One Minute Mentor: Nitriding Austenitic Stainless Steel
June 9, 2017
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The graph shows thermogravimetric uptake curves for nitriding of AISI 316 at 445°C (830°F) in different NH3-H2 gas mixtures. Nitriding potentials are indicated in the figure. The nitrogen uptake as a function of time for the various nitriding potentials is depicted.
Clearly, the applied nitriding potential plays a major role on the growth rate of nitrogen uptake in the solid state, because nitrogen uptake increases with nitriding potential (nitrogen activity).
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 thermogravimetric uptake curves for nitriding of AISI 316 at 445°C (830°F) in different NH3-H2 gas mixtures.
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One Minute Mentor: Solubility of Nitrogen in Expanded Austenite
May 12, 2017
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The graph shows the solubility of nitrogen in expanded austenite (γN) as a function of the imposed nitrogen activity (aN) in a gas mixture of NH3 and H2 for the stainless steels and temperatures indicated in the legend.
Fully nitrided and denitrided conditions are indicated by aN = ∞ and aN = 0, respectively. The master curve can be applied for estimating the surface content of nitrogen for a gas mixture of known nitrogen activity aN = K(T) · K, with K(T) the temperature-dependent equilibrium constant of ammonia dissociation.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Physical Metallurgy of Expanded Austenite
The graph shows the solubility of nitrogen in expanded austenite (?N) as a function of the imposed nitrogen activity (aN) in a gas mixture of NH3 and H2 for the stainless steels and temperatures indicated in the legend.
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One Minute Mentor: Physical Metallurgy of Expanded Austenite
April 27, 2017
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The microstructural feature responsible for the favorable technological properties resulting from low-temperature gaseous thermochemical treatment is a supersaturation of nitrogen and/or carbon in austenite. This is called expanded austenite.
An investigation of homogenous nitrogen-expanded austenite with adjusted contents of nitrogen (as controlled by gaseous nitriding) showed that nitrogen-expanded austenite has a face-centered cubic (fcc) Bravais lattice. The study also showed that systematic hkl-dependent peak shifts with respect to the perfect fcc peak positions can be explained from the presence of stacking faults .
This is the ultimate proof that the designation "S-phase" is inappropriate, because no new phase develops, but rather a diffusion zone where the austenite lattice is expanded as a consequence of an interstitial supersaturated solid solution.
The graph shows the lattice parameter of (expanded) austenite as a function of the interstitial content of nitrogen and carbon. The data were obtained with x-ray diffraction on nitrided or carburized thin foils.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel
The microstructural feature responsible for the favorable technological properties resulting from low-temperature gaseous thermochemical treatment is a supersaturation of nitrogen and/or carbon in austenite. This is called expanded austenite.
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One Minute Mentor: Low-Temperature Surface Hardening of Stainless Steel
April 18, 2017
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The graph shows a schematic time-temperature-transformation (TTT) diagram of austenite with a high nitrogen or carbon content. In order to prevent the development of chromium nitrides/carbides, a long-time low-temperature treatment could be applied. Alternatively, a high-temperature treatment (above 1050 °C, or 1920 °F) combined with fast cooling can be chosen.
The corrosion-resistant nature of stainless steels is due to the presence of a very stable passive layer, mainly formed by the alloying element chromium and oxygen from the environment. Unfortunately, many stainless steels are susceptible to galling, which often leads to expensive materials solutions in applications where stainless steel would otherwise have been the preferred material choice for its combination of strength and anticorrosion properties.
Surface engineering is common practice for improvement of wear performance. The improvement of hardness and wear properties relies on the development of a surface-adjacent case consisting of either a compound layer and an underlying diffusion layer, as for nitriding/nitrocarburizing; or a martensitic transformation, as for carburizing.
Stainless steel poses two challenges in this respect. First, the native passive layer is impenetrable by nitrogen and carbon atoms. Second, nitrogen and carbon react with chromium to form nitrides/carbides, thus withdrawing chromium from solid solution, with the result that corrosion protection is lost.
A remedy for dissolving nitrogen or carbon into stainless steel without the formation of chromium nitrides or carbides would be choosing a high temperature at which nitrides/carbides are no longer stable; or a low temperature at which the incubation time for nitrides/carbides is longer than the treatment time to achieve a sufficiently thick case.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel
The graph shows a schematic time-temperature-transformation (TTT) diagram of austenite with a high nitrogen or carbon content. In order to prevent the development of chromium nitrides/carbides, a long-time low-temperature treatment could be applied.
