Methods of Protection of Steel | Oxidation | Metallurgy

Unless special precautions are taken, heating the steel results usually in certain amounts of oxidation or scaling and may also result in decarburisation. Both are usually undesirable. To prevent oxidation and decarburisation of steels, various methods from the simplest, but not completely reliable, to complex and expensive but almost full proof, are used depending on the criticality of the part, practice, desired quality and cost allowed.

Various methods can be broadly divided into two classes:

1. Isolate the steel parts being heated from the furnace atmosphere.

2. Partial or complete elimination of oxidising and decarburising gases from the furnace atmosphere, or control of these gases, or use protective atmospheres.

1. Isolation of Small Parts from Furnace Atmosphere:

The parts made of plain carbon steels, low alloy steels having carbon content 0.6% to 1.0%, or even high-chromium steels are put in a not, or box packed with cast iron chips and turnings (so that the air inside the box, or sir that is sucked into the box on account of temperature variations preferentially oxidises the chips, and these chips are inert to above materials in normal heat treating temperatures. But chips have slightly carburising action on hot-work tool steel containing around 0.3% carbon. These chips start sintering at about 1000°C, and thus should not be used at temperatures above 1050°C.

The tools are wrapped in newsprint paper, before packing with cast iron chips. The surface of the tools is protected against mechanical damage), or in a mixture of charcoal and sodium carbonate, or used carburiser. These methods are fairly good for annealing of parts, but are less effective, when used for hardening purposes, because the cast iron chips or carburiser is carried with the parts to the quenching tank and thus, pollutes the quenching medium.

Annealing of thin rods is done by inserting them into tubes and blocking the holes with fireclay. However, dies are placed with the impression downwards on a metal plate, the gap between the die and the base is sealed with fireclay having 10 to 20% ground asbestos.

At high temperatures, the packing material such as carburiser may lead to carburisation; wood-charcoal, or coke fines act as decarburiser on plain carbon and low-alloy steels containing medium to high carbon, but act as carburiser on hot work steels and high-alloy chromium steels; fire clay for blocking air may develop cracks, and thus, may not be able to effectively and adequately isolate the parts. These compounds are frequently very inconvenient to use the cost of boxes, or tubes, having short span of life, is additional burden.

Borax may be used in some cases to protect against decarburisation. The sprinkled borax powder over the surface to be protected melts to form a protective film. It must be brushed off before plunging the part in a coolant, like oil as this film reduces the rate of cooling. Borax film spalls off when the steel is water- hardened (during plunging) leaving behind a bright clean surface.

There are some commercially available protective pastes to be applied to the protected surface prior to heat treating. It is always good to make a test trial for the recommended steel and if found reliable, then use it. Sometimes, steel parts are wrapped in sheets, or bags made of heat resisting steel foil to protect the parts against decarburisation. Sometimes, copper plating, 0.015 to 0.025 mm thick may be given to protect steels. Ceramic coats have also been used.

2. Modification of Furnace Atmosphere:

The following requirements must be fulfilled, if an atmosphere is to be practically and economically successful in general heat treatment of steels:

i. Lowest possible cost

ii. Simplicity in operation

iii. Neutral reactions with steel being heat treated

iv. Minimum oxidation characteristics

v. Freedom from explosion risks and absence of toxic characteristics

vi. Easy availability of raw materials

vii. Atmosphere should be stable at heating temperature, and if possible with high degree of turbulence as it helps in quick transfer of heat to parts.

Some simple methods are described below:

I. A slightly oxidising atmosphere in the furnace is often desirable when freedom from decarburisation is important at low cost of atmosphere. The oxidised layer is machined off. Machining allowance has got to be a bit more in this method. Here rate of oxidation is more than rate of decarburisation.

II. In simple reverberatory furnaces, the combustion of carbonaceous fuel is regulated to decrease the O₂ and other oxidising gases in the furnace atmosphere to obtain a neutral or reducing atmosphere.

The colours and the appearance of the flame indicate the character of the furnace atmosphere:

(a) Oxidising atmosphere has luminous and transparent flame.

(b) Reducing atmosphere has non-luminous and dark flame.

(c) Neutral atmosphere has half-transparent, white-coloured flame with violet strips.

It is desired in these furnaces to obtain minimum amount of scale. A reducing atmosphere produced by reducing the supply of air in the fuel/air mixture can prevent the oxidation of the steel, but causes decarburisation as the H₂ of the atmosphere causes this (equation 2.10 and 2.14), that is, a reducing atmosphere could be decarburising.

It is possible to have three distinct combinations, namely:

(a) Oxidising and decarburising

(b) Reducing and decarburising

(c) Reducing and carburising.

III. It is possible to produce commercially an atmosphere in which high carbon and similar steels are practically free from decarburisation. Scaling is materially reduced by the presence of 4%, or more of CO in the furnace atmosphere.

