Ferrite Decarburization of High Silicon Spring Steel in Three Temperature Ranges

Abstract

Surface decarburization of high silicon spring steel in ambient air was studied. The experimental results confirmed the decarburized mechanism under AC1 temperature, in the temperature range of AC1-AC3 and AC3-G. Under AC1 temperature, pearlite spheroidization and surface decarburization are carried out simultaneously and pearlite spheroidization is reinforced. Considering the oxidation loss depth, the “true ferrite decarburized depth” at 850 °C (AC3-G) is still smaller than that at 760°C (AC1-AC3). That is because an “incubation period” must pass away before ferrite decarburization occurs in the temperature range of AC3-G, and the ferrite decarburized rate is limited to being equal to the partial decarburized rate.

Surface Decarburization of the Hypo-Eutectoid Carbon Steel C45 during Annealing in Steady Air at Temperatures T > AC1

“…The measured maximum thicknesses of the decarburized layers after different annealing times are: Complete decarburization of 160 µm, including 80 µm of ferrite layer (t a = 1 2 h); complete decarburization is equal to the 3 µm thick ferrite layer (t a = 1 h); and complete decarburization of 280 µm, including the 70 µm of ferrite layer (t a = 2 h). The largest anomalies regarding the visible decarburization are found in this temperature range (the characteristic coarse-grained ferrite surface layer is the thickest after t a = 1 2 h, the decarburization is practically not visible after t a = 1 h, a large part of the surface is left non-decarburized even after t a = 2 h, and constant thicknesses of the partially decarburized layer are not observed; the results are therefore different to those in, e.g., [9]), but they can be explained by the kinetics of oxidation and decarburization and with different fitting of the oxide, which affects the kinetics of these processes. As mentioned before, in general, there is a tightly fitting oxide layer present on the surface of the samples after cooling, especially on the parts that were not decarburized, showing the effect of fitting of the oxide layer on the decarburization of the steel.…”

“…The Van-Ostrand-Dewey equation is also used and corrected for creating a diffusion model of decarburization that is based on microhardness measurements [21]. For annealing temperatures T a < T G the equations for the calculation of the completely decarburized surface ferrite layers are known [5,9,22,24]. Some equations also take into account the oxidation of the surface [6,9,16,22,24,25].…”

“…This partially decarburized ferrite-pearlite layer shows an increasing content of pearlite as we move towards the interior, until it reaches its equilibrium content at the depth where the carbon content is C 0 . It is known that the thickness of a partially decarburized layer does not change very much during annealing, regardless of the annealing time [9]. The thickness of the partially decarburized layer in this temperature range is therefore controlled by the ratio of the diffusion rates of the carbon in the surface ferrite layer and in the austenite.…”

“…Both strong oxidation and decarburization of the steel's surface occur when annealing at these temperatures in an oxidizing atmosphere [3][4][5][6][7][8][9]. The decarburization occurs as the steel shifts towards the thermodynamic equilibrium between itself and the atmosphere, but in an oxidizing atmosphere this equilibrium cannot be achieved.…”

“…In practice, it is not necessary that all the mentioned microstructures will be visible after cooling. The reason is the velocity of the simultaneous reactions of oxidation and decarburization of the surface at the annealing temperature [5][6][7][8][9][10][11]. If the oxidation lags behind the complete decarburization then there is a reduced, completely decarburized layer as well as a whole, partially decarburized layer visible after cooling.…”