What is Stainless Steels?
Stainless steels are iron alloys with added chromium and nickel. A minimum of 10-12% chromium content is required in order to the produce rusting resistance for stainless steel. As more Chromium is added beyond this level a higher degree of corrosion resistance is maintained. However, the lattice becomes less ductile and nickel is usually added in order to restore toughness. Nickel content at 8% and chromium at 18% then becomes the well-known 18-8 stainless steel, as often seen stamped on cutlery, for example.
Austenitic Stainless Steels
18-8 type stainless engineering steels have the designation 300 series and the most common alloys are the 301, 302, 303 & 304 grades which are referred to as austenitic stainless steels, since their crystal lattice structure in metallurgically known as face-centred cubic (fcc).
What stainless steel MIG wire do I use for these austenitic grades
301, 302 & 304 grade stainless steels are welded using 308 stainless steel MIG wire . No matching chemistry welding wire is produced. 308L welding wire contains 2% extra chromium content in order to counter the 2% Cr that is lost in the welding arc. Thus, the weld deposit then ends up as type 304.
Carbon content may form chromium carbides and denude the grains of chromium. This reaction occurs at grain boundaries and can lead to localised severe corrosion. To counter this liability, stainless austenitic alloys have a controlled low carbon content. This can be further protected with an addition of titanium in type 321 or an addition of niobium in type 347. Since 60-70 % of the added titanium can be lost in the welding arc, usually type 321 alloy is welded using 347 welding wire. The suffix L is used to denote a carbon content usually of less than 300ppm (0.03%)
304 grade stainless steel can be protected for pitting and general corrosion by an addition of molybdenum at around 2.5% .This version is designated 316 or 316L welding wire.
Type 347 can sometimes be susceptible to liquation-cracking and/or centre-line cracking. To minimise this susceptibility the welding wire chemistry is adjusted to produce 3-7% delta ferrite in the weld deposit. The islands of delta ferrite absorb carbon and tramp elements within the centre of the austenite grains and thus leave clean grain boundaries.
Welding of Ferritic Type 409 & 430 Stainless Steels
The 400 series of plain chromium stainless steel materials includes ferritic-martensitic and some precipitation hardening stainless alloys. They are iron alloys that contain more than 10% of chromium. The chromium forms a self-healing transparent continuous protective Nano-film.
Usually if the carbon content is kept below 0.10% the alloys are single- phase ferritic alloys, whereas beyond this level of carbon, the materials are martensitic. A popular ferritic and lower cost grade is the 10% Chromium very low carbon content type 409, which is widely used for corrosion resistant car exhaust systems 409Ti is further stabilised with up to 0.5% Ti addition. 409 & 409Ti grade stainless welding wires possess excellent weldability and are generally welded with matching chemistry stainless steel MIG wire.
The other popular ferritic stainless steel is type 430 with 16-18% Cr content. The higher Cr level provides extra corrosion and scaling resistance. It is also welded with matching 430 grade stainless MIG wire. A lower welding heat input safeguards against excess grain growth. A range of other applicable welding wire include; 308, 309, 316, 312.
There are a number of fine variants of 409 and 430 alloys and the above welding data applies.
Welding of Martensitic Grade Stainless Steels
- 410 & 410NiMo
- 420, 430, 440 & variants
- Jethete
- 416 free-maching grade
- 431 variant
The 400 series of hardenable Martensitic stainless engineering high-alloy-iron materials typically contain 11.5 to 18wt% chromium with a variety of carbon levels. Unlike the austenitic and ferritic grades, the Martensitic series have a body centred tetragonal crystal lattice which responds to a range of heat-treatment regimes. These materials are ferro-magnetic.
The most commonly used alloys in this series are
- 410, 410NiMo, & original Staybright alloy.
- 420, 430, 440, with micro alloyed variations, used for tools and wear applications
- 416 free-matching 410 variant grade; specialised advice required for welding applications.
- Jethete, jet engine alloy, widely used for critical containment components. Jethete is a variant on 410 & 410NiMo and welded using high purity AMS 5823 and specialised procedures.
410 is a hardenable martensitic stainless alloy used for highly stressed parts needing good corrosion resistance and strength. Can be heat-treated to obtain high-strength properties with good ductility. It is the most widely used Martensitic grade. Welding is achieved with 410, 309L, 312, FM82 and 409Ti welding wires is also adopted.
420 to 440 grades are high carbon, martensitic, straight chromium high-hardenability stainless steels. Characterized by good corrosion resistance in mild domestic and industrial environments, including fresh water, organic materials, mild acids, various petroleum products, coupled with extreme high strength, hardness and wear resistance when in the hardened and tempered condition. Welding requires specialised procedures and welding wires, in order to minimise HAZ defect-formation.