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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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One Minute Mentor: Plasma Nitriding
January 3, 2017
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Plasma nitriding is a surface-hardening process in which glow-discharge technology introduces nascent (atomic) nitrogen into the surface of a metal part for subsequent inward diffusion. The process takes place in a vacuum chamber, with the workpiece as the cathode (negative potential) in a nitrogen/hydrogen atmosphere at pressures between 100 and 1000 Pa (0.015 and 0.15 psi).
A voltage in the range of 500 to 1000 volts is applied between the metal part and the chamber wall, resulting in the formation of a plasma through which positive nitrogen ions are accelerated against the negative-biased part to be treated. This ionic bombardment heats the piece to the treatment temperature, cleans the surface by sputtering, and provides active nitrogen species for the nitriding process.
The diagram shows the schematic of a device used for glow-discharge plasma nitriding. 1 and 2, gas introduction; 3, thermocouple; 4, cathode; 5, pressure sensor, 6 to 8, needle valve; 9, diaphragm valve; 10, solenoid; 11, vacuum pump; 12, sample holder; 13, sample; 14, anode; 15, sealing ring; 16, stainless tube; 17, window and 18, insulator.
Volume 4D, Heat Treating of Irons and Steels ⇒ Nitriding of Stainless Steels ⇒ Thermochemical Nitriding Treatments
Plasma nitriding is a surface-hardening process in which glow-discharge technology introduces nascent (atomic) nitrogen into the surface of a metal part for subsequent inward diffusion
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One Minute Mentor: Carburizing Tool Steels
April 22, 2016
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P20 mold steel is seldom carburized for molds used in compression molding, particularly for molding the more abrasive plastics. Carburizing temperatures no higher than 1650°F are recommended for this steel because higher temperatures may impair polishability; otherwise, conventional carburizing practice is used, and molds may be quenched in oil directly from the carburizing temperature. A common working range is 54 to 58 HRC.
The graph shows tempering characteristics of carburized mold steels.
(a) Upper curve represents steel carburized in hardwood charcoal 1675 to 1700°F for 8 hours, air cooled in pack, reheated at 1725 to 1750°F, cooled in air, and tempered. Middle curve represents steel carburized in cast iron chips at 1725 to 1750°F, removed from pack, cooled in air, and tempered.
(b) Surface hardness after heating at temperature for 2 hours in carburizing compound, followed by oil quenching and tempering.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Mold Steels and Corrosion-Resistant Tool Steels ⇒ Recommended Heat Treating Practices
P20 mold steel is seldom carburized for molds used in compression molding, particularly for molding the more abrasive plastics.
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One Minute Mentor: How Austenitizing Temperature Affects High-Temperature Hardness
February 18, 2016
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The austenitizing temperature affects not only the final hardness but also the grain size. Owing to the high amount of alloying elements dissolved in the matrix, prior austenite boundaries are preferentially etched instead of martensite structure, enabling the quantification of austenite grain size. A continuous increase in austenite grain size with the hardening temperature is observed.
The graph shows the reduction in high-temperature hardness of M2 tool steel that results from austenitizing at relatively low temperatures.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of High-Speed Tool Steels ⇒ Recommended Heat-Treating Procedures
The austenitizing temperature affects not only the final hardness but also the grain size.
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One Minute Mentor: Heat Treatment of a Premium Grade H13 Die
February 4, 2016
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The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F). It shows the effect of intermediate cooling rates on the microstructure and toughness of H13 tool steel austenitized at 1075°C (1970°F). (a) Continuous cooling transformation (CCT diagram) shows variation in microstructure with varying cooling rates. (b) Variation in toughness as a function of carbides ejected from austenite. Hardness was maintained at a constant value.
Hot-work die steels containing 5% Cr are normally used (H13 is the most common in the United States) because they have:
• A good combination of toughness and strength for safety and service
• Good hardenability in thick sections
• Good hot hardness and strength to resist cyclic stressing at operating temperatures up to 400 °C (750 °F)
• Good resistance to tempering during service that gradually lowers the surface hardness and the fatigue strength
Carbide precipitation, other than that produced during tempering, is known to be deleterious to the fatigue life of H13 tool steel. Fast quenching rates required to suppress intermediate precipitation of carbides (such as proeutectoid grain-boundary carbide, pearlite, or bainite) also produce large thermal gradients within the tool. Fast quenching rates can cause distortion or even cracking. The larger the tool, the greater the risk of cracking.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Hot-Work Tool Steels ⇒ Examples of Heat-Treating Procedure for Hot-Work Tools
The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F).
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