Stainless Steel Properties
Stainless Steel refers to a category of corrosion and oxidation resistant metals developed from iron-chromium alloys. Typically, more than 10% chromium is required to produce stainless steel. Stainless steel is generally a high strength material, with enhanced corrosion resistance and high temperature oxidation resistance.
Applications for stainless steel are wide ranging. Water, food and beverage, pulp and paper, cryogenic fluid and chemical/petrochemical processing, medical applications, electronics, power generation, aerospace/defense, transportation—many different processes, industries and types of equipment make use of stainless steel.
The first stainless steels were developed in 1821. In later years, nickel was added to further modify the material’s properties. Metallurgists have since developed a wide variety of stainless that are distinguished by their mechanical strength, microstructure and chemical composition. They can be categorized into five main types: austenitic, ferritic, martensitic, duplex, and precipitation hardening.
Energy Beam Welding Stainless Steel
For critical joining applications demanding very high quality welds, precise penetration, enhanced mechanical properties and minimal part distortion, conventional fusion (arc) welding processes, like TIG and MIG welding, often cannot satisfy the requirements. The shortcomings of these more conventional processes—high heat input, the need for multiple passes in thicker joints, a tendency for excessive distortion, increased amount of residual stress, and limits on welding speed—are well known.
Energy Beam technology—Electron Beam or Laser Beam—mitigates many of the above mentioned shortcoming, and has been used for decades to produce high quality, high strength welds in most stainless steel alloy grades.
Austenitic stainless steels (referred to as 18-8 , also known as 300 series ) are the easiest to weld with Electron Beams or lasers, and the resulting welds have strength properties similar to those of the base metal (not cold-worked). Free-machining grades, which have sulfur, selenium, lead, and/or copper added, should be avoided. These added materials result in low melting compounds in the grain boundaries with higher risk for hot cracking during welding.
Martensitic stainless steel grades (common grade 410) can also be welded. However, upon cooling, the heat-affected zone (HAZ) and the fusion zone (FZ) transform to a narrow high hardness band exhibiting higher risk of cracking and reduced ductility from cold deformation. In some applications, preheating before weld and a post weld heat treatment may mitigate these conditions.
The Ferritic stainless steel grades (common grade Type 430 / 405 ) are more tricky to weld than austenitic 300 series. This is due to reduced amount of nickel compared to the austenitic grades. When welding ferritic stainless steel, grain growth in the heat-affected zone (HAZ) will occur. This phenomena results from high peak temperature in the narrow region immediately adjacent to the weld joint, and will contribute to reduced ductility and increased brittleness. This can be avoided by using stabilized ferritic grades, where minor additions of niobium, titanium, and zirconium form precipitates that prevent grain growth.
Duplex stainless steels contain higher chromium content (20–28%), higher molybdenum (up to 5%), lower nickel (to 9%) and 0.05–0.50% nitrogen compared to more familiar austenitic stainless grades. These stainless steels are characterized by a two-phase microstructure containing approximately equal volumetric fractions of austenite and ferrite. Like the 300 series, a variety of Duplex grades are available. Duplex material is generally weldable. However, when welding in the vacuum conditions of an EB chamber, there can be a reduction in the percentage of nitrogen, which can result in some impairment of mechanical properties compared to base metal levels. This must be evaluated during the Weld Procedure Qualification testing. Welding with a laser beam does not require a vacuum, and if needed, a suitably alloyed welding filler wire can be used to achieve critical levels of alloy element in the weld zone.
Precipitation Hardening stainless steels provide a combination of the properties of martensitic and austenitic grades. Like martensitic grades, have high strength through heat treatment and also the corrosion resistance of austenitic stainless steel. The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steel grade 630. An advantage of precipitation hardening steels is that they can be supplied a readily machinable, “solution treated” condition. After machining, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion.
The technical staff at EB Industries includes a welding metallurgist with several decades of expertise working with all grades of welded stainless steel applications and can provide support to your design and engineering staff with regard to material selection, joint design, tooling and developing weld parameters that lead co consistent, high quality welds.