431 martensitic alloy is a 0.2C 18Cr variant and is often welded using austenitic superalloy filler metals such as 80/20 , 625, FM82, unless absolutely matching properties are essential.
Filler Metals for Welding Austenitic Stainless-Steel Tube and Pipe
Like any application, welding austenitic stainless steel or 300 series tube and pipe presents specific challenges. It requires special attention to heat input and post-weld cooling to achieve optimal results as well as requiring the right filler metal. Matching the appropriate filler metal to this material can help ensure that the welds maintain their corrosion resistance, toughness, and mechanical properties. All factors that are critical for austenitic stainless steel to enter successfully into service in pharmaceutical, energy, and food or chemical processing applications where this material is commonly found.
The importance of filler metals
Hot cracking is a defect that occurs immediately upon completion of the weld, is the biggest potential threat when welding austenitic stainless steel, including the 304L and 316 grades commonly used in tube and pipe applications. This material has a particularly large grain structure, making it more susceptible to this defect than other types of materials.
The combination of the right filler metal, as well as proper control of heat input and post-weld cooling can help prevent hot cracking. These factors offer the added benefit of protecting the material’s corrosion resistance during the welding process.
Filler metals for welding austenitic stainless steel, as with any welding application, must match the base material according to its chemical and mechanical properties. The process of making this match, however, is slightly easier with austenitic stainless steel than many other materials. That’s because the intended service condition dictates what grade of austenitic stainless to use for the application. In turn, the grade of material dictates what filler metal should be used. Achieving the proper chemical and mechanical properties is, thereafter, a given.
To weld 304L austenitic stainless-steel tube and pipe, the proper filler metal match is one with an American Welding Society (AWS) 308 or 308L designation. A 316L filler metal is the appropriate choice for welding 316 austenitic stainless steels. For welding a carbon steel valve or bracket to an austenitic stainless-steel tube or pipe (a common industry practice), a 309 grade filler metal should be used.
These filler metals are designated for welding austenitic stainless steel due to the alloys that they contain, including chromium, molybdenum, nickel, and silicon. Each element provides a distinct attribute in the completed weld.
- Chromium provides corrosion resistance
- Molybdenum offers corrosion resistance, along with good high temperature performance
- Nickel increases weld toughness
- Silicon helps maintain weld pool fluidity
Engineers and designers often refer to a property in stainless steels known as the ferrite number. The ferrite number is a measurement of the amount of ferrite versus austenite in the structure of the material. The ferrite number is an indication of a stainless-steel weld’s ability to resist cracking, and in some cases is an indication of its corrosion resistance. The amount of ferrite present is determined largely by the chemistry of the material. In general, the goal when welding austenitic stainless steel is to reach a maximum ferrite number between 3 and 7, however, special applications may call for higher ferrite numbers to achieve the best results.
Options of stainless-steel filler metals
Filler metals with 308(L), 316 and 309L designations are available in a variety of types, used according to the specific welding process. Most filler manufacturers offer:
- Shielded metal arc welding (SMAW) electrodes (commonly referred to as stick electrodes)
- Gas metal arc welding (GMAW) wires, including solid and metal-cored wires
- Flux-cored arc welding (FCAW) wires
- Gas tungsten arc welding (GTAW) cut lengths
Each type of filler metal offers specific attributes and benefits and has its best uses. Each has its limitations as well. Understanding these aspects is an important part of gaining good weld quality when welding austenitic stainless steel.
SMAW or stick electrodes
These electrodes are readily available in the marketplace, usually in diameters ranging from 2.5mm to 5.0mm and familiar among welding operators, making them frequently chosen filler metals for welding austenitic stainless-steel tube and pipe. They also aid in achieving proper ferrite numbers.
Stick electrodes work well when welding out of position or on applications that require access into complex joints where a welding gun cannot reach. They offer the advantage of being portable as well since they generate their own shielding gas during the welding process and do not require an external shielding gas cylinder. That is particularly helpful for field applications.
There are some limitations to welding with stick electrodes for austenitic stainless-steel applications, namely that they’re less efficient to use than other filler metals. Stick electrodes are only twelve inches long, so they require frequent changeover when welding. Also, welding operators typically only weld to within three inches of the end before changing to a new one. The downtime for changeover and the stub loss can cost time and money. These filler metals also generate slag, which requires downtime for clean-up, and the weld pool can be quite fluid, depending on the product, making it difficult for some welding operators to use.
