Gas Nitriding

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Introduction to Gas Nitriding

Gas nitriding represents an advanced thermal surface treatment process whereby nitrogen is introduced into the surface layer of ferrous alloys. This process is achieved by holding the metal at a controlled temperature range of 495-565°C (925-1050°F) in the presence of a nitrogenous gas, most commonly ammonia. The introduction of nitrogen creates a hardened case with exceptional properties without requiring subsequent quenching.

The principal advantages of gas nitriding include:

• Development of extremely high surface hardness
• Significant enhancement of wear resistance and antigalling properties
• Substantial improvement in component fatigue life
• Enhanced corrosion resistance properties
• Creation of a surface that maintains its hardness at elevated temperatures up to the nitriding temperature

A particularly valuable characteristic of gas nitriding is the minimal distortion and deformation it produces compared to carburizing or conventional hardening processes. This reduced distortion results from the relatively low processing temperatures and the absence of a quenching requirement that would typically cause volume changes in the treated components.

Nitridable Steels for Optimal Results

Steel composition plays a crucial role in determining nitriding effectiveness. Among the common alloying elements, aluminum, chromium, vanadium, tungsten, and molybdenum provide significant benefits during nitriding by forming stable nitrides at nitriding temperatures. Molybdenum offers dual advantages by not only forming nitrides but also reducing the risk of embrittlement during the process. Conversely, elements such as nickel, copper, silicon, and manganese have minimal impact on nitriding characteristics.

Though all steels can theoretically form iron nitrides when exposed to nascent nitrogen at appropriate temperatures, those containing one or more major nitride-forming elements produce superior results. Aluminum-containing steels (0.85-1.50% Al) deliver exceptional nitriding performance due to aluminum's strong nitride-forming capability. Chromium-containing steels can achieve comparable results when their chromium content is sufficiently high. Standard carbon steels without alloying elements are generally unsuitable for gas nitriding as they develop extremely brittle cases that tend to spall, while showing minimal hardness improvement in the diffusion zone.

The following steel categories are particularly well-suited for gas nitriding applications:

• Aluminum-containing low-alloy steels such as 7140 (Nitralloy G, 135M, N, EZ)
• Medium-carbon, chromium-containing low-alloy steels (4100, 4300, 5100, 6100, 8600, 8700, and 9800 series)
• Hot-work die steels with 5% chromium content (H11, H12, and H13)
• Low-carbon, chromium-containing low-alloy steels (3300, 8600 and 9300 series)
• Air-hardening tool steels (A-2, A-6, D-2, D-3 and S-7)
• High-speed tool steels (M-2 and M-4)
• Nitronic, ferritic, martensitic, austenitic, and precipitation-hardening stainless steels

It's important to note that aluminum-containing steels produce cases with exceptional hardness and wear resistance but limited ductility. This trade-off requires careful consideration during material selection. In contrast, low-alloy chromium-containing steels develop nitrided cases with enhanced ductility but somewhat lower hardness. Tool steels such as H11 and D2 consistently achieve high case hardness while maintaining excellent core strength.

Gas Nitriding Process Fundamentals

Prior Heat Treatment Requirements

All hardenable steels must undergo hardening and tempering before the nitriding process begins. The tempering temperature selection is critical and must exceed the maximum nitriding temperature by at least 30°C (50°F) to ensure structural stability during nitriding. This temperature differential prevents undesirable microstructural changes that could compromise the final properties.

For certain alloys, particularly 4100 and 4300 series steels, the case hardness directly correlates with core hardness – a reduction in core hardness results in decreased case hardness. Therefore, to maximize case hardness in these materials, metallurgists typically establish maximum core hardness by tempering at the minimum allowable temperature prior to nitriding.

Single-Stage and Double-Stage Nitriding Approaches

Gas nitriding with anhydrous ammonia can be performed using either single-stage or double-stage processes, each offering distinct advantages for specific applications.

The single-stage process operates at temperatures between 495-525°C (925-975°F) with ammonia dissociation rates ranging from 15-30%. This approach creates a brittle, nitrogen-rich white nitride layer at the surface of the nitrided case, which is desirable for certain applications requiring maximum surface hardness.

The double-stage process begins identical to the single-stage method but introduces a modified second stage. During this second stage, the process may continue at the initial temperature or increase to 550-565°C (1025-1050°F). The key difference is the significantly higher ammonia dissociation rate of 65-80% (preferably 75-80%). Achieving these elevated dissociation rates typically requires an external ammonia dissociator.

The effects of implementing a higher temperature during the second stage include:

• Reduced case hardness
• Increased case depth
• Potential reduction in core hardness (dependent on prior tempering temperature and total cycle time)
• Possible reduction in effective case depth due to core hardness changes

The selection between single and double-stage processes depends on the specific property requirements of the finished component, including desired case depth, hardness profile, and surface characteristics.

Surface Preparation and Operating Procedures

Surface Conditioning for Optimal Results

Proper surface preparation is crucial for achieving uniform nitriding results. After hardening and tempering operations, components must undergo thorough cleaning before nitriding. While most parts can be successfully nitrided immediately following vapor degreasing, certain finishing processes create surface conditions that inhibit effective nitriding.

