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Welding

Welding methods can be divided into two types: fusion methods and pressure methods.

Fusion methods

Fusion methods, where the two edges or surfaces being joined are heated and joined with or without a filler material. These comprise:

  • Manual Metal Arc (MMA)
  • Metal Inert Gas (MIG)
  • Metal Active Gas (MAG)
  • Flux Cored Arc Welding (FCAW)
  • Tungsten Inert Gas (TIG)
  • Plasma Arc Welding (PAW)
  • Submerged Arc Welding (SAW)
  • Laser Beam Welding (LBW)
  • Laser Hybrid Welding (LHW)

Pressure methods

Pressure methods, where two clean surfaces are brought into close contact to form a metallic bond between the two surfaces. These comprise:

  • Resistance Spot Welding
  • Seam Welding
  • High Frequency Welding

The microstructure in stainless steel welds is largely dependent on the chemical composition. Constitutional diagrams such as the Shaeffler-De Long can be used as a rough predictor of weld microstructure, but the thermal history, particularly the cooling rate after welding, also has a major influence. It is important to realize that weld microstructure should not be considered as a material property. Rather, technological properties such as mechanical strength, ductility/formability, and corrosion resistance in different environments should be taken into consideration for a fabricated welded construction.

Other important aspects for a fabricator are productivity and the risk of unexpected defects or imperfections when welding new steels or product forms. For this reason, a welding procedure qualification should always be performed before a new material or process is introduced to identify unexpected deviations. More information about welding stainless steels can be found in the Outokumpu Welding Handbook.

Weldment properties

Ferritic stainless steels

The main limitation of ferritic weldments compared to their austenitic counterparts is the lack of toughness in thicker sections. Sheet materials are typically used to ensure sufficient weldment toughness and ductility.

Weldment properties are strongly affected by welding parameters. It is recommended that ferritic grades be welded using minimum heat input to prevent excess grain growth in the heat-affected zone (HAZ). Moist electrodes and shielding gases that contain hydrogen or nitrogen should be avoided. Due to their lower thermal expansion and higher thermal conductivity, distortion and buckling during welding is lower for ferritic stainless steels compared to austenitic or duplex grades.

Martensitic stainless steels

Being hardenable, martensitic stainless steels are more difficult to weld than the other types of stainless steel. Regardless of prior condition, welding produces a hard martensitic zone adjacent to the weld that is prone to cracking. The hardness increases with the steel’s carbon content and makes welding more complex. The presence of hydrogen increases the risk of hydrogen-induced cold cracking.

Nevertheless, martensitic steels can be successfully welded provided the right precautions are taken to avoid cracking in the HAZ. Preheat and post-weld heat treatments (PWHT) are normally required to obtain reliable weldments, and matching fillers should be used to maximize the strength of the welded joint. If PWHT is not possible, austenitic or duplex fillers can be used for improved ductility.

Duplex stainless steels

The weldability and welding characteristics of duplex stainless steels are better than those of ferritic stainless steels, but generally not as good as those of austenitic steels. Modern duplex steels with significant nitrogen content are readily weldable. Weldment properties are strongly affected by welding parameters such as the heat input range, so appropriate procedures should be followed to obtain a correct weldment structure.

Duplex stainless steels commonly solidify with a fully ferritic structure, with austenite nucleation and growth during cooling. Filler metals are specially designed with higher nickel content to produce a phase balance similar to that of the base material. Autogenous welding (without filler) is generally not recommended for duplex steels. The duplex microstructure is more sensitive to the effect of subsequent passes compared to, for example, standard austenitic grades. To reduce the effect on the microstructure from previous passes, the interpass temperature should be a maximum of 150 °C for standard or lean duplex and 100 °C for superduplex steels.

Austenitic stainless steels

In general, austenitic stainless steels have excellent weldability. The final weld metal structure normally contains a few percent of delta ferrite, which is the sign of a sound weld. The level of heat input for most common austenitic grades could be up to about 2.5 kJ/mm. If the welding is carried out on stabilized or fully austenitic grades, somewhat lower levels may be needed to avoid solidification cracks (≤1.5 kJ/mm). Austenitic steels have about 50% higher thermal expansion compared to ferritic and duplex steels. This means that larger deformation and higher shrinkage stresses may result from welding.

For high alloy austenitic grades, pitting corrosion resistance can be reduced due to microsegregation, primarily of molybdenum, during solidification. Therefore filler metals are, in most cases, overalloyed with chromium, nickel, and molybdenum to enhance corrosion resistance. Nickel-base fillers are used for the highest alloyed austenitic grades. The manganese-alloyed austenitic grades offer relatively high strength at a moderate cost. These steels show lower weldability than the standard Cr-Ni grades, primarily because they are more susceptible to hot cracking – cracks in the weld formed during solidification.

For some applications, metastable austenitic steels are used in a cold-rolled condition, in which they are temper rolled to very high strength levels. Welding will naturally have a softening effect in the weld zone, and this should be taken into consideration at the design stage.

Weld defects

Typical examples of weld defects include:

  • Incomplete penetration
  • Undercut
  • Lack of fusion
  • Pores
  • Slag inclusions
  • Weld spatter
  • Arc strikes

These defects have negative impact on mechanical properties and resistance to local corrosion, and also make it difficult to maintain a clean surface. They should therefore be removed, normally by grinding, although repair welding may sometimes be necessary.

 

Glossary

Interpass temperature
The recommended temperature of the material between the weld passes. Can be specified as a minimum or maximum temperature.

Delta ferrite
A non-magnetic form of ferrite, stable between 1403 °C and 1535 °C, which is the melting temperature.

 

The Outokumpu Welding Handbook

The Outokumpu Welding Handbook aims to contribute to existing knowledge on welding and offer hands-on advice on welding methods and processes together with design in which stainless steel is the base. The handbook also discusses appropriate filler material and post-weld treatments, as well as provides information about the impact of welding on the overall properties of the construction.

Björn Holmberg, Senior Technology Adviser, from Outokumpu's Avesta Research Center says: "Our ambition is to provide a comprehensive picture of all welding-related areas. We hope to provide answers to fundamental questions about the welding of our stainless steel – whether the reader is an architect, designer, welder or inspector – as the interest of all is to produce and deliver a good fit for purpose construction."

Contact a Outokumpu sales representative near you to get a printed copy of the Outokumpu Welding Handbook.

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Feb 01, 2018