The single most important thing to remember when fabricating a product from stainless steel is to ensure that the fabrication processes retain and do not compromise the intrinsic properties of stainless steel. Fabrication of all stainless steel should be done only with tools dedicated to stainless steel materials.
Stainless Steel Grades Explained
Tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolour the surface of the fabricated product. Tools and blades must be kept sharp and clearances, for example between guillotine blades, must be tighter than for carbon steels.
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Apart from the commercial reasons for choosing stainless steel as a material, like heat and corrosion resistance, another advantage is its fabrication properties. The austenitic grades in particular can be fabricated by all standard fabrication methods. Some techniques are more suited to particular stainless grades than normal carbon steels. This is well demonstrated for severe deformation procedures such as deep drawing.
Due to stainless steel tending to have higher strength and work hardening rates than carbon steel, some alteration to tooling and equipment may be required:
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Cold drawing can produce components in 301, 302 and 304 with tensile strengths in excess of 2000 MPa due to work hardening. Producing components at these strengths is limited to very thin sections and fine wires.
With increasing section thickness the amount of cold work required to produce these strengths also increases to the point that it cannot be done practically. This is due to the surface of stainless steel work hardening more rapidly than the interior of the material. Work hardening the entire cross section of larger diameters requires exceptional and impractical forces.
Austenitic stainless steel can be used to produce deep drawn components that require very high elongations and thus stainless steel is widely specified for the production of hollowware.
Unlike carbon steels, work hardening rates for stainless steel mean that more severe deformation is possible at slower forming speeds. For forming operations normally performed at high speed, like cold heading, it is recommended that the process is slowed.
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Most grades of stainless steel can be cut using standard cutting methods employed for other metals. The work hardening rate of stainless steel sometimes means heavier equipment and specialist blades or cutting edges are required. Consideration must also be given to changes in the heat affected zone when the cutting method generates high heats along the cut.
Common cutting methods include:
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Like many other fabrication methods for stainless steel, bending can be done using the same equipment used to bend other metals. A difference is that working hardening rates may mean a need for more rigid equipment and higher power levels. Equally, equipment capacity will be much lower than for Carbon steels.
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Round bar, flat bar, sheet and plate can be bent using a press brake, bending machine or ring-rolling. Due to work hardening, bending should be done quickly. Some over-bending will be required to counteract spring-back of the bend. The inside bend radius should not be smaller than the thickness of the material being bent.
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Tube bending is often done for architectural and other applications. Bending of stainless tube can be difficult unless done by persons experienced in the field. Rotary bending and hydraulic press bending can both be employed to bend stainless steel tube.
Centre line bend radii should not be smaller than twice the tube diameter.
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Most grades of stainless steel can be welded by all traditional welding methods. However, the weldability of different grades can vary considerably. Some austenitic grades are considered to be the most readily welded metals. Weldability is generally low for ferritic and martensitic grades. For some grades, such as 416, welding is not recommended at all.
Recommended filler rods and electrodes vary depending on the grade being welded.
A commonly held belief is that stainless steel is difficult to machine. Machining can be enhanced by correct grade selection and using the following rules:
Stainless steel like 303 and free machining grades have Sulphur included in the composition. This produces a marked increase in machinability but reduces both corrosion resistance and weldability. Over the years manufacturers of stainless steel bar have greatly improved the machinability of standard 304 and 316 grades. This has been achieved by careful control of compositions and production process variables that avoid the detrimental effect seen with Sulphur-bearing grades. This means the point has now been reached where many specifiers and machine shops have switched away from 303.
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Although stainless steel have excellent corrosion resistance, and are often selected for this property, proper finishing is required to maintain it. After any fabrication process that alters the surface condition of the material, the stainless steel needs to be degreased, cleaned and finished appropriately.
Finishing methods may include one or more of the following:
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Pickling uses an acid or mixture of acids to remove scale produced in high temperature operations like welding, heat treatment or hot working. Acids and procedures depend upon the grade of stainless steel being treated. A great deal of care must be taken during the pickling process and with disposal of the waste as the acids used include sulphuric acid, nitric acid and hydrofluoric acid.
Pickling also removes rust due to corrosion of the stainless steel or corrosion of contaminant iron and steel particles.
The scale is removed as it retards the corrosion resistance of the underlying stainless steel.
Pickling is commonly done using baths or “Pickling Paste”. Pickling paste is a specially prepared stiff paste of strong acids. In this form it can be applied to vertical or overhanging surfaces and localised areas. Pickling paste is often employed to remove post-weld discolouration.
These are strong acids and appropriate caution must be taken when handling them. The same applies with acids used in passivation.
