Sunday, February 26, 2012

Stainless Steel Fabricating Processes

Due to its versatility, there are a wide range of stainless steel fabricating techniques. It may be helpful to review some of the processes involved in order to better understand the benefits and disadvantages of the different types of stainless steel fabrication.

Although it has relatively high strength and work hardening rates, stainless steel is malleable enough to be bent, folded, machined, welded, deep drawn, or spun. This guide will focus on three common stainless steel fabricating processes: work hardening, machining, and welding. Work Hardening with Stainless Steel

Work hardening is the process of strengthening material through deformation. Relative to other varieties of steel, stainless steel work hardens quickly, although the exact rate depends on the specific grade of the alloy. The austenitic family of stainless steel, for example, has a hardening rate slightly higher than that of carbon steel and is a frequently used grade series in fabrication processes.

It is also helpful to match the steel grade with a work hardening treatment that is well-suited for it. Since austenitic stainless steel can usually be hardened only through cold working, a thermal treatment process would be better applied to other grades, such as those in the martensitic family. Work Hardening Rates of Stainless Steel

Since ferritic stainless steels yield lower work hardening rates, austenitic and martensitic stainless steels are the series that benefit most from work hardening applications. In some instances, austenitic stainless steel can be cold worked up to 1,000 MPa, although the maximum is typically in the 800 MPa range. When cold drawn, stainless steel may reach tensile properties of 2,000 MPa or greater, but such high strength levels are usually limited to fine wire sizes. Size is a concern due in part to the quick work hardening rate of stainless steel, which yields diminishing tensile strengthening at the core of a wire as its diameter increases.

A fast work hardening rate provides certain benefits, as well. For example, stainless steel’s hardening rate renders the alloy effective in projects that require high strength and corrosion resistance. Common applications include the manufacturing of nuts and bolts, machine parts, cryogenic machinery and hospital equipment. The martensitic group, in particular, displays the highest levels of hardness and corrosion resistance among the stainless steel categories, making it a popular alloy for the production of tools, valve parts, bearings, and cutlery. Other Characteristics of Stainless Steel

Work hardening can also increase the magnetism of stainless steel. While its magnetism tends to be relatively slight, grades with higher work hardening rates exhibit higher levels of magnetic capacity after treatment. Stainless steel also undergoes greater deformation at slower forming speeds, requiring some high-speed forming processes to be slowed down to improve their work hardening efficiency. Machining Stainless Steel

Grades 303, 430, 410 and 416 achieve resistance to chipping when alloyed with manganese sulfide. Due to the decreased ductility and corrosion resistance that comes with the addition of manganese sulfide, these grades have limited machining applications, though several free-machining grades have been developed to address the problem.

To overcome the limits of machining certain types of stainless steel, some companies have incorporated exclusive steel melting techniques to improve the machinability of commonly-used austenitic grades. These specialized processes can improve machining efficiency, and may contribute to longer tool life. Tips on How to Machine Stainless Steel

Due to several concerns, such as the possibility for chipping, machining stainless steel can be a complex process. Here are some suggestions that may help with machining this alloy:

Apply coolants or lubricants to the equipment Use large tools to help dissipate heat Maintain light cuts and constant feeds Use chip breakers to deflect debris Select a machine tool that reduces vibration Keep the cutting edge sharp at all times

Welding Stainless Steel

Most types of stainless steel can be welded, but the degree of efficiency depends on the grade. Here are some tips for welding each category of stainless steel:

Austenitic: most grades of the austenitic family, except for free-machining Grade 303, are well-suited for welding purposes. However, this group is susceptible to sensitization and inter-granular corrosion on thicker products. For projects that involve welding thick materials, it may be better to use low carbon content grades such as 304L or 316L. Some stabilized grades, such as Grade 347, may also be effective.

Martensitic: these grades are also good options for welding, but may be prone to cracking. To help reduce the likelihood of fractures, pre-heat and post-heat the material, or use supplementary austenitic filler rods.

Ferritic stainless steel: though relatively less suitable for welding, some stabilized ferritic grades, such as Grade 409, may be appropriate for certain projects. Issues such as sensitization, low ductility, and high grain growth may be overcome with the help of austenitic stainless steel fillers or by post-heating the welded material.

Duplex: this set of grades works well for low thermal expansion and is welding efficient. Some suitable welding grades such as Grade 2205, have higher nickel content to improve ductility, strength, and corrosion resistance.


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