superplasticity at high temperatures is shown by ferrous alloys with compositions, for example, Fe = 98.6%, Mn = 1.16%, P = 0.5%, V = 0.1%, C = 0.14%. The composition of some typical steels are listed in Table 12.56. 

Motohashi, Y., and Shibata, T. (1986). Hot dipping of. steel sheet into Al-Zn eutecoid-base superplastic alloy and the properties of the hot dip coated steel. J. Met. Finish. Soc. Japan, 37(10), 625-630 (in Japanese). 

Transage 175 has exceptional hardenabUity. The alloy readily age hardens to strength levels of 1170 MPa (170 ksi) or higher, following the slow cooling rates imposed by hot isostatic processing facilities and by superplastic forming operations. In steel terms, the ideal round size exceeds 200 mm (8.0 in.). 

If these limitations are recognized and established guidelines for hot and cold forming are followed, titanium and titanium alloys can be successfully formed into complex parts. Titanitun and its alloys can be formed to standard machine tolerances similar to those obtained in the forming of stainless steel. Very complex shapes can be formed to tight tolerances by superplastic forming, discussed in de-t in the next Tbchnical Note in this Volume. 

Another common die alloy for tem-peratxires up to about 1000 °C (1830 °F) is a wrought stainless steel alloy of nominal composition Fe-22Cr-4Ni-9Mn. This alloy exhibits both creep and oxidation resistance for satisfactory operation up to 1000 °C (1830 °F) and is appropriate for forming of most titanium alloys. For higher temperatures, such as is necessary for superplastic forming of y-TlAl, ceramic dies must be used. 

---