The surface modification of stainless steels by nitriding has been known for many years. The critical step in the process sequence is that of cleaning and preparation of the stainless steel prior to the nitriding process to create a suitable surface condition for both nucleation and diffusion of nitrogen to take place.

Pulsed plasma nitriding system for forging dies, compliments of SECO/WARWICK


The method of preparation – precleaning and surface preparation – for the ion-nitriding process takes place in the same chamber as the process itself, but it is a separate operation. It is accomplished by control of the internal retort pressure, gas composition and current density. It is essential to commence the nitriding process as soon as possible after the stainless steel surface is prepared.

Introduction

The method of surface hardening of stainless steels by using the glow-discharge method in combination with pulsed DC-power technology has become a mature technology since the early 1980s.

The heat treater is able to use the process with greater flexibility than a more conventional process such as gas nitriding and salt-bath nitriding. Stainless steel has been difficult to nitride successfully using the conventional nitriding techniques that make use of materials such as anhydrous ammonia and cyanide/cyanate salts. The techniques operate in a small band of chemical compositions combined with temperature to produce the surface metallurgy of the compound zone.

By using partial-pressure techniques of vacuum and pulsed DC power, very precise amounts of nascent hydrogen, nitrogen and carbon can be presented to the metal surface to develop an appropriate metallurgy that will best suit the steel’s performance.

The objective of the investigation was to observe the case formation in relation to:
  • Surface preparation
  • Nitride nucleation
  • Compound zone
  • Case depth
  • Case hardness
The well-known methods of gas nitriding and salt-bath nitriding of stainless steels use anhydrous ammonia (2NH3) for gas nitriding or a cyanide-based salt for liquid nitriding that will produce a compound zone also known as the “white layer.” The two-stage nitriding process developed by Dr. Carl Floe of MIT can reduce but not eliminate the compound zone or white layer. Care needs to be taken with the two-stage process to not form nitride networking, which can lead to premature failure of the part.

Nucleation

When a steel is nitrided using the plasma or ion technique, the case includes a diffusion zone with or without the formation of a compound zone. Formation diffusion begins with early nitride nucleation at the surface as the steel approaches the selected nitriding temperature. As a result of the nucleation, the compound zone begins to form. Many factors converge to determine the type of compound zone that will be formed during the process. They are as follows:
  • Alloy content
  • Nitrogen concentration
  • Time
  • Temperature
The formation of the gamma prime (g¢) nucleation takes place easily due to the higher concentration of nitrogen as a result of plasma than is seen with gas or salt-bath nitriding. Because of the excitation of the nitrogen-gas species, it is already dissociated and will be absorbed more readily than with gas nitriding.

Gamma Prime Compound Zone
This is the outer layer, which when polished and etched with nital etchant shows as a white band on the top of the nitrided surface. This is known as the compound zone or the white layer. This is the zone in which there are g¢ (gamma prime), Fe4N, and e (epsilon), Fe2-3N, intermetallics formed. Because conventional steels and tool steels usually have a higher carbon concentration than stainless steels, there is a tendency for the steel to form epsilon. The use of a higher concentration of hydrogen will tend to catalyze the Fe2N. The compound zone or white layer can be a mono phase of either g¢ or e or dual phase – a combination of both g¢ and e. The dual-phase layer is very hard and brittle and is susceptible to impact and cyclical fracture.

Diffusion Zone
The diffusion zone is located immediately below the compound zone, or if no compound zone has been formed, it extends inward from the surface to the core of the material. The hardness of the diffusion zone is determined by the nitride-forming elements such as chromium, molybdenum, aluminum, titanium, manganese and tungsten. The depth of the diffusion zone is a function of time, temperature, alloying elements and the method of presenting atomic nitrogen to the surface of the steel.

In a ferrous material, the nitrogen will exist within the lattice structure as individual separate atoms up to the limit of solubility of nitrogen in iron.

