Evaluation of surface decarburization depth by magnetic Barkhausen noise technique

Decarburization is an important process during the heat treatment of steels. It can be divided into three separated events: dissolution of carbides, diffusion of carbon through the iron matrix, and surface reactions. The process is very sensitive to temperature. During intercritical annealing, austenite nucleates in the cementite-ferrite interface and grows at the rate determined by the diffusion of carbon in austenite. The presence of a decarburizing atmosphere during annealing guides the carbon diffusion in ferrite toward the surface, generating a flux of carbon from austenite toward ferrite, disturbing the austenite growth. In the presence of pearlite, the ferrite-austenite interface can be assumed to remain static until pearlite is completely dissolved, reducing then the carbon flux in austenite, consequently diminishing the austenite formation rate. At intercritical temperatures, the cementite-free ferrite layer at the surface reaches a greater width due to the combination of the thermodynamic fraction of austenite, dissolution rate of cementite, and the diffusivity of carbon in austenite and ferrite. In this study, an experimental investigation of the effects of the carbide morphology and distribution and the $$\alpha -\gamma $$
 phase transformation in the decarburization kinetics on hypo-eutectoid steels is presented. It is suggested that the change of the dissolution kinetics of the carbides due to its morphology will affect the austenitization kinetics. Thus, the distribution of the carbon in the microstructure may determine the rate of decarburization in combination with the carbon diffusion through the phases or the gas-metal reactions.

Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels

The ferrite decarburization behavior of 60Si2MnA spring steel wires for automotive suspensions, including the forming condition and the influence of heating time and cooling rate after hot rolling, was investigated comprehensively. Also, a control strategy during the reheating process and cooling process after rolling was put forward to protect against ferrite decarburization. The results show that ferrite decarburization, which has the strong temperature dependence due to phase transformation, is produced between 675 and 875°C. The maximum depth is found at 750°C. Heating time and cooling rate after rolling have an important influence on decarburization. Reasonable preheating temperature in the billet reheating process and austenitizing temperature in the heat-treatment process are suggested to protect against ferrite decarburization.

Forming condition and control strategy of ferrite decarburization in 60Si2MnA spring steel wires for automotive suspensions

The characteristics of complete and partial decarburizations in spring steel 60Si2MnA were systematically investigated, including the microstructure, the hardness gradient, and the formation mechanism. The relation between decarburized types and heating temperature of the steel was comprehensively discussed. It is found that heating temperature has an important influence on the decarburized types. With the rise of heating temperature, the decarburized types change from no decarburization to complete decarburization, complete and partial decarburizations, partial decarburization, and no decarburization.

Surface decarburization characteristics and relation between decarburized types and heating temperature of spring steel 60Si2MnA

Surface decarburization of steel was non-destructively and quantitatively investigated by magnetic hysteresis. Flat samples were prepared from spring steel 54SiCr6 and decarburized layers of different thickness were produced on their surfaces by annealing at 800 °C in air for 1, 4, 8 and 20 h. Three types of treatment were applied on different samples in order to remove the simultaneously appearing surface oxides. The decarburized layers were examined magnetically by hysteresis loops measurements and the results were related to their optically determined depths. The magnetic measurements showed high sensitivity with respect to free ferrite and mixed ferrite/pearlite layers detection even without removal of the oxide layer. The free ferrite layers were easily and quantitatively detected by minor loops measurements with low field amplitude.

Detection of spring steel surface decarburization by magnetic hysteresis measurements

Decarburization and oxidation have considerable influence on the product properties of spring steels. The investigations in part I of this paper concentrate on the experimental determination of the influence of different thermal cycles on the decarburization process. With the thermo-mechanical simulator Gleeble 1500, the influence of different process parameters, such as the time between furnace reheating and hot rolling, the hot rolling temperature, finish rolling temperature, laying temperature, and the α→γ phase transformation temperature range, is systematically investigated. In part II of this paper [15], numerical simulation techniques are applied to simulate the decarburization behavior under the experimental conditions, which are described in the present part I.

Investigation of Decarburization in Spring Steel Production Process – Part I: Experiments

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Investigation of Decarburization in Spring Steel Production Process – Part I: Experiments