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St 52 modified cylinder tube characteristics show double-sided ductile fracture behavior. |
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Burst behavior of conventional St52 cylinder tubing exhibits double-sided brittle crack propagation. |
Mobile hydraulic equipment often must work in extreme environments — in the blazing sun of desert-like conditions or subzero temperatures of the arctic. It's under these frigid conditions where some steels can become brittle. If hydraulic cylinders working in temperatures down to 4° F or lower use conventional steel for their barrel, they can suffer catastrophic failure from brittle fracture.
Heaters can be used to keep components above critical temperatures. However, this practice is not always practical, and the potential for catastrophic failure still exists. Equipment can also be allowed to warm up before subjecting it to substantial loads, but, again, catastrophic failure could still occur if specific procedures are not followed.
The problem is, steels commonly used in hydraulic cylinder barrels lack the toughness to be used at these low temperatures. As a result, when failure occurs, brittle failure could cause the barrel to fracture into multiple pieces that could become projectiles. However, a modified St 52 steel now is available that retains its toughness at temperatures 40° C below that of its unmodified counterpart.
The achievement of 50% fractured surface in Charpy impact test can be used as an appraisal. The upper photo shows the burst behavior of a tube made of modified St 52. Modified cylinder tube characteristics show double-sided ductile fracture behavior. However, the burst behavior of a conventional cylinder tube shows a double-sided brittle crack propagation.
Charpy impact energy of longitudinal and transverse specimens is determined for quality control of modified cylinder tubes. However, only the value of the transverse specimen correlates with the highest stress in the cylinder tube. The minimum impact energy of the transverse solid Charpy impact specimen, made of modified material, is 80 J at -4°F. The impact energy in this case causes a double-sided ductile fracture.
Requirements of manufacturers pushed the development of high-strength and ductile cylinder tube grades. The modified material not only offers reliable operation of hydraulic cylinders down to 4°F and lower, but also allows up to 30% thinner tube walls, which reduces cylinder weight.
This material was submitted by Sven Herzig, director of R & D, and Wolfgang Mussmann, MHP Mannesmann Prazisohr GmbH, Mulheim, Germany. Visit www.mhptubes.de for more information.