Solid wires
Solid wires are a common filler metal choice for welding thicker-walled austenitic stainless-steel pipe, for example, over 1/2-inch wall thickness. They work well for welding fill and cap passes in the flat or horizontal position while the pipe is being rotated (e.g., after completing a GTAW root pass), and they can be used to weld out of position for root, fill and cap passes using a pulsed or advanced short-circuit process. These wires are typically available in diameters ranging from 0.030 to 1/16 inches, and they provide good deposition rates when welding on austenitic stainless-steel pipe.
Solid wires for austenitic stainless steel can be more difficult to alloy than other types of filler metals, which can affect the availability of these filler metals and the size of the lots that filler metal manufacturers make. The raw materials needed to manufacture stainless steel filler metals are available in large heats and by a single chemistry. If an application requires very specific levels of an element in the wire (chromium, for example), it may be necessary to purchase very large batches of the solid wire and it may take more time for a filler metal manufacturer to develop the product.
Metal-cored wires
Metal-cored wires are becoming an increasingly popular alternative for welding thicker-walled austenitic stainless-steel pipe. Like solid wire, these wires require pulsing or advanced short-circuit processes to weld out of position, but with these processes it’s possible to weld from the root to the cap with the single filler metal, a GTAW root is not necessary.
Metal-cored wires offer several advantages for welding austenitic stainless-steel tube and pipe.
- They provide faster travel speeds and greater deposition rates than solid wires, allowing welding operators to place more filler metal in the weld joint faster
- They are also very good at bridging gaps, which helps with successfully welding tube or pipe joints that have misalignment or poor fit-up. They produce minimal spatter which minimises post-weld cleaning
- They are more easily alloyed than solid wires and can be made in smaller batches, decreasing the waiting period for specialty orders, and reducing the amount companies need to purchase
One disadvantage to stainless steel metal-cored wires is their cost. They tend to be more expensive than solid wires, but many companies find that the productivity and quality improvements outweigh the wire’s unit cost. These filler metals are available in 0.9mm to 1.6mm diameters.
Flux-cored wires
Flux-cored wires are good for achieving high deposition rates and good productivity when welding out of position on thick-walled austenitic stainless-steel pipe. Unlike solid or metal-cored wires, this filler metal does not require a special power source to weld out of position, the flux coating on the wire acts like a dam that helps hold the molten weld pool in place.
Flux-cored wires do produce higher levels of smoke than other filler metals, so it’s important to have the proper ventilation or weld fume source capture in place when using them. They also generate a slag that welding operators will need to chip or grind in between passes and after the cap pass, which can lead to greater downtime for clean-up.
GTAW cut lengths
The GTAW process, paired with stainless steel cut lengths, offers very clean, precise, and high-quality welds, making it a good choice for welding thin-walled tube and pipe. These filler metals also do not produce any slag so there is no need for post-weld cleaning. Cut lengths are available in a range of diameters (from 0.8mm to 3.2mm) and in industry standard lengths of 1000mm.
Generally, a single welding pass with a GTAW cut length as filler is adequate to complete a weld on thinner-walled tube and pipe. A GTAW root pass is also prevalent when multi-pass welding on thicker-walled pipe (particularly for high-pressure, critical applications).
One disadvantage to the use of the GTAW process is its slow speed. It also requires back-purging with shielding gas, which can add to costs and downtime for setup.
New technology with the MasterWeld 300T with MDA (MasterWeld Dynamic Arc) and the AWT300
This new welding process combines all the benefits of TIG welding, offering clean, precise, and high-quality welds, with the speed of MIG welding, especially on this wall stainless pipe, this is achieved with the combination of three unique MasterWeld technologies.
MasterWeld Dynamic Arc (MDA)
MDA function makes it possible to keep the product of voltage x current constant. The power source increases the welding current as the arc voltage decreases and reduces the welding current if the arc voltage increases. The MDA value can be adjusted from a minimum of 10 amps to a maximum of 50 amps at each 1-volt variation, whether positive or negative.
Welding benefits of the MDA function
- Faster welding - less distortion of the welded part, increased vertex angle penetration
- Heat input concentrated exclusively on the weld and not on the surrounding area
- Less oxidation of the part and hence reduced post-welding reworking costs
- Improved control of the first root pass
- Reduced risk of the electrode sticking when it touches the weld puddle
- Facility to work with the electrode very close to the weld puddle to concentrate the arc
This technology, combined with the MasterWeld MWT300-E Auto TIG Feeder, and the MasterWeld AWT300 Auto-feed TIG Welding torch, with unique, through-the-handle wire feed design, offers a semi-automatic TIG welding process that is second to none, especially on thin wall stainless steel pipe, the weld time is drastically reduced from that of traditional TIG welding.
We trust you find this guidance essential to choose the right stainless steel MIG wire for your welding application, however if you need further technical support, please contact us free on 0800 975 9710.