STAINLESS STEEL INFORMATION SERIES

COLD FORMING OF STAINLESS 1400 STEEL 1050 40 Cold forming stainless steel is generally different to processing low-alloy and % ELONG plain carbon (mild) steels, primarily 700 30 because stainless steels are stronger, MPa harder and more ductile, work harden more rapidly and must maintain their 350 15 inherent corrosion resistance. These characteristics necessitate greater power requirements, an allowance for a greater 0 0 wear rate of processing equipment and 10 20 30 40 the application of working procedures GRADE 430 % COLD WORK that will avoid surface damage and GRADE 301

contamination or impairment of FIGURE 1 Effect of cold work on mechanical properties corrosion resistance. The grade of stainless steel being processed will FORMABILITY OF 200 series. The higher nickel grades generally dictate the type of process to STAINLESS STEEL such as 304DDQ and 305 (10-13% Ni) be used. AUSTENITIC STAINLESS STEEL work harden the least of the austenitic Specific grades of stainless steel are These are the chromium nickel (Cr/Ni) grades. The lower work hardening rate usually chosen on the basis of specific and chromium, nickel manganese (Cr/ is characteristic of a more pronounced inherent properties such as corrosion Ni/Mn) stainless steels the 300 and 200 convergence of the ultimate tensile or heat resistance, strength, ductility series respectively. stress and yield stress curves with cold etc. The response of the steel to work Nickel (Ni) and manganese are the work. This convergence means that hardening and the subsequent effect alloying elements which promote the total deformation prior to fracture on the mechanical properties will play the formation of, and stabilise, the would be less than for a work-hardening a significant part in selecting a steel for austenitic crystal structure. The higher grade such as grade 301. A reduction formability. The formability of a steel the Ni:Cr ratio the more stable the in thickness is hence not so effectively is largely determined by the rate at austenite, and hence a difference in countered by a higher developed which the yield strength approaches the mechanical properties and response to strength in the location of the thinning. ultimate tensile strength as the material work hardening both of which in turn A lower yield stress, as with grade 305, is cold worked. affect the formability. means that much less force is required to Figure 1. shows a narrowing of the Grade 301 has the lowest nickel initiate deformation. band between the yield strength and (6.5%Ni) content of the austenitic stain- The cold forming operations used the ultimate tensile strength curves as less steels and has thus the highest rate for austenitic stainless steels are the the material is cold worked indicates of work hardening. Although having a same as those used for ferritic stainless that the formability is limited for grades fully austenitic microstructure in the steels. However, the forming conditions such as 430. The narrowing shows annealed condition the lower nickel differ. The austenitic stainless steels are that most of the available yielding is content results in a greater proportion of capable of greater deformation due to expended and any further deformation martensite being formed during plastic their high ductility and thus a greater will result in rupture. On the other deformation which helps the metal resist amount of reduction in a single given hand, for steels that do not show a necking and assists in a more uniform operation can be tolerated. Among the great convergence such as grade 301, deformation. An advantage of the high austenitic stainless steels, the greatest a tendency for greater work hardening work-hardening rate is that appreciable deformation in a single operation can is indicated and, together with a higher increases in strength and stiffness can be withstood by the more rapidly work- ductility for the same degree of cold be achieved which are useful for cold- hardening grades such as 301 or 304. The work, will permit severe deformation formed structural sections. Similar formability of cold-worked austenitic during forming. properties are seen in many of the stainless steels is adequate to permit

