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Cold Forming Stainless Steel

FABRICATION - COLD FORMING

All Columbus Stainless Steels supplied in the annealed condition (No.1, HRA, 2D, 2B or BA) can be cold formed by any of the conventional processes. These processes include blanking, bending, piercing, roll forming, coining, embossing, pressing, spinning, flow forming and deep drawing. In general the equipment such as presses, press brakes, guillotines, etc. used for carbon steels can be used for stainless steels. However, as more power is required to work stainless steels, the capacity of the equipment is effectively reduced, eg. Guillotines and press brakes that are rated up to 6mm thickness for carbon steel are restricted to 4mm for stainless steels. In operations where austenitic grades are cold worked, eg. Pressing and deep drawing the capacity of the equipment can be effectively reduced by up to 60% due to rapid work hardening characteristics of these materials.

In addition to the extra power requirements to form stainless steels, greater demands are made on the form tooling. Stainless steels not only have greater strength than carbon steels but work harden more, cause more wear, are susceptible to pressure welding, exhibit more springback and have lower heat conductivity. As a result of this, tool materials must be harder, smoother and better designed than tooling for carbon steel.

Higher grade tool steels are required and frequently tungsten carbide or aluminium bronze are utilized. However, in the case of the latter these materials can be used as "inserts" in critical areas rather than the complete tool. In many cases tools designed for carbon steels are used for stainless steels, in such cases one must expect to make changes in "setting", use higher quality lubricants, have more frequent maintenance and reduced life.

As a result of the above, lubrication requirements are critical and high quality products are essential. Heavy duty pastes and emulsions, sulphurised or sulphurchlorinated oils, chlorinated oils or waxes are used depending on the forming operation. Mineral oils, soap solutions and general purpose soluble oils are not usually used. Ease of removal after forming is also a consideration since all traces of lubricant must be removed prior to subsequent heat treatment or before putting into service. Surface contamination can increase finishing coasts and adversely affect corrosion resistance. In some cases, especially pressing or deep drawing of ferritic grades, material is PVC or PE coated prior to forming. This not only acts as a very effective lubricant, but increases the formability, prevents galling and/or pressure welding and minimises damage to the surface finish.

The following is a summary of the differences between carbon steel and stainless steel for some of the main cold forming processes.

BLANKING AND PIERCING

Due to its higher shear strength, more power is required along with greater tool wear when working with stainless steels. This can be minimised by using angular shear on the punch and die or by punch staggering during multi hole operations.

With regard to the die materials, these should be both hard and tough, for intricate jobs shock resistant tools must be used. These materials include D2, D4 and carbide (HRC 60-65) and S1 or S5 (HRC 57-60) for the latter. Clearance between punch and die has always been a contentious issue when blanking thin materials. Some manufacturers recommend clearances below 0.025mm per side, others prefer more generous clearances. However, for cutting plate the general rule appears to be 10 to 15% of material thickness per side. The choice of clearance is always a trade off between degree of burring, risk of "shear break" and tool life.

A cut edge, especially a sheared edge is usually rough or burred, and the material is in the work hardened condition. The resulting burrs are not only dangerous for the operators, but can lead to damage and excessive wear to form tools. In addition the notch effect and the work hardened edge can reduce the materials ability to be formed in subsequent operation. Burrs can be removed by manual grinding or filing or by various automatic deburring equipment.

Usually blanking and piercing are done without lubrication. However, to prolong tool life or to minimise burr or shear break lubrication may be used. Sulphurised or chlorinated oils are usually used. However, care must be taken to ensure their removal especially if the parts are to be welded or heat treated.

PRESS BREAKING AND ROLL FORMING:

In both cases the equipment and tooling used for carbon steels can be used for stainless steels. However, there are several differences that need to be taken into account, viz.:

1. Due to its inherently higher strength and work hardening tendency, "springback" is much greater for stainless steels. Therefore, a greater degree of bending is required. This will depend on the grade of steel, its harness, thickness, the bend radius and bend angle.

2. When running both carbon steel and stainless steel on the same equipment, it is advisable to plan operations carefully to avoid carbon steel "pick-up" on the stainless steel. It is recommended that the stainless steel be run before carbon steels or the equipment be thoroughly cleaned when changing from carbon steel to stainless. When changing from stainless to carbon steel, cleaning is not usually necessary. In cases where the equipment itself is old and worn and there is a danger of "pick-up" from this source, it is recommended that the working surfaces be covered with material such as PVC or PE.

3. Lubrication is not normally required, except in cases where galling and scoring occur. In such cases heavy duty emulsions can be used.

4. Prolonged working of stainless on equipment designed for carbon steel may lead to abnormal tool wear.

COINING AND EMBOSSING:
These operations are usually done on the softer austenitic (304, 304L) and ferritic grades (409, 430). Again due to the higher hardness, harder, stronger dies than those required for carbon steels must be used. Lubrication is essential, heavy duty pastes and emulsions are generally used. Due to the rapid work hardening, interstage annealing may be required if multiple operations are involved. For example, in the manufacture of coinage, material is blanked, coined and annealed prior to embossing.

SPINNING AND FLOW FORMING:
For the sake of simplicity we will define spinning as the process whereby a circular blank is centred and held against a form block in a rigidly built lather. The part is rapidly rotated and pressure is applied to the metal surface by means of a spinning tool. This induces material flow, which is accompanied by limited thinning of the material and cups, cones, etc. are produced. The process can be both manual or automatic. Flow forming is a similar process but in this case extensive thinning of the material (over 50%) is achieved. Due to the loads required this process is either fully or semi automatic. Manual spinning due to its low tooling costs is extensively used to produce small quantities of any given part. Flow forming on the other hand due to its higher tooling and equipment costs is used for a larger number of a smaller range of items. Figure 4 and 5 details arrangement and tooling for manual spinning.

