A material advantage for hydraulic piston rods

29 August, 2019

Cost reduction

Since rod diameter is the main cost factor, the strength of the rod material is the main lever to reduce cost. Compared with the most commonly used material for hydraulic cylinder rods (C45E), the cost reduction potential by using stronger materials is of the order 20 to 30%.

However, not all high-strength steels are suitable for piston rods. The main problems derive from the inherent impact of slag inclusions which intersect the surface and segregation at the centre of the rod. Some chemical compositions, aimed at promoting strength or machinability, can cause problems in welding and especially friction welding. Moreover, slag inclusions at the material surface can dissolve during the chrome-plating process, causing porosity which results in weakness in the chrome layer.

It is a complex process to assess the true cost effects of different steel materials for a piston rod. But a simplified approach is to first select several grades with sufficient impact toughness and suitable machinability/welding characteristics and then to choose the grade offering the lowest price per MPa of yield strength (see Figure 3).

Surface plating

To ensure that the piston rod will last its economic life in different environments, the surface can be adapted with the thinnest possible layer of chrome or nickel-chrome. Plating with chromium is the most common treatment due to its combination of cost efficiency, wear resistance, low friction and corrosion resistance. The characteristics of the chrome layer are very dependent on the base material properties, such as the distribution and size of inclusions. Steel grades with amenability to the chroming process and with high and repeatable precision in chemical composition are crucial to obtain consistent quality. Ovako Cromax has a unique position of in-house production, from steel mill to plating, which facilitates the development and supply of such steel grades.

Corrosion resistance in the field is ensured not only by the chrome layer, but also by a film of hydraulic oil. In corrosive environments or in safety critical applications exposed to risk of mechanical damage, chromium can be combined with an underlying layer of nickel. The nickel layer dramatically improves the corrosion resistance independent of the oil film.

Machine less - manufacture more

A major cost driver for cylinder parts like pistons, cylinder heads or endcaps is machining time and tool wear. As these parts also often are subject to high stress with risk of fatigue in threads or grooves, material choice also has to take fatigue properties and sometimes weldability into consideration. With the right choice of material, machining costs can be reduced both due to direct costs and the fact that repeatable machining properties facilitate a higher degree of automation in production. Moreover, a material with good chip-breaking helps in the deburring process and promotes high final cleanliness levels.

The Ovako M-steel has been developed for superior machinability without compromise in fatigue strength. It offers consistent machinability from heat to heat and enables higher cutting speeds and improved productivity. For example, customers switching from conventional S355 steel to Ovako 520M have reported 30 to 40% reduction in machining cost, which is reflected in the true cost of the component. M-steel metallurgy also leads to spheroidised non-metallic inclusions, meaning that transversal properties in both toughness and fatigue strength are significantly improved as compared to elongated inclusions of standard steels.

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