Deformation processing of stainless steel

This report covers the state of the art of both primary and secondary fabrication methods for stainless steel. Methods currently employed for primary fabrication of these alloys include: rolling, extrusion, forging, and drawing of tube, rod, and wire.

Secondary metalworking operations are those processes that produce finished or semi finished parts of sheet, bar, or tubing using additional metal forming operations. The following secondary forming processes are discussed: brake bending, deep drawing, spinning, shear forming, drop hammer, trapped rubber, stretch forming, roll forming, dimpling, joggling, and sizing. Equipment and tooling used for the various operations are discussed and illustrated wherever possible.


PREFACE

This report on practices used to deform the stainless steels into useful shapes is intended to provide information that may be of use to designers and fabricators. Recommendations are considered to be reliable guides for selecting conditions, tools, and equipment for specific operations and the causes for many of the problems encountered are identified and precautions for avoiding them are mentioned.

The report summarizes information collected from equipment manufacturers, technical publications, reports on government contracts, and by interviews with engineers employed by major stainless steel fabrication companies in the United States. A total of 134 references are included, most of which cover the period since 1960.

This report is a summation of current information available on deformation processing of stainless steels. Reports of experience in forming stainless steel from various Government and industrial sources have been compiled and summarized to offer assistance and guidance in performing many of the deformation processes. More detailed information on many of the processes is available by consulting the extensive reference list.

Stainless steels find wide use in applications requiring high strength and good corrosion resistance at room temperature. The superior corrosion resistance of the stainless steels compared with carbon steels has made them very attractive in such applications as automobile trim and curtain wall panels where the surface of the material is exposed to the atmosphere for long periods of time. They are also used in applications requiring oxidation resistance at elevated temperatures. Aircraft skins made of stainless steel are used around hot areas of aircraft.

Stainless steel, sheet, strip, and plate are produced by rolling stainless steel. Wire, rod, and tubing are made by drawing. Both tubing and structural shapes have been produced by hot extrusion process and special shapes are made by forging. Sometimes the extruded shapes, such as tees and angles, are cold drawn to improve' tolerances and to achieve thinner webs. Stainless steel tubing is extensively used for piping in the petroleum and food processing industries as well as for hydraulic equipment and de-icing systems in aircraft.

Stainless steel rod and wire are used for fasteners and sometimes springs where corrosion or oxidation resistance is important. Stainless steels are formed by secondary deformation methods at room temperature. Brake bending, deep drawing, spinning, and other sheet metal forming processes are used in the manufacture of stainless steel parts throughout the metalworking industry.

The commercial stainless steels have been used extensively in the manufacture of parts resistant to atmospheric corrosion or chemical corrosion. The corrosion resistance of these alloys results in minimum maintenance of structures fabricated from them. In applications at slightly elevated temperatures, up to 600 F, stainless steels retain a significant fraction of their room-temperature strength. Consequently, they have been used for skins in high-speed aircraft.

All of the commercial stainless steels are iron-base alloys that develop their properties either through cold work, as with the austenitic and ferritic types, or through heat treatment as with the martensitic types. They may be grouped according to the final structure of the material as either austenitic, ferritic, or martensitic grades. In the annealed condition, the materials may be cold worked at room temperature. The austenitic types work harden very rapidly, however, and may require frequent intermediate anneals for severe forming. Working at elevated temperatures is generally not desirable due to the high cost of this operation. The hot-working temperatures are generally above the mill-anneal temperature for the stainless steels. Dimensional stability of most stainless steels during thermal cycling is comparable to commercial steels at room temperature, consequently no special thermal processes are required as with the precipitation hardenable stainless steels to assure dimensional control on large parts.

The purpose of this report is to summarize the present status of primary and secondary deformation processes for commercial stainless steels. Primary deformation processes are designed to reduce an ingot or billet to a standard mill product such as sheet, plate, bar, forging, extruded shapes or drawn rod, tube, or wire. Secondary deformation processes produce semi-finished or finished parts by additional forming operations on such primary shapes as sheet, bar, or tubing. Deep drawing, spinning, brake bending, etc., are typical secondary deformation processes.

All of the stainless steels have much higher tensile strengths in the annealed condition than carbon steels with the same carbon contents. They are work hardened by cold deformation processes to various degrees, some at a greater rate than carbon steels. The thermal conductivity of these materials is considerably lower than that of carbon steels, by as much as one-third; so is the electrical conductivity. The coefficient of thermal expansion is considerably higher. Most of the stainless steels have very good corrosion resistance provided the surface of the material has been properly cleaned and no iron from forming tools is left on the surface. The stainless steels with high chromium contents have good resistance to oxidation at elevated temperatures.

Stainless steels may be grouped by their metallurgical structure into ferritic, austenitic, and martensitic types. The ferritic and martensitic types are magnetic while the austenitic steels are not. Some of the austenitic types, upon cold working or being cooled to sub-zero temperatures, can transform partially to a martensitic or ferritic structure which makes them slightly magnetic.

The good ductility of stainless steel results from the austenitic structure. The face-centered- cubic structure of austenite is typical of very ductile metals such as gold, copper, nickel, and aluminum. As would be expected, the austenitic stainless steels are generally quite ductile, withstanding tensile elongations up to 50 percent.

Additions of chromium without nickel to iron will not maintain the austenite structure. The presence of nickel, which is a strong austenite stabilizer, causes the 18-8 grades of stainless steel to be austenitic at room temperature. In Type 200 stainless steels, some of the nickel is replaced by manganese which is also a austenite stabilizer. The replacement of nickel by manganese reduces the cost of stainless steel at the sacrifice of some resistance to corrosion under certain conditions.

Cold working of the austenitic stainless steels will cause some gradual transformation of the structure to ferrite which is magnetic. This tendency decreases as the nickel content increases. The original austenitic structure and mechanical properties can be restored by proper annealing treatments.

The presence of precipitated carbides in stainless steel reduces their resistance to corrosion, particularly to intergranular attack. The corrosion resistance is impaired bacause the formation of chromium carbide is accompanied by a depletion of chromium in adjacent regions. This undesirable condition can be avoided by suitable heat treatments.

There has been a strong trend toward the use of ferritic stainless steels in recent years because they offer a slight price advantage. The total alloy content of these steels is substantially less than equivalent austenitic stainless steels but they can be used when certain qualities of the latter grades are not essential. The ferritic stainless steels, which contain about 17 percent chromium and no nickel, form a thin adherent scale at temperatures as high as 1550 F and resist further oxidation. This characteristic permits these grades to be used in applications involving intermittent heating. Care should be exercised in forming material that has been heated intermittently because the ductility is impaired when the metal is cooled after being heated at a temperature of about 900 F for long periods. To restore ductility, the material should be given an annealing treatment at a temperature between HOOF and 1400 F.

The corrosion resistance of ferritic steels is more limited than that of the austenitic stainless steels but they are resistant to a wide range of reagents. The resistance is improved if the surface of the material is properly polished and cleaned. The ferritic stainless steels resist normal atmospheric conditions when dirt is not permitted to accumulate on the material surface. Ferritic stainless steels are generally lower cost than the austenitic grades since they do not contain as much alloy addition. They are widely used for auto trim and kitchen sinks. No aerospace applications were found.


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Deformation processing of stainless steel