information series 6.indd 1 20-Feb-17 10:58:14 AM cold forming operations without prior and the lower elongation means less to stress corrosion cracking. They have annealing. However, during a sequence plastic deformation can be tolerated a two-phase (duplex) microstructure of operations, the extent of cold work prior to fracture. Although higher consisting of about equal proportions of achieved may necessitate intermediate initial deformation forces are required, austenite and ferrite. annealing, to return the steel to its the force/load does not need to be Duplex stainless steels can be readily original ductility increased as deformation progresses, cold formed by the same methods used The better formability of the austenitic because ferritic stainless steels do not for austenite stainless steels. stainless steels is particularly apparent work harden to the same extent as the Duplex stainless steels have higher in such processes as stretch bending austenitic stainless steels. The poor proof strengths than the conventional where a greater tensile deformation will notch ductility of the ferrite stainless austenitic stainless steels and therefore be sustained, and in severe drawing steels in heavy sections requires that more power is required to initiate operations where a high ductility is the speed at which the load is applied forming. When forming grades such as required. will have to be slower than for low alloy 2101, 2304 and 2205, the capacity of a However, because of the higher or plain carbon (mild) steels. Ferritic press brake will be reduced by 50% when annealed strengths and response to work stainless steels will tend to fracture compared to austenitic stainless steels, hardening, greater forces are required under shock loads at low temperatures. although once the yield stress has been for austenitic stainless steels compared As can be seen in Figure 1, the yield attained, the duplex stainless steels flow to ferritic stainless steels. Not only are strength for grade 430 converges as easily as the austenitic stainless steels. higher deformation forces necessary, rapidly to the ultimate tensile strength Because of the higher proof stress but the initial force must be increased as as the cold work progresses. Since of the duplex stainless steels, greater the metal deforms to accommodate the the yield point must be exceeded for springback can be expected. Over- effects of work hardening. plastic deformation (and hence for cold bending by approximately 10% on a In general, the austenitic stainless forming) to occur the close convergence 90° bend should compensate for this. steels are more difficult to form as the of the yield and ultimate tensile stresses Hydraulically operated presses are nickel (Ni) content decreases, as in is conducive to rupture. This response is preferred. grade 301 (approximately 6.5% Ni). The typical of the ferritic stainless steels. This Duplex stainless steels such as 2101, presence of stabilising elements such as effect, plus the rapid drop in ductility 2304 and 2205 require larger inner titanium (Ti), niobium (Nb) and tantalum with increasing cold work, necessitates bend radii — typically 3 to 4 times (Ta) as well as higher carbon (C) contents the use of fully annealed sheet together plate thickness. Severe bending should have an adverse effect on the forming with intermediate annealing, where always be carried out transverse to the characteristics of the stabilised grades. necessary, during processing. rolling direction due to directionality of This is due to the formation of second Severe bending should always be the as-rolled microstructure. phase particles in the microstructure carried out transverse to the rolling Where heavy cold forming has been such as titanium carbides, carbo-nitrides direction due to directionality of the as- done, consideration should be given to etc. Forming of grades 321 and 347 is rolled microstructure. heat treatment — particularly if severely thus less favourable than grades 302, 304 The decreasing ductility with corrosive service conditions are to be and 305. increasing work of the ferritic stainless expected. The 200 series austenitic stainless steels requires more inter-annealing steels (i.e. those in which nickel (Ni) is steps than is necessary for plain carbon POWER REQUIREMENTS partially replaced by manganese (Mn) (mild) steels when spinning or roll Power requirements for forming require more power due to their greater forming. Nevertheless, grades such stainless steels, mainly because of the initial strength and a high response as 409, 441 and 430 are often used for high yield strength, are greater than for to work hardening. They also suffer a applications that require forming by equivalent thicknesses of low alloy and greater degree of springback than the blanking, bending, drawing or spinning. plain carbon (mild) steel. equivalent 300 series. As a general rule, approximately twice MARTENSITIC STAINLESS as much power is required for forming FERRITIC STAINLESS STEELS STEELS stainless steels. Not only is more power These are plain chromium stainless These are plain chromium stainless required initially, but because the steels which have low carbon (C) steels which have relatively high carbon austenitic and duplex stainless steels contents (< 0.1% C). They are contained (C) contents (0.15-1.2% C). They are also work harden rapidly, increasingly more in the 400 Series. contained in the 400 Series. power is required as forming proceeds. The mechanical properties of the The forming characteristics of grades Most of the ferritic grades behave in a ferritic stainless steels compared with 403, 410 and 414 (the lower carbon similar manner to the carbon steels with the plain carbon (mild) steels indicate grades) in the fully annealed condition regard to work hardening, although that different cold forming methods are may very similar to those of the ferritic more power is required to initiate the required for these materials. stainless steels. forming process, due to the higher initial The higher yield strength of ferritic The remaining martensitic grades yield strength of the ferritic stainless stainless steel implies that more with higher carbon contents are not steels. power is required for a given amount recommended for cold forming. of deformation; the high ultimate LUBRICATION tensile strength indicates that higher DUPLEX STAINLESS STEELS The lubrication requirements for loads can be applied before rupture; These steels have excellent resistance forming stainless steels are more

information series 6.indd 2 20-Feb-17 10:58:14 AM TABLE 1 a permanent deformation of the workpiece, the metal on either side of the neutral axis suffers either compressive

FATTY OILS FATTY SOAP-FAT PASTES WAX-BASED PASTES DUTY HEAVY EMULSIONS C FILM SULPHURIZED OR SULPHO- OILS CHLORINATED HIGH VISCOSITY CHLORINATED OILS LOW VISCOSITY CHLORINATED OILS GRAPHITE MOLYBDENUM DISULPHIDE or tensile plastic deformation. Refer to

