Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating as defined by the Iron-Carbon Phase Diagram.
|Fig. 6. Typical gas-nitrided load|
|Fig. 1. Simplified iron-iron carbide phase diagram|
Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating (Ac1) as defined by the Iron-Carbon Phase Diagram (Fig. 1). The material typically is placed in contact with ammonia, which allows the transfer of nitrogen to the surface during its thermal decomposition to nitrogen and hydrogen. Other special nitriding processes are also used for certain types of stainless steels involving the decomposition of nitrogen gas at high temperatures, but these will not be the focus of this discussion.
Several unique features of nitriding are:
- Nitriding is a (relatively) low-temperature process compared to other case-hardening processes (Fig. 2).
- Quenching is not required for a hard case.
- Part distortion is typically less than other case-hardening processes.
- Nitriding is relatively easy to control in terms of process parameters.
- In the gas nitriding process, a crystal structure that is ferritic rather than austenitic is highly desired.
|Fig. 2. Temperature for typical case-hardening processes|
Purpose of Nitriding
Nitriding is a diffusion-related surface treatment (Fig. 3) with the objective to increase surface hardness (among other properties) by the creation of a case on the surface of the part (Fig. 4).
One of the appeals of this process is that rapid quenching is not required. Therefore, dimensional changes are kept to a minimum. It is not suitable for all applications. For example, one of its limitations is that the extremely high surface-hardness case is more brittle than that produced by the carburizing process.
A typical manufacturing sequence for gas nitriding (Fig. 5) consists of several heat-treatment steps, including pre-treatments and (optionally) stress relief between machining steps.
Nitriding creates a component that has the following properties:
- High surface hardness (typically > 67 HRC)
- Resistance to wear
- Anti-galling properties (for applications in poor lubrication conditions)
- A minimum of distortion and deformation (less than, for example, carburizing/hardening)
- Resistance to tempering (that is, resistant to softening)
- Stability of the nitrided case
- Improved fatigue life and other fatigue-related properties
- Reduction in notch sensitivity
- Resistance to corrosion (except for 300- series stainless steels)
- Small volumetric changes (some growth does occur)
Properties that are considerably improved by nitriding are fatigue strength (resistance to dynamic loading), friction and resistance to wear, and cor-rosion resistance.
|Fig. 3. Diffusion-related processes|
Types of Nitriding
Three methods of nitriding are commonly used in the industry today: gas nitriding (Fig. 6), plasma nitriding (Fig. 7) and salt-bath nitriding (Fig. 8). Each method is unique and has both advantages and limitations. Only gas niriding will be discussed here.
|Fig. 4. Typical nitrided case showing a surface compound (white) layer and subsurface diffusion zone|
Prerequisites for Nitriding
To ensure the best nitriding results, the following precautions and recommendations should be followed. First, the steel should be hardened, quenched and tempered prior to nitriding so as to possess a uniform structure. Tempering temperature has an influence on the hardness of the case as well as the depth of nitriding (Fig. 9). The tempering temperature must be sufficiently high to guarantee structural stability at the nitriding temperature. The minimum tempering temperature should be 50°F (10°C) higher than the maximum temperature to be used for nitriding.
|Fig. 5. Typical nitriding manufacturing sequence|
In addition, the following is recommended:
- Before nitriding, the steel must be free from decarburization. Precleaning is mandatory; residue on the parts will result in spotty cases.
- If freedom from distortion is of paramount importance, the internal stresses produced by machining or heat treating should be removed before nitriding by performing a stress-relief operation, that is, heating to and holding at a temperature of 1000-1300°F (538-705°C) followed by slow cooling.
- Since some growth takes place on nitriding, this should either be allowed for in the final machining or grinding operation prior to nitriding or removed by lapping or careful grinding. If required, the removal of a slight amount of the nitride case should be anticipated in the nitriding case-depth calculation.
- If maximum resistance to corrosion is desired, the parts should be used as processed (with white layer intact).
- Nitrided steels of the Nitralloy type should not be used where resistance to the corrosion of mineral acids is encountered or where resistance to sharp abrasive particles at high velocities is required (as in sand nozzles).
- If straightening is required after nitriding, it should be done hot, if possible, in the temperature range of 1200°F (650°C). Cold-straightening techniques should be carefully reviewed as microcracking is a concern.
- If maximum hardness and maximum resistance to impact are desired, and the question of maximum corrosion resistance is not of vital importance, the removal of 0.001-0.002 inch (0.025-0.050 mm) of the nitrided case is desirable. The amount to be removed depends on the original case depth. This operation will remove the most brittle surface layer.
