Ferroalloys & Alloying Additives Online Handbook

Hydrogen
Atomic number	1	CONTENTS
Density, 20° C (68° F)	0.07 g/cm³	BACK
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GENERAL

Hydrogen is an inevitable impurity in steel and cast iron. It can be quite harmful in certain applications and, where necessary, should be avoided or removed as required.

ENTRY

Hydrogen can enter steel by any of several routes. The first occurs during the steelmaking process itself where water, contained in the charge as damp scrap, fluxes and ferroalloys, in the furnace gases or in inadequately dried refractories, dissociates on contact with liquid steel and allows hydrogen to be absorbed by the bath. This hydrogen can be largely removed by the purging action of the carbon boil, but enough can remain to be troublesome later on. Contact between the liquid steel and moisture in ladle refractories and/or humid air is another common source of hydrogen. Since this hydrogen cannot be purged by furnace reactions, special techniques are required to remove it.

Water vapor contained in furnace gases is obviously more prevalent in practices utilizing the combustion of hydrocarbon fuels. However, the level of hydrogen in the steel at any instant is set by a balance between the competing reactions of entry (from the gases) and removal through the carbon boil and degassing. At low carbon levels, the rate of hydrogen absorption is greater than the rate of removal. Dissolved hydrogen content drops to its lowest level at the end of the boil but can increase again with additional alloy and synthetic slag additions. The hydrogen content of steel melted under an oxidizing acid slag is lower than that under a reducing slag. High frequency induction melting produces lower hydrogen contents than electric furnace practice. At tap from the EAF, the hydrogen content of the liquid steel will be between 1-10 ppm, with 4-6 ppm the most common. Once solidified, steel can absorb hydrogen through the action of electrochemical reactions taking place on the steel's surface. The most common of these are pickling, electroplating, cathodic protection and corrosion. Hydrogen, liberated during these reactions, is in part absorbed by the steel before it has the opportunity to recombine to harmless bubbles of H2. Absorption is favored by the presence in the electrolyte of certain "poisons", such as sulfides, arsenides, phosphides, and selenides, which inhibit the recombination reaction. Hydrogen can also enter steel through exposure to the gas at high temperatures and pressures, a condition not uncommon in chemical and petrochemical processing equipment. Water vapor and hydrocarbons are also harmful in this regard. In any event, hydrogen dissolves in steel interstitially as a monatomic species, but whether it does so as atoms or protons is not known.

SOLUBILITY

The solubility of hydrogen in steel is strongly dependent on crystal structure, temperature and composition. Hydrogen is much more soluble in austenite than in ferrite, for example. In all cases, solubility increases with temperature, ranging from less than 1 ppm at room temperature to about 8 ppm at 704 C (1300F).

Mention should be made of the units used to express hydrogen content in steel. The most commonly encountered are parts per million (ppm) and millimeters or cubic centimeters of hydrogen, corrected to standard temperature and pressure, per 100 g of steel. The relationship between the two is 1 ppm = 1.11 ml/100 g.

Carbon generally increases the solubility of hydrogen, but the situation is made more complex at high temperatures by the formation of methane, CH4. Manganese also has a complex effect, which may be based on crystal structure. Silicon lowers hydrogen solubility, as may aluminum. Chromium contents up to 10% increase hydrogen solubility, but higher concentrations decrease it. The effect is explained in terms of crystal structure, since about 10% Cr closes the g-loop and higher concentrations cause the steel to be fully ferritic up to the melting point. Nickel increases hydrogen solubility, with solubility being proportional to nickel content. Molybdenum has no effect on solubility, while tungsten decreases it. Vanadium, titanium, columbium, zirconium and tantalum all increase hydrogen solubility, particularly at low and moderate temperatures.

Cold work has no effect on hydrogen solubility in pure iron, but the presence of carbides causes a marked increase. It is believed that hydrogen migrates to, and collects in, internal voids formed next to carbide and inclusion particles. Thus, when a cold worked steel is annealed, some, but not all, of the hydrogen is removed by diffusion.

PERMEABILITY

Although the diffusivity of hydrogen is an important physical property, it is more common to speak in terms of the permeability, defined as the product of diffusivity and solubility, P=S x D. In contrast to solubility, hydrogen permeability is higher in ferrite than in austenite. This is fortunate since it facilitates the removal of hydrogen from steel by heating, as described below.