PRESS BRAKE Figure 2. FORMING On either side of the neutral axis, plastic

PRESS FORMING stresses remain in those parts of the material that have passed the elastic MULTIPLE SLIDE limit (yield point) and those parts lying FORMING nearer the neutral plain that have not DEEP DRAWING reached the elastic limit may have residual elastic stresses. These residual SPINNING elastic stresses cause “springback”, i.e. CONTOUR ROLL the tendency of a deformed piece of FORMING metal to return partway to its original shape. EMBOSSING Springback increases if: The elastic limit of the material TABLE 1 BEST GOOD NOT RECOMMENDED increases (due to cold work - refer to the austenitics). The plastic strain increases (due to critical than in forming plain carbon carbon can diffuse into the steel and the increased amount of plastically (mild) and low alloy steels. cause sensitization, with subsequent deformed material in the workpiece). This is due to the necessity of impairment of corrosion resistance. The amount of plastic strain involved preserving the high quality surface on Table 1 lists the suggested lubricants depends on both the thickness of the stainless steels. In addition, stainless for a particular forming process. workpiece and the radius of the bend, steels have higher strength, greater Mineral oils, soap solutions and not the total bend angle. Thus the hardness, lower thermal conductivity water emulsions or general amount of springback per degree of and higher coefficients of friction. purpose soluble oils are omitted bend increases as the ratio of bend Galling and scoring may also be since they are ineffective in most radius to sheet thickness increases. expected, especially where high stainless steel forming operations. There will therefore be less springback pressures are to be employed. Regardless of the operation being in a given thickness sheet when it is bent The selection of a particular lubricant performed, selection of a lubricant through a small radius than when it is for a forming process is based on two can affect both quality and cost of the bent through a large radius. Springback primary considerations: formed product. The lubricant must can be controlled by reducing the punch the lubricant must be efficient ie: to have the properties required for the job, radius or by slightly overbending. assist in the forming process and but use of a heavier duty lubricant than minimise scrap and excessive is required is poor economy because of BEND RADII scoring. the cost incurred in its removal. The smallest radius for any given metal the lubricant must maximise the that can be formed without cracking is tool life, which is determined by SPRINGBACK called the minimum bend radius. It is wear and pick-up. In any bending operation that produces generally found that the minimum bend In addition, a suitable lubricant is radius increases almost in proportion to not determined solely by the particular the sheet or strip thickness. forming operation, but also by other In extremely soft metals or ductile factors such as material and tool surface steels such as annealed austenitic roughness. stainless steels, it may be possible to For medium and severe forming, a form a bend with zero radius when the pickled surface after cold rolling and seams are bent through 180° to form an annealing (i.e. 2D finish) has been found interlocking joint along the edges of two to be the most suitable. The relatively sheets. This can be done provided the rough surface retains lubricants better tool edges are not sharp, otherwise the than a smooth or polished surface and die will cut the steel while forming the lubricant is thus drawn in between the bend. contacting surfaces. Generally, for annealed material, a It is also important to consider the bend radius (R) equal to the material ability and ease of removal of the thickness (T) will meet most engineering lubricant after forming, particularly if requirements. Cold worked material the part is to be subsequently exposed however, will require larger bend radii, to high temperature (eg: an annealing eg: or stress relief heat treatment). If FIGURE 2 1/4 Hard tempers R = 1 - 1.5T lubricants are not totally removed, 3/4 and Full Hard tempers R = 3 - 6T

information series 6.indd 3 20-Feb-17 10:58:14 AM The bend radius selected must be done to press brake forming, but like the fracture because of the greater forces with the grade of steel in mind ie. some austenitic stainless steels require more employed. steels such as duplex stainless steels, power to initiate the bend. Die materials used for long service in require larger radii. For ordinary press brake forming, mass production include high strength lubricants are generally not used, aluminium bronze, high quality tool FORMING PROCESSES although the use of lubricants or pvc steels and carbide tools. A brief overview of some different films may reduce scoring on sensitive The austenitic stainless steels are forming processes is given below: finishes. generally more easily press formed than other grades of stainless steel. FIGURE 3 Their work hardening characteristics which result in higher strength are often used to advantage in press formed components, the greater stiffness giving an improved resistance to fatigue. Stainless steels, and the austenitic grades in particular, have high ductility but are susceptible to wrinkling in press forming if the workpiece is stressed in compression. It is therefore recommended that in terms of metal flow, the part be stretched rather than compressed. Press forming of the ferritic stainless steels can be improved by slightly warming the blanks to about 120-200°C. At this temperature the metal is more ductile, so less power is required for FIGURE 4 forming and, in addition, the tendency for the workpiece to crack is lowered. Chlorinated lubricants are often used in press forming because the viscosity and activity can be adjusted over a wide range. These are also easily removed with solvents and degreasers. A typical press-formed austenitic stainless steel component is illustrated in Figure 5 (on the following page). The severe forming induces work hardening that will increase the rigidity and fatigue resistance.

