Historically, the processing temperatures used for ferritic nitrocarburizing in molten salt baths have been in the range of 1000 to 1075 F (540 to 580 C). This is based on kinetics and thermodynamics of the reactions required to produce particular nitride phases, which, in turn, develop the desired engineering properties. Recent work is directed toward nitrocarburizing at temperatures outside the normal range using plasma at low temperatures and salt bath nitrocarburizing (SBN,) at higher temperatures.
Kolene has investigated the response of popular grades of carbon, alloy and stainless steels to SBN within an expanded temperature range of 750 to 1160 F (400 to 625 C). The effect of SBN temperature on metallurgical characteristics and engineering properties was studied, specifically on diffusion rates, phase composition, hardness, and resistance to corrosion and wear.
Carbon steels
AISI 1018 and 1045 carbon steels were investigated only at the high-temperature side of the processing range because no technical or economical benefit of low-temperature nitrocarburizing was apparent.
A comparison of diffusion characteristics at a temperature of 1160 F (625 C) with those at the conventional SBN temperature of 1075 F (580 C) shows a significant increase in diffusion rates at the higher temperature for both steels. A typical SBN cycle for low carbon steel is 90 min at 1075 F, developing a 0.0006-in. (0.015 mm) thick compound layer (Fig. 1). By comparison, it takes less than 30 min to produce the same depth at a temperature of 1160 F.
The microstructures of the compound layers of both steels at 1160 F SBN contain a typical light-etched structure of nitride plus an additional new microstructural phase at the compound layer/substrate interface for 1018 steel (Fig. 2).
This phase is presumed to be the result of transformation of austenite formed at 1160 F when quenched in a 750 F (400 C) salt bath. The thickness of the phase in 1018 steel ranged from 0.0001 to 0.0004 in. (0.0025 to 0.01 mm) for 15 and 60 min cycles, respectively. Hardness of the phase is HK10 430 compared with HK10 869 for the adjacent compound layer. The phase is not apparent in the 1075 F SBN samples (Fig. 3)
Phase composition of the compound layers for 1018 steel processed at 1160 F/30 min and 1075/75 min are similar (Fig. 4). The 1075 F process produces some gamma prime, which is not apparent in the 1160 F process microstructure. The greater depth of the compound layer of the high-temperature sample precluded positive definition of the new phase. Phase composition of the compound phases for 1045 processed at the same two temperatures also was similar.
The traverse hardness profile for the 1045 steel processed at 1160?F/45 min is equivalent to that for 1045 processed at 1075 F/90 min (Fig. 5). The same correlation exists for 1018 steel; i.e., the near-surface hardness patterns were equivalent.
Results of wear resistance and frictional properties for 1018 steel at the 1075 F and 1160 F cycles and for an AISI 4620 carburized and hardened standard are shown in Fig. 6. Running (wear) properties of 1018 steel was determined using the block-on-ring test method in accordance with ASTM G 77 (modified). No significant difference in running properties was evident, with both wear resistance and frictional coefficients notably better than those of the 4620 carburized and hardened standard test block.
Corrosion tests were conducted in accordance with ASTM B-117, with the end point arbitrarily defined as the time when 5% of the surface area exposed to salt spray displays products of corrosion. Exposure time was terminated at 1000 hours. The 1018 steel processed at 1160 F/30 min has approximately twice the corrosion resistance of that for steel processed at 1075 F/90 min (Fig. 7). There was no significant difference in corrosion results for the 1045 steel processed at the different temperatures.
Alloy steel
AISI 4140 was selected as a representative alloy steel. Two factors considered to define the scope of the investigation were that an economic benefit may be possible using shorter cycles at 1160 F (as with carbon steel), and because 4140 is a hardenable steel grade, base material properties are sensitive to tempering temperatures (Fig. 8) including the temperature of the final SBN process. Therefore, it was of interest to determine to what extent strengthening of the subsurface was possible, and consequently, any possible engineering benefits that may result from low temperature (750 to 1000 F, or 400 to 540 C).
The response of 4140 steel to nitrocarburizing at cycle temperatures ranging from 750 F to 1160 F is shown in Fig. 9. The significant influence of temperature on diffusion is apparent and confirms that SBN at 1160 F may offer a means to reduce cycle time. Conversely, at temperatures of 750 and 850 F (400 and 455 C), nitrogen diffusion rates were insufficient to produce compound depths considered minimum within a reasonable time period. SBN at a temperature of 950 F (510 C) demonstrated reasonable rates of diffusion and was defined as the low temperature limit for further study of 4140.
There were some differences in the microstructure and phase composition between the 1075 and 1160 F specimens. The compound layers from both cycles consist primarily of epsilon nitride with evidence of Fe3O4 and no gamma prime. At the compound layer/core interface for the 1160?F specimen, fine lines of precipitate 0.0001 to 0.0002 in. (0.0025 to 0.005 mm) long appear to emanate and extend toward the surface, presumably developing as a result of an interrupted quench (750 F) from the 1160 F SBN temperature. This condition was not apparent in the specimen processed at 1075 F.
One of the significant end results of SBN is the enhancement of contact fatigue strength. The improvement in response to higher contact loads relates to the strength level of the near-surface region of the component, a depth wherein maximum stresses occur under contact loading. Strengthening of this region occurs from the solid-solution effect of nitrogen in the diffusion zone plus the strength level of the base material.
Comparing traverse hardness profiles of specimens processed at 950 and 1075 F SBN temperature cycles shows the benefit of processing at the lower temperature (Fig. 10). A significant increase in hardness (strength) is apparent at the anticipated depth of maximum stress (arbitrarily noted as 0.004 in., or 0.1 mm). This primarily is a result of the higher base material strength made possible because of the lower SBN temperature of 950 F. An extended cycle time of six hours is necessary to achieve the necessary depth of diffusion.
A comparison of traverse hardness patterns for high-temperature SBN (1075 and 1160 F) indicates that a reduced dwell time of 45 min at 1160 F produces a hardness profile comparable to that resulting from a 90 min cycle at 1075 F (Fig. 11). The influence of temperature on diffusion appears to be similar for both carbon and alloy steels.
In ASTM B-117 salt spray tests, corrosion resistance of 4140 steel processed at 1160 F/45 min is two times that for 4140 processed at 1075 F/90 min (Fig. 7).
Stainless steel
SBN of stainless steels primarily is specified to provide antigalling and wear resistance to the surface. This generally is accomplished using a conventional SBN temperature of 1075 F. This significantly degrades the corrosion resistance of the austenitic grades, such as AISI 304 and 316, which were studied in this work.
The response of 304 and 316 to SBN in a temperature range of 750-1160 F was determined by measuring the surface layer (compound layer), defined as the total uninterrupted depth of indiscernible microstructures including compound(s) observed microscopically. Diffusion characteristics of both austenitic grades are comparable (Fig. 12). The rate of diffusion at 750 F SBN is insignificant, particularly between cycle times of 4 and 8 hours, with the resulting layer depths not exceeding 0.0002 in. (0.005 mm). As the SBN temperature is increased, there is a corresponding increase in diffusion, and compound layers about 0.0025 in. (0.06 mm) deep can be produced in 2 hours at temperatures of 1075 and 1160 F.
The microstructures of compound layers vary with SBN temperature. At 750 F, the microstructures for both steels consist totally of a white phase with evidence of subsurface microcracks at mid-depth running parallel to the surface. This structure has been noted by others during investigations of plasma nitrocarburizing at low temperatures[2, 3] and has been identified as a metastable phase called expanded austenite, or "S" phase. Subsurface cracking also was apparent after the 750 F plasma process and was attributed to the presence of very high compressive stresses developing in the S phase.
At the 850 F SBN temperature, the microstructure of 304 showed the onset of a dark phase identified as nitride, which becomes more prevalent with increasing SBN temperature. However, the 316 microstructure (SBN at 850 F) consists totally of the S phase (Fig. 13) having a hardness of HK10 1395. At SBN temperatures between 950 and 1160 F, the microstructures of both steels consist totally of the dark nitride phase.
Results of ASTM B-117 salt spray tests show that temperature is the most significant processing parameter influencing the stainless qualities of SBN austenitic stainless steels. At SBN temperatures between 750 and 850 F, microstructures consist primarily of S phase, thereby providing excellent corrosion resistance. As the microstructure changes to nitrides with increasing SBN temperatures, the corrosion resistance decreases. The transition is most evident in 304. For example, at the 750 F SBN temperature, a compound layer of 100% S phase provides excellent corrosion resistance, which is reduced to about 600 hours protection with conversion of S phase to nitrides at the 850 F SBN temperature.
By comparison, the microstructure of 316 at the 850 F SBN temperature consists entirely of S phase with no nitrides, and there is no loss in corrosion resistance. The persistence of S phase at 850 F is attributed to the 2% molybdenum in the composition, which stabilizes the metastable low-temperature S phase and raises the decomposition temperature.
All SBN cycles at temperatures above 850 F; that is, at 950 to 1160 F, produced extensive degradation of corrosion resistance for the austenitic grades in less than 100 hours of exposure. Thus, there is no discernable benefit of deeper compound layer to maintain stainless qualities.
Conclusions
Carbon steels (1018 and 1045) and alloy steel (4140) can be salt bath nitrocarburized at elevated temperatures (1160 F) and thereby develop specified compound layers depths in significantly less process time and without notable adverse affect on the resulting engineering properties.
Salt bath nitrocarburizing at a temperature below 1000 F is an effective way to increase hardness within the near-surface zones of hardened and tempered 4140 by allowing the use of lower tempering temperatures. This should produce an increase in contact fatigue strength.
Austenitic stainless steels (304 and 316) can be salt bath nitrocarburized at predetermined temperatures within a range of 750 to 850 F to produce a monophase surface layer having an equivalent hardness greater than HRC 70 without degradation of corrosion resistance.