When a steel part is heated to high temperatures for heat treatment, its surface reacts chemically with the surrounding medium in the furnace. The furnace atmosphere may consists of various gases depending on the conditions of combustion and the temperature The common furnace gases are O2, CO2, CO, H2, H2O (water vapour), N2, CH4, etc.
As a result, two undesirable surface phenomena of special importance occur:
1. Oxidation (Scale formation),
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2. Decarburisation.
These two processes are diffusion based phenomena and thus, the extent of both oxidation and decarburisation depends on:
a. Temperature of heating,
b. Time of heating,
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c. Composition of the steel being heated,
d. Composition of the surrounding medium.
The rates of both processes are increased at high temperatures and with increased time of heating, though temperature is more effective. The presence of element, like chromium in steel decreases the tendency of both the processes particularly of decarburisation, as it reduces the diffusion of carbon.
Various gases in the furnace atmosphere react differently with steel. For example, H2 decarburises; O2, CO2 and water vapor oxidise and decarburise; CO and CH4 carburise the steel. Even salt or lead baths cause decarburisation, due to their contamination with oxides through contact with the atmosphere.
Oxidation of Steel:
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It is caused by chemical reaction of oxygen, carbon-di-oxide and/or water vapor with the surface of steel.
The main reactions during oxidation are:
As these reactions are reversible in nature, the equilibrium relationship between iron and iron oxide in contact with CO and CO2, or H2 and water vapor at the heat treatment temperature control the extent of oxidation (apart from the composition of steel, temperature and time). The water-gas reaction in which these four gases interact can help in analysing the process of oxidation.
where, Pco2 PH2o and PCO are the partial pressures of carbon-di-oxide, hydrogen, water vapour and carbon mono-oxide respectively. The furnace atmosphere at a heat treating temperature, adjusts as per equation (2.6), i.e., K1 being constant at a temperature, a change in one component causes changes in other components. If the amount of water vapour is decreased, and if Pco2/Pco fixed, then the amount of H2 should decrease to came backward reaction (2.3) i.e., oxidation must reduce.
Due to oxidation, first an oxide film and then, a thicker scale forms on the surface of the steel. As oxidation is a diffusion controlled process, the thickness of the oxide layer increases with the rise of temperature, and with the passage of time at a temperature, though temperature is more effective in increasing the thickness of the oxidised layer. At a temperature of around 180°C, a tight, adhering straw-coloured film forms which, becomes porous and loose scale at a temperature of around 425°C.
Oxidation tarnishes the lustre of steel surface of parts, and the scale formed is a loss of the precious iron metal, thus, it is an undesirable (though a natural) process. It is a harmful process, when it is necessary to have close tolerance in dimensions of the parts being heat treated. Oxidation reduces the dimensions of the steel parts. Scale prevents rapid removal of heat during quenching, resulting in soft spots, or lower hardness values on surface, or less hardened depth of parts.
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Scale being hard itself, reduces tool life during machining. It the thickness of the oxidised scale is less than the allowance for machining, then machining removes it and still gives parts of correct dimensions. But if the thickness of the oxidised layer is greater than the machining allowances, which happens more often if oxidation is not controlled, then the size of the part is too small to find an application and then, the part is just a scrap.
The burning of steel means heating the steel very close to the solidus temperature (which is always avoided) and manifests as a layer of partially oxidised steel lying under the layer of scale, and where oxygen has penetrated along the grain boundaries of the steel and where, it causes not only decarburisation but formation of iron oxide. Proper temperature control of heat treatment furnaces avoids burning of the steel as it is also an irrevocable damage and part becomes a scrap.
If proper precautions are taken, or if proper furnace atmosphere is provided so that at the heat treating temperature, the rate of oxidising-reducing and carburising- decarburising are same in both directions, that is, the metal will not react with the atmosphere and its composition will not change, that is, atmosphere is said to be inert or neutral, by reactions such as-
where iron oxide is reduced back to iron.
Decarburisation of Steel:
It is a process of selective (preferential) oxidation of the carbon in steel when heated for heat treatment.
The carbon is removed from the surface layers while the iron does riot get oxidised. Thus, there is a thickness from the surface towards interior, in which, the carbon content is less than the original carbon content of the steel. Decarburisation of the steel takes place when it is heated above about 650°C, as then the rate of decarburisation is more than the rate of oxidation of the steel. The main furnace gases causing decarburisation are O2, H2, CO2 and H2O.
It being a diffusion controlled phenomenon depends on:
(1) Temperature of heating,
(2) Time of heating,
(3) Nature of furnace atmosphere,
(4) Composition of the steel.
The main reactions causing decarburisation are:
I. When the temperature of the steel in below A1 (≈ 727°C) temperature, when the steel has carbon present mainly as cementite (Fe3 C):
II. When the temperature of the steel is above A1 (≈ 727°C) temperature, when the carbon is present dissolved in austenite:
where Feγ (C) is the carbon dissolved in austenite (gamma iron).
The producer gas reaction as given below is important here:
where, C is the carbon dissolved at the surface of the steel. The equilibrium constant for this reaction can be written as,
where, ac is the activity of carbon in the atmosphere, and Pco and Pco2 are the partial pressures of carbon mono-oxide and carbon dioxide respectively in the furnace atmosphere. Based on thermodynamic data, the equilibrium constant of this reaction (2.17) can also be written as,
where, T is temperature of heat treatment in absolute scale. By knowing the partial pressure of CO and CO2, or H2 and H2O and knowing K2 at the heat treating temperature (from equation 2.19), the carbon potential of the furnace atmosphere can be calculated. If it is less than the carbon content of the steel, then decarburisation takes place and if it is more than carburisation takes place. But if it is equal to the carbon content of the steel, the atmosphere is inert as for as decarburisation is concerned.