The e-phase nucleation commences more readily than the g¢. The e phase of a compound zone will usually grow on top of the g¢ phase with g¢ between the nitride diffusion zone and the e phase.

Sputter Cleaning
The preparation of the stainless steel surface is accomplished by the process known as “sputter cleaning.”

The purpose of the process is to break down and remove the oxide layer, which forms the corrosion-protection barrier on stainless steels. Sputter cleaning is accomplished by the ionization of a gas or gas mixtures such as hydrogen and argon. The choice of gas for sputter cleaning is based upon the degree of surface oxidation or cleanliness of the steel surface. Hydrogen, being the lightest gas and the easiest to ionize, discharges its ion very readily. Argon, on the other hand, is a dense gas that can very easily cause surface etching if used indiscriminately.

The result of a poorly prepared sputter-cleaned stainless steel surface will manifest itself in the form of an irregularly formed case. The irregularity will be clearly visible by microscopic examination and, most probably, by surface hardness tests.

The time period for sputter cleaning seems to be arbitrary. Studies have shown that if the time at sputter-cleaning temperature is extended beyond two hours with a normal hydrogen/argon mix, surface etching can occur. It should be noted that even at nitriding temperature the hydrogen is maintaining an adequately prepared surface. The nitrogen concentration will be highest at (or near) the surface. As the concentration of nitrogen increases, precipitates will be seen as the limit of solubility is approached and it becomes a saturated solution of nitrogen. The hardness is believed to be increased by the lattice distortion caused by nitride precipitation.


    Studies of Specific Stainless Grades

    Three different stainless grades – AISI 422, 440A and 630 – were chosen to investigate the results of ion nitriding. Pre-heat-treatment parameters for each grade can be found in Table 1. The nitride specifications are found in Table 2. Process parameters to meet these specifications are given in Table 3, and the actual results can be found in Table 4.

    AISI 422
    Stainless steel AISI 422 was ion nitrided considering its design for service temperature up to 1200°F and its ability to perform in a corrosive environment. Four cycles were run in accordance with the parameters given in Table 1 and 3 to meet the requirements of Table 2.

    The microhardness survey on all four cycles of pulsed ion nitriding exhibited similar results. Gas nitriding had exhibited a surface hardness of 852 VPN, which was the minimum requirement of the specification. Using the pulsed ion-nitriding process, the surface hardness increased significantly to 960 VPN (Table 4), which is a significant improvement.

    The surface finish deteriorated 230% from 15 Ra to 35 Ra with gas nitriding. This necessitated two further grinding operations after gas nitriding. The deterioration in the surface finish after pulsed ion nitriding was 13.3%. This was within the accepted surface-finish requirement and did not require any further machining.

    The part was sensitive to distortion, and prior measurements were taken before treatment. The growth/distortion was well within the set tolerance levels, voltage, pressure, amperage and gas flow.

    AISI 440A
    AISI 440A stainless steel was treated using the pulsed ion-nitriding process to establish what the minimum nitriding temperature would be. Martensitic stainless steels begin to decrease their high, as quenched hardness, at temperatures as low as 300°F (148°C).

    A process temperature of 350°F (195°C) was too low a temperature for nitrogen diffusion to occur. A further examination was made of the tempering curve, and it was concluded that a process temperature of 700°F (371°C) could be used. Treatment above 700°F (371°C) causes a rapid deterioration of the corrosion resistance of the 440A stainless steel.

    The results of the 700°F (371°C) cycle shown in Table 3 can be found in Table 4. It was concluded that by using a higher process temperature (greater than 350°F) and extending the cycle time at temperature, nitrogen diffusion will take place and form stable nitrides. It was also concluded that by selecting a process temperature higher than 700°F, serious deterioration of the surface-corrosion characteristics would occur.

    AISI 630
    AISI 630 stainless steel was processed using the pulsed ion-nitriding process. It is used extensively in the aerospace industry for gear manufacture and other critical-performance items as a precipitation-hardened steel. It exhibits good tensile strength and impact values by solution and precipitation treatment.