- If nitrided parts exhibit a shiny-gray surface after their removal from the furnace, the results should be viewed with suspicion. Invariably, the case will be shallow and below hardness. The parts should have a matte-gray appearance, although a slight discoloration does not indicate faulty nitriding. The opening of the furnace at too high a temperature or the presence of air leakage on cooling will account for the slight discoloration.
|Fig. 7. Typical plasma-nitrided load|
Prior Heat Treatment
In certain alloys, such as the 4100 and 4300 series, hardness of the nitrided case is modified appreciably by core hardness (Fig. 10). Observe that a decrease in core hardness results in a decrease in case hardness. In order to obtain maximum case hardness, these steels are usually provided with maximum core hardness by tempering at the minimum allowable tempering temperature.
All hardenable steels must be hardened and tempered before being nitrided. The minimum tempering temperature is usually at least 50°F higher than the maximum temperature to be used in nitriding. Typical tempers range from 1150-1350°F (620-730°C).
|Fig. 8. Typical salt-bath-nitrided load|
Nitriding is to be considered a white (clean) glove treatment, that is, all residuals including oils and grit must be cleaned off the surface of the parts prior to nitriding. Even skin oils from handling parts without clean gloves can be problematic. If parts are not absolutely clean, spotty case depths will result.
One acceptable way to clean parts is by vapor degreasing and abrasive (aluminum-oxide grit) cleaning just prior to nitriding.
Another method involves a light phosphate coating. The steps involved are:
- Cold-water rinse
- Oxalic-acid bath dip
- Cold-water rinse
- Warm-water rinse
- Phosphate solution dip
If a decarburized surface is not removed before nitriding, the case will spall very readily.
|Fig. 9. Typical influence of tempering temperature on hardness and depth of nitrided case (nitrided for 60 hours at 950°F/510°C)|
The following results can be expected from the gas nitriding process:
Parts gas nitrided in ammonia should have a dull, matte-gray color (Fig. 11).
Structure of the Nitrided Case
In the nitriding process, the nitrogen that diffuses into the steel reacts with the nitride-forming elements present in solid solution. The hardening results from the reaction. The depth of case depends on how far beneath the steel surface nitrogen is able to diffuse during the nitriding period. The principle involved is that as the alloy elements are removed from solid solution, nitrogen (which is constantly being supplied from the surface) diffuses farther into the alloy and, thus, produces an increasingly deep case. The case depth for any given treatment time and temperature depends upon the amounts of alloy elements with which nitrogen must react before it can diffuse farther.
|Fig. 10. Case depth as a function of hardness gradient for 4140 steel|
Nitrogen ConcentrationThe nitrided medium needs to contain only sufficient active nitrogen to maintain the white layer (Fig. 12). Any increase beyond this point serves to increase the depth of white layer and does not affect the thickness of the inner (diffusion) layer.
The white layer has excellent corrosion resistance. In certain applications, it does not need to be eliminated.
During nitriding, parts increase slightly in size because of the increase in volume that occurs in the case. This change causes a stretching of the core, which results in tensile stresses in the core that are balanced by compressive stresses in the case after the parts are cooled to room temperature (e.g., notch-sensitivity reduction – a localized surface effect).
|Fig. 11. Surface appearance of nitrided valve springs|
Tensile StressesTensile stress originates at imperfections cancelled by compressive stresses.
Growth and Distortion
Dimensional change in nitrided parts are governed largely by composition, tempering temperatures, time/temperature of nitriding, relative thickness of case/core, shape of the part and areas marked off to prevent nitriding. The amount of growth is usually constant for identical parts nitrided in different batches by a fixed processing cycle. After the amount of growth for a particular part has been determined experimentally, allowance for it can be made during final machining (prior to nitriding).
Sharp corners or edges should be avoided on parts to be nitrided, because the projections formed at sharp corners are high in nitrogen content and susceptible to chipping. Sharp edges nitride through the section and are without support from a soft ductile core.
Parts nitrided by the two-stage process and not ground after nitriding have excellent dimensional stability. IH
|Fig. 12. Nitrided case|
For more information: Dan Herring is president of THE HERRING GROUP Inc., P.O. Box 884 Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: email@example.com; web: www.heat-treat-doctor.com. Dan’s Heat Treat Doctor columns appear monthly in Industrial Heating, and he is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.