Nitriding

Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of steel at a temperature range (500 to 550°C, or 930 to 1020°F), while it is in the ferrite condition. Thus, nitriding is similar to carburizing in that surface composition is altered, but different in that nitrogen is added into ferrite instead of austenite. Because nitriding does not involve heating into the austenite phase field and a subsequent quench to form martensite, nitriding can be accomplished with a minimum of distortion and with excellent dimensional control.

The mechanism of nitriding is generally known, but the specific reactions that occur in different steels and with different nitriding media are not always known. Nitrogen has partial solubility in iron. It can form a solid solution with ferrite at nitrogen contents up to about 6%. At about 6% N, a compound called gamma prime (γ’), with a composition of Fe₄N is formed.

At nitrogen contents greater than 8%, the equilibrium reaction product is ε compound, Fe₃N. Nitrided cases are stratified. The outermost surface can be all γ’ and if this is the case, it is referred to as the white layer. Such a surface layer is undesirable: it is very hard profiles but is so brittle that it may spall in use. Usually it is removed; special nitriding processes are used to reduce this layer or make it less brittle. The ε zone of the case is hardened by the formation of the Fe₃N compound, and below this layer there is some solid solution strengthening from the nitrogen in solid solution.

Principal reasons for nitriding are:

• To obtain high surface hardness
• To increase wear resistance
• To improve fatigue life
• To improve corrosion resistance (except for stainless steels)
• To obtain a surface that is resistant to the softening effect of heat at temperatures up to the nitriding temperature

Nitridable Steels

Nitrided steels are generally medium-carbon (quenched and tempered) steels that contain strong nitride-forming elements such as aluminum, chromium, vanadium, and molybdenum.

The most significant hardening is achieved with a class of alloy steels (nitralloy type) that contain about 1% Al. When these steels are nitrided the aluminum forms AlN particles, which strain the ferrite lattice and create strengthening dislocations. Titanium and chromium are also used to enhance case hardness although case depth decreases as alloy content increases.

Of the alloying elements commonly used in commercial steels, aluminum, chromium, vanadium, tungsten and molybdenum are beneficial in nitriding because they form nitrides that are stable at nitriding temperatures. Molybdenum in addition to its contribution as a nitride former also reduces the risk of embrittlement at nitriding temperatures. Other alloying elements such as nickel, copper, silicon and manganese have little, if any, effect on nitriding characteristics.

Although at suitable temperatures all steels are capable of forming iron nitrides in the presence of nascent nitrogen, the nitriding results are more favorable in those steels that contain one or more of the major nitride-forming alloying elements. Because aluminum is the strongest nitride former of the common alloying elements, aluminum containing steels (0.85 to 1.50% Al) yield the best nitriding results in terms of total alloy content.

The following steels can be gas nitrided for specific applications:

1. Aluminum-containing low-alloy steels
2. Medium-carbon, chromium-containing low-alloy steels of the 4100, 4300, 5100, 6100, 8600, 8700 and 9800 series
3. Hot-work die steels containing 5% chromium such as HI1, HI2, and HI3
4. Low-carbon, chromium-containing low-alloy steels of the 3300, 8600, and 9300 series
5. Air-hardening tool steels such as A-2, A-6, D-2, D-3 and S-7
6. High-speed tool steels such as M-2 and M-4
7. Nitronic stainless steels such as 30, 40, 50, and 60
8. Ferritic and martensitic stainless steels of the 400 and 500 series
9. Austenitic stainless steels of the 200 and 300 series
10. Precipitation-hardening stainless steels such as 13-8 PH, 15-5 PH, 17-4 PH, 17-7 PH, A-286, AM350 and AM355.

Nitriding processes

Process methods for nitriding include:

• gas (box furnace or fluidized bed),
• liquid (salt bath),
• plasma (ion) nitriding.

The advantages and disadvantages of these techniques are similar to those of carburizing. However, times for gas nitriding can be quire long, that is, from 10 to 130 h depending on the application, and the case depths are relatively shallow, usually less than 0.5 mm. Plasma nitriding allows faster nitriding times, and the quickly attained surface saturation of the plasma process results in faster diffusion. Plasma nitriding can also clean the surface by sputtering.

Gas Nitriding

Gas nitriding is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature in contact with a nitrogenous gas, usually ammonia. Quenching is not required for the production of a hard case. The nitriding temperature for all steels is between 495 and 565°C.

Because of the absence of a quenching requirement with attendant volume changes, and the comparatively low temperatures employed in this process, nitriding of steels produces less distortion and deformation than either carburizing or conventional hardening. Some growth occurs as a result of nitriding but volumetric changes are relatively small.

Prior Heat Treatment. All hardenable steels must be hardened and tempered before being nitrided. The tempering temperature must be high enough to guarantee structural stability at the nitriding temperature: the minimum tempering temperature is usually at least 30°C (50°F) higher than the maximum temperature to be used in nitriding.

Single-Stage and Double-Stage Nitriding. Either a single- or a double-stage process may be employed when nitriding with anhydrous ammonia. In the single-stage process, a temperature in the range of about 495 to 525°C is used and the dissociation rate ranges from 15 to 30%. This process produces a brittle nitrogen-rich layer known as the white nitride layer at the surface of the nitrided case.

The double-stage process, known also as the Floe process, has the advantage of reducing the thickness of the white nitrided layer.