    Test results are given in Table 4. The microhardness survey exhibited a surface hardness of 960 VPN, which gradually diminishes to the core of the material. The transition hardness from case to core was taken at 500 VPN. The results were as expected, showing no compound zone. The low nitriding temperature of 900°F (482°C) was chosen to improve the core hardness value, which it did with a slight increase in the core hardness of almost one Rockwell C.

    The core metallurgy tended to show an increase in hardness of the martensite formation resulting from the additional “precipitation” treatment of the pulsed ion nitriding. A much denser structure of ferrite stringers emerged in the martensite matrix.

    Conclusion

    Nitriding of stainless steels is not new, and the use of pulsed ion nitriding is gaining wider acceptance by metallurgical engineers. A wider range of stainless steels can be nitrided than in the past.

    Ion nitriding allows engineers to process stainless steels with a repeatable consistency and accuracy and facilitates the manipulation of the surface metallurgy by control of the gas chemistry. There is a loss of corrosion stability, which is dependent upon the steel analysis, sputter cleaning, gas chemistry, current density and gas pressures.

    The surface preparation is the most important step in ion nitriding of stainless steel. Very simple precleaning steps are taken to remove excessively heavy hydrocarbon contaminants. However, these precleaning steps will not remove embedded steel particles from the work surface. If these particles are not removed prior to nitriding, the restriction of nitrogen diffusion will occur. Thus, the sputter-cleaning procedure is an important step in processing.

    The quality of surface finish is governed by the quality of sputtering that takes place. Therefore, care should be exercised when selecting both the sputter-clean parameters and the voltage so as not to chemically damage the surface by aggressive cleaning or arcing. Growth and distortion are two factors of concern from the pulsed ion-nitriding process.

    Growth is dependent on gas chemistry, cycle time, temperature, steel composition and case depth. As atomic nitrogen begins to diffuse into the steel surface at temperature, stable nitrides will form in the stainless steels. The growth will be dependent on the composition of the nitride formers within the steel. The increase in volume of the surface of the steel, therefore, will be dependent on the case depth formed, which is a function of time, temperature, gas composition and steel composition.

    Distortion is dependent on part geometry, prior mechanical working, pre-heat treatment and masking arrangements. Distortion can be reduced by minimizing the risk of thermal shock. With ion nitriding, temperature ramps and holds can be built into the cycle in combination with sputter cleaning. It cannot be said of the process that it will be distortion free, only that the risk of distortion can be greatly reduced.

    The separation of the heating requirement from the plasma power has meant that one is able to uniformly preheat the workpiece under controlled conditions without the use of high power-input voltages for plasma generation. Plasma voltage is started at a suitable temperature – usually 480-500°F (250-260°C).

    The process of plasma ion nitriding is accepted throughout Europe and the Far East, and because of its ability to control the resultant metallurgy on a repeatable basis, it is becoming a preferred method. The control technology of pulsed-plasma ion nitriding has reached a high level of maturity to the point where all of the process variables can be both controlled and monitored without high levels of skill.

    There is an increasing industrial awareness of the risk of pollution either by legislation or social awareness. Pulsed ion nitriding offers a method of heat treating by using non-pollutant or non-hazardous material due to the extremely low emission levels.

    Pulsed ion nitriding is a proven technology and can be the vehicle that carries the industrial community into the next generation of surface treating by thermo-chemical means. IH

    The author acknowledges the assistance of Robin Maloney and Bruce Kramer of SECO/WARWICK Corporation, Meadville, Pa., and Lynn Pye of Pye Metallurgical Consulting.

    For more information: David Pye is president of Pye Metallurgical Consulting, P.O. Box 1349, Meadville, PA 16335; tel: 814-337-0194; fax: 814-337-5939; e-mail: davidpye@pyemet.com; web: www.pyemet.com

    Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: nucleation, diffusion, gas nitriding, salt-bath nitriding, diffusion zone, sputter cleaning, lattice distortion, precipitation harden, pulsed ion nitriding