One Minute Mentor: Nitriding Potential

February 17, 2017
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The nitriding potential controls the amount of nitrogen in the metal. The graph shows the atomic concentration of nitrogen as a function of nitriding potential (K_n) and depth.

One Minute Mentor: Reactor for Gas Nitriding

February 3, 2017
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This diagram shows the major parts of a gas nitriding reactor. 1, muffle furnace; 2, outer shell; 3, heater; 4, internal container (retort); 5, gas inlet pipe; 6, exhaust pipe; 7, motor; 8, fan; 9, metal-made jig; 10, gas guide cylinder.

11, inverted funnel; 12, vacuum pump; 13, effluent gas combustion facility; 14, carbon donor compound gas cylinder; 15, ammonia gas cylinder; 16, nitrogen gas cylinder; 17, hydrogen gas cylinder; 18, flow-rate meter; 19, gas control valve.

Volume 4D, Heat Treating of Irons and Steels â‡’ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments

One Minute Mentor: Plasma-Immersion Ion Implantation

January 23, 2017
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The fundamental difference between plasma nitriding (glow discharge) and plasma-immersion ion implantation (PIII) is that the latter relies on high-energy pulses (up to 45 kV) that promote a low-energy ion implantation in the target,  but that is still less than the energy used in conventional ion implantation (>100 kV). A combination of target temperature and pulse voltage achieve the desired implantation profile.

The PIII process can be seen as a modification of the plasma nitriding process that provides control of individual operational parameters and thus improves the plasma treatment. Most systems use an RF plasma at frequencies up to 2.45 GHz to generate a nitrogen microwave plasma, independent of temperature and bias voltage at the target, in the same beneficial low-pressure atmosphere as the regular RF plasma treatment process.

Volume 4D, Heat Treating of Irons and Steels â‡’ Heat-Treated High Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments

One Minute Mentor: Radio-Frequency Plasma

January 16, 2017
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Glow-discharge plasma nitriding is carried out in nitrogen-hydrogen atmospheres at pressures in the range of 100 to 1000. At pressures below 10 Pa, the glow-discharge technique is not used, but plasma can still be generated by radio-frequency (RF) excitation, microwave energy, or electrons produced by a heated filament. These low-pressure plasmas diffuse through the treatment chamber with a large number of active species, which increases the nitriding efficiency. The uncoupling of plasma generation from the part being treated allows independent control of the energy and flux of ions against the substrate surface, and the temperature can be kept low.

In the RF plasma process, plasma is generated by high frequency, in the range of 13.56 MHz, is applied to an inductive antenna inside the vacuum chamber. The part is heated by heating the chamber and the working table with standard electrical resistance. One of the limiting factors for the commercial exploitation of the process is the high vacuum required, and the subsequent cost. The diagram shows a schematic of RF plasma equipment.

Volume 4D, Heat Treating of Irons and Steels â‡’ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments 

One Minute Mentor: Active Screen Plasma Nitriding

January 6, 2017
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Recently, a process called active screen plasma (ASP) has been developed for the purpose of minimizing or avoiding some of the limitations of conventional plasma nitriding. One such limitation is the edge effect, in which the regions near the edges of the pieces may not be nitrided to the same standard as the rest of the part.

In the conventional glow-discharge process, the plasma is produced by the voltage between the wall of the chamber (positive potential) and the workpieces (negative potential). By contrast, in the ASP process, the plasma is produced by the voltage between the chamber (positive) and a screen (negative) placed around the parts. The nitriding species produced on the screen are then transferred and deposited on the part, which is then heated to the treatment temperature by radiation from the active screen, which itself is under ion bombardment.

The schematic shows active-screen plasma nitriding equipmernt. (a) Floating potential (b) Anodic potential.

Volume 4D, Heat Treating of Irons and Steels â‡’ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments 

One Minute Mentor: Distortion in Plasma-Carburized Stainless Steel

December 19, 2016
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The graph shows the XRD patterns of low-temperature plasma-nitrided (PN), plasma-carburized (PC), and untreated AISI 316 stainless steel samples. It illustrates the displacement of the peaks corresponding to the γ-phase to the left, with increasing nitrogen or carbon. The shift is smaller for the PC sample than for the PN sample, indicating less lattice expansion and less distortion in the carbon S-phase.

Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Stainless Steels 

One Minute Mentor: The S-Phase

December 9, 2016
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The success of plasma and gas nitriding at low temperatures (lower than 840°F) and solution nitriding at high temperatures is associated with the absence of chromium nitride or carbide precipitation, and thus with the formation in the surface layer of a particular phase called the S-phase. This nitrogen- and/or carbon-rich phase presents higher concentration of the interstitials when treated at lower treatment temperatures and when high-chromium alloys are used, resulting in better properties. 

The figure shows the XRD patterns of untreated and plasma-nitrided AISI 316 stainless steel at 790°F. It illustrates the displacement of the peaks corresponding to the γ-phase to the left, with increasing nitrogen, as well as the broadening of these peaks, compared to the γ-phase peaks of the untreated steel.

Volume 4D, Heat Treating of Irons and Steels â‡’ Heat Treated High-Alloy Steels ⇒ Nitriding of Stainless Steels ⇒ Stainless Steels