Liquid Nitriding

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Introduction to Liquid Nitriding

Liquid nitriding, or nitriding in a molten salt bath, operates at temperatures similar to gas nitriding—typically between 510 and 580°C. The process employs a molten, nitrogen-bearing fused salt bath containing cyanides or cyanates as the case-hardening medium. Unlike liquid carburizing and cyaniding, which use similar bath chemistries, liquid nitriding is a sub-critical case-hardening method. This means finished parts can be treated without significant dimensional changes, preserving geometric stability. Additionally, liquid nitriding introduces more nitrogen and less carbon into ferrous materials compared to higher-temperature diffusion treatments.

Applications of Liquid Nitriding

Liquid nitriding is primarily used to improve surface wear resistance and increase fatigue endurance limits in steels. It often enhances corrosion resistance as well. Although it is not suitable for applications requiring deep case depths and hardened cores, liquid nitriding has successfully replaced other heat treatment processes on both performance and economic grounds. The applications of liquid nitriding closely mirror those of gas nitriding, with gas nitriding preferred when deeper cases are needed. Both methods offer benefits including improved wear resistance, antigalling properties, increased fatigue strength, and reduced distortion compared to higher-temperature case-hardening processes.

The extent of property improvement depends on the specific nitriding process used and the chemical control of the salt bath.

Liquid Nitriding Salt Bath Systems

The term "liquid nitriding" encompasses several salt bath processes conducted at sub-critical temperatures, relying on nitrogen absorption and reaction rather than significant carbon uptake. Typical commercial baths are mixtures of sodium and potassium salts.

• Sodium salts constitute 60–70% by weight and typically include 96.5% NaCN, 2.5% Na₂CO₃, and 0.5% NaCNO.
• Potassium salts form 30–40% of the mixture and contain approximately 96% KCN, 0.6% K₂CO₃, 0.75% KCNO, and 0.5% KCl.
• The standard operating temperature of this bath is about 565°C.

This bath is widely used for nitriding tool steels, including high-speed steels, and various low-alloy steels such as aluminum-containing nitriding steels.

An alternative bath composition for tool steels includes 60–61% NaCN, 15.0–15.5% K₂CO₃, and 23–24% KCl.

Several proprietary liquid nitriding processes introduce gaseous or solid additives to enhance bath activity, broaden steel compatibility, and improve nitrided layer properties.

Cyanide-free salt baths have been developed, though small amounts of cyanide (up to about 5%) form during operation. These baths have gained acceptance due to reduced environmental impact.

Specialized Liquid Nitriding Processes

Liquid Pressure Nitriding

This process introduces anhydrous ammonia into a cyanide-cyanate salt bath operating between 525 and 565°C. The ammonia diffusion simplifies bath aging, allowing immediate operation with recommended cyanide-cyanate ratios (30–35% cyanide and 15–20% cyanate). The continuous ammonia supply helps maintain bath composition by counteracting cyanide depletion.

Aerated Bath Nitriding

Measured air is bubbled through the molten bath to agitate the mixture and stimulate chemical activity. The bath typically contains 50–60% cyanide (as sodium cyanide) and 32–38% cyanate, with potassium content ranging from 10 to 30% (preferably about 18%). Potassium may be present as cyanate, cyanide, or both, with the remainder sodium carbonate. This process forms a compound layer for wear resistance and a diffusion zone to enhance fatigue strength.

Only steels alloyed with chromium, titanium, or aluminum respond well to conventional bath nitriding. Plain carbon steels respond poorly to conventional nitriding but well to aerated bath nitriding, which should be specified for such steels to achieve adequate hardness.

Aerated Cyanide-Cyanate Nitriding

This proprietary process employs high cyanide (45–50%) and cyanate (42–50%) levels in potassium-based salts, oxidized by aeration. Precise sodium-to-potassium ion ratios are crucial for optimizing the integrity of the compound and diffusion zones.

Aerated Low-Cyanide Nitriding

Developed in response to environmental concerns, this cyanide-free process uses potassium cyanate mixed with sodium/potassium carbonates or chlorides. Minor cyanide forms during use but is mitigated by an oxidizing quench salt that decomposes cyanide and cyanate, reducing pollution and distortion compared to water quenching.

Effects of Steel Composition on Nitriding

Although alloy steels benefit from improved compound and diffusion layers, plain carbon steels with low to medium carbon content often see relatively greater enhancements. For example, fatigue strength of 1015 steel nitrided for 90 minutes at 565°C and water quenched can improve by roughly 100%. Similarly treated 1060 steel shows 45–50% fatigue strength improvement.

Operating Procedures for Liquid Nitriding

Salt Bath Preparation and Aging

Salt baths must be properly prepared and heated, with aging performed when necessary to oxidize cyanide to cyanate. Aging depends on temperature and surface-area-to-volume ratio and is typically done at 565–595°C for at least 12 hours without workpieces in the bath.

Prior Heat Treatment

Alloy steels generally require prior heat treatment similar to gas nitriding. Hardening parts before nitriding enhances dimensional stability. Tempering temperatures should be at or slightly above nitriding temperature to optimize core properties and minimize distortion.

Starting and Maintaining the Bath

Case-producing salt mixtures vary but are generally based on sodium and potassium cyanides and cyanates. Melting occurs at 540–595°C, with precautions like covering the retort or venting to prevent spattering or explosions.

Externally heated salt baths require limited startup power (about 37% capacity) until melting is uniform. Internally heated baths often use moderate natural gas flames to create conductive molten salt pools between electrodes.

Bath compositions must be monitored and maintained, with cyanide-to-cyanate ratios varying by process:

• Commercial NaCN-KCN baths reach 21–26% cyanide to 14–18% cyanate after aging.
• Liquid pressure nitriding baths use 30–35% cyanide to 15–20% cyanate.
• Aerated baths maintain 50–60% cyanide to 32–38% cyanate.
• Aerated low-cyanide processes operate around 36–38% cyanate to 17–19% carbonate.

Overheating baths above 595°C should be avoided to preserve bath integrity.