Introduction to Gas Nitriding Titanium

For all of the known benefits of using titanium alloys, including high strength-to-weight ratios and excellent corrosion resistance, titanium unfortunately is known to exhibit poor tribological properties. That is, it has a high coefficient of friction (COF) when in moving contact with many mating materials, resulting in poor sliding and adhesive wear resistance that leads to failure by galling. Owing to this, metal-to-metal applications encountering friction and wear considerations require a surface treatment for adequate serviceability. This is increasingly a topic of investigation as various segments of industry are searching to reduce moving mass in machinery components (1, 2).

One treatment for enhancing the tribological properties of titanium that has been researched for years is nitriding, which is performed by plasma or gas processes (3). Nitrogen has great affinity and solid solubility in alpha titanium, which greatly increases hardness/strength (1, 3). Gas nitriding, which is the topic of this study, is classified as a diffusion process, whereby nitrogen gas dissociates and nascent nitrogen is adsorbed and diffused into the titanium matrix. This nitriding process is not a “coating”; it is a thermochemical heat treatment where nitrogen becomes an integral part of the titanium matrix. So unlike a coating, it does not have to initially bond well to function well; the case will not peel/flake away from the base metal.

The titanium/nitrogen phase diagram shows that higher levels of nitrogen will form compounds of titanium and nitrogen that can be thought of as a hard ceramic. Provided the compound layer is not too thick and depending on its chemical makeup and service application, it can provide greatly enhanced wear performance. Below the surface compound layer is a diffusion zone that is solid solution strengthened by nitrogen. This adds support to the surface for sustaining higher application loads, and like other diffusion processes, the depth of the diffusion zone is dependent on the time and pressure of the treatment.

Titanium has an even stronger affinity for oxygen than nitrogen, making the purity of the nitriding atmosphere highly influential to the end results (1, 3). Investigation into the formation of oxygen enriched surfaces (at high levels forming alpha case) in vacuum heat treating titanium using a graphite insulated vacuum furnace revealed that even at good commercial vacuum levels of 5 x 10^-4 Torr, titanium became surface enriched with oxygen during heating to 1450°F (4) . The study of the “gettering effect” of titanium indicates that oxygen may be a constituent of the case structure when gas nitriding titanium in commercial vacuum furnaces.  Depending on the amount present, oxygen in the nitride case is not necessarily a detriment. Oxygen interstitially strengthens titanium like nitrogen, and at a certain level may contribute to enhanced wear performance in a given application. Ti-6Al-4V has been purposely thermally oxidized to increase wear properties and reportedly performed comparatively well in a wear test study involving numerous surface engineering treatments (2).

51���� is developing processes for gas nitriding titanium in a vacuum furnace using partial pressure nitrogen gas at elevated temperatures.  Of interest are the effects of partial pressure on the case hardness, case depth, and the surface chemistry/crystalline phases of titanium alloy Ti-6Al-4V.

Experimental Details for Titanium Gas Nitriding

Ti-6Al-4V sheet was cut into approximately 1.5 inch squares. The surface of the coupons was used as received following a rigorous alcohol wipe; no surface blasting or polishing was performed prior to processing.

An all metal (molybdenum) vacuum furnace with a cylindrical, vertical hot zone, 10″ diameter x 18″ high, was used for this study. Three titanium coupons used for each run were hung on separate molybdenum wires attached to the lid of the furnace. All cycles began after an initial pump down to 2 x 10^-5 Torr to assure vacuum integrity. The parts were heated in vacuum to 854°C (1570°F) and partial pressure nitrogen was introduced at 70 microns, 1 Torr (1000 microns), or 630 Torr. Nitriding time was 10 hours for each pressure.

Hardness profiles for each run were measured using a Struers Durascan and 25 gram load.  Specimens were etched using 2% HF for metallographic analysis and total optical case depth measurement. The RJ Lee Group in Monroeville, PA performed surface analyses using x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The intent of this study was to determine the influence of pressure selection as a means to control the formation of TiN, Ti₂N and Ti oxide.

