EoW November 2013

Focus on Scandinavia

Spring wire grade sets new standard for fatigue resistance

The corrosion properties of duplex steels are also an advantage and will benefit the plunger spring application. There appears to be some evidence that low quality diesel fuel with high levels of water content has led to stress corrosion cracking in common rail plunger springs. Common rail fuel injection is designed to inject fuel into an engine’s combustion chamber under high pressure. A common fuel pipeline – the common rail – supplies the engine’s fuel injectors. This results in virtually complete combustion of the fuel with high-energy conversion and reduced emissions. As well as the fatigue resistance challenge of plunger springs, Sandvik believes that the new material may find applications in other markets. Typically, applications where conventional stainless steels are used but greater fatigue resistance is required; where other material grades are used but better corrosion resistance is required; or as an alternative to coated surface springs made from carbon, Cr-Si or Cr-Si-V steels.

Spring makers and designers looking for a high level of fatigue resistance performance will be interested in the new duplex stainless steel spring wire from Sandvik. The new grade, called Sandvik Springflex™ SF – the SF stands for Super Fatigue resistant, is produced using a patented innovation in processing technology. Designed to meet the extreme fatigue performance demands of springs in common rail fuel injection systems in motor vehicles, these ‘plunger’ springs need the necessary strength to carry high repetitive loads and to last the lifetime of the motor, without risk of failure. Space is limited and so the spring also needs to be small – about 40 grams of wire. Testing as a compression spring has confirmed that Sandvik Springflex SF wire can resist more than 300 million highly stressed cycles, which has led to line qualification for the new material by a major manufacturer of common rail systems. In order to meet these demanding physical requirements and maximise fatigue performance, Sandvik has introduced a key manufacturing innovation. A new, patented process has been developed, which includes peeling the hot-rolled wire rod, prior to cold

drawing. This has the effect of reducing significantly the number of surface defects, and subsequently improving fatigue resistance. The material’s duplex microstructure means that the beneficial properties of both austenitic and ferritic structures are exploited and remain stable throughout the wire drawing, spring coiling and heat treatment processes. Sandvik Springflex SF is, therefore, less sensitive to crack propagation than austenitic steels, such as ASTM 302 and 17-7PH steels. The chemical composition and manufacturing route mean that ductility remains throughout the entire production process. ▲ ▲ A selection of springs made from Sandvik Spring wire

Sandvik Materials Technology – Sweden Website : www.smt.sandvik.com

54 Photo: bigstockphoto.com –‘Snogebaek, Bornholm, Denmark’Photographer – foto kate September 2012 S Nov 3

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