Again, alloying elements display individual effects. Carbon decreases permeability, but hydrogen will decarburize Fe3C at high temperatures. Manganese is believed to play only a small role, and molybdenum has no effect at all. Silicon decreases permeability. Chromium has no great effect in austenite, but decreases hydrogen permeability in ferrite. Permeability increases with nickel content up to about 6% Ni, than decreases thereafter.

There are no known iron hydrides, although iron is an endothermic (negative heat of absorption) occluder. Hydrogen expands the ferrite lattice as it takes up interstitial positions, but the degree of lattice expansion is difficult to measure by X-ray diffraction techniques. Thin foils charged with hydrogen from one side only will bow visibly, and this effect can be used to estimate the degree of distortion caused by the dissolved atoms.

EFFECTS

Hydrogen is generally harmful to steel, but can be tolerated in many instances and by most steels. Hard, high strength steels and those used under severe service conditions are more sensitive to hydrogen and here, steps must be taken to remove the impurity or add protective alloying elements.

Hydrogen remaining after steelmaking migrates to internal defects where it recombines to form gaseous H2. The pressures exerted by this precipitated hydrogen can be substantial. For example, if liquid steel contains 10 ppm hydrogen, pressures exceeding the yield strength will be generated before the steel has cooled to room temperature. This so-called hydrogen flaking is particularly damaging in heavy section forgings, and has led to many catastrophic failures in items such as large crankshafts and turbine rotors. Nickel steels are particularly susceptible to flaking, but in general hydrogen contents below 2.5 ml/100 g are considered safe.

The hydrogen that enters solid steel can also collect at internal voids. As pressures build up in these voids, the familiar hydrogen blistering occurs. Chromium-molybdenum steels are resistant to this form of attack (as well as to graphitization) at elevated temperatures and are therefore widely used where potential hydrogen hazards are known to exist.

Dissolved interstitial hydrogen is also very harmful, causing an increase in yield strength and a corresponding decrease in ductility and impact properties. This is one form of hydrogen embrittlement. More important, though, is the effect known as delayed failure or static fatigue. This occurs in high strength steels that have been cathodically or otherwise charged with hydrogen and loaded in tension to stresses below their yield strengths. After a period that may extend from minutes to several weeks, depending on hydrogen content, temperature and stress level, the steel fails in a completely brittle manner.

Finally, hydrogen is known to cause cracking in welds, especially in high strength steels with tensile strengths exceeding 1690 MPa (240 ksi). The mechanism is related to the delayed failure described above and is prevented through the use of low-hydrogen electrodes or a post-weld heat treatment.

PREVENTION/CURE

Hydrogen content in liquid steel can be minimized by assuring that all charge materials, furnace and ladle additions and refractories are thoroughly dried. While contact with furnace gases or atmospheric moisture is more difficult to avoid, practices used to prevent reoxidation should also be helpful in keeping hydrogen from entering the steel.

Several techniques have been developed to remove hydrogen from liquid steel. These include argon bubbling, the AOD, and a number of vacuum processing operations. The latter two have been found to be most effective and are now widely used. The steel may be degassed in the ladle, the AOD, or as a stream of fine droplets passing from the ladle to another held in a vacuum chamber. Alternatively, the steel may be cast into consumable electrodes that are subsequently arc remelted under vacuum. In all cases, the aim is to reduce the dissolved hydrogen content to below the harmful threshold of about 2.5 ml/100 g.

Hydrogen can be removed from solid steel by baking or annealing. The rate of removal depends on the temperature and the square of the diameter of the part being treated. Hydrogen removal is roughly 250-400 times faster at 205 C (400 F) than at room temperature, but annealing temperatures may not be too high as hydrogen solubility increases with temperature. Small parts such as plated screws can be baked at 190-205 C (375-400 F); higher temperatures may damage the plated surface or, in the case of cadmium, lead to liquid metal embrittlement. Baking times for small to moderate sections extend to about 24 hours.

Large forgings and slabs, which are susceptible to flaking, require more extensive treatment. Forging ingots should be slowly cooled to allow as much hydrogen as possible to diffuse out of the steel. Then, depending on residual hydrogen content and section size, the forging will be further degassed by soaking at 650 C (1200 F). Five-inch diameter billets containing 3 ml/100 g require 20 hours at this temperature to reduce hydrogen content to 2 ml/100 g; 30-in forgings containing 10 ml/100 g must be soaked for 880 hours to achieve the same effect.

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