Gas Nitriding Titanium: Results and Discussion

The hardness profiles for the three pressures evaluated are presented in Figure 1. This shows that the hardness of the diffusion zone drops off sharply for the 630 Torr (A) and 1Torr (B) nitrided specimens. The 70-micron (C) results are comparable to the near surface hardness (0.0005 inches [12.7 µm]) of the 630 Torr (A) results, but show a deeper, more gently sloping diffusion zone hardness. Note that the diamond indenter of the Durascan is too large for the very thin compound layers (10 µm range) to be accurately measured. (A nano-indenter hardness tester is needed and will be used in further study along with scratch hardness testing.)

After etching to reveal the microstructure, optical measurements were performed for each pressure and the results are presented in Table 1. Tests A and B have comparable case depths while Test C has a case depth close to double the other two. The photomicrographs revealing the total case depths are shown in Figure 2. Higher magnification micrographs of Figure 3 show the thickness of the compound layer. The compound layer was comparable for all three pressures, yet the total case depth was deepest for the lowest pressure, corroborating the microhardness profile results.

Based on prior investigations of the heat treatment of titanium alloys (4), it was surmised that the deeper case at the lowest pressure may be attributable to a  higher presence of residual water vapor in the hot zone owing to a “stale atmosphere” (relatively poor quality vacuum). This would result in oxygen enrichment within the case when water vapor adsorbs and dissociates on the titanium surfaces. To confirm this hypothesis, surface analyses using x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) were performed (5). The main crystalline phases detected with the XRD scans at each pressure are listed in Table 2. Oxygen is present on the surface as the titanium suboxide, Ti₂O.

Normalized x-ray counts, Table 3, can be used determine the relative ratios of the compounds TiN, Ti₂N and Ti₂O. This reveals that the compound layer of Sample C, processed at the lowest pressure, has the highest oxide concentration, 33%, of the three samples. The oxide concentration of Sample A is 14% and 11% for Sample B.

Table 3

When processing at a low pressure of 70-microns, very little nitrogen flow is pumped through the furnace. Water vapor is inherently adsorbed in the hot zone insulation and thus the low nitrogen flow becomes concentrated with water vapor upon heating. Further, 70-micron flow results in low furnace volume turnover. This ultimately results in higher levels of oxygen in the case. This result supports the hypothesis.

Samples A (14%) and B (11%) oxygen concentrations are comparable owing to increased nitrogen pressures and gas flow rates resulting in greater dilution of water vapor by nitrogen. In order to maintain a pressure of 630 Torr (A), however, an almost completely closed vacuum at times leads to a decrease in the volume turnover at the highest pressure. This results in somewhat higher oxygen concentration at 630 Torr compared to 1 Torr.

Gas Nitriding Titanium: Conclusions

This preliminary study revealed that the partial pressure of nitrogen when gas nitriding Ti-6Al-4V in a vacuum furnace can have a significant effect on the nitrided case characteristics. Processing with very low partial pressure of 70-microns resulted in a considerably higher amount of surface oxides being present in the compound layer compared to processing at higher pressures of 1 Torr and 630 Torr. The pressure and gas flow/turnover rate of nitrogen influences the “gettering ability” of titanium for oxygen during the nitriding process. The lowest pressure process resulted in a higher hardness profile and total case depth. This is considered attributable to oxygen being an interstitial strengthening element in titanium, just as it is nitrogen’s purpose in diffusion zone strengthening of the case.

At this point in the investigation it is unknown to what extent oxygen may enhance or detract from the wear performance of the nitrided case. Wear requirements for a product tend to be very application specific, but some standardized wear tests are planned for further study of vacuum nitrided Ti-6Al-4V. Reference (3) mentions nitriding of race car steering racks (rack and pinion gearing) as well as engine valves and spring retainers. 51���� has nitrided titanium alloy engine valves and spring retainers, in addition to gears designed for the next Mars expedition land rover. This latter application is germane to the gear industry and the hope is that there is an increasing focus on reducing moving mass in machinery components, including gears.

