Stainless Steel Fabrication Process
When it comes to the stainless steel fabrication process, all stainless steels, particularly the austenitic grades, can be fabricated. The commonly used austenitic grades can be deep drawn, hot forged, cold forged, folded, roll formed and spun.
With a high strength and work hardening rate, austenitic stainless steels are known for their high ductility. This in turn enables these stainless steel grades to be very heavily cold formed into into deep drawn products.
The stainless steel manufacturing process also includes the production of other cold formed products from other grades of stainless steels without the occurrence of splitting.
Fabrication processes for stainless steel includes:
- Work Hardening
- Soft Soldering
- Silver Soldering (Brazing)
Base on the type of stainless steel grade used in the manufacturing process, metals can be work hardened upon cold forming.
The austenitic grades of stainless work harden at a rapid rate, but the cold working rate of the 400 series stainless steels are slightly higher than that of plain carbon steels. Because of austenitic grades’ fast cold working rate, it is a suitable metal for applications where a high resistance to corrosion and strength is a requirement.
Note however that austenitic grades can only be hardened through work hardening processes, whereas martensitic grades can be hardened by using thermal treatments such as tempering and quenching. Ferritic grades can be hardened through cold working processes, but the rate is low which in turn makes it difficult to achieve high strength requirements.
Also note that the stainless steel grades that posses the highest rate of work hardening usually also has the highest magnetic permeability (for the amount of cold work processes).
Cold drawing, a cold working technique, is capable of increasing the tensile properties of stainless steel grades (commonly 304, 302 and 301) to reach up to 2 000 MPa. Note that high tensile strengths are limited to thin sections and fine wires.
As the size of the material increases, the cold working required to achieve high tensile characteristics will become unsuitable. This is because of the rapid work hardening rate of large surfaces.
Work hardening rates in the stainless steel manufacturing process:
Here it is important to note that the 400 alloy series of stainless steel posses magnetic properties at room temperatures, resulting in the same work hardening rate as that of low carbon steels. Wire products made from the 400 series can be cold worked in order to achieve tensile strengths of up to 1 000 MPa, whereas bar products are regularly cold worked at higher rates than 850 MPa.
While Ferritic stainless steel grades cannot be heat-treated, Martensitic grades can. By using hardening and tempering processes on Martensitic grades, manufacturers are capable of achieving maximum mechanical and corrosion resistance properties.
As temperatures increase, the work hardening rates tend to decrease. Stainless steels undergo severe deformation during cold working processes at slow forming speeds.
Austenitic stainless steel grades are generally difficult to machine, but stainless steel manufacturers have developed a free-machining stainless steel grade – 303. Note that free-machining versions for both Ferritic and Martensitic grades are also available.
Due the the presence of manganese sulphide, serving as chip breakers, in grades 430F and 416 also exhibit an improved machinability. High levels of sulfur results in a reduced ductility, in turn resulting in the prohibition of welding, bending and cold forging techniques.
The presence of non-metallic inclusion in free-machining grades means that these grades do in fact offer a lesser resistance to corrosion, thus making these grades inadequate for use in severe applications such as those required by marine industries.
Improved machinability versions of austenitic stainless steels (such as grade 316 and grade 304) is produced through steel melting processes – a process that has a sufficient chip breaking effect as well as the capability of significantly improving the stainless steel’s machinability without taking away from the metal’s mechanical properties such as weldability, resistance to corrosion and formability.
Machining Stainless Steel:
- When machining stainless steel, it is of the utmost importance that the machine tool is both free of vibration and sturdy. A sufficient amount of power is required for the proper operation of the machine tool.
- A constant feed is necessary in order to ensure the proper placement of the work.
- Machining of stainless steel requires the proper use of both coolants and lubricants. For example, when machining austenitic alloys, a large amount of heat is created at the cutting edges of the tool.
- Large tools can be applied in the stainless steel fabrication process in order to help in the dissipation of generated heat.
- Tool cutting edges need to be kept sharp. Using dull tools in the stainless steel manufacturing process can lead to the metal surface work hardening as well as glazing.
- The balance between light cuts and depth is extremely important, as a sufficient depth is necessary in order to prevent the tool from promoting work hardening.
Even though almost all stainless steel grades can be welded, each stainless steel has a different weldability, with Austenitic grades being the most readily welded. The weldability of each stainless steel depends on the type or group it belongs to.
Martensitic grades have a high carbon content, meaning that extreme care is required during welding and, while Ferritic grades do have an excellent weldability, welding should be done with the utmost care.