Machine finishing operations such as buffing, finish grinding, lapping, and burnishing may produce surfaces that impede nitrogen penetration, resulting in uneven case depth and potential distortion. Several effective surface conditioning methods can overcome these challenges.

One recommended approach involves vapor degreasing followed by abrasive cleaning using aluminum oxide, garnet, or silicon carbide grit. Any residual abrasive material must be thoroughly removed by brushing before furnace loading. Clean gloves should be used when handling prepared parts to prevent contamination.

Alternatively, parts may undergo preoxidation in an air atmosphere at approximately 330°C (625°F). This process can be performed either as a separate operation or integrated into the heating portion of the nitriding cycle with appropriate precautions.

Furnace Purging Procedures

The nitriding cycle begins with a critical purging step to remove air from the retort before temperatures exceed 150°C (300°F). This prevents oxidation of both the components and furnace internals while avoiding potentially explosive gas mixtures when using ammonia.

While nitrogen is the preferred purging medium, ammonia can also be used with proper safety protocols. A typical ammonia-based purging sequence follows these steps:

1. Close the furnace and initiate anhydrous ammonia flow at the maximum practical rate
2. Set furnace temperature control to 150°C (300°F) and heat to this temperature without exceeding it
3. Once the atmosphere contains 90% or more ammonia with 10% or less air, proceed with heating to the nitriding temperature

When incorporating preoxidation into the process cycle, nitrogen must be available as a purging medium at the conclusion of the 330°C (625°F) oxidizing stage. Under no circumstances should ammonia be introduced into a furnace containing air at elevated temperatures due to explosion hazards.

Purging is equally important at the conclusion of the nitriding cycle during furnace cooling. The standard practice involves displacing residual ammonia with nitrogen to minimize ammonia release when unloading. This dilution reduces worker exposure to ammonia vapors. Nitrogen introduction can be delayed until components have cooled below 150°C (300°F).

Purging Gas Selection: Nitrogen versus Ammonia

Nitrogen offers several advantages as a purging gas, including enhanced safety, simplified handling, and precise control. However, implementing nitrogen purging requires additional equipment and piping infrastructure.

Ammonia purging requires no supplementary equipment and remains relatively safe when properly handled. The primary safety concern involves the explosive potential of air-ammonia mixtures containing 15-25% ammonia if exposed to ignition sources.

Process Parameters and Control

Dissociation Rates and Cycle Management

The nitriding process fundamentally relies on the affinity between nascent nitrogen and iron or other specific metallic elements. This nascent nitrogen is generated when gaseous ammonia dissociates upon contact with heated steel components.

While various dissociation rates can yield successful nitriding results, process optimization requires careful control of these parameters. The nitriding cycle should commence with a dissociation rate between 15-35%, maintained for 4-10 hours at approximately 525°C (975°F). This initial phase develops a shallow white layer that serves as the foundation for nitrogen diffusion into the main case structure.

Ammonia flow rate typically targets a minimum of four complete atmosphere changes in the retort per hour. At dissociation rates of 15-35%, control is achieved primarily through ammonia flow rate adjustment. However, when operating at higher dissociation rates of 75-80%, it becomes necessary to introduce completely dissociated ammonia to maintain process stability.

Managing Distortion and Dimensional Changes

Nitriding-related distortion can originate from multiple sources:

• Relief of residual stresses from prior manufacturing operations (welding, hardening, machining)
• Stress introduced during nitriding due to inadequate support, non-uniform heating, or rapid temperature changes
• Volume expansion in the case, which induces stretching of the core

The volume increase during nitriding creates a stress distribution where the case develops compressive stresses balanced by tensile stresses in the core after cooling to room temperature. The magnitude of permanent dimensional changes depends on the material's yield strength, case thickness, and the amount and nature of nitrides formed during processing.

Despite these challenges, nitriding produces significantly less distortion than other case-hardening processes that require quenching to form martensite. This dimensional stability represents one of the primary advantages of the nitriding process.

Stabilizing Treatment for Precision Components

Nitrided components establish an equilibrium between compressive stresses in the case and tensile stresses in the core. When this balance is disrupted, such as when a portion of the case is removed by grinding, slow dimensional changes may occur as the stresses redistribute toward a new equilibrium state.

To prevent these undesirable dimensional changes in precision components, a stabilizing treatment is recommended. Components are first ground nearly to final dimensions, then heated to 565°C (1050°F) for one hour, and finally finish ground or lapped to precise specifications. Components that are not ground after nitriding demonstrate excellent inherent dimensional stability without requiring this additional treatment.

Cost Considerations in Finishing Operations

The minimal distortion characteristic of nitriding offers significant economic advantages. The increased cost of the nitriding operation and suitable nitriding steels is frequently offset by the ability to finish components to size prior to nitriding, thereby reducing or eliminating costly post-hardening finishing operations.

This cost-benefit consideration makes nitriding particularly attractive for applications requiring precise dimensions and exceptional surface properties, especially when post-hardening machining would otherwise be extensive and expensive.