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Passivation is a process used to remove any free iron contamination of the stainless surface. Iron in the form of elemental iron, cast iron, carbon steel, mild steel or other non-stainless alloys can cause problems with stainless steel. This material is normally deposited from tools or work surfaces during fabrication. The iron particles promote corrosion on the surface of the stainless steel. At it’s mildest form, the discolouration caused can be unsightly. More serious corrosion is isolated pitting corrosion at the point of contamination.
Passivation involves treatment of the stainless steel with nitric acid or a nitric acid and sodium dichromate combination to remove the iron contaminants.
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Stainless steel are readily polished and ground if standard techniques are slightly modified. A build up of material on abrasive media occurs due to the high strength of stainless steel. Low thermal conductivity means that there is also a build up of heat. The result can be heat tinting of the surface of the material.
Using low grinding and feed speeds combined with specifically selected lubricants and grinding media can alleviate these complications.
Corrosion resistance of stainless steel tends to increase with the extent of surface polishing.
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The reverse of electroplating is electropolishing. This is an electrochemical process that removes the peaks of the rough surface of a metal. Electropolishing smooths the material surface making it brighter.
The resultant finish is very corrosion resistant, hygienic and attractive. On some stainless steel the surface finish appears frosted rather than smooth and reflective.
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Any mechanical polishing procedures must be carefully done to ensure contamination by iron based materials doesn’t occur.
Sand blasting must be done with clean silica or garnet sand. Shot, grit and cut wire blasting must use stainless steel media of equal or greater corrosion resistance than the metal being cleaned. Barrel and vibratory finishing are often used to polish fittings and small parts.
Light heat tint can be removed by wire brushing but the brushes must be stainless steel and never be used on other materials.
Mechanically cleaned parts are not as corrosion resistant as pickled stainless steel. This is due to mechanical cleaning not removing all the chromium depleted material from the surface and the retention of some scale residue. Mechanical cleaning is often used to prepare the surface of stainless steel before pickling.
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Occasionally a highly polished surface is not wanted for stainless steel. In this case blackening is used to produce a non-reflective black oxide surface. Several methods can be used to achieve this finish.
Some treatments are proprietary but two common methods are immersion in a solution of sulphuric acid and potassium dichromate solutions or immersion in a molten salt bath of sodium dichromate.
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Stainless steel can be given a range of surface colours for architectural applications. These colours include bronze, blue, gold, red, purple, black and green. A range of shades can also be produced.
The colouring is done by a proprietary process that involves immersing the stainless steel in a hot chromic/sulphuric acid solution. This is followed by a cathodic hardening treatment in another acidic solution. The base material reacting with the hot acid produces a transparent film. Although the film is colourless, light interference imparts a colour to the layer. If the underlying metal surface is highly polished the effect will be a strong metallic lustre. For matt and satin finished stainless steel, the result will be a matt finish.
DISCLAIMER
This Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn. In particular, the mechanical property requirements vary widely with temper, product and product dimensions. The information is based on our present knowledge and is given in good faith. However, no liability will be accepted by the Company is respect of any action taken by any third party in reliance thereon.
As the products detailed may be used for a wide variety of purposes and as the Company has no control over their use; the Company specifically excludes all conditions or warranties expressed or implied by statute or otherwise as to dimensions, properties and/or fitness for any particular purpose.
Any advice given by the Company to any third party is given for that party’s assistance only and without liability on the part of the Company. Any contract between the Company and a customer will be subject to the company’s Conditions of Sale. The extent of the Company’s liabilities to any customer is clearly set out in those Conditions; a copy of which is available on request.
This information has been sourced, reviewed and adapted from materials provided by Aalco - Ferrous and Non-Ferrous Metals Stockist.
For more information on this source, please visit Aalco - Ferrous and Non-Ferrous Metals Stockist.
What Is Sheet Metal Bending？
Sheet metal bending involves using machines and tools to form metal into a specific shape.
This can be achieved through the use of a press brake, punching machine, ironworker, or other machinery.
These machines utilize a power system to drive the tooling and apply pressure on the metal sheet, causing it to deform.
To ensure accurate results in sheet metal bending, several parameters must be determined before the process begins.
These parameters include the material thickness, bending radius, bending allowance, bending deduction, K factor, and others.
It is important to keep in mind that different materials have varying properties such as tensile strength and ductility.
Different machines may use different bending methods to produce the same profile from a metal sheet.
Therefore, it's crucial to choose the right machine and approach based on the specific requirements and parameters of the project.
What Are the Methods of Sheet Metal Bending?
The sheet metal bending process results in different bending shapes based on the angle and radius of the bend.
To ensure precision in the bending process, standard bending methods are employed. These methods vary, but they all aim to produce uniform standards in the final profiles.
Let's take a look at some of the main bending methods in sheet metal bending:
V-Bending - This is the most common bending process and is named so because of the V-shaped punch and die used in the process. The punch presses the metal sheet into the lower die, resulting in a V-shaped workpiece.
