Parameters affecting sag resistance in spring steels
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I.
INTRODUCTION
A U T O M O B I L E suspension springs are designed so that yielding anywhere under sustained loading is prevented. However, during repeated loading in service and with time, the spring can progressively deform plastically or sag, especially if the working stresses are a high fraction of the yield stress of the material. Recent trends toward reducing the weight of automobile suspension springs have led to the development of a number of microalloyed steels and a variety of processing treatments which have claimed to improve the sag resistance of the springs while increasing their operating stresses. More often than not, however, the subtle effects of varying levels of hardness and prior austenite grain size, as well as small but significant differences in critical elements such as Si, are overlooked in comparing these new steels with the conventional grades. In addition, it is not always clear from the literature if the spring steels have been tested in their optimum condition. Another difficulty relates to the various testing techniques used to characterize sag resistance, which may be influenced by particular spring designs and other test variables, making direct comparison of the results rather difficult. It has long been known that Si improves the relaxation resistance of springs, tL21 The beneficial effects of other alloying additions, such as Mn (for hardenability), Cr, Mo, Ni (for better ductility and surface finish), and V (for grain refinement), have also been reported from as early as 1946 m and 1950/21 The strengthening by precipitation of alloy carbides of Cr, Mo, V, Nb, W, e t c . , often referred to as secondary hardening, is also wellknown. 13-ml In recent years, however, a number of patents have been awarded 111'~2'131 for spring steels containing microalloying elements such as V and Nb, which are said to improve the relaxation resistance of the spring M. ASSEFPOUR-DEZFULY, Senior Research Officer, and A. BROWNRIGG, Senior Principal Research Officer, are with The Broken Hill Propietary Co. Ltd., Melbourne Research Laboratories, P.O. Box 264, Clayton, Victoria 3168, Australia. Manuscript submitted December 19, 1988. METALLURGICAL TRANSACTIONS A
by grain refinement and secondary carbide precipitation. tl41 The general statements t1~,141 regarding the beneficial effects of V and Nb on the sag resistance of spring steels, in the hardness range of 45 to 55 H R C , however, are not supported by the experimental results presented, Il< if proper consideration is given to the error or scatter bands in the data (not shown on any of the graphs) and to the very small differences which were highlighted. Furthermore, the thermodynamics/kinetics considerations for the precipitation of V and/or Nb carbonitrides (secondary hardening) are not consistent with the claims, particularly at hardness levels of 50 HRC and above (tempering temperatures < 450 ~ In fact, these were only identified at tempering temperatures greater than 510 ~ (
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