Welding of Stainless Steels

Stainless steels or, more precisely, corrosion-resisting steels are a family of iron-base alloys having excellent resistance to corrosion. These steels do not rust and strongly resist attack by a great many liquids, gases, and chemicals. Many of the stainless steels have good low-temperature toughness and ductility. Most of them exhibit good strength properties and resistance to scaling at high temperatures. All stainless steels contain iron as the main element and chromium in amounts ranging from about 11% to 30%. Chromium provides the basic corrosion resistance to stainless steels. There are about 15 types of straight chromium stainless steels.

Nickel is added to certain of the stainless steels, which are known as chromium-nickel stainless steel. The addition of nickel reduces the thermal conductivity and decreases the electrical conductivity. The chromium-nickel steels belong to AISI/SAE 300 series of stainless steels. They are nonmagnetic and have austenitic microstructure. These stainless steels contain small amounts of carbon because this element has tendency to make chromium carbides, which are not corrosion resistant. Carbon is undesirable particularly in the 18% chromium, 8% nickel group.

Manganese is added to some of the chromium-nickel alloys. Usually these steels contain slightly less nickel since the chromium-nickel-manganese alloys were developed originally to conserve nickel. In these alloys, a small portion of nickel is replaced by manganese, generally in a two-to-one relationship. The AISI/SAE 200 series of stainless steels are the chromium-nickel-manganese series. These steels have an austenitic microstructure and they are nonmagnetic.

Molybdenum is also included in some stainless steel alloys. Molybdenum is added to improve the creep resistance of the steel at elevated temperatures. It will also increase resistance to pitting and corrosion in many applications.

Stainless steels can be welded using several different procedures such as shielded metal arc welding, gas tungsten arc welding, and gas metal arc welding.

These steels are slightly more difficult to weld than mild carbon steels. The physical properties of stainless steel are different from mild steel and this makes it weld differently. These differences are:

The properties are not the same for all stainless steels, but they are the same for those having the same microstructure. Regarding this, stainless steels from the same metallurgical class have the similar welding characteristics and are grouped according to the metallurgical structure with respect to welding.

Austenitic Type. Manganese steels are not hardenable by heat treatment and are nonmagnetic in the annealed condition. They may become slightly magnetic when cold worked or welded. This helps to identify this class of stainless steels. All of the austenitic stainless steels are weldable with most of the welding processes, with the exception of Type 303, which contains high sulphur and Type 303Se, which contains selenium to improve machinability.

The austenitic stainless steels have about 45% higher thermal coefficient of expansion, higher electrical resistance, and lower thermal conductivity than mild-carbon steels. High travel speed welding is recommended, which will reduce heat input and carbide precipitation, and minimize distortion.

The melting point of austenitic stainless steels is slightly lower than melting point of mild-carbon steel. Because of lower melting temperature and lower thermal conductivity, welding current is usually lower. The higher thermal expansion dictates that special precautions should be taken with regard to warping and distortion. Tack welds should be twice as often as normal. Any of the distortion reducing techniques such as back-step welding, skip welding, and wandering sequence should be used. On thin materials it is very difficult to completely avoid buckling and distortion.

Ferritic Stainless Steels. The ferritic stainless steels are not hardenable by heat treatment and are magnetic. All of the ferritic types are considered weldable with the majority of the welding processes except for the free machining grade 430F, which contains high sulphur content. The coefficient of thermal expansion is lower than the austenitic types and is about the same as mild steel. Welding processes that tend to increase carbon pickup are not recommended. This would include the oxy-fuel gas process, carbon arc process, and gas metal arc welding with CO₂ shielding gas.

The lower chromium types show tendencies toward hardening with a resulting martensitic type structure at grain boundaries of the weld area. This lowers the ductility, toughness, and corrosion resistance at the weld. For heavier sections preheat of 200°C is beneficial. To restore full corrosion resistance and improve ductility after welding, annealing at 760-820°C, followed by a water or air quench, is recommended. Large grain size will still prevail, however, and toughness may be impaired. Toughness can be improved only by cold working the weld.

If heat treating after welding is not possible and service demands impact resistance, an austenitic stainless steel filler metal should be used. Otherwise, the filler metal is selected to match the base metal.

Martensitic Stainless Steels. The martensitic stainless steels are hardenable by heat treatment and are magnetic. The low-carbon type can be welded without special precautions. The types with over 0.15% carbon tend to be air hardenable and, therefore, preheat and postheat of weldments are required. A preheat temperature range of 230-290°C is recommended. Postheating should immediately follow welding and be in the range of 650-760°C, followed by slow cooling.

If preheat and postheat are not possible, an austenitic stainless steel filler metal should be used. Type 416Se is the free-machining composition and should not be welded. Welding processes that tend to increase carbon pickup are not recommended. Increased carbon content increases crack sensitivity in the weld area.

Welding filler metals

The selection of the filler metal alloy for welding the stainless steels is based on the composition of the stainless steel. The various stainless steel filler metal alloys are normally available as covered electrodes and as bare solid wires. Recently flux-cored electrode wires have been developed for welding stainless steels.

Filler metal alloy for welding the various stainless steel base metals are: Cr-Ni-Mn (AISI No. 308); Cr-Ni-Austenitic (AISI No. 309, 310, 316, 317, 347); Cr-Martensitic (AISI No. 410, 430); Cr-Ferritic (AISI No. 410, 430, 309, 502). It is possible to weld several different stainless base metals with the same filler metal alloy.

Welding procedures

For shielded metal arc welding, there are two basic types of electrode coatings. These are the lime type indicated by the suffix 15 and the titanium type designated by the suffix 16. The lime type electrodes are used only with direct current electrode positive (reverse polarity). The titanium-coated electrode with the suffix 16 can be used with alternating current and with direct current electrode positive. Both coatings are of the low-hydrogen type and both are used in all positions. However, the type 16 is smoother, has more welder appeal, and operates better in the flat position. The lime type electrodes are more crack resistant and are slightly better for out-of-position welding. The width of weaving should be limited to two-and-one-half (2,5) times the diameter of the electrode core wire.

Covered electrodes for shielded metal arc welding must be stored at normal room temperatures in dry area. These electrode coatings, of low hydrogen type, are susceptible to moisture pickup. Once the electrode box has been opened, the electrodes should be kept in a dry box until used.

The gas tungsten arc welding process is widely used for thinner sections of stainless steel. The 2% tungsten is recommended and the electrode should be ground to a taper. Argon is normally used for gas shielding; however, argon-helium mixtures are sometimes used for automatic applications.

The gas metal arc welding process is widely used for thicker materials since it is a faster welding process. The spray transfer mode is used for flat position welding and this requires the use of argon for shielding with 2% or 5% oxygen or special mixtures. The oxygen helps producing better wetting action on the edges of the weld. The short-circuiting transfer can also be used on thinner materials. In this case, CO₂ shielding or the 25% CO₂ plus 75% argon mixture is used. The argon-oxygen mixture can also be used with small-diameter electrode wires. With extra low-carbon electrode wires and CO₂ shielding the amount of carbon pickup will increase slightly. This should be related to the service life of the weldment. If corrosion resistance is a major factor, the CO₂ gas or the CO₂-argon mixture should not be used.

For all welding operations, the weld area should be cleaned and free from all foreign material, oil, paint, dirt, etc. The welding arc should be as short as possible when using any of the arc processes.