Effect of Modifying the Chemical Composition on the Properties of Spring Steel
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INCREASING demands from the automotive industry for performance improvement, environment preservation, weight reduction, and cost savings put a lot of pressure on vehicles’ components, which require new design concepts and the use of new, advanced materials.[1] In this respect, weight reduction is very important as it reduces costs, but more importantly it also reduces fuel consumption and CO2 emissions.[2] The biggest fuel consumers and polluters are trucks, where the redesign and use of high-strength leaf springs can bring considerable economic and energy-related benefits. The parabolic leaf springs used in the suspension systems of trucks’ front axles are usually made of two leafs, which support the weight of the trailer and provide the spring function in the suspension system. With new spring designs, eventually aiming at a mono-leaf solution, savings of over 20 pct of the total weight of the spring can be expected.[3] Besides a lower weight, the reduced dimensions also allow for an increased design flexibility, making room for additional safety components. However, the mono-leaf spring design requires high- or even ultra-high-strength steel, with a yield strength of over 2000 MPa,[4,5] where an improvement in the tensile
B. PODGORNIK, F. TEHOVNIK, and J. BURJA are with the Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia. Contact e-mail: [email protected] B. SENCˇICˇ is with SˇTORE STEEL d.o.o., Zˇelezarska cesta 3, Sˇtore, Slovenia. Manuscript submitted November 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
strength should not degrade other properties such as the toughness and the fatigue resistance.[6–8] Moreover, maintaining high reliability should be the first priority. The typical yield and ultimate tensile strengths of commercial Si-Cr-V-type spring steels, quenched and tempered at about 400 C, are 1500 to 1600 and 1700 to 1900 MPa, respectively. In the case of spring steels, the emphasis in materials research has been focused on increasing the strength while maintaining good ductility, toughness, and fatigue properties.[9] An improved strength of the spring steel can be achieved by controlling the alloying composition, an effective heat treatment, micro-alloying, a thermomechanical treatment, and shot-peening.[9–14] Important gains in terms of strength improvement are achievable by lowering the tempering temperature; however, this also reduces the material’s ductility and toughness.[11,15] Thus, commercial steel grades cannot be used above certain strength levels because they do not fulfill the ductility and toughness levels imposed by the spring’s design requirements.[6] Another way of improving the spring steel’s strength is by grain refinement,[6,11,12] as part of an optimized thermomechanical treatment and possible micro-alloying. Furthermore, the addition of certain alloying elements has been reported to effectively improve spring steel’s strength as well as the sag resistance.[9] However, if done wrongly it can result in a degradation rather than an improvemen
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