IV. The muffle and electric furnaces may be made to have slightly reducing atmosphere created by charging charcoal on the bottom of the furnace near the door, which on burning gives rise to CO. Springs are commonly hardened in such furnaces.

V. In another simple method, though no attempt is made to get bright finish, but a commercially acceptable finish, free from decarburisation, which on hardening shall be file-hard and would give the silvery while finish after shot-blasting (which is a simple and best test indicating freedom from decarburisation of steels in hardened condition, because a decarburised and soft area is shown as dull against a silvery white background on shot blasting of hardened steels of carbon around 1%).

Ordinary paraffin, or kerosene in a ‘Paragen burner’ with an air pressure of 70-140 kNm^-2 is burnt and the products of combustion which are at high temperature pass through a slot in a specially designed sill brick and into the furnace, where after passing around the parts, leaves through a flue in the back wall of the furnace. This method is cheap, simple, free from sulphur compounds and no effect takes place by changes in gas, or air pressures. The gases have sufficient CO to suppress decarburisation by CO₂ and H₂O.

The above discussed simple methods are found to be insufficiently effective and are practically impracticable if large tonnage of steels, like rods, wires, and strips, etc. are to be heat treated. Thus, heat treating furnaces with protective gas atmospheres are used to obtain more effective results. Normally, a gas, or a mixture of gases are used in the furnace atmospheres and to avoid air infiIteration, the protective gas is present under a slightly positive pressure.

It is easy to control the protective gas atmospheres in muffle and electric furnaces. Steels given heat treatment in such furnaces retain completely bright surface, and thus, it is called bright heating or clean heating, and after a particular heat treatment is called, say bright annealed or bright hardened. Before discussing in details about protective furnace atmospheres, another practice, that is, salt bath heating may be mentioned here.

The greatest advantage of heating parts in salt baths is that it gives negligible decarburisation and prevents surface carbon content variation. It is because salt baths have neutral character and require short heating lime as heat transfer is fast in them. The bath must be kept is good condition. Some salt baths are carburising or nitriding.

Table below gives composition of some salt baths of neutral character with their working temperatures:

Normally, to prevent decarburisation of tools to be heated in BaCl₂ bath, borax covering is done by methods such as- (1) Sprinkle borax powder on surface of tools heated to 800- 850°C, where it melts to give a thin covering layer, or (2) Slightly warmed tools are immersed in hot saturated borax solution and taken out. Water on evaporation leaves a thin layer behind which melts on heating.

The salt baths pick up iron from steels being heat treated, which being in contact with the atmospheric air, gets oxidised. The iron oxide thus formed, has a decarburising action on steel charges. Thus, the bath should he regenerated. Few pieces of silica brick when added combines with the iron dissolved in the bath. Sludge, thus formed, must be removed regularly, or periodic deoxidation of salt bath is done once, or twice in a shift.

A small amount of finely crushed ferrosilicon (1 – 1.5 by weight % of total salt in the bath) having 75% ferrosilicon is added in intensely heated bath. Bath is agitated and then, allowed to settle for half an hour. To check the presence of iron oxide in bath, a graphite rod is immersed in the bath.

Graphitic carbon shall reduce the iron oxide, if present in bath and small bright beads of iron will form on the rod. The carburising, or decarburising action of the bath can be detected by immersing a steel foil in bath for few minutes and then quenched. If the foil is softer now after quenching, bath is decarburising. If it is more brittle, then it is carburising. Foils of varying carbon content used can help to know the carbon potential of the bath even.

Aims of Furnace Atmospheres:

Control of furnace atmospheres has gained critical importance for successfully heat treating parts requiring more precise metallurgical specifications.

The aims of properly controlled and applied furnace atmosphere could be:

(a) As a source of elements in heat treatment like, carburising, nitriding, etc.

(b) As surface cleaning of parts.

(c) As a protective medium against oxidation, decarburisation, etc.

2. Gases in Furnace Atmospheres and their Reactions with Steels:

Most common gases in common furnace atmospheres are- O₂, N₂, CO₂, CO, H₂, H₂O (water vapor), hydrocarbons, inert gases, etc. Sulphurous gases are harmful particularly to high-nickel, high chromium steels and alloys and thus, are avoided. They come from industrial fuels, furnace refractories and cutting oils sticking to steel parts. They accelerate the rate of scaling which intensifies with the rise of temperature.

Lithium vapours have been used as a furnace atmosphere, for protecting against scaling the steel surfaces when steel is heated to forging temperatures by reactions:

2 Li + H₂O —> Li₂O + H₂

4 Li + O₂ —> 2 Li₂ O

Air, has 79% N₂ and 21% 0₂ (traces of CO₂), is a common atmosphere in a furnace having no protective atmosphere (and in which carbonaceous fuel is not burnt) and behaves like oxygen atmosphere. Oxygen forms scale as well as decarburises the steel. Nitrogen is good for annealing low carbon steels, but even residual water vapours with it cause decarburisation. N₂ reacts with many stainless steels, whereas atomic nitrogen reacts with steels to form at the steel surface hardness producing nitrides.

