Stainless Steel MIG Welding Wire

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Guidance on what MIG Wire to use on different stainless steels

What are 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.

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.

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