Question:
In a recent test of our heat-treating knowledge, we were asked several questions concerning nitrogen gas. The answers surprised us. Can you shed some light on them? Here are the questions.

1. Since nitrogen is an inert gas, it can be used as a protective atmosphere for neutral-hardening tool steels. (The answer was false.) If it is true can you explain how that is?

2. Some gases are inert to some metals but not to others. (The answer was true.) Again, can I get an explanation for this?

Answer:
It will be my pleasure to try and shed some light on these questions. Many people view nitrogen as an inert (non-reactive) gas, and for many heat-treat applications this is true. However, unlike truly inert gases such as argon or helium (the Group-8 elements in the Periodic Table), under the wrong circumstances nitrogen can be reactive. I believe (but do not know for certain) that these two questions were designed to test the heat treater's knowledge of this fact.

Question 1 requires awareness of the fact that above 1850°F (1010°C), a percentage of molecular nitrogen (N2) will break down into atomic nitrogen (N) that can be absorbed into the surface of the steel being processed. This phenomenon has been studied by various gas companies during the heat treatment of stainless steels, for example. Since many tool steels are heat treated above 1850°F (1010°C) and since they contain a higher percentage of alloying elements, for example chromium (a known nitride former) and molybdenum, atomic nitrogen will react with the surface of certain tool steels to create a potentially brittle nitrided layer.

Question 2 requires an awareness that some materials, titanium and tantalum being examples that come quickly to mind, will absorb nitrogen if present in the atmosphere during processing (or in a partial- pressure gas such as present in vacuum processing), again forming nitrides that can be detrimental to mechanical properties. Hence, nitrogen is not recommended/permitted. Other metals, such as low-carbon steel when processed under normal heat-treat conditions in a nitrogen atmosphere, do not absorb nitrogen from the atmosphere.

Here are some other facts you should know about nitrogen:

Nitrogen makes up 78.03% of air (by volume), has a gaseous specific gravity of 0.967 and a boiling point of -195°C (-320.5°F) at atmospheric pressure. It is colorless, odorless and tasteless. Commercially, nitrogen is produced by a variety of air-separation processes, including cryogenic liquefaction and distillation, adsorption separation and membrane separation. Nitrogen used for vacuum applications is, in most cases, adequately supplied with industrial-grade gas. The typical impurity levels in the specification for industrial-grade nitrogen are 10 ppm oxygen (maximum) and a minimum dew point of -68°C (-90°F) or lower (3.4 ppm by volume). The actual levels are usually in the 2 ppm range for both oxygen and water vapor. It is often the case that there is more “pick up” of impurities in the piping to the equipment than in the supply product itself.

Nitrogen is the most common backfill gas used in vacuum heat treatment. It is also commonly used as a partial-pressure gas in the range of 500-5,000 microns (0.67-6.67 mbar) or in convection heating where it has been found to be extremely effective in reducing heat-up/cycle time. In most instances, it is not recycled.