Research Progress on Surface Oxidation Phenomena of Piercing Plugs for Seamless Steel Tubes
Introduction:
Piercing plugs are widely used in the production of seamless steel tubes, especially in oil and gas industry pipelines that transport oil and gas over a long distance. They are composed of a piercing rod and an alloy, commonly referred to as the plug body and die, respectively. The piercing rods are usually made of tool steel, while die is made of forging steel, such as Ni-based or Fe-based alloys. Owing to the high temperature in the heating zone of the piercing process (around 1,600 °C) and the subsequent rapid cooling of the tubes, the die and the punching claws will undergo oxidation and silicidation on their surfaces, i.e., forming a oxidized layer on their surface.
Prolonged exposure to surrounding environment during and after the piercing process will further accelerate the oxidation of the plug. This surface oxidation phenomenon, if not properly controlled, can lead to distortion of the die, cracking of the claws, blockage of the tube hole, and degradation of the plug-tube joint properties. Moreover, oxidation might also cause severe corrosion when the plug is exposed to acid-base media. Therefore, understanding and preventing surface oxidation phenomenon is important to improve the reliability and safety of the product.
Influencing Factors
Temperature presents a major influence on the oxidation rate of piercing plugs. Generally, oxidation rate increases as temperature increases; high-heat oxidation of plugs is normally observed at temperatures above 800 °C, while low-heat oxidation may appear at temperatures lower than 400 °C. Pressure is another factor that can influence the oxidation rate of piercing plugs. Higher pressure rate leads to thicker oxide layers, which improves oxidation resistance.
Oxidation-Resistant Alloys
Several studies have focused on developing oxidation-resistant alloys for piercing plugs. In 2004, Uchino et al. developed a high-steel matrix aluminum (HSMA) alloy used for die and claws of piercing plugs. The alloy demonstrated high oxidation resistance when tested at 1,150 °C for two hours, indicating its potential application in the production of seamless steel tubes. Moreover, in 2007, Sakane et al. developed another Ni-based alloy containing 20% Cr, 12% Ni, and 4% Mo. That alloy showed good oxidation resistance when tested at temperatures up to 800 °C, achieving excellent corrosion resistance in hydrochloric acid solutions.
Surface Coating and Protection Materials:
Surface coatings can protect the die and bearing claws from oxidation, and also improve the overall performance of the piercing plug. Laser cladding and building-up welding can be used to coat the surface of piercing plugs with some oxidation-resistant materials, such as chromium, nickel, aluminum or titanium. In 2008, Chakraborty et al. successfully clad a mix of chromium and molybdenum onto the surface of die and bearing claws of piercing plugs, resulting in excellent oxidation resistance in simulated environments of temperature range 850–1,050 °C.
In addition to coating the plug surfaces, surface protection materials can also be used to improve the oxidation resistance of the plug materials. These materials include ceramic materials, such as silicon nitride and silicon carbide, which have been used in a variety of applications to protect steel materials from oxidation and wear. Ceramic materials can be applied to the surface of the dies and bearing claws, and then heated in a furnace at a temperature of around 1,150 °C. This process can form a strong protection layer, which will significantly improve the oxidation resistance of the dies and bearing claws.
After-Treatment Approaches:
After the piercing process, cooling and heat treatment techniques can be used to improve the oxidation resistance of the piercing plugs. These techniques include slow cooling, quenching, and aging. In slow cooling process, the die and claws of the plugs are cooled to a low temperature at a slow rate, as this will reduce the formation of oxides and therefore increase the oxidation resistance of the material. Meanwhile, quenching and aging can also be used to improve the oxidation resistance of the die and claws, by promoting increased precipitation hardening reactions. Quenching the die can form a strong oxide film on its surface, and the effect can be further enhanced by adding a small amount of nickel to the alloy. In addition, aging the die can lead to increased homogenization in the alloy, resulting in improved oxidation resistance as well.
Surface oxidation of piercing plugs is a major concern in the oil and gas industry, as it can lead to severe deterioration and degradation of this important component of pipelines. In recent years, substantial progress has been made in the understanding, analysis, and prevention of the surface oxidation phenomenon. This paper reviews the major progress in terms of oxidation-resistant alloys, surface coating and protection materials, and after-treatment approaches. Although further investigation is needed, this paper provides an updated overview of the current research trend, which will guide future development in this field.
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