SPINNING PRESS BRAKE FORMING The bracket shown in Fig. 4 is typical of Stainless steel components such as cups, Stainless steel sheet, strip and plate press brake forming. The punch angle in cones and dishes can be readily formed of virtually all types and tempers are the second operation has been reduced by manual or power spinning. Again, formed in press brake equipment. This to set the final angle by overbending. more power is required than for low method of forming is usually limited carbon (mild) steel. to small quantities and relatively PRESS FORMING Manual spinning can be carried out simple parts. Press brakes are generally Stainless steels are press formed with the same equipment as that used for GRADE MAX GRADE MAX equipped with standard or sharp Vee STRETCH(%) STRETCH(%) dies, as shown in Figure 3. plain carbon (mild) steel. However, 301 25 316 35 Springback is controlled by adjusting more power is required. 302 40 316 35 the angle of the dies. The ram force is approximately 60% All of the austenitic stainless steels in greater than that for an equivalent mild 304 40 410 20 the annealed condition can be bent 180° steel workpiece. 305 45 430 30 over one stock thickness (1T), but need Press frames must have sufficient up to 50% more power to form than bulk and rigidity to withstand the on all the 300 series stainless steels. plain carbon (mild) steels. Springback higher forces. The lower work hardening grades such must be allowed for as it is more severe Alternatively the press capacity must as 304 and 304 DDQ enable greater in the austenitic stainless steels. be de-rated accordingly. Dies used for reductions before inter-annealing The plain chromium (400 series) press forming stainless steels wear becomes necessary. All annealing stainless steels vary in their response out faster and are more susceptible to operations must be followed by

information series 6.indd 4 20-Feb-17 10:58:15 AM FIGURE 5 to make it smooth and bright. The roller usually imparts a helical or spiral groove to the surface of the workpiece which may detract from the aesthetic appeal of the finished component. Lubricants are used to reduce friction, minimise galling and tool drag, and also to provide cooling. For manual spinning, firmly adherent lubricants are preferred, whilst for power spinning the cooling action of the lubricant is more important. Chlorinated or sulphurised lubricants pickling to restore the clean, smooth are usually avoided as they are difficult FIGURE 6 surface and to remove any carbon steel to remove and traces left on the steel contamination. could have harmful, corrosive effects. The approximate maximum limits of stretch in manual spinning are: THREE-ROLL FORMING The amount of stretch is not There are different configurations of necessarily uniform over the entire three-roll forming machines but the component, as it varies with the shape principles are similar – see Figure 7. of the form. A second stretch after The position of the top roll is fixed annealing of approximately 8% less than while the bottom roll adjusts vertically the initial stretch can be attained. to compensate for material thickness. A typical sequence of operations for The rear roll is adjustable angularly manual spinning of a cone is illustrated and this determines the diameter of the in Figure 6. The process starts with the cylinder formed. Usually two rolls are drilling of a hole in the centre of a blank driven although in some machines all for location of the centre mounted on three rolls can be driven. a tailstock. The blank is then mounted Springback can be a major problem on a wooden (or metal) mandrel and during rolling of the austenitic and the forming process accomplished by duplex stainless steels due to the large applying pressure to the surface with a radii involved and work hardening of bar or roller-type spinning tool. the steel during the forming process. It The 400 series ferritic stainless is therefore advantageous to have the steels, because of their relatively low equipment set up so that the desired ductility, do not lend themselves to curvature can be achieved in one pass. manual spinning. The high pressure Lubricants are not generally used in of the forming tool causes severe local this type of roll forming. deformation of the workpiece resulting in early thinning and fracturing. CONTINUOUS ROLL Power spinning of the 300 and 400 FORMING series stainless steels is readily carried This is a process used for production out with the low work hardening types of long lengths of formed sheets, being superior. Much larger reductions such as roof sheeting, cable racking, of type 430 can be made by this method components for rail carriages, tube of spinning. FIGURE 7 forming, etc. The process involves the Cracking at the edge of austenitic passage of the steel from a coil through stainless steel blanks during spinning a number of rotating rolls, each forming is a major problem. This is due to the the material gradually in succession localised work hardening and minute until the final form is achieved. imperfections induced in the cut edge. Most of the grades of stainless steel Dressing the cut edge by grinding prior can be roll formed successfully, but to spinning will overcome the problem. care should be taken with the duplex Cracking and distortion can be prevent- and ferritic grades as directionality ed by keeping a narrow flange on the in the rolling direction can result in workpiece. This flange can be trimmed cracking if tight bend radii are used. off in the last operation if it is incompat- As with many of the other forming ible with the component design/ operations, tool wear and scoring of function. the stainless steel surfaces are an issue, The surface of a power-spun form is and correct selection of tool materials rough and requires extensive finishing and lubricants is critical.

