—Stainless steels are reknowned for their shining bright surfaces coupled with resistance to rust or stain in spite of moisture or the action of corrosive environments.
—Certain stainless steels are very hard. Others, possess unusual strength and can retain the same at extremely high and low temperatures.
—Because of the above and many other excellent properties, stainless steels find many applications both industrial and household.
—Stainless steels may be classified as
(i) Ferritic stainless steels;
(ii) Martensitic stainless steels, and
(iii) Austenitic stainless steel, the much used one. (For more details refer Chapter 5).
Stainless Steel Making Processes
— Various processes used for making stainless steel are explained below:
(a)Electric arc furnace process,
(b)Argon-Oxygen-Decarburization (AOD) process,
(a) Electric Arc Furnace Process
—The furnace is charged with stainless steel scrap, nickel, chromium and other charge. As the charge melts, oxygen is blown through a lance onto the melt surface to reduce carbon to the desired level. Carbon oxidizes to CO.
—Due to high chromium content of stainless steel, there is oxidation of chromium to chrome oxide which goes into slag.
—Molten bath temperature of 1900°C with attendant significant furnace refractories are required to minimize the oxidation of chromium to economically feasible levels.
—After the oxygen blow, the slag is reduced with ferrosilicon etc., to partially recover chromium.
—The reduced slag is analyzed and required final additions including low carbon ferrochrome are made.
—Arc furnace production of stainless steels has been difficult especially for very low carbon grades.
(b) AOD Process
—Argon-oxygen process is a widely used method for making quality stainless steel.
—The charge is melted in an electric arc furnace and after a slag-off it is transferred to the AOD vessel for refining, alloying and final finishing,
—Fig. 27.12 shows an AOD vessel. The tuyers, arranged in a semicircle around the side near the bottom, blow process gases inside the vessel.
—The slag is reduced and final additions are made after a preliminary analysis.
—The vessel is turned down for sampling, temperature measurement, slag off and tapping and the heat is poured into a ladle for further use.
—In AOD process there is continual thorough stirring of heat.
—The method provides a very close control of heat chemistry.
—Computer control ensures repetitive uniform melting and refining.
—AOD refining reduces the hydrogen content to about half the value in electric furnace melting.
—There is minimal loss of chromium to the slag.
(c) The CLU Process
—This process was developed jointly by Creusot-Loire of France and the Swedish firm Uddeholm.
—The process uses a mixture of oxygen and steam as the refining agent.
—The charge is melted in an electric arc furnace. Pig iron or hot melt can be used directly from blast furnace.
—The CLU process is carried out in a converter (Fig. 27.13) with bottom or side placed tuyeres for injection of gas mixture.
—A mixture of oxygen and superheated steam is blown into the melt in the converter.
—With CLU process, it is possible to refine to low carbon levels without excessive oxidation of chromium.
—The CLU process in the converter can be divided into following steps:
(1)The carbon refining step [Fig. 27.13 (a)].
(2)The reduction step [Fig. 27.13 (&)].
(3)The hydrogen refining step [Fig. 27.13 (c)].
(1) The carbon concentration is reduced from the initial level to the specified final content by blowing oxygen and steam into the melt.
Initial blowing of pure oxygen is done if the carbon content of the charge is higher than 3%. Steam is used when a carbon level of approximately 0.8% is reached.
The refining temperature of 1675°C can be kept fairly constant until the desired low carbon percentage is reached.
(2) In the carbon refining step, (1-above) some chromium and manganese are oxidized.
In this reduction step, a mixture of lime and ferro-silicon is therefore added for reduction of the slag. As a result, the chromium recovery is almost 100% and that of manganese about 80%. During this step, the molten bath is stirred by injection of steam.
(3) During oxidation and reduction, a small portion of injected hydrogen from the steam is dissolved in the molten metal. In this
hydrogen refining step, inert gas such as a mixture of argon and nitrogen is bubbled through the bath and as a result the hydrogen concentration decreases rapidly.
Desulphurisation if required may be done by adding lime during this step.