Low-Pressure Carburizing for Aerospace Steel Alloy: The Heating Treatment of P675

Many low-alloy steel sheets are heat-treated using low-pressure carburizing (LPC), especially in the automobile industry.

FREMONT, CA: Today, LPC is gaining in popularity for many important reasons: increased precision in targeting the effective case depth (ECD), repeatability of the same carburizing recipe, higher heat-treatment temperatures that permit shorter cycle durations, improved distortion control, and the lack of intergranular oxidation (IGO) or decarburization.

Higher heat-treatment temperatures during LPC can save cycle time by up to fifty percent compared to conventional carburizing. If the same temperature is selected for both standard carburizing and LPC heat treatments, fewer hydrocarbon gases are required for LPC. An additional benefit of LPC is the cleanliness of the items following heat treatment and the improvement of the working environment.

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Hydrocarbon gases normally blend with air or nitrogen at a predetermined ratio in conventional atmospheric carburizing procedures. This mixture of gases, known as endogas, consists of carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2) (CO2). If air is a component of the endogas, water vapor is also present in the furnace's environment. Carbon dioxide and water vapor are possible oxidizing agents that contribute to the formation of IGO on the surface of components.

Unlike standard atmospheric carburizing, LPC heat treatment utilizes solely oxygen-free hydrocarbon gases, such as acetylene. IGO and decarburization are, therefore, not an issue during LPC heat treatment.

LPC can also be used to heat treat aerospace-grade, high-alloy steels. Common alloys include stainless steel—Pyrowear® 675 (P675) and X13VDW (ASTM XM32)—and high-alloy steels such as M50NiL and 9310. Aerospace industries have unique requirements for the installation of furnaces (according to AMS 2750F) and heat treatment in vacuum (per AMS 2769C).

The fact that high-alloy steels can readily generate carbides is one of the difficulties associated with heat-treating steels for the aerospace sector. The development of carbides from components that should be present in the matrix of the steel alloy is damaging to its function and performance. For example, chromium can readily create carbides. Due to carbide production, chromium depletion from the stainless-steel matrix can increase the likelihood of undesired corrosion.

Furthermore, producing large carbides or carbides in a network is detrimental to fatigue and fracture performance. LPC enables improved carbon injection control in the furnace. Therefore, the development of large or network carbides is easy to regulate. The inclusion of microalloying elements such as titanium (Ti), niobium (Nb), or vanadium (V) can also reduce unwanted carbide formation by creating TiC, NbC, and VC, which also refines the grain and enhances the mechanical properties.

LPC is accomplished by injecting acetylene (for carburizing) and nitrogen (for diffusion) sequentially into the furnace. During the acetylene boost, acetylene dissociates into carbon atoms and hydrogen molecules, enriching the carbon content on the surface of the heat-treated components. The subsequent nitrogen boost permits the carbon (absorbed on the component's surface) to diffuse into deeper levels. The duration of each boost and the number of boosts depend on the desired temperature and ECD. This procedure is carried out at a constant temperature and pressure of around 10 mbar.

Varied recipes were established for P675, X13VDW, M50NiL, and 9310 steel alloys with different ECDs and temperatures for the ECM Flex vacuum furnace. More compact loads can be heat treated in furnaces with smaller cells that are also consistent with AMS 2750 standards, such as the ECM Nano vacuum furnace.

In these furnace systems, the hydrocarbon gases employed for heat treatment were acetylene and propane. The metallurgical specifications required homogeneous ECD, the absence of large or network carbides, and a surface carbon content between 0.8 percent and 0.9 percent by weight.

The Heating Treatment of P675

The P675 steel alloy LPC formula was designed to achieve an ECD of 0.055 inches (1.4 mm) and a hardness of 50 HRC. For this formulation, LPC was carried out at 1680°F (915°C) by injecting acetylene (C2H2) boosts for a total of 39 minutes at a volume flow rate of 3,500 NL/hr. The injection of acetylene accounts for 1.5 percent of the total duration of the heat treatment. The diffusion time was 43 hours and 42 minutes, or 98.5 percent of the overall treatment time. LPC lasted a total of 44 hours and 21 minutes.

High-pressure gas quenching (HPGQ) was carried out utilizing 19 bars of nitrogen. The complete heat treatment consisted of LPC, hardening, cryogenic, and tempering. For the LPC (carbon injection + diffusion) and the complete heat treatment, ECD measurements concerning different specifications (50, 52, and 58 HRC) were obtained.

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