Bull. Mater, Sei., Vol, 12, Nos 3 & 4, September 1989, pp. 263 270. © Printed in India. Ferritic stainless steel- A coinage material V P SARDANA and D A PIKLE Salem Steel Plant, Salem, India Abstract. Generaleconomic and metallurgical requirements, and processing methods for coinage materials are reviewed. The Indian effort at the development of ferritic stainless steels for coinage is discussed. The results of several trials at the Salem Steel Plant towards establishing the appropriate composition and processing route are presented and evaluated. Keywords. Coinage; ferritic stainless steel. I. Introduction Traditionally coins have been made out of gold, silver, copper and brass. With the passage of time, these materials have become scarce and expensive. Substitutes were found in nickel and its alloys. Coins have also been made of aluminium. Inflation increases the value of the metal in the coin continuously as compared to its face value. The coins may therefore be melted down for use as metal and not be used as coins. Thus it becomes imperative to look for a suitable substitute which is economical, stable in price and easily available. Ferritic stainless steel fills this gap well. 2. Demands on coinage material The main demands on coinage material from the metallurgical point of view are linked with the requirements of blanking, coining and wear-resistance during use. The volume of metal in the work piece is equal to the volume of the die space when the die is in the closed position. The simplest means of ensuring volume control in a coin blank is by carefully controlling the weight. It is here that uniform thickness plays a vital role. The coinability of a metal is difficult to quantify although the conditions under which a ductile metal cannot be coined can be stated in terms of the compressing loads that the die system can exert on the work piece. Soft metals can be easily coined, but they wear out easily in use and also pose problems in blanking. The blanks are not fiat and edges are rounded, In coining, deformation of work metal is accomplished largely in a compression strain cycle which leads to a progressive increase in the compression flow strength as the deformation progresses. This deformation cycle results in a product with good wear-resistance, but in coining operation it can raise the yield strength to a level beyond the permissible maximum die load, resulting in stoppage of coining action. Therefore a definite hardness range and strength is specified for coinage material. After the coins are produced, the material does not undergo any further surface treatment. Therefore it is necessary that the surface finish of the 263 264 V P Sardana and D A Pikle material used for producing coins should be the same as that required in the finished coins. Defects like slivers, deep scratches etc. on the surface of the material are not permitted as these can also lead to rejection of the coins. Coarse grain structure will not only give an improper edge after blanking, but may also result in improper surfaces after coining. Finer grain structure is therefore insisted upon. Material selection for coinage depends not only on metallurgical factors but also on the economy. If the face value of the coin is lower than the material value, the coin may be melted down. If it is the other way round, there is an incentive for making spurious coins. The value of the coinage material should also be stable over a ceasonable period; otherwise the coins will again disappear for other uses. This is an important aspect while considering a material for coinage. 3. Process of coining Coins are produced by close die-forming in which a prepared blank is compressed between the coining dies. Strips rolled to accurate thickness form the starting material for making coins. Generally the mints produce their own strips of coining metals. Stainless steel strips are, however, mass-produced in big steel plants and can be a bought-out item as far as the mint is concerned. Blanking is the first stage in the manufacture of coins. The strip passes through a straightener and then into a blanking press of high capacity. In older units, the blanks were barrel-tumbled to deburr and to control weight. With proper adaptation of the blanking process for the material being used, the blanks obtained do not need any further processing before they are subjected to rimming. Rimming is the process of making a raised edge which protects the image on the coins during their circulation. Rimming is done by rotating the blanks under pressure between a stationary surface and a moving disk. The blanks are then cleansed of the residual oils. In the case of cupro-nickel, the blanking is done on cold-worked material and the blanks are annealed to soften them and make them suitable for coining. Ferritic steel strips are already annealed and do not need further heat treatment. The hardness of the strip is VPN 138 to 146 and of cut blanks VPN 142 to 148 indicating no appreciable change during the blanking operation. Each blank is channeled into a coining press by a feed mechanism. While held in the collar which has the same configuration and size as the finished coin, up to 150 tons of load presses the blanks simultaneously between the obverse and reverse dies of the coin. The coins are finally inspected, counted and packed. In general, coins are needed in large quantities. To facilitate production and minimise die wear, the details incorporated in the coin design are in low relief. 