Flow forming generally requires bigger, more robust and sophisticated purpose built lathes and tooling. These processes can be used to produce items from as small as
Domestic mixing bowls to dished ends several metres in diameter. The simple tooling used in spinning can be used for a range of materials including stainless steels. In flow forming the chucks are usually manufactured from cast iron or steel. Other tools are used not only to flow, but to flange, bevel, trim, etc. and the materials used will depend on the function of the tool.

The most common lubricants used in spinning are adherent bar soaps and waxes. These must be used to reduce friction, galling and tool drag. In flow forming, it is important that the lubricant has a coolant action and that specialised proprietary brands are used.

All austenitic grades can be formed by both processes, but the lower work hardening types 304, 304L and 304DDQ can be spun to greater reduction, before intermediate annealing becomes necessary. The ferritic grades are more readily flow formed than manually spun.

Generally, parts produced by the processes have rough surfaces with characteristic spiral or helical grooves. Unless being used for non aesthetic applications, the parts require extensive finishing operations t make them smooth and bright.

PRESSING:
Press forming is an operation where a blank is pressed between a set of shaped dies to produce a finished or semi-finished part. Presses can be set to produce a single part from a single set of dies in one operation, or a complex part produced in a series of operations in a progression tool. Due to the capital costs of the equipment and tooling, presses are usually employed for mass production. Again, as in most other forming processes, the inherent high strength of stainless steel increases the power requirements - up to 60% more force may re required than for carbon steel.

Due to the above, the tooling used in press forming stainless steels wear out faster and are more susceptible to fracture as a result of long runs at high loads. High strength tool steels such as D2 are used as they offer good resistance to shock and wear. However, in cases where galling and pressure welding is prevalent aluminium bronzes are used, and in cases where high wear is present, carbide is used. In practice, it is quite common for a tool to be subjected to the above factors. In such cases compound tools are made in tool steels with carbide and/or aluminium bronze inserts in critical areas. Care must be taken to allow for factors such as "springback" in design of tools. Also due to their ductility, stainless steels (particularly the austenitic grades) tend to be susceptible to wrinkling when subjected to compressive force. If a particular type of pressing is susceptible to this phenomenon, then a blank holder or pressure pad may be incorporated into the tooling. Figure 6 illustrates the above.

With regard to lubrication, chlorinated or sulphurised oils, pigmented pastes, waxes and soaps are used. In high speed operations, heavy duty emulsions are used due to their superior coolant effect. In some cases PVC and PE coatings are used, not only as lubrication, but also to protect the surface finish.

All the Columbus Stainless steels supplied in the annealed condition can be press formed, although the performance of the ferritic grades can be enhanced by PVC or PE coating.

DEEP DRAWING:
Deep drawing is a process by which flat sheet metal is formed into a cylindrical or box shaped component by means of a punch and draw ring or die. These actions are usually performed in a double action press with a blank holder to prevent wrinkling in the flange as metal is pulled into the die (see Figure 7).

Initially the workpiece is stretched over the punch nose as it moves into the die. This stretching continues until the force is sufficient to overcome the force exerted by the hold down ring which is controlled to allow the material to compress and flow into the die. The major mode of deformation changes from pure stretching to drawing at this stage, and is accompanied by an increase in material thickness. This thickening can be as high as 40% for austenitic grades and 10-15% for ferritic grades.

The mechanics of the drawing process are extremely complex and the drawing characteristics of stainless steels differ considerably to those of carbon steels. Some of the factors involved in the process are listed below:

1. Equipment used for deep drawing stainless steels should be up to twice as powerful as that used for carbon steel.

2. Double action hydraulic presses are preferred, single actions can be used if external "cushion" pressure can be applied. Mechanical presses can be used in they can be run at slower speeds than used for carbon steels.

3. Tool materials are similar to those used in press forming, viz. D2, aluminium bronzes and carbide. Due to the large masses of such tools, extensive use is made of cast iron as a backing material, with the more expensive material used as inserts in the appropriate areas (especially in the draw ring). The choice of material for the hold down ring is not usually critical, and cast irons and steels are usually used.

4. Die clearances are similar to carbon steel for ferritic grades (blank thickness plus 10-15%); for austenitic steels clearances are much greater blank thickness plus 35-40%).

5. Draw ring radius should be 4 to 8 times material thickness. If it is too small it can cause fracture of the material and if too large increases tendency to wrinkling.

6. Punch radius should be 4 to 6 times material thickness, again if too small it leads to tearing and if to large it leads to wrinkling.

7. The limiting drawing ratio, i.e. the ratio of blank diameter to punch diameter is between 2.1 and 2.2 for stainless steels compared to 2.15 and 2.5 for carbon steels.

8. Lubricants are selected based on two factors, the most important being to provide a stable film between the workpiece and the tooling to prevent welding galling and seizing as well as reduce friction. Secondly, it must be readily and completely removable after the drawing operation.

9. Lubricants include chlorinated or sulphurised oils and waxes; pigmented pastes; heavy duty emulsions and drawing soaps. PVC and PE coatings are sometimes used, especially with ferritic grades, as they improve formability and protect the surface finish.

10. Stainless steels can be re-drawn after initial drawing. This process is used either to reduce the diameter and increase the height for the full length or for a specified length or to decrease the radii and/or produce flat bottomed components.

11. A spinning operation is sometimes carried out on a drawn component to reduce the "thickening" effects and increase the overall wall height.

12. As a general rule stainless steels cannot be re-drawn to the same extent as carbon steels without the inclusion of interstage annealing.

Columbus produce two materials, viz. 304 DDQ and 430 DDQ specifically for deep drawing. When these materials are ordered, it is advisable to state the exact end use so that the material most suited to the process can be supplied.

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