Choosing a Protective Atmosphere for Tool Steel Heat Treatment

The four principal protective atmospheres

The four are distinguished by how much air is fed to the generator, which is called the air factor. Endogas is made with the greatest air deficiency, an air factor below 0.5, and because the reaction between the gas and that little air absorbs heat rather than releasing it, the retort has to be heated and a catalyst is used. Exogas is made with an air factor between 0.5 and 0.8, and the combustion releases enough heat to keep itself going. Lean exogas, made with an air factor between 0.8 and 0.99, contains enough carbon dioxide and water vapour to be useful only for certain non-ferrous treatments, and it is usually purified instead to give the prepared nitrogen base gas that the source calls monogas. Dissociated ammonia is made on a different principle altogether, by cracking ammonia over a catalyst above 800 °C, and it comes out as 75% hydrogen and 25% nitrogen.

Composition and dew point of the four principal protective atmospheres. The prepared nitrogen base and dissociated ammonia are the two driest of the four, and endogas carries the most hydrogen and the most carbon monoxide. Dew point is the temperature at which the gas is saturated with water vapour, so a lower figure means a drier gas.

The dew point column is the one that matters most for tool steel. A dew point of −30 °C, which the prepared nitrogen and the dissociated ammonia both reach, is the condition under which a high chromium grade can be heated without its surface oxidising. The endogas range of −15 to +5 °C and the exogas range of −10 to +20 °C are wetter, and that sets the limit on which grades they are safe for.

What each atmosphere is used for

Fields of application of the four principal atmospheres. The list is the source’s own and is not restricted to tool steel. Note where the atmosphere is named as suitable for alloy steels containing chromium, aluminium or silicon, and where it is named for carburising.

Two rows in that table point straight at tool steel work. The endothermic atmosphere is the one named for carburising, for annealing and hardening, and for sintering hard materials without decarburisation, and it is the atmosphere a gas carburising furnace runs on. The ammonia-base atmosphere is the one named for alloy steels that contain chromium, aluminium or silicon, which is the family most tool steels belong to, and it is the reason dissociated ammonia appears again in the sections below on hardening high speed steel and on hardening high chromium grades in vacuum.

What each atmosphere costs and what it demands of the furnace

Cost is dominated by two things, the fuel the generator burns and the purification the gas needs before it is usable. The exothermic base atmosphere is the cheapest because the reaction pays for its own heat and the gas needs little post-treatment. The nitrogen base and the endothermic base cost about two and a half to three times as much, and dissociated ammonia several times more again. The two figures worth noting at the top of the tail are purified hydrogen and argon, both of which sit an order of magnitude above everything else and are chosen for what they protect rather than for their price.

Relative costs of producing the atmospheres, in pounds per normal cubic metre. The figures are the source’s 1984 cost basis and are reproduced to show the ranking between the gases rather than as current prices.

The three properties below decide the furnace and the safety provision, and they are the reason a shop cannot simply pick the cheapest gas. A high explosion hazard means the furnace, the pipework and the purging procedure all have to be built for it, and it is the single most common reason a converter moves a job from dissociated ammonia to a nitrogen base atmosphere. Threat of toxicity tracks the carbon monoxide content, which is why the exothermic and endothermic gases are rated for it and the pure hydrogen and nitrogen mixtures are not.

Main characteristics of the principal atmospheres. Read across a row for one atmosphere and down a column for one property. The nitrogen base atmosphere is the safest of the four to handle and the endothermic atmosphere the most demanding.

Bright, reducing, scale-free and low-scale annealing

The source sorts every controlled atmosphere annealing treatment into four classes by how perfect the surface has to be, and the class is what really decides the gas. Bright annealing keeps the work bright after cooling, and because the oxidising constituents act during the cooling stage as well as the heating stage, they have to be held down through the whole cycle. Reducing annealing uses an atmosphere that cleans up a slightly oxidised or rusty surface, but it does not reach a perfect brightness and it fails on high chromium alloys because chromium oxide is very difficult to reduce. Scale-free annealing allows temper colours to form, so a gas with some oxidising power is acceptable. Low-scale annealing allows a slight scale, and it is the class used for heating before forging, where a scale-free surface is not achievable in practice once the part leaves the furnace.

The practical value of the four classes is the price ladder behind them. True bright annealing of a stainless grade needs dissociated ammonia or hydrogen, while scale-free annealing of the same part is satisfied by lean exothermic gas or monogas. Naming the class before naming the gas usually brings the cost down.

Choosing the atmosphere from the composition of the steel

Where the choice is open, the source gives the rule by alloy content, and the threshold is chromium and manganese. If the chromium, manganese or tungsten content of the steel is below 2 to 2.5%, an exothermic, endothermic or nitrogen base atmosphere is suitable. If the manganese content, and more particularly the chromium content, exceeds 2%, then every constituent that contains oxygen has to be removed to a very low level. That means carbon dioxide and water vapour, and also carbon monoxide, because even the chemically bonded oxygen can oxidise or decarburise the surface. For those grades the source names hydrogen, dissociated ammonia or high vacuum as the suitable atmospheres.

Two further restrictions are worth carrying into a quotation. The hydrogen and nitrogen mixtures, the 75/25 grade and the 80/20 grade, are suitable only where the steel is not a nitriding steel, so an atmosphere that is correct for one tool steel can nitride the surface of another. And when the steel is a chrome-nickel heat resistant or acid resistant grade, cracked ammonia or hydrogen is used with a dew point below −50 °C, which is drier than any of the four principal atmospheres delivers from the generator.

Carbon content then splits the lower alloyed steels. A low carbon steel can be run in exogas provided the water vapour is held low enough that the 3 to 10% carbon dioxide does no harm. A steel of higher carbon content should be run in endogas or monogas, because those are the two that will not decarburise it, and a steel whose carbon and chromium are both high is where the choice is effectively made for you.

Where this leaves a tool steel order

The order of the decision is to fix the surface class first, then read the alloy content off the certificate, then pick the gas. A cold work grade in the D or A group carries enough chromium that the oxygen-bearing constituents have to be out of the gas, which points at prepared nitrogen, dissociated ammonia or vacuum. A hot work or high speed grade is the case the source treats at length in vacuum and in dissociated ammonia. The carbon and low alloy tool steels are the ones that can be run in the cheaper endothermic and exothermic atmospheres without harm.

The furnace practice around that decision is on the tool steel heat treatment guide, and the vacuum route with its own pressure and cooling data is on the vacuum heat treatment page. The carburising side of the endothermic atmosphere is measured on the gas carburising carbon penetration page, and the ammonia side on the tool steel nitriding page. Decarburisation as a defect and how much stock has to come off to clear it is on the decarburisation and stock removal page.

Source: Controlled Atmospheres for Heat Treatment, R. Nemenyi and G. H. J. Bennett, Pergamon Press, 1984.