The austenitisation and decarburisation of high silicon spring steels
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The 1970’s saw the demand for more fuel efficient vehicles. This necessitated lighter automobiles, and was partly attained by lowering the weight of the coil springs used for the suspensions of these vehicles. Substantial weight reductions were achieved with new steels such as SUP7, with their elevated carbon and silicon contents allowing the springs to operate at higher stresses. However, these steels are prone to decarburisation during spring manufacture, which sharply reduces the sag resistance and the fatigue strength of the springs. The literature pertaining to the decarburisation of high silicon spring steels is limited, despite the subsequent reduced sag and fatigue properties. The research detailed in this thesis was therefore undertaken to investigate the decarburisation of spring steels with 0 to 3wt.% silicon for a range of temperatures and heat treatment atmospheres. The austenitisation of these steels was also investigated to provide complementary information. Austenitisation and decarburisation heat treatments were undertaken with 0, 1 and 3wt.% silicon experimental steels, and the commercially available 2wt.% silicon SUP7 and SUP7NV steels. These five steels have similar carbon, manganese and chromium contents. Two dimensional austenite growth occurred for the different steels, with the austenite grain shape inherited from the initial ferrite grain structure. The austenitisation accelerated with increasing temperature, with the higher silicon steels exhibiting faster transformation kinetics at comparable intercritical fractional superheat temperatures due to the higher Ac₁ and Ac₃ temperatures. The austenite growth was controlled by the substitutional diffusion of the alloy elements at low intercritical temperatures, and by carbon diffusion through the austenite at higher temperatures. This resulted in the faster transformation of ferrite/spheroidal cementite initial microstructures than pearlitic microstructures at low intercritical temperatures, and the opposite at higher temperatures. Austenite nucleated earlier in ferrite/spheroidal cementite initial microstructures when the cementite particles were located predominantly on the ferrite grain boundaries of the initial structures rather than within the ferrite grains. The austenite growth eventually became faster for the latter structure. An increasing silicon content up to 2wt.% increased the rate of carbon removal at comparable temperatures. Multiple linear regression analysis demonstrated that the mass of carbon removed during decarburisation increased with silicon contents up to 2wt.%, with temperature, and with PH₂O/PH₂ ratios of 0.01 to 0.25, while manganese decreased the decarburisation. However, slower decarburisation resulted for the 3% Silicon steel, especially at higher temperatures and lower PH₂O/PH₂ ratios, where a dense oxide layer strongly inhibited carbon removal. The decarburisation of the 1% Silicon, SUP7 and SUP7NV steels was fastest at their respective Ac₃ temperatures. Columnar free ferrite grain structures were obtained for the 0% Silicon and SUP7 steels, compared with equiaxed grains for the 1% Silicon, SUP7NV and 3% Silicon steels. The decarburisation of SUP7 was fastest when the specimen was heated in the furnace with the heat treatment atmosphere already established, while slower decarburisation resulted when the specimens were annealed in inert atmospheres prior to the introduction of the decarburisation atmosphere. Different initial SUP7 microstructures influenced the carbon release kinetics as the specimens heated to temperature. Prolonged decarburisation of SUP7 and SUP7NV at 900°C yielded decreasing free ferrite depths after 4hr, despite continued carbon removal from the steel. This research contributes to the understanding of the austenitisation and decarburisation of spring steels. The information obtained is directly applicable to automobile coil spring production, but is also pertinent to the austenitisation and decarburisation of steel.