Roll Bending - This process is used for bending workpieces with large curls and involves the use of three rolls driven by a hydraulic system to bend the sheet.
U-Bending - This method involves using a U-shaped die to bend the workpiece. The punch is powered by a system to press the metal sheet into the U-shaped die, resulting in U-shaped profiles.
Rotary Bending - This method can bend the workpiece with a degree of more than 90. The final profile is similar to a V-bend, but the profile surface is smoother.
Edge Bending - This method is used in panel bending and involves the use of upper and lower molds that move up and down for bending. It's usually used for shorter metal sheets to reduce sharpness and prevent damage to the bending edge.
Wipe Bending - This method is similar to edge bending. The metal sheet is placed on the lower die and pressure is applied to the protruding metal by a pressure pad and punch, resulting in bending."
What Materials Are Fit for Sheet Metal Bending?
The choice of material for bending is crucial for achieving desired bending results.
Some materials may not be suitable for bending and could result in fracture or damage to the tooling. Materials with low ductility can be heated to reduce the risk of fracture.
When selecting materials for bending, it's important to consider their characteristics.
Here are some common materials used in sheet metal bending and their properties:
Stainless Steel Sheet Bending
Features of Stainless Steel
Steel is a combination of materials, including small amounts of carbon, manganese, silicon, copper, phosphorus, sulfur, and oxygen.
It is classified based on the carbon content as high, medium, low, and ultra-low carbon steel.
Steel can be bent easily as the tools used for bending steel plates are also made of steel.
However, bending stainless steel requires a relatively larger force due to its high yield strength, hardness, and poor ductility.
Additionally, the springback of stainless steel after bending is significant, hence requiring a larger bending radius to avoid cracking the workpiece.
Considerations for Bending Stainless Steel Sheet
Plate Thickness and Bending Tonnage Before bending stainless steel, it is essential to determine the thickness of the plate. Thicker plates require a larger bending machine.
Bending Angle and Bending Radius
The bending angle and radius are crucial to consider.
A larger bending radius may result in excessive springback, while a smaller radius may cause cracking.
Generally, the bending radius is around 0.2. For materials like high-carbon steel, a larger inner radius is necessary to prevent cracking.
Stainless steel has high resilience, and the bending angle and radius cannot be too small.
Bending Springback
The springback of a metal plate is proportional to the material's yield strength and inversely proportional to its elastic modulus.
Low-carbon steel has less springback and is ideal for high-precision workpieces, while high-carbon steel and stainless steel have significant springback.
The larger the bending radius, the greater the springback.
Smaller bending radii result in higher accuracy.
Calculating the Bending Allowance
The bending allowance, which is the expansion of the outer side of the sheet, can be calculated with the knowledge of the sheet thickness, bending angle, and inner radius.
This calculation determines the required length of the sheet for bending.
The formula for calculating the bending allowance is: BA=(π/180) x B x (IR+K x MT), or use a bending allowance gauge.
Bending with Machines
Finally, a machine like a press brake can be used for bending processing.
If the metal sheet is prone to cracking, it can be hot-formed or annealed.
Annealing improves the ductility of metals by softening them, and hot bending involves heating the metal to a red state and then bending it.
Conclusion
This blog post provides an overview of the basics of sheet metal bending, with a focus on important considerations for bending stainless steel.
Metal bending can be achieved using various machines, including press brakes. For simple bending tasks, a vise can also be used.
ADH is a manufacturer of sheet metal processing machines with 20 years of experience in the industry.
If you are in need of press brakes or other such machinery, you can reach out to one of our sales representatives for more information on the products and their prices.
FAQ
How to Bend Stainless Steel Sheet Without a Brake?
First, gather the necessary materials including stainless steel plates, hammers, vises, rulers, protractors, and markers.
Use the ruler to measure the plate's thickness, calculate the K factor and inner radius, and then determine the bend allowance using the formula BA=(π/180) x B x (IR+K x MT).
Use the protractor and marker to mark the bending line and radius on the plate.
Cut the stainless steel plate to the appropriate size, and use the vise to bend the plate to the desired angle.
Ensure an even bend by striking the metal with a wooden hammer.
Check the bend angle and bend allowance for accuracy. If necessary, you can assist the bend by heating the metal.
How to Calculate Bend Allowance?
Remember that bending the metal under pressure will result in internal compression and external stretching.
When calculating the bend dimension, be sure to take into account the bend allowance which is dependent on the sheet thickness, inner radius, K factor, and bending angle.
The formula to calculate the bend allowance is BA=(π/180) x B x (IR+K x MT), where K is the K factor, B is the bending angle, IR is the internal radius, and MT is the plate thickness.