information series 6.indd 5 20-Feb-17 10:58:15 AM STRETCH FORMING Stretch forming is the forming of sheet, bars and rolled sections over a block of the required shape while the workpiece is in tension. The workpiece is stretched just beyond its yield point (usually 2-4% elongation) to retain the contour of the form block.

EXPLOSIVE FORMING Explosive forming is a method of changing the shape of a metal blank by the instantaneous high pressure that results Stainless steels may not behave the STRETCH DRAWING same in a high-energy, high rate forming method as when formed at a slower, more conventional rate. In some instances, The amount of metal moved certain grades of stainless steel can be determines the severity of the forming accurately formed into intricate shapes process and is usually referred to as without cracking by explosive forming, the number of diameters in the upset. whereas the same grades would crack if In cold-heading, a diameter is a length subjected to the equivalent severity in of wire/rod equal to its diameter and conventional processes. The converse hence the severity of an operation is may also be true. measured in diameters, or the volume of metal deformed to the length of rod TUBE BENDING AND deformed. FORMING With the exception of the free Equipment similar to that used for machining grades such as grade 303 and carbon (mild) and low alloy steels the higher carbon martensitic grades is used for stainless steels although, such as 420 and 440, virtually all of the again, greater power is needed. 300 and 400 grades can be headed to a austenitic stainless steel tubes can be severity of 1.5 diameters in one or two EXPLOSIVE FORMING bent to a centre-line radius of 1.5 times blows. the tube diameter. As the ratio of the In two or three blows, most grades can tube diameter to wall thickness (D:t) Tube spinning can be used to reduce be cold-headed to a severity of about 1.7 increases, it becomes more important the wall thickness of tubes and to increase diameters. More severe heading may to provide both internal and external their strength. Producing specific shapes require the blank to be heated. support to prevent wrinkling, buckling from tubing is a major function of tube For cold upsets as severe as 2.25 and flattening. spinning, eg. flanges can be spun at diameters, grade 430 is the only 400 Numerical controlled or robotic substantial savings compared with other series grade that can be considered. Of operations are regularly used for high processes such as machining. The lower the austenitic types, grades 304 and 305 volume tube bending operations, such work hardening grades such as 304 and are best suited to cold-heading. as in auto exhaust pipe production 305 are more suited to spinning. For mild upsets up to 1.5 diameters, Mandrel lubricants should be fairly ordinary machine oil applied to the die heavy, viscous oil-based lubricants with COLD-HEADING usually will suffice. For the most severe emulsifiers for easy removal. Very light This method of forming is used to form a upsets a solid lubricant such as copper chlorinated mineral oil can be used in head-like shape on the one end of a wire plating or dry lime on the stock together some bending operations between the or rod blank. Cold-heading is usually with machine oil is recommended. wiper die and the tube. associated with the production of nails, Tubing can be flared to increase the screws, bolts, rivets and other fasteners CONCLUSION diameter 25-30% if in the annealed or more elaborate parts such as ball The austenitic, ferritic and duplex condition. The diameter can be reduced studs, pinion shafts etc. Heading is fast, stainless steels can be readily formed by rotary swaging or increased by scrap free and produces strong, uniform in all of the conventional cold forming bulging or beading. Rubber punches are parts and is best suited to high volume equipment. often used for this purpose. production. The austenitic stainless steels, with their high ductility, can be pressed or formed into complex shapes. AUSTENITIC STAINLESS STEELS, WITH THEIR Careful selection of lubricants and HIGH DUCTILITY, CAN BE PRESSED attention to the extra power require- ments will result in the achievement of OR FORMED INTO COMPLEX SHAPES. uniform, high quality products. SS

information series 6.indd 6 20-Feb-17 10:58:15 AM