Carbon-dioxide and carbon mono-oxide are important gases in furnace atmospheres. At the austenitising temperatures, CO₂ reacts with the carbon of the surface of steel to give CO, as-

where, Fe_γ(C) is the carbon dissolved in austenite.

This reaction proceeds forward till there is no CO₂ in the atmosphere, or, the steel surface is completely free of carbon, and then, the following reactions take place:

where, FeO is generally formed above 555°C, whereas Fe₃O₄ is formed below 555°C as illustrated in Fig. 2.5. Hydrogen reduces iron oxide to iron, but can decarburise the steel depending on temperature (generally above 705°C), water vapours present (water vapours decompose to give nascent hydrogen and oxygen) and carbon content of the steel (higher is the carbon content of steel, more is the decarburisation) react by reaction 2.14. Molecular hydrogen can decarburise high carbon steels by reaction 2.25,

Water vapours oxidise iron (reaction 2.3) and decarburise the steel (reaction 2.12) at very low temperatures and, even when present in small amounts. Water vapours cause ‘blueing’ during cooling cycle.

The common hydrocarbons present could be methane (CH₄), ethane (C₂H₆) propane (C₃H₈) and butane (C₄H₁₀), which increase the carburising power of the atmosphere. Butane and propane having large number of carbon atoms per molecule, generally cause soot formation, whereas methane and ethane are used as the main source of carbon (methane being more commonly used) for gas carburising, because these gases on thermal decomposition give nascent carbon (reaction 2.26 for methane), which reacts with steel surface, the latter also acts as a catalyst-

CH₄ à 2 H₂ + C (nascent) …(2.26)

Of the inert gases, argon finds more use than helium (as argon is cheaper), and is used when almost absolute protection is desired and other atmospheres cannot provide, such as for bright hardening of stainless steels and heat treatment of titanium alloys. These gases should be free from H₂, O₂ and carbon-bearing gases.

Reactions between Gases:

In a furnace, where a hydrocarbon fuel is burnt, the combustion products contain some, or all the gases like- CO₂, CO, H₂, O₂, N₂ and H₂O and in proportion depending on the oxidising/reducing conditions, burner efficiency, tightness of the furnace and the size of door openings. The excess air combustion conditions give excess air, CO₂ and water vapours, which quicken the formation of loose scale on the steel surface.

Air-deficient combustion conditions give more CO and H₂, and thus cause decarburisation of steel due to the reducing conditions, and produce tight oxide on the surface of steel. The amount of scale formed in both Cases depends on temperature and time at that temperature. Specially designed direct-fired furnaces can have truly neutral atmospheres, which give fully acceptable results. It is essential to control water vapour as it has high oxidising power, or decarburising power for steel.

The reaction (2.1) is irreversible and cannot be controlled, but reactions 2.2 and 2.3 are reversible and thus, can be controlled to advantage:

In reactions 2.2 and 2.3, the four gases involved are CO₂, CO, H₂ and water vapour (H₂O). Of these four main gases in a furnace, H₂O and CO₂ are oxidising, whereas H₂ and CO are reducing in nature.

In the furnace atmosphere, one type of gas cancels the effect of other type of gas and thus, a neutral, oxidising or reducing atmosphere can be created there by controlling them by the water gas reaction:

and whether the oxidation, or reduction takes place at the surface of the steel, depends on equilibrium condition depending on the temperature, composition of steel and composition of the atmosphere. Below 830°C, water vapour has stronger oxidising power than CO₂, and whereas CO has stronger reducing power than H₂. At 830°C, the oxidising power of CO₂ and water vapour are equal and so are the reducing power of CO and H₂. Thus, at 830°C, the equilibrium constant of the water-gas reaction (2.4) is unity. Above 830°C, the oxidising power of CO₂ is more than water vapour, whereas H₂ has more reducing power than CO.

Let us rewrite the equations:

It is assumed that H₂ content of the system is constant at 40%, and combined CO and CO₂ remains constant at 20%, then by varying the temperatures of reactions, changes take place in equilibrium constants, the composition of gases as well as the dew point as given for few temperatures in table 2.3.

The carbon potential may be defined as that carbon content which as specimen of carbon-steel foil acquires when equilibrium conditions have been established between the carbon potential of the carburising medium and the carbon content of the foil.

The required carbon potential of the gas atmosphere should be about the same as that of the carbon dissolved in the steel at the heat-treatment temperature. If the potential is less, then decarburisation takes place and vice versa.