The Lehrer diagram often serves as a guide for selecting gas mixtures for nitriding alloy steels, but it is only accurate for ammonia gas nitriding processes when hydrogen is used as the diluting gas. This paper presents the results of a study showing that the use of pure nitrogen as a diluent has a marked effect on the phase boundary lines of the standard Lehrer diagram, essentially shifting them to the left. The paper also includes examples showing where the use of nitrogen is advantageous and where it is not.

For decades, industrial gas providers have tried to develop a compelling reason for metal thermal processing shops to switch from the dominant dissociated ammonia atmosphere technology to hydrogen/nitrogen blended gas “synthetic” atmospheres. Two problems interfered with this business approach – ammonia for dissociation was generally very cheap, and the alternatives that the industrial gas companies proposed did not solve all of the issues faced by the thermal processors. The offerings of the industrial gas providers failed to displace dissociated ammonia in most installations because the cost of the atmosphere was high as compared with dissociated ammonia, and the solution proposed by the industrial gas providers simply replaced one highly hazardous gas delivery and storage problem – ammonia – with another – hydrogen. Users and their local Authorities Having Jurisdiction did not find the tradeoff attractive to swap ammonia storage for hydrogen storage. Onsite hydrogen generation technology makes it possible to replace delivered, stored hazardous ammonia with “zero-inventory” onsite generated hydrogen and stored or generated nitrogen. This approach eliminates the hazardous material objection to ammonia replacement for thermal processors and makes it much more interesting to consider replacement of dissociated ammonia with hydrogen/nitrogen. While economic issues remain, a look at total costs of operation makes hydrogen/nitrogen generation a viable and growing solution for thermal processors. This paper reviews results for several customers that transitioned successfully from dissociated ammonia to hydrogen/nitrogen. The discussion addresses costs, regulatory compliance, and process results.

A software simulation tool, CarbonitrideTool, has been developed by Center for Heat Treatment Excellence (CHTE) to predict the Nitrogen and Carbon concentration profiles in selected steels. In this paper, the introduction of the software will be presented. In addition, enhancements have been made to improve the CarbonitrideTool. The diffusion of nitrogen increases the amount of retained Austenite (RA) by changing the Ms temperature. In this paper, the modification has been made to calculate the RA fraction. The empirical prediction of microhardness profile will also be presented. The results of verification experiments will be presented and discussed.

Nitrogen (N 2 ) atmospheres with different, not always optimized levels of reducing and carburizing gases are often used to prevent decarburizing and oxidation of steel parts during annealing in continuous furnaces. The type and concentration of these additives in N 2 should correlate to the extent of air leakage into furnace, entrainment of air with loaded parts, steel composition, and complex reaction kinetics in the gradients of oxygen (O 2 ) and temperature existing between the entrance and hot zones of the furnace. This study explores the effect of small, 0.1 vol.% - 0.4 vol.% propane (C 3 H 8 ) additions on composition of air-contaminated N 2 atmosphere in the temperature range of 500°C - 860°C. Microstructures are presented for AISI 1045 steel exposed to the atmospheres produced. Atmosphere compositions compared include those produced by a new type of plasma activated, in-situ reformer for N 2 -diluted C 3 H 8 . The latter method extends the atmosphere protection to the lower range of annealing temperatures. Present results may assist heat treaters in optimizing their neutral hardening operations.

Plasma nitriding is a thermochemical surface heat treatment of steel components to produce nitride layers which increase wear-, corrosion- and fatigue resistance. Research into plasma nitriding lately showed that there is a significant and characteristic amount of ammonia formed off the process gases nitrogen and hydrogen. This research paper is aimed to analyze the influence of plasma treatment parameters, such as pressure, voltage, temperature and nitrogen to hydrogen ratio on the atmosphere and the formation of ammonia during plasma nitriding. The ammonia content is measured in the exhaust gas. By correlating the measured ammonia with the treatment parameters and modeling the nitriding process, the ammonia content can then be predicted. Further a plasma nitriding potential, comparable to the gas nitriding potential, based on ammonia content is calculated and its practicability as process control parameter is shown by correlating the potential with the nitriding results, e.g. the formation of ε and γ’ nitride phases.

