Hydraulic head for high pressure pumps

Hydraulic head in IDI isothermal ductile iron for high-pressure piston pumps

The component described in this case history derives from the existing head for high flow rates, but differs from this version in terms of the type of suction and supply valve units, now optimised for high pressure. It has been designed to reach a pressure of 400 bar compared to the 250 bar of the current version.

The customer needed to launch the new version of the hydraulic head as quickly as possible. To succeed in doing this, we worked on a co-design with the customer whilst respecting the foundry requirements and thus, introduced some size modifications as well as opting to change the material to achieve the higher performance required.

The goal of increasing performance meant that the GJS600-3 spheroidal graphite iron, a material previously used for high-capacity heads, was no longer suitable for the new stress levels. The decision was, therefore, taken to upgrade the material, opting for a spheroidal graphite iron with a yield stress of around 500 Mpa.

These piston pumps are capable of working at pressures of up to 1500 bar with absorbed power up to 150 HP. Choosing high-performance and high-quality materials for the construction of the head, the key component, therefore, to achieving these levels of performance, is of fundamental importance. Isothermal ductile iron IDI800-6 proved to be the best material for the versions with pressure up to 400 bar.

Starting material for high flow rates (pressure of 250 bar)

The new solution assessed for achieving the high pressure goal (400 bar)

Tensile and fatigue test performed on test specimens obtained from separately cast samples Ø25 mm. The fatigue limit on smooth surfaces (R-1) was obtained by testing Ø6.5 mm smooth test specimens machined with the Short Stair Case method (run out at 5ML of cycles). In reality, the workpiece has a variable thickness.

Isothermal ductile iron IDI800-6 ensures greater fatigue performance and mechanical features compared to GJS600-3 spheroidal graphite iron. Given the absence of binding agents and, therefore, segregation in the metal matrix, these features were uniformly maintained on all the thicknesses after the thermal treatment. Moreover, the hardnesses proved to be more uniform in the thickness variations, so the hardness range was narrower and only just higher than that of GJS600-3, thus ensuring an excellent compromise in terms of machinability. The absence of alloying agents also meant the cost was only slightly higher than that of the original spheroidal graphite iron solely due to the additional thermal treatment.

The working requirements requested by the customer and bench tested were:

• Working pressure of 400 bar
• Uniformity of the micro structure between the surface and core and all the thicknesses
• Internal and surface integrity
• Good machinability
• Compliance with the deadlines for the market launch
• Low cost increase compared to the GJS600-3 solution
• 1500 bench working hours (the equivalent of 5·10⁷ operating cycles)
• On test bench at a rotation speed of 550 rpm (max absorbed power 150hp)
• No “cavitation pitting”

IDI800-6, in this case, proved to be the best material compromise between technology and cost effectiveness for production of the hydraulic head. Nickel plating was introduced as an alternative to other corrosion protection treatments during the validation stage.

The co-design work, managed entirely Zanardi Fonderie’s staff, played a key role in producing a hydraulic head in IDI for the industrial sector which resulted in a 60% increase in working pressure.

An exchange of ideas between the customer’s and Zanardi Fonderie’s technical departments proved important right from the preliminary design stages. This enabled the designers to make comparisons and share, on the one hand, the stress resistance requirements, and the industrialisation requirements of the foundry, on the other. The goal, which was duly achieved, was to produce a product sample in as little time as possible (whilst complying with the size and integrity requirements requested by the customer), minimising the risk of having to make radical design changes to achieve the goal.

From a mechanical point of view, an in-depth examination was performed on the tensile state of the component, EAE (Environmentally Assisted Embrittlement) induced by contact with fluids, contact fatigue phenomena, the effect of surface treatments and, lastly, the requirements for internal and surface integrity.

Table comparing the two solutions