4. Ferritic stainless steel Stainless steels of type AISI 301,302, 304, 410 and 430 are considered suitable for coining. Coins in the 430 grade have been made in a number of countries such as Ferritic stainless steel - A coinaffe material 265 Brazil, Mexico, Italy, Turkey and Iraq. Stainless steels are relatively hard to coin and are consequently preferred in a soft, annealed condition in the range of RB 75-85. In Indian conditions, ferritic grades are particularly attractive because they do not contain any nickel which is scarce in the country and is an imported item. 4.1 Composition Common ferritic stainless steel is of AISI 430 grade. The standard specification of this material prescribes 16-18% chromium and 0.12% (max) carbon. This range is farily wide and there exists the possibility of choosing the chemistry to get the desired end properties. If we refer to the Fe-Cr phase diagram, the austenitic loop shifts to the right with increase in C, Mn and Ni. These elements are therefore restricted to values as low as possible. Coupled with this, higher Cr content ensures total ferritic structure eliminating the possibility of formation of austenite which may transform to martensite during annealing. This ensures a soft single-phase material. Increased Cr and low carbon contents can, however, lead to increased roping tendency and deterioration of the surface. The correct composition is therefore a compromise between these conflicting requirements. Si favours ~ain growth and therefore is kept at a low value. 4.2 Processing Ferritic stainless steel, like other stainless steels, is produced in an electric, arc furnace and refined by an AOD or a VOD process. Slabs are also produced, mostly by continuous casting. Great care is required in rolling ferritic stainless steels as these are susceptible to surface defects. Hot rolling temperatures as well as coiling temperatures play a vital role in the process, The hot rolled coils are cooled amt sent to the cold-rolling complex where they are first annealed in a bell-annealing furnace, The annealed coils are shot-blasted and pickled. The coils are thoroughly inspected at the exit end of the annealing and pickling line. If the coils have surface defects, they are marked for strip-grinding. The coils are then cold-rolled to the final thickness. Final annealing and pickling is done in a continuous line. For pickling of the final product, only electrolytic pickling is used. The product is finally rinsed, dried and coiled. The annealed and pickled coils are skinpassed with, 0.8 to I% elongation. The coils are then slit or cut to the required dimensions. At ewry stage of pro~essing, interleaving paper is used. 4.3 Properties Ferritic steel of the 17 Cr variety is ductile and has good forming and fabrication characteristics. Though it does not have as good corrosion resistance as chromium- nickel steels, it is satisfactory under less severe corrosive conditions like mild and freshwater, industrial atmospheres and mild oxidizing chemicals. It has also good abrasion resistance. Important material characteristics of standard AISI 430 grade steel and other coinage materials are given in table 1. Roping is a characteristic peculiar to ferritic stainless steel. This is a phenomenon, involving the alignment of crystals, which leads to a directional appearance in the form of thin parallel lines. The appearance of directionality increases when the material 266 V P Sardana and D A Pikle "fable I. Material characteristics of ferritic stainless steel and other coinage materials. Ferritic 70°,, Copper stainless Property + 30% nickel Aluminium steel Hardness Rb = 40 Rockwell Rh 75 85 H = 45 ",, Elongation 36 35 22 Coinability Good Good Good Corrosion resistance Good Good Good Life 1.~ unit I unit 3 units is elongated. The roping lines are seen in the rolling direction. Although the appearance is ridge-like, the thickness is uniform. Except for appearance, this does not affect the performance of the material. The roping effect depends on the composition as well as the processing parameters. It is observed that material which ensures total ferritic structure has more roping tendency.