Thermochemical surface engineering by nitriding/carburizing of stainless steel causes a surface zone of expanded austenite, which improves the wear resistance of the stainless steel while preserving the stainless behavior. As a consequence of the thermochemical surface engineering, huge residual stresses are introduced in the developing case, arising from the volume expansion that accompanies the dissolution of high interstitial contents in expanded austenite. Modelling of the composition and stress profiles developing during low temperature surface engineering from the processing parameters temperature, time and gas composition is a prerequisite for targeted process optimization. A realistic model to simulate the developing case has to take the following influences on composition and stress into account: - a concentration dependent diffusion coefficient - trapping of nitrogen by chromium atoms - the effect of residual stress on diffusive flux - the effect of residual stress on solubility of interstitials - plastic accommodation of residual stress. The effect of all these contributions on composition and stress profiles will be addressed.

A study was conducted on a set of H13 steels to enhance their performance as matrices and pins. The steels were austenitized in a high-pressure vacuum furnace at 1015 °C for 180 minutes, followed by nitrogen quenching in a high vacuum (2 bar). Two tempering treatments were applied: one at 540 °C and another at 580 °C, each for 180 minutes, with subsequent nitrogen cooling to room temperature. The nitrocarburizing process was carried out in a liquid bath salt furnace at 580 °C for varying durations of 45, 60, 90, 120, 150, and 180 minutes to assess the impact of treatment time on the quality of the nitrocarburizing layer. Post quenching and tempering, the steels exhibited hardness values ranging from 550 to 570 HV. After nitrocarburizing, the surface hardness increased to between 740 and 810 HV, with a nitrocarburizing layer thickness of less than 14 μm. The microstructural evolution of the compound layer was analyzed using scanning electron microscopy and X-ray diffraction. The characterization revealed a continuous nitrocarburizing ε-Fe 2–3 (C,N) layer. Specimens treated for 45 to 60 minutes demonstrated superior wear performance compared to those treated for 90 to 180 minutes.

Titanium use in aerospace and medical applications continues to grow. Alpha case formation is a diffusion reaction that occurs at the surface of titanium when processing at elevated temperature in atmospheres containing oxygen, nitrogen, and/or carbon, with oxygen being the prominent element associated with alpha case. Oxygen is solution strengthening at low concentrations, but greatly decreases ductility and forms alpha case at higher concentrations. Thus, alpha case is brittle and has a detrimental effect on part performance and longevity. Higher temperatures increase alpha case depth. Temperatures less than 550°C (1022°F) limit oxygen mobility and keep case depth from increasing. Above 480°C (896°F), air or water vapor will start to produce alpha case.

Historically, this carburizing has been performed in an endothermic gas consisting of CO 2 , CH 4 , CO, etc, but carburizing in low pressure with the proper gas mixture changes the landscape. Using C 2 H 2 , the process is no longer endothermic as C 2 H 2 is a catalytically decomposable hydrocarbon and dissociates in the presence of an iron catalyst. LPC is a recipe driven in contrast to the constant monitoring of the carbon potential in atmospheric gas carburizing, and with the wide acceptance of simulation programs, recipes are no longer created by trial and error. Introduction of nitrogen to the steel, followed by carbon with higher temperatures, can dramatically reduce cycle times and still control grain growth.

Atmospheric pressure carburizing and neutral carbon potential annealing in nitrogen containing small additions of hydrocarbon gases can offer cost and steel surface quality alternatives to the comparable, endothermic atmosphere or vacuum operations. An experimental program was conducted for refining real-time process control methods in carburizing of AISI 8620 steel under N 2 -C 3 H 8 blends containing from 1 to 4 vol% of propane at 900°C and 930°C. Multiple types of gas analyzers were used to monitor residual concentrations of H 2 , CO, CO 2 , H 2 O, O 2 , CH 4 , C 3 H 8 , and other hydrocarbons inside furnace. A modified shim stock technique and the conventional oxygen probe (mV) were additionally evaluated for correlation with gas analysis and diffusional modeling using measured carbon mass flux values (g/cm 2 /s). Results of this evaluation work are presented.