Evaluation and Comparison of High-Temperature Coatings for Decarburization Prevention in Low-Alloy Steels

Abstract High-temperature heat treatment can cause carbon loss in steel due to reactions with oxygen, reducing strength and durability. High-temperature protective coatings offer a cost-effective solution to minimize decarburization and its effects. In this study, seven commercially available high-temperature coatings (WBC, ASC, AS, ASV, HTG, HTR-Zr, and HTR-Si) were applied to AISI 4340 low-alloy steel via spraying or dipping at room temperature, and their decarburization protection was evaluated at 1000°C, 1100°C, and 1200°C for 30, 60, 90, and 120 min under atmospheric conditions. The results were also compared with those obtained by wrapping stainless-steel foils on AISI 4340 low-alloy steel substrates. Optical microscopy and micro-indentation hardness measurements were used to determine the decarburization depth/maximum affected depth and to investigate the formation of free ferrite depth and partial decarburization depth during heat treatment, while profilometry was conducted to study surface irregularities after heat treatment. The samples were subsequently characterized using scanning electron microscopy/energy dispersive spectroscopy to examine surface morphology and elemental composition. WBC showed the best decarburization protection by reducing the decarburization depth by 21% to 44%, followed by zirconia oxide-based (HTR-Zr) coating, which reduced the depth of decarburization by 20% to 37%. This was attributed to the formation of thicker and more consistent Al 2 O 3 and SiO 2 protective layers. Additionally, decarburization modeling was also performed using Fick’s second law to observe the variation between experimental and modeled depth values, where the same hardness values are observed. Good agreement was observed between experimental and theoretical depth values. These findings can be applied to other steels with similar compositions.