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. High hot-rolling temperature promotes roping, whereas two-stage rolling reduces the same. Roping can be minimised by high temperature normalising treatment. This, being an additional operation adding to the cost of production, is resorted to only if the end use demands low roping value. Roping is quantified by way of comparison and indicated by roping index number.

4.4 Adt'antages of ferritic stainless steel Ferritic stainless steels have definite advantages as a coinage material. These are both economic and technical. The prices of various coining materials over the years can be seen in figure I. The cost of ferritic stainless steel is about one-seventh that of cupronickel, Besides, its cost over the years has been stable as compared to that of nickel, copper and aluminium. Inflation in cost has been 32~o over the last four years in case of nickel, 500~, in case of copper and -9.5°,o in case of 430 grade stainless steel. The good mechanical properties and the corrosion resistance of ferritic stainless steel increase the life of coins many-fold. Its hardness and wear resistance are almost twice that of copper and cupronickel, and thrice that of aluminium. Comparison between the various materials used for coinage is made in table 1. Stainless steel has been used as coinage material in some countries and the relief obtained has been good. Selection of proper forming tools overcomes the slightly higher hardness of the material as compared to that of non-ferrous materials. The increased cost of tooling is, however, offset by the very high life obtained with these coins. Ferritic stainless steel is free of nickel. Nickel and copper are imported into the country whereas ferrochrome is available in plenty. Ferritic stainless steel is available in sheet and strip forms in accurate thicknesses and widths, thus reducing some of the steps in the conventional manufacture of coins. Being a mass production item, its availability is assured. Ferritic stainless steel has a good bright appearance, and being hard and corrosion resistant, maintains this appearance for a long time. Due to its high wear resistance, Ferritic stainless steel - A coinage material 267

COMPARATIVE PRICES OF COINING MATERIALS

120

110 1

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_z 7O

~-X 60 ~" SO a. 40 _u 3O E 2O 10 0 1981 1982 1963 196& lgss YEARS l/fl$30 $S STRIP J"%~JCOPPER CATHODE HI ORIQUETTE

Figure I. Comparative prices of coinage materials.

it also retains the relief on the surface intact. The modern highly sophisticated mass production techniques ensure uniform, reproducible and high levels of quality, both in terms of appearance and durability. Stainless steel is a high temperature melting alloy and loses chromium during melting. It is also hard. These properties make counterfeiting difficult and unattractive.

5. Trials for coinage material at the Salem Steel Plant

5.1 Basis of trials Although ferritic material has been used for coinage in different countries, the composition selected is generally kept secret for obvious reasons. Some compositions were however obtained from Indian Mint, Canadian Mint and Japanese steel suppliers. These are indicated in table 2. It can be seen that for coining purposes, steel with slightly higher chromium (17"5-18-5~) and lower carbon (0.025~) contents is generally used. To study the effect of composition and also to gain some experience in the processing of the material for coinage particularly from the point of view of low hardness and good surface finish, we decided to conduct trials with the normal ferritic material available with the plant. This material (designated sample 1) was then converted into coins in the Government of India Mint, Bombay. The second set of trials was made with material of higher chromium and lower carbon contents. This material was sent to the Royal Canadian Mint for coining trials and is designated sample 2. 268 V P Sardana and D A Pikle

Table 2. Chemical composition of ferritic grade coining materials.

Element

Material (from) C Si Mn P S Cr Ni

Standard AIS1 430 0.12 1.0 1-0 0.04 0-03 16-18 -- Initially indicated by Indian Mint 0.05 0-4 0.5 17.5-18.5 0-5 Usinor Inox. France 0.037 0.27 ------17.95 0.18 Nissho lwai, Japan 0.08 0.6 0"6 0.035 0.020 17.2-18.5 0.5 Royal Canadian Mint Canada 0.03 0.4 05 0.02 0.02 18+0.5 0.5 SSP sample I 0.06 0-4 0.23 0.021 0-005 16.18 -- SSP sample 2 0-025 0.26 0.27 0.019 0.002 17-62 0.17

Sample 1 Sample 2

4 x 1025 3 x 1025

Cold-rolled to 1 '31 mm Cold-rolled to 2-6 mm

Annealed (continuously) Ground at 850"C (10 m/min) 1 Cold-rolled to 1-31 mm Pickled Annealed (continuously) at Resquaring shear 850°C (12 m/min

Pickled

Skinpassed with 08-1 '0% elongation Figure 2. Processing sequence durmg trials.