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References:

1. Bloyce, A., Morton P.H., Bell T.; Surface Engineering of Titanium and Titanium Alloys, ASM Handbook Volume 5, Materials Park, OH, 1994; 835-851.
2. Bansal, D.G., Eryilmaz, O.L., Blau, P.J.; Surface Engineering to Improve the Durability of Ti-6Al-4V Alloy, Wear 271 (2011); 2006-2015.
3. Rolinski, Ed; Nitriding of Titanium Alloys, ASM Handbook 4E, Materials Park, OH, 2016; 604-621.
4. Jordan, D. and Osterman V.; Minimizing Alpha Case during Vacuum Furnace Heat treating, HTPro, Advanced Materials and Processes, March 2016; 46-48.
5. RJ Le Group, XRD Analysis of Nitrided Titanium Report presented to Don Jordan, July 15, 2016.

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For all of the known benefits of titanium alloys in all sorts of applications, from medical to aerospace to automotive, titanium is also known to exhibit poor tribological properties.  That is, it has a high coefficient of friction (COF) when in moving contact with essentially all structural metals, resulting in poor sliding and adhesive wear resistance that leads to failure by galling (cold welding).  Because of this, metal-to-metal applications encountering friction and wear considerations require a surface treatment for adequate serviceability.  One such treatment is solution nitriding, which is performed in a vacuum furnace using partial pressure nitrogen gas at elevated temperatures in the annealing range. Solution nitriding is classified as a diffusion process where nitrogen gas dissociates and nascent nitrogen is adsorbed and diffused into the titanium matrix.  Like other diffusion processes, the depth of the diffusion zone is dependent on the time of the treatment.  For alloy Ti-6Al-4V with a core hardness of 30 HRC, 51���� has generated hardnesses as high as the mid-60’s to upper-60’s HRC (converted from HV 25gf) at a depth of 0.0076mm (0.0003”), followed by a gradual decrease in hardness to the core over a distance of 0.25mm (0.01”).  Shorter cycle times have produced hardnesses in the mid-50’s to high-50’s HRC and shallower total case depths.

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Originally Published: October 2013

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Recent advancements in instrumentation and other process controls have allowed engineers, chemists, and metallurgists to continually strive to develop new applications for the modern vacuum furnace.  Heat treating cycles continue to be developed for materials and processes not previously thought to be applicable to vacuum equipment.

One of the more recent successes was to Oxynitride steel product in a continuous process in one furnace and one cycle.  Although this trial was completed in a laboratory type R & D furnace on a rather small scale,  it is anticipated that equipment such as the modern Vacuum Gas-Nitriding Furnace described in a recent ASM Publication[[1]] would be easily  applicable and provide production load capabilities.

Oxynitriding can best be described as a process where the material has first been Nitrided and is then purposely oxidized to form an additional performance enhancing layer.  We realize that existing efforts for providing an Oxynitrided-type product are usually processed in salt bath or gas retort type equipment with varying success.[[2]] Our effort was to try to advance and improve the process by using a vacuum furnace to provide an environmentally  clean,  “Green” method with faster and more consistent results.

The resulting complex oxidized surface layer improves part corrosion resistance, while still maintaining the excellent wear resistance imparted by the Nitriding.  The end product has an attractive dark, matte gray finish.  We believe varying the oxidizing time will result in several different shades of gray that can approach black if desired. Certainly, the application of supplemental corrosion inhibiting polymer treatments could potentially allow for a variety of colors to be produced, including an aesthetically pleasing dark, lustrous black.

The development of this Oxynitriding process in a vacuum furnace and its benefit regarding improved corrosion resistance consisted of the following:

• Three samples of H-13 Tool Steel were prepared for processing.
• One piece of the material was set aside to represent the Non-Treated virgin state.
• One piece was vacuum gas Nitrided and set aside to represent the Nitrided only condition.
• The remaining piece was gas Nitrided in the vacuum furnace and then oxidized in the same cycle by the introduction of a partial pressure of wet inert gas. The wet inert gas provided the oxidizing agent (dissociated H₂O at the Nitriding temperature) to achieve the desired end result.
• This piece was set aside to represent the Oxynitrided condition of the testing.
• The three samples were then subjected to an ASTM B117 salt spray test for 156 hours by an independent commercial testing laboratory. The objective was to compare the corrosion resistance of the Oxynitrided part to the Nitrided and Non-Treated virgin part throughout the salt spray testing time. The results were photographically recorded at specific time intervals of testing to determine how each part and treatment compared.
• The laboratory reported the % of red rust on each sample surface after each time interval (see Table 1)

Oxynitride in a Vacuum Heat Treat Furnace Results:

The comparison of rust progression on the Non-Treated virgin, Nitrided, and Oxynitrided parts is detailed in Table 1.  Figures 1-3 are visual views of corrosion development for time periods of 0, 84, and 156 hours, respectively.  Additionally,   Figure 4 is a photomicrograph of the Oxynitrided part after the 156 hours salt spray test, clearly showing that no observable corrosion attack has occurred on the surface.

