Welding of Heat Resisting Alloys

There are many different environments where Heat Resisting Alloys are utilised and a various number of alloys have been developed that have optimum characