Improving the Nitriding Process Using ZeroFlow® Gas Nitriding Technology
November 14, 2017
Source: ASM International
Gas nitriding has been performed for almost a century with many variations over the years. The benefits of nitriding are well documented and nitriding has gained amazing industrial acceptance. However, the equipment and control of the nitrided layer has continued to be challenging, and as such, nitriding is still slowly being adopted by industry, but the rate of change is improving.
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One Minute Mentor: Effects of Nitrocarburizing on Stainless Steel
September 29, 2017
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Electrochemical measurements show that transpassivity (which is necessary for the onset of pitting) of expanded austenite occurs at appreciably higher applied voltages than for austenite with the same metallic composition but without nitrogen. A potential disadvantage of the presence of pitting-resistant expanded austenite at the surface of a stainless steel that is not itself pitting-resistant, is that on the introduction of damage to the expanded austenite zone, accelerated corrosion of the underlying stainless steel may occur.
The graph shows polarization curves for AISI 316 in as-delivered and in the low-temperature nitrocarburized state. Measurement was carried out in an aqueous 3.5% NaCl solution at room temperature. For the nitrocarburized sample repassivation occurs in the transpassive region, an indication that the material is pitting resistant.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Low-Temperature Nitriding and Nitrocarburizing of Other Stainless Steels
Electrochemical measurements show that transpassivity (which is necessary for the onset of pitting) of expanded austenite occurs at appreciably higher applied voltages than for austenite with the same metallic composition but without nitrogen.
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One Minute Mentor: Hardness Depth Profiles of Stainless Steel after Nitriding and Nitrocarburizing
September 18, 2017
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X-ray diffraction analysis indicates that the hardened case consists of a mixture of expanded austenite and expanded martensite. Dissolving nitrogen into cubic martensite causes a tetragonal expansion leading to nitrogen-expanded martensite. In addition, E-carbonitride is present close to the surface. By comparing the hardness profiles in the graph, it can be concluded that nitrocarburizing generally leads to a thicker case than nitriding, and that a slightly higher maximum hardness is obtained after nitrocarburizing.
Martensitic stainless steels have been successfully nitrided and nitrocarburized. An example is given for molybdenum-stabilized AISI 420-type austenitic stainless steel 7C27Mo2.
The graph shows hardness-depth profiles after nitriding (solid lines) and nitrocarburizing (dash-dot lines) for 16 hours of 100 μm strip 7C27Mo2 (molybdenum-modified AISI 420) at the temperatures indicated. Hardness profiles were fitted assuming a sigmoidal function.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Low-Temperature Nitriding and Nitrocarburizing of Other Stainless Steels
X-ray diffraction analysis indicates that the hardened case consists of a mixture of expanded austenite and expanded martensite.
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One Minute Mentor: Nitrocarburizing in a Carrier Gas
August 31, 2017
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The graph shows (a) Micrograph of low-temperature nitrocarburized austenitic stainless steel AISI 316 in an atmosphere of (partly) decomposed urea (in situ activation). The sample was heated to 490°C (910 °F) in 45 minutes and thereafter immediately cooled.
(b) Glow discharge optical emission spectroscopy (GDOES) concentration-depth profiles of low-temperature nitrocarburized stainless steel AISI 316 in an atmosphere of (partly) decomposed urea; see microstructure in (a).
Nitrocarburizing in a carrier gas to which (initially solid) urea was added led to a fast growth of the expanded austenite zone. The gaseous decomposition products of urea that develop in the furnace are highly activating, which means that activation and nitrocarburizing are achieved simultaneously. The hardness measured directly at the surface (not in a cross section) is higher than 1300 HV. It is obvious that the expanded austenite zone consists of a nitrogen-rich and a carbon-rich part.
The near-surface zone consists of nitrogen-expanded austenite, and the carbon-expanded austenite is located underneath. The sum of the two interstitial profiles yields a gradual transition from the surface into the substrate in terms of total interstitial content.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Gaseous Low-Temperature Surface Hardening of Austenitic Stainless Steel
Nitrocarburizing in a carrier gas to which (initially solid) urea was added led to a fast growth of the expanded austenite zone. The gaseous decomposition products of urea that develop in the furnace are highly activating, which means that activation and nitrocarburizing are achieved simultaneously.
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One Minute Mentor: Residual Stress Profiles as Determined by X-Ray Diffraction
August 18, 2017
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For nitrocarburizing or consecutive carburizing and nitriding, the typical stress profile is smoother than for nitriding only. The graph shows residual stress profiles determined with destructive X-ray diffraction methods in carburized/nitrided AISI 316. Carburizing/nitriding: 2 hours at 970°F in an atmosphere consisting of 30% CO-70% hydrogen, followed by nitriding (23 hours at 820°F, at KN = 1.14.
For comparison a profile for a carburized (2.5 hours at 970°F, in 90% CO/10% H2) sample and for a nitrided (22 hours at 830°F, in 100% ammonia) sample are given. Residual stress values were calculated assuming that the same elastic constants apply for expanded austenite and austenite. The presented values are therefore only an indication that very high compressive stresses are present.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Gaseous Low-Temperature Surface Hardening of Austenitic Stainless Steel
For nitrocarburizing or consecutive carburizing and nitriding, the typical stress profile is smoother than for nitriding only.
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One Minute Mentor: Residual Stress Profiles
August 3, 2017
Source: ASM International
Residual stress profiles of nitrided austenitic AISI 316 are shown in the graph.
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One Minute Mentor: Nitrogen-Expanded Austenite
July 7, 2017
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The graph shows hardness-depth profiles of nitrided AISI 316. A profile for carburizing is shown for comparison.
Lines obtained by fitting a sigmoidal function are given to guide the eye. The hardness values measured directly at the surface (with the same load) were 1585 HV for KN = 2.49; and 1417 HV for KN= 0.293.
The effect of the nitriding potential is also reflected in the hardness-depth profiles. A smooth decrease near the surface is followed by a sharp drop at a position close to the transition from γN to γ. Nitrogen-expanded austenite is very hard, and the transition from the hardened case to the (soft) core occurs within a few microns.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Gaseous Low-Temperature Surface Hardening of Austenitic Stainless Steel
The graph shows hardness-depth profiles of nitrided AISI 316. A profile for carburizing is shown for comparison.
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One Minute Mentor: Control of Nitriding Potential at Low Temperatures
March 31, 2017
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The graph shows the case thickness for fluidized-bed nitrided 316L samples after 8 hours of treatment. The treatment temperature must be kept, ideally, below 430°C (805°F). The dissociation of NH3, and thus the nitriding potential, is controlled by the addition of H2, N2, and CH4 (which dissociates into carbon and hydrogen and also promotes nitrocarburizing) in the treatment atmosphere.
The addition of H2 and CH4 decreases the NH3 dissociation, and the addition of N2 increases the NH3 dissociation according to the following relationship:
%N = k[p(NH3)/p(H2)3/2]
where k is the equilibrium constant, and p is the partial pressure of NH3 and H2.
The final result is that, after 8 hours of nitriding treatment, the fluidized bed is able to produce a case approximately 8 µm at its thickest, as shown in the graph. In contrast, plasma nitriding of 316L samples for 8 hours would result in a layer thickness on the order of 18 to 20 µm.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
The graph shows the case thickness for fluidized-bed nitrided 316L samples after 8 hours of treatment.
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One Minute Mentor: Nitriding in Fluidized-Bed Reactors at Low Temperatures
March 16, 2017
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In nitriding treatment furnaces, the heating is done electrically, and the fluidizing gas composition is chosen to provide the optimum chemistry for the desired surface treatment (usually a mixture of NH3 + N2, with CH4 added when nitrocarburizing is desired). The granular solid is considered inert and does not react with the part being treated but is fundamental in establishing the fluidized bed and its outstanding properties.
For stainless steel nitriding, treatment temperature must be kept below 840°F to prevent the precipitation of chromium nitride. The lower treatment temperature increases the efficiency of the Cr2O3 barrier against nitriding; even after the acid treatment, there is still some oxide layer left at the steel surface.
The result is lower diffusion rates and lower nitriding potential (less dissociation of NH3, which is the mean supplier of atomic nitrogen needed for the nitriding treatment), as shown in the graph. The NH3 dissociation effect is controlled by changing the composition of the atmosphere, but the oxide layer would dissociate or become permeable only at much higher temperatures, which would cause the precipitation of nitrides and carbides in the case.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
In nitriding treatment furnaces, the heating is done electrically, and the fluidizing gas composition is chosen to provide the optimum chemistry for the desired surface treatment (usually a mixture of NH3 + N2, with CH4 added when nitrocarburizing is desired).
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One Minute Mentor: High-Temperature Nitriding for Deeper Case
March 2, 2017
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High-temperature nitriding or solution nitriding at temperatures of 2010 ± 90°F is a new treatment available to produce cases with nitrogen concentrations in the range of 0.4 to 0.9 wt%, which is low compared to plasma nitriding treatments at low temperature (up to 16.9wt% N), but the resulting case is up to 100 times deeper (2 mm, or 0.08 in.). The lower nitrogen concentration implies a lower surface hardness (360 HV for solution nitriding of austenitic steels).
The graph shows a time-temperature transformation (TTT) curve for the precipitation of Cr2N, from which it can be estimated that, to avoid CrN precipitation, the temperature must drop from 2010°F to below 1650°F in approximately 20 seconds. That is feasible only in relatively thin parts (large area/volume ratio) that cool sufficiently to cope with the TTT curve time limitations to avoid precipitates, and only for a small number of parts at a time.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
High-temperature nitriding or solution nitriding at temperatures of 2010 ± 90°F is a new treatment available to produce cases with nitrogen concentrations in the range of 0.4 to 0.9 wt%, which is low compared to plasma nitriding treatments at low temperature (up to 16.9wt% N), but the resulting case is up to 100 times deeper (2 mm, or 0.08 in.).
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