How to Weld Stainless Steel
Introduction to Stainless Steel
Before dealing with welding stainless steel, one should offer a loose description of the material. Stainless steels represent a class of iron-base materials that have a certain resistance to rusting and corrosion in some environments, due to the presence of chromium in their composition. Chromium helps produce a tough, impervious layer of chromium oxide on the material’s surface, which shields the surface from rust and corrosion. One should be aware that the expression “stainless steel” represents a huge class of different materials. It is not a technical term identifying any specific metal and cannot be used for practical purposes like purchasing.
The three more general classes of stainless steels – Austenitic, Ferritic, and Martensitic – are indicated by reference to their metallurgical structure. More specifically, they use an identifier that refers to the appearance of their micro-structure, as seen under the microscope or by x-ray diffraction. These micro-structures may be present in certain steel, so they are used to indicate the prevailing structure in the stainless steel. The properties of each class can affect the welding process in different ways, so it is important to identify which type is being used ahead of time.
When welding stainless steel, Austenitic stainless steels are considered the most easily welded of the three classes. They are known as the "300 series", which refers to a standard classification originated by the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE). An important sub-class, known as "18/8", has alloying elements that are 18 percent chromium and eight percent nickel.
Main characteristics of Austenitic stainless steels are as follows:
- Not magnetic or only slightly magnetic
- Not attacked by a 10 percent solution of nitric acid (HNO3) in alcohol
- Does not harden by any heat treatment
- Strain hardening - Quite ductile and easily deformable by mechanical working which increases both hardness and strength
- Easily welded, with the necessary precautions
- Thermal conductivity between one third and one half that of other steels
- Coefficient of thermal expansion by 30-40 percent, even 50% at times
When welding stainless steels, the two last characteristics affect the outcome in various ways, producing larger distortion than as found in other steels.
Not all austenitic stainless steels of the 300 series have equal weldability. The addition of sulfur or selenium used to improve machinability (as in Type 303) results in severe weld hot-cracking, which makes this particular material "non-weldable".
Use caution when welding austenitic stainless steels. The corrosion resistant characteristics of these stainless steels may be adversely affected by the sensitization process, which occurs at temperature intervals from 600 to 900 Celsius (1100 to 1650 Fahrenheit). This promotes the gathering of chromium carbides at grain boundaries and the parallel loss of anticorrosive chromium from the base metal.
The above temperature range occurs naturally, not in the weld zone where temperature is higher and lasts only for a short time, but in the two strips of metal on each side of the weld bead. This is the Heat Affected Zone (HAZ) where the harmful effects take place.
In a sensitized joint, the chromium, which is the main "stainless" ingredient, becomes sequestered or taken out of play and locally unavailable for the protective action. If not addressed correctly, welding stainless 18/8 steels may cause the loss of their protective property along sensitized paths, and the welded material becomes prone to intergranular attack in a corrosive environment.
There are three strategies that can be employed to cut down on the adverse effects of the sensitization process in series 300 stainless steels. One is to use a very low carbon version (i.e. 304L where L stands for low-carbon) where not much carbon is available for making chromium carbides.
Another strategy is to use a different type of base metal that includes titanium (type 321) or columbium (type 347), which will form titanium or columbium carbides, causing the carbon to become unavailable for chromium during the sensitization process. This leaves the chromium free to perform its anti-corrosive tasks.
Note: The filler metal for this material, if required, should always be columbium. Why? Because titanium is reactive and is not readily recovered during deposition. This means it would not be available when needed most. Columbium however is not reactive. It will stay put during the melting process, and when the material is heated to the sensitization temperature range, it will do the job of producing columbium carbides, saving the day.
The third strategy is to perform a solution heat treatment at elevated temperature (1050 Celsius or 1900 Fahrenheit), which would repair corrosion susceptibility. This strategy puts in a solid solution of chromium carbides, which originated during the welding sensitization process of regular 18/8 stainless steel. This process, however, contends with problems like heavy oxide formation if not performed in a vacuum or other protective atmosphere, free from contaminates. Type 309 and 310, used for elevated temperature applications, and type 316 or better type 316L used for enhanced corrosion resistance, are generally not prone to sensitization and are used with filler wires of similar composition.
