Flapper valves are one of the most important components in compressors used, for example, in automotive air conditioning systems and in refrigerators. Fundamentally a spring made from pre-hardened steel, flapper valves are subjected to very high cyclic loads as the valve opens and shuts. In recent years, environmental and economic considerations have led to changes in service conditions and demands for increased compressor efficiency, which can be achieved by increasing the valve lift or using thinner valves. Flapper valve materials with a higher performance capability are critical to such developments.
Sandvik’s answer has been the development and launch of Sandvik Hiflex™ - a new generation of martensitic stainless chromium strip steel and a modified AISI 420 type material. It offers better fatigue and impact fatigue strength than existing materials as well as good ductility. Cyclic bending stresses and impact stresses are the two main factors that cause a ‘suction and discharge’ flapper valve to fail. Sandvik Hiflex displays up to 10% improvement in bending fatigue strength and up to 25% improvement in impact fatigue strength compared with the best available competitive materials.
Now and in addition to these previously known benefits, Sandvik’s R & D organization has established the important influence on the impact fatigue strength of the ‘damping’ capacity of flapper valve materials. Damping is the phenomenon or property which governs how quickly a flapper valve stops vibrating after impact. A high damping capacity contributes to high impact fatigue strength. Tests show that stress waves, which can lead to valve failure, decay or diminish more quickly in materials with high damping capacity. Sandvik Hiflex shows the highest damping capacity of all materials tested suggesting why it also has the highest impact fatigue strength.
In order to meet industry’s demands and in summary, a flapper valve steel with high impact fatigue strength must have a high tensile strength, high ductility and high damping capacity. Sandvik Hiflex has all of these properties. It is particularly suitable for use in applications with a high valve lift and in carbon dioxide compressors for automotive air conditioning, where high temperatures and pressures exist and place heavy demands on the valve material. Its stainless steel metallurgy means that Sandvik Hiflex is also suitable where corrosion resistance is required.