April/May 2019

HYDRAULICS 30 HYDRAULICS & PNEUMATICS April/May 2019 www.hpmag.co.uk Fig. 2 is shown as a function of effective length in Fig. 3. In addition to the benefits of weight reduction, there are economic advantages due to reduced material costs. As an illustration of the potential – a piston rod in steel C45E, 63 mm diameter and effective length 1000 mm can be replaced by a rod in Cromax 180X with diameter 55 mm giving a weight reduction of 5.8 kg or 24 %. As an alternative, if a rod in steel C45E is replaced by one made of Cromax 180X with the same diameter, then it can transmit a greater load while keeping the same margin of safety against buckling. Obviously, a redesign will be required to enjoy the benefit of the increased load rating, involving increased diameters of the tube, piston, gland and so on, which may not always be realistic or feasible. Improved fatigue life The fatigue failure of piston rods occurs either at locations where a reduced section is combined with stress-concentration, at threads for example, or at welds. Fusion-welded piston rods are particularly susceptible to fatigue failure because the welds contain crack-like defects, are characterised by unfavourable tensile residual stresses and very often do not penetrate the entire cross-section of the rod. It is for this reason that piston rods, which are subjected to high alternating loads and which carry an elevated risk for fatigue failure, are usually friction welded. A properly-executed friction-welded joint is virtually defect-free and extends over the entire cross-section of the rod; it is therefore characterised by far greater fatigue strength than a corresponding fusion weld. Tests have shown that the higher tensile strength of Cromax 180X (min. 750 MPa) offers a better fatigue performance than rods made of the conventional grade C45E with minimum tensile strength 590 MPa. Hence, the desired fatigue resistance can be achieved using a smaller diameter rod with positive benefits for weight and cost. Good and consistent machinability The machinability of Cromax 180X has been documented by testing in turning and threading. This shows that the steel can be processed efficiently by machining, compared with hard- chrome bars based upon steel C45E even though its strength and hardness are considerably higher. Fusion and friction welding The weldability of steel used for piston rods is an important characteristic. In fusion welding, the primary metallurgical consideration is that the formation of hard, brittle constituents in the weld heat affected zone (HAZ) should as far as possible be avoided. Cromax 180X rods up to 60mm in diameter can be fusion welded without preheating; for larger diameters, preheating is recommended in order to counteract fast cooling after welding and so lessen the risk that brittle constituents are formed in the HAZ. Welded piston rods subjected to heavy loads, especially reversed compression-tension loads, are best manufactured via friction welding. Correctly executed, such a weld has equivalent strength and fatigue resistance to the base steel in the rod. Cromax 180X is designed to mitigate the risk for HAZ-embrittlement in connection with friction welding. Suitability for induction-hardening and cold-drawing Cromax 180X in an induction-hardened execution is available for applications where there is risk for damage to the piston rod as a result of external impact. For piston-rod applications requiring extremely high strength, Cromax 180X can also be offered in a cold- drawn execution having a minimum yield strength of 690 Mpa which is on-a-par with quenched-and-tempered low-alloy steel with considerably higher alloy content, grade 42CrMo4 for example. Summary – a new steel grade for piston rods A new steel grade, 180X, is now available for piston-rod applications in hydraulic cylinders. In hard-chrome-plated execution, Cromax 180X is characterised by higher strength than traditional grades used for hydraulic- cylinder piston rods. The improved yield strength means that rods can be downsized without compromise to the margin of safety against buckling under axial compressive loads and against fatigue if the loads are alternating. This offers positive benefits for both weight and cost. www.ovako.com/Products/Cromax Figure 2 – Demonstrating downsizing achievable when piston rods with diameters 45, 63 and 90 mm made from steel C45E (dotted lines) are replaced by Cromax 180X (full lines). Figure 3 – Percentage weight reduction achievable when piston rods with diameters 45, 63 and 90 mm made from steel grade C45E are downsized via replacement with Cromax 180X.