Generally both these surface phenomena, oxidation and decarburisation take place simultaneously. If the rate of both these processes are equal, or even when the rate of oxidation is faster than decarburisation, then the steel surface lying immediately below the scale layer is not decarburised and has the same composition as the interior of the steel.
This happens at low temperatures of heating (≈ 650 °C). But usually, the rate of decarburisation is higher than the rate of oxidation. The surface layer of steel beneath the scale loses its carbon, causing decarburisation and the surface may show only ferrite grains.
Decarburisation is normally a harmful phenomenon. Parts, which are machined after the heat treatment and if, the thickness of the decarburised-layer is lesser than the machining allowance, the decarburisation does not pose a problem as it is completely removed during machining operation. Decarburisation causes problems in cases, where the machining is not done after the heat treatment, such as in rolled stocks, springs, tools, etc., or if the thickness of the decarburised layer exceeds the machining allowance.
A decarburised-surface-layer has lower tensile strength, elastic properties, hardness, wear resistance and fatigue strength (endurance limit) than those of interior portions of the part. The part gets easily worn off and fails easily by fatigue, because decarburisation causes development of tensile nature of internal-stresses in surface-layers of the part.
We also know from our elementary knowledge of the strength of materials, that the terminal and flexural stresses are highest in the surface layers of a part when it is stressed, that is, the surface-fibres of the part have to bear the maximum stresses, but decarburisation reduces the stress-bearing capacity of the surface-layers causing early and easy failures.
Decarburisation depends on the chemical composition of the steels. Chromium, if present in steel, makes it less sensitive to decarburisation, as it reduces the diffusion of carbon, and itself produces thin, adhering and impermeable film of scale, which physically prevents interaction of carbon and atmosphere of the furnace. Elements like Si, W, V and Mo increase the tendency of the steel to decarburisation.
Method of Measuring Decarburised Thickness:
Decarburisation cannot be detected by visual examination of steel parts and tools. It is revealed by microscopic examination, or by measurement of hardness.
Microscopic Method:
A transverse micro-section is prepared by usual method of grinding, polishing and etching and then, the microstructure is examined at a magnification of 100 X A hardened steel is first annealed (taking all the precautions to prevent oxidation and decarburisation during annealing) and then, prepared for the microscopic examination.
Two clear zones of decarburisation are:
(a) Zone of complete decarburisation- it has only ferrite grains.
(b) Zone of partial decarburisation- here the amount of pearlite is less than in the interior.
Normally, the depth of decarburisation is defined as the sum total of the depths of zones of complete as well as partial decarburisation. It is the thickness in which carbon gradient is established. The carbon content of decarburised layer generally increases progressively and exponentially from a low value at the surface to the value of the core of part.
The depth of the point at which the core carbon content is reached is the total depth of decarburisation but the latter point (and thus, the depth) is difficult to determine with reliability because of asymptotic manner in which the core carbon content is approached. But as the carbon has a range in the specification of the steel, it is acceptable in this range. Commonly in practice, depending on the specification, the effective-depth of decarburisation is used.
Ms Method:
The decarburised layer of highly-alloyed tool-steels has lesser amount of carbides embedded in scorbutic structure, but is difficult to establish the difference in amount of carbides under microscope.
The Ms method, i.e. temperature at which martensitic transformation begins, is based on the principle that Ms temperature is raised as the carbon content of the steel decreases. A thin slice that includes the decarburised layer is cut, austenitised under condition of no decarburisation and quenched in oil, or in fused salt heated to a Te temperature, 5° to 10° above Ms temperature of the actual steel. The decarburised layer having lower carbon but higher Ms transforms to martensite at this temperature of bath. The specimen is then annealed at 580-600°C immediately after quenching in oil.
Martensite of decarburised layer gets transformed to troostite and which etches dark under microscope, while no changes take place in the central part and which will be of light colour. Thus, the decarburised depth can be measured clearly now. This method is reliable, if core carbon is more than 0.6% but not less than 0.35%°C.
Hardness Method:
Total depth of decarburisation can be estimated reliably by determining the hardness variation with depth in a quench-hardened part by using micro-hardness tester. The hardness increases continuously in decarburised zone till the point where it just becomes equal to the hardness of the un-decarburised core. It is already known that hardness is less when the carbon is less.
Fick’s Second Law for Calculating Decarburised Thickness:
Fick’s Second law also called diffusion transport equation, deals with the transient-state diffusion. It relates to change in concentration with time (∂c/∂t) and is based on the fact that matter is conserved within the system.
The solution is given by equation 1.65:
This is Grube solution in which Co is the initial concentration of the carbon when t = 0, and ‘erf refers to ‘error function’.
When values are such that:
This equation could be used for carburising and decarburising of steels.
Effects of Temperature:
Fick’s Laws say that movement of diffusing material is proportional to the diffusivity, D. As the atomic motion and the number of vacancies both increase with increase of temperature, so does D and is found to obey the equation
D = Ae-Q/RT …(2.21)
where, A = constant; Q = constant activation energy, R = gas constant = 1.987 cal/m.K T = Temperature absolute scale, or
In D = In A – Q/RT…(2.22)
This equation could be used to estimate the effect of changing a furnace temperature for heat treatment for decarburisation or for carburising a gear etc. We can calculate the time needed at the new temperature to obtain equivalent results.