The second class of stainless steel is called ferritic stainless steel. This steel is ferromagnetic, but cannot be hardened by heat treatment. This is a common type of stainless steel used in car exhaust components. A limited amount of ferritic structure, when present in an otherwise mainly austenitic structure, is considered beneficial in that it reduces the chances of hot cracking. Welding stainless ferritic steels can readily be performed using arc welding processes, either with ferritic or austenitic filler metal. A post weld heat treatment may be needed to improve properties.
Martensitic stainless steels are magnetic and fully able to be hardened through heat treatment. Welding stainless steel of this type is not recommended, although feasible with special techniques. Welding may produce cracks, especially if carbon content is not sufficiently low. Preheat and post heat treatments may be necessary.
One Final Class
To complete the welding stainless steel overview, one should mention a fourth class of materials not listed above – precipitation hardenable (PH) stainless steels, which are quite readily weldable. However precise instructions should be followed concerning heat treatments in order to develop the required properties.
Welding Processes for Stainless Steel
There are many different types of welding that can be used when welding stainless steel. They all have their advantages and disadvantages, and they all require specific instructions to ensure a proper, strong weld every time.
Friction welding stainless steels presents almost no problems, except for the free cutting types of stainless steel that should not be welded at all. It is used for welding stainless steel not only to other stainless steel work pieces, but also to different metals like copper and aluminum. One should always be aware of the material type and condition before welding, as well as the effects of heat near the joint. Some elements like sulfur or selenium can compromise the final soundness of welded joints.
Resistance welding can be used in most stainless steels. 300 series austenitic steel of the 300 series can readily use resistance welding, as can ferritic steels. However, martensitic stainless steels can pose a problem because the weld result is brittle, if not softened adequately by a post weld tempering treatment.
The resistance welding process is currently used on stainless steel with adaptations to deal with the differences in the electrical resistance and low thermal conductivity, as well as high coefficient thermal expansion, higher melting temperature, and high strength at elevated temperatures. Electrode force is more elevated, while time and current are less for low carbon steels.
All stainless steels must not only be cleaned of dirt, oil, grease, or paint before the resistance welding process (or any welding process), but they must also be cleaned of the naturally forming chromium oxide layer. This has to be removed with a stainless steel wire brush.
Arc welding can be used when welding stainless steel, as long as the proper flux is used. This makes the process much less viable to TIG welding, unless there is no other choice available. TIG welding of stainless steel requires eliminating all traces of residual flux on the part after the welding process, which lengthens the operation and increases costs. Arc welding is commonly used, paying close attention to the class and the condition of the material being welded, as well as keeping an eye on sensitization and deformations.
All types of arc processes can be employed for welding stainless steels with due attention to joint shape, dimensions and preparation. In particular Shielded Metal Arc Welding (SMAW) is widely used because of its flexibility. It should be noted that electrodes come in two types concerning the cover composition, which may influence the choice of the current employed.
There are many filler metals that can be used during the arc process. The classification for stainless steel filler metals can be found in the American Welding Society’s AWS A5.9/A5.9M:2006 – Specification for Bare Stainless Steel Welding Electrodes and Rods.
Electron Beam Welding
Electron beam welding (EBW) of stainless steels is readily performed with good results, even in very deep welds. The remarkably high depth to width ratio permits EBW to join configurations not possible with other means. With the heat input being low and the heat affected zone having limited extent, there is often no remarkable damage to the mechanical properties so that further heat treatment is not required.
Laser welding can also be used when welding stainless steels, as long as precautions are in plate to insulate the welding from the air and to limit the damage properties obtained during the heating treatment.
In the end, stainless steel welding is not a complicated process at all. It just takes some attention to detail when it comes to work piece materials, filler metals, and the type of welding being used. If all of that is up to par, you can be successful welding your stainless steel parts.
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