Material science and mechanics both figure into success

November 2013 - What does it take to weld stainless steel—is it a secret or applied know-how? As a short answer: It’s no secret, and it takes applied know-how.

It’s a matter of applied skills in welding to master this challenging type of material. So, what is stainless steel all about and why the name stainless steel?

Harry Brearley, of Sheffield, England, was an early 20th century expert in steel and steel production. In 1913, as part of his experimentation to find a steel with better resistance to erosion, he melted steel with 0.24 percent carbon and 12.8 percent chromium. Using nitric acid to etch the material and examine its grain structure under the microscope, he found the nitric acid left no “stains” on the surface of the new material. The first use of the new material was in the cutlery industry. Originally called rustless steel, the material was soon renamed stainless steel.

Independent of Brearley’s material research in England, a material scientist with the German company Krupp added nickel to the melt. Whereas Brearley’s stainless steel could be supplied only in hardened or tempered condition, the new stainless steel from Krupp was softer and ductile, and consequently easier to work with. It also had higher acid resistance.

Stainless steel’s corrosion resistance is due to the high percentage of chromium and alloying elements, which help develop a passive surface oxide film.

In industry today, equivalent stainless steels frequently have equivalent grade number designations from three different organizations, the American Iron and Steel Institute, SAE International (originally the Society of Automotive Engineers) and ASTM International, which uses the unified numbering system. Examples of some common equivalent designations are shown in the table below. Other grades designate stainless steels for specific applications, such as castings.

How can someone on the shop floor identify ferritic versus austenitic stainless steel? It is as simple as testing with a magnet, because only ferritic stainless steel is magnetic.

For welding purposes, everyone in production needs to understand that each material designated stainless steel has its own grain structure. In general, it expands 2.5 times more than mild steel when heat is applied. That difference applies to the cooling process as well but differs among martensic, ferritic and austenitic stainless steels. Therefore, the type of filler material or welding wire must match the base metal type.

For example, welding ferritic stainless steel with an austenitic weld wire creates fine cracks in the weld bead and a weld with unacceptable quality. This happens because the material being welded has more material substance than the weld bead. When the materials expand and contract differently, the base material always develops more force in the process of expanding and contracting, which is then applied to the weld bead layer. The weld bead cracks simply because of the tensile stress.

A similar effect can happen in a workpiece with very different component thicknesses. For example, welding a 6-mm-thick mounting bracket onto a 2-mm-thick ferritic end cap creates unequal forces in the weld. As a rule, when welding stainless steel, the base material and the added part should be the same thickness to guarantee equal material expansion and contraction of the weld and base material. If one or the other is thicker, more heat and filler material will be applied and will result in unequal expansion and contraction toward the weld, forming fine cracks and weld failure.

One way to avoid this type of welding failure is to chamfer the thicker material. Other useful practices include:

• Developing appropriate welding parameters, such as calculated feed and speed of the applied weld wire.

• Ensuring high-quality form, shape and edges of the workpieces to be welded together.

• Proper clamping of the workpieces to each other.

• In automated welding production, simultaneously welding with two robots such that torches are applied on opposites sides, if possible.

How to successfully weld stainless steel is not a secret. But it does take additional know-how to create quality welds in stainless steel. FFJ

Udo O. J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.