Welding Austenitic Stainless Steels
Using all conventional electric welding procedures, these group of stainless steels are readily welded. Grades with a low carbon content are used for heavy sections, resolving the issues of sensitization and inter granular corrosion.
Grade 303 (a free-machining grade) is subject to hot cracking and is thus not a chosen or preferred grade when it comes to welding.
Ream more about Austenitic Stainless Steels.
Welding Martensitic Stainless Steels
Except for the 416 grade (which is a free-machining grade with high a sulphur content), this group of stainless steels can be welded with Austenitic filler rods. The use of these types of rods leads to an improved ductility.
Note however that hard and brittle zones are formed adjacent to the weld itself when using Austenitic filler rods. Extreme caution is needed in both the pre-heating and post-welding processes as cracking can occur in the above mentioned zones.
Read more about Martensitic Stainless Steels.
Welding Ferritic Stainless Steels
Due to sensitization, a lack of ductility and an excessive grain growth, these group of stainless steels do not have a good weldablity. However, post-weld heat treatments are applied in order to solve some of these problems.
Filler metals, which helps to enhance the toughness, can be from Austenitic grades, and, because excessive grain growth is such a problem and very difficult to overcome, most grades are only welded in thin gauges.
Stabilized Ferritic stainless steels also offer a better weldability. While normal grades in this group make use of Austenitic fillers, Grade 3CR12 (which posses a low carbon content) can be readily welded and can even be used in heavy section plates.
Read more about Ferritic Stainless Steels.
Welding Duplex Stainless Steels
Though not having as good weldability as that offered by Austenitic grades, Duplex stainless steels do posses a good weldability. Standard welding processes can be used on this group of stainless steels and, as an advantage, this group has lower thermal expansion coefficient when compared to the Austenitic group of stainless steels.
Welding Dissimilar Metals
It is possible to weld different grades of stainless steel (such as grade 430 and grade 304). Note that precautions during this stainless steel fabrication process is vital.
In order to reduce the dilution effects on stainless steel, the use of over alloyed Austenitic welding rods are recommended. An example of this would be the use of the grade 309.
You can learn more about welding by visiting our TIG Welding Stainless Steel page.
A stainless steel manufacturing process used by manufacturers, soft soldering involves the use of a lead-tin solder, used for soldering all the stainless steel grades. In cases where the manufactured product will be used in food processing industries or transport industries, the use of lead solders should be avoided.
Also, in cases where the mechanical strength depends on the soldered joint, soft soldering should not be used as the type of joints created using this process is relatively weak.
Strength of joints can be improved by:
- Riveting edges
- Lock-seaming edges
- Spot welding edges
When applying soft soldering processes, it is important that:
- The surface of the steel to be soldered is both clean and oxidation free.
- Here, any type of solder can be used, but the use of at least 50% tin is recommended. A better color match and strength can be achieve by using a solder of 30% - 40% lea, and 60% - 70% tin.
- Flux should only be done in the area being soldered, making use of a brush.
- Because rough surfaces enhances the solder adherence, roughening is performed using files, abrasive paper or grinding wheels.
- Stainless steels has a low thermal conductivity. The temperature of soldering should be maintained, as with soldering carbon steel, but will require a longer time. Here, a large, hot iron can be used.
- When it comes to soft soldering, the use of a phosphoric, acid-based flux is recommended instead of a hydrochloric acid-based flux, as these types of fluxes do require neutralization after the soldering process has been completed.
Silver Soldering (Brazing)
Also know as brazing, silver soldering is applied in cases where a strong joint is required, but welding processes cannot be performed.
The joint created by using this technique will have a slightly lower resistance to corrosion than that of the stainless steel, but it is important to note that these types of joints are capable of reaching the requirements set by mildly corrosive operating conditions as well as the requirements set by atmospheric conditions.
When applying silver soldering (or brazing), the following aspects should be kept in mind:
- When using silver soldering as a stainless steel fabrication process, make use of silver brazing alloys that has a melting point of between 590°C and 870°C.
- In cases where grade 430 alloys are being brazed, it is important to use silver solder that contains 3% nickel, as this alloy, when combined with austenitic grades, helps to reduce crevice corrosion.
- The use of a slightly reduced flame is used in order to uniformly heat the joint.
- After all oxides and dirt has been removed from the surface to be soldered, flux should be applied.
- In the cases of high production work, the use of induction heating or controlled atmosphere furnaces are common place.
- After the brazing process has been completed, all remaining flux should be removed. This can be done by using high-pressure steam or hot water.