The first stage of the double-stage process is, except for time, a duplication of the single-stage process. The second stage may proceed at the nitriding temperature employed for the first stage or the temperature may be increased to from 550 to 565°C; however, at either temperature, the rate of dissociation in the second stage is increased to 65 to 80% (preferably 75 to 80%). Generally, an external ammonia dissociator is necessary for obtaining the required higher second-stage dissociation.

The principal purpose of double-stage nitriding is to reduce the depth of the white layer produced on the surface of the case. Except for a reduction in the amount of ammonia consumed per hour, there is no advantage in using the double-stage process unless the amount of white layer produced in single-stage nitriding cannot be tolerated on the finished part or unless the amount of finishing required after nitriding is substantially reduced.

To summarize, the use of a higher temperature during the second stage:

• Lowers the case hardness
• Increases the case depth
• May lower the core hardness depending on the prior tempering temperature and the total nitriding cycle time
• May lower the apparent effective case depth because of the loss of core hardness depending on how effective case depth is defined.

Operating Procedures. After hardening and tempering and before nitriding, parts should be thoroughly cleaned. Most parts can be successfully nitrided immediately after vapor degreasing.

Bright Nitriding

Bright nitriding is a modified form of gas nitriding employing ammonia and hydrogen gases. Atmosphere gas is continually withdrawn from the nitriding furnace and passed through a temperature-controlled scrubber containing a water solution of sodium hydroxide (NaOH). Trace amounts of hydrogen cyanide (HCN) formed in the nitriding furnaces are removed in the scrubber thus improving the rate of nitriding.

The scrubber also establishes a predetermined moisture content in the nitriding atmosphere reducing the rate of cyanide formation and inhibiting the cracking of ammonia to molecular nitrogen and hydrogen. By this technique control over the nitrogen activity of the furnace atmosphere is enhanced and nitrided parts can be produced with little or no white layer at the surface. If present, the white layer will be composed of only the more ductile Fe₄N (gamma prime) phase.

Pack Nitriding

Pack nitriding is a process analogous to pack carburizing. It employs certain nitrogen-bearing organic compounds as a source of nitrogen. Upon heating, the compounds used in the process form reaction products that are relatively stable at temperatures up to 570°C.

Slow decomposition of the reaction products at the nitriding temperature provides a source of nitrogen. Nitriding times of 2 to 16 h can be employed. Pans are packed in glass ceramic or aluminum containers with the nitriding compound, which is often dispersed in an inert packing media.

Ion (or Plasma) Nitriding

Since the mid-1960s, nitriding equipment utilizing the glow-discharge phenomenon has been commercially available. Initially termed glow-discharge nitriding, the process is now generally known as ion, or plasma, nitriding. The term plasma nitriding is gaining acceptance.

Ion nitriding is an extension of conventional nitriding processes using plasma-discharge physics. In vacuum, high-voltage electrical energy is used to form a plasma, through which nitrogen ions are accelerated to impinge on the workpiece. This ion bombardment heats the workpiece, cleans the surface, and provides active nitrogen.

Metallurgically versatile, the process provides excellent dimensional control and retention of surface finish. Ion nitriding can be conducted at temperatures lower than those conventionally employed. Control of white-layer composition and thickness enhances fatigue properties. The span of ion-nitriding applications includes conventional ammonia- gas nitriding, short-cycle nitriding in salt bath or gas, and the nitriding of stainless steels.

Ion nitriding lends itself to total process automation, ensuring repetitive metallurgical results. The absence of pollution and insignificant gas consumption are important economic and public policy factors. Moreover, selective nitriding accomplished by simple masking techniques may yield significant economies.

Comparison of Ion Nitriding and Ammonia-Gas Nitriding

Ammonia-gas nitriding produces a compound zone that is a mixture of both epsilon and gamma-prime structures. High internal stresses result from differences in volume growth associated with the formation of each phase. The interfaces between the two crystal structures are weak. Thicker compound zones, formed by ammonia-gas nitriding, limit accommodation of the internal stresses resulting from the mixed structure.

Under cyclic loading, cracks in the compound zone can serve as initiation points for the propagation of fatigue cracks. The single-phase gamma-prime compound zone, which is thin and more ductile, exhibits superior fatigue properties. Reducing the thickness of the ion-nitrided compound zone further improves fatigue performance. Maximization occurs at the limiting condition, where compound zone depth equals zero.

Case Hardness. The bulk of the thickness of the nitride case is the diffusion zone where fine iron/alloy nitride precipitates impart increased hardness and strength. Compressive stresses are also developed, as in other nitriding processes. Hardness profiles resulting from ion nitriding are similar to ammonia-gas nitriding but near-surface hardness may be greater with ion nitriding, a result of lower processing temperature.

Advantages and Disadvantages of Ion Nitriding. Ion nitriding achieves repetitive metallurgical results and complete control of the nitrided layers. This control results in superior fatigue performance, wear resistance, and hard layer ductility. Moreover, the process ensures high dimensional stability, eliminates secondary operations, offers low operating-temperature capability and produces parts that retain surface finish.

Among operating benefits are:

• Total absence of pollution
• Efficient use of gas and electrical energy
• Total process automation
• Selective nitriding by simple masking techniques
• Process span that encompasses all sub-critical nitriding
• Reduced nitriding time

The limitations of ion nitriding include high capital cost, need for precision fixturing with electrical connections, long processing times compared to other short-cycle nitrocarburizing processes, and lack of feasibility of liquid quenching for carbon steels.