Table 1

H–13 Sample Results Versus Salt Spray Time Exposure

% Rust Observed

Oxynitride in a Vacuum Heat Treat Furnace Conclusions:

• A Nitrided only part will improve corrosion resistance of H–13 steel by more than 50 % in the short term and more than 25 % over a long period of use.
• An Oxynitrided H-13 steel part processed in a vacuum furnace will greatly improve corrosion resistance by more than 90 % over a long term of application.
• It is anticipated that further improvements in corrosion resistance can be achieved on an Oxynitrided part with the additional application of polymeric anticorrosion coatings or corrosion inhibiting oils. Future testing will continue to confirm these expected improvements.
• Examination of the Oxynitrided part microstructure revealed no indication of corrosion attack.

Although there are certainly many advantages of the Oxynitriding process using the vacuum furnace, the prime result is to be able to consistently achieve the desired Nitrided case depth and white layer control and to be able to add extended corrosion resistance to the product in one continuous cycle.  There is no need for other equipment or a second step in the process.  The cycle is a relatively low temperature process (typically 950 – 1000° F) and thus greatly minimizes the possibility of part distortion.  The vacuum furnace application of the Oxynitriding process allows for precise, shorter, and repeatable cycles resulting in high quality parts exhibiting unique surface attributes all produced in a non-contaminating Green environment.

In summary, specific advantages of the Oxynitriding Process in a vacuum heat treat furnace include:

• Vastly improved corrosion resistance.
• Significantly improved wear resistance, particularly sliding-contact wear.
• Improved fatigue strength.
• Aesthetically pleasing dark gray/black finish.
• Significantly higher surface hardness for long durable service life.
• Confidence that the process is reliably repeatable owing to the precise controls of the vacuum furnace.
• Can be considered as a replacement for expensive plating requirements.

[1]. Don Jordan; Harry Antes, Vacuum Gas-Nitriding Furnace Produces Precision Nitrided Parts, Published in ASM Heat Treating Progress, September 2009

[2]. Totten, George, Steel Heat Treatment; Metallurgy and Technologies, CRC Press, Boca Raton, 2007, 496.

Author: Don Jordan; Vice President / Corporate Metallurgist

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51���� has established a method of controlling the amount and depth of white layer resulting from Gas Nitriding.  This procedure was accomplished following extensive testing using AISI 4140 Steel in a 51���� Gas Nitriding Vacuum furnace. Various applications requiring Nitriding often require specific white layer limits which can now be provided by this process.

Following an initial rapid pump down to produce an Oxygen free, vacuum environment, the Nitriding cycle consisted of a pre-heat at a partial pressure of Nitrogen followed by Nitriding at a slightly positive pressure using an Ammonia/Nitrogen mixture.  Many cycles were performed varying the time and gas flow parameters at temperature and the resulting white layer composition and thickness determined.  The key to controlling the white layer formation was the introduction of a Boost-Diffusion technique during the Nitriding phase.  Surface hardness and depth of nitride zone were then recorded from microhardness measurements and metallography.  All this data was compiled to establish Nitriding procedures that provide the final desired structure in the minimum cycle time.  This includes processes that produce the minimum depth or complete absence of white layer as dictated by the final application of the parts.

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Author: Donald Jordan, 51����

Originally Published: Heat Treating Progress, September 2009

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Originally Published: Heat Treating Progress, March 2008

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Authors: Robert Lamont, Jr., Atch-Mont Gear, Inc. and A. Bruce Craven, 51����

Originally Published: Gear Technology, November 1996

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Author: A. Bruce Craven, 51����

Originally Published: Industrial Heating, May 1989

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