5.2 Trials with normal material (sample 1)

The processing sequence is given in figure 2. Material supplied to the Government of India Mint, Bombay, was in the form of 80ram wide strips in cut lengths. Its chemical analysis and properties as tested in our laboratory are given in table 2. Strips supplied by the Salem Steel Plant (SSP) were checked for hardness by the Mint and found to be between VPN 138 and 146. The strip was then fed into the mechanical blanking press to be cut into blanks of 19mm dia for 25 paise coins. It was observed that though the blanking operation was quite satisfactory, the blanks had marginally more burrs as compared to cupronickel. The blanking tools were also getting blunt quicker. The yield at the blanking stage was of the order of 70%, which is very good as compared to 55% in the case of cupronickel for the same size coin. The variation in the weight of the blanks was 1% as compared to the 2.5% max prescribed. The Ferritic stainless steel - A coinage material 269

blanks were washed with detergent in a tumbling barrel to remove the oil used during blanking. The blanks were then rimmed in automatic rimming machines. Stainless steel blanks performed very well during rimming operations similar to those of cupronickel. The rejection at this stage was negligible. After rimming, sorting was done to get good blanks. The final step was that of stamping proper. Good blanks were fed into automatic coining presses. Shine and relief on the coins was good. Rejection was low amounting to 1-1.5~o only. The finished coins were getting slightly hotter as compared to cupronickel. The die material used was the same as that for cupronickel. This resulted in the die life being only one-fifth of that for cupronickel. The overall average yield with stainless steel material was estimated to be 659/o as compared to 60~o obtained on the cupronickel material presently used by the mints. The yield at the minting stage was higher in case of cupronickel.

5.3 Trials with special material (sample 2) If we compare the various specifications of the composition of the ferritic stainless steels used for coinage, we see that the normal material basically has its chromium content on the lower side and carbon content on the higher side. The composition obtained from M/s Nissho Iwai, Japan, however was with higher carbon. The next trials were conducted with low carbon and high chromium content. The processing sequence of the material is given in figure 2 and the mechanical properties obtained are given in table 3. As can be seen, the material turned out to be softer before skinpassing than the earlier one, but the roping effect was very prominent. The material was supplied to the Royal Canadian Mint in coil form after skinpassing. The material has been successfully coined. The surface has been found to be acceptable. The hardness after skinpassing was comparable to the normal 2D material.

5.4 Observations Both the materials with which trials were conducted were found to be good for coining. Low-carbon, high-chromium material, however, gave a lower hardness and

Table 3. Properties of ferritic stainless steel--AISl 430 (Annealed).

Property Std SSP sample 1 SSP sample 2

2D 2D 2B UTS N/mmz minimum 450 466 432 452 0-2% proof stress (N/mm) minimum 205 318 304 317 % Elongation (rain) 22 31 35 30 Hardness HRB {max) 88 75 73 75 Abrasion resistance Good .... Cold-forming Fair .... Weldability Fair ------Melting range C 1430-1510 ------Scaling temperature 8000 C ------Grain size 6-7 -- 8/8-5 8 Ra/Rt 0.08/2-5 -- -- 0.05/1.32 Roping index .... 3 270 I/ P Sardana and D A Pikte better surface brightness. Also, the low-carbon high-chromium material was more prone to the roping effect. This defect, however, diminishes in severity after the rimming and minting operations and does not affect the appearance of the finished coins. The second observation was that the high-chromium low-carbon material had more surface defects such as shells and slivers. This was possibly due to the fact that this material had been produced through the ingot route. There is no basic difference or difficulty in the manufacture of both these grades in the cold-rolling or finishing stages. Reproducible results in the matter of properties were possible. The grain structure showed a fully annealed crystallised structure of the ferritic matrix and finely distributed carbides.

6. Conclusion

Ferritic stainless steel is very attractive as coinage material by virtue of its low cost, long life, price stability, good appearance and easy indigenous availability. Though low-carbon high-chromium material gives a softer and brighter material, it is prone to higher levels of roping. A compromise is therefore necessary. The right choice of composition coupled with standardised process parameters can give reproducible and acceptable results, as seen in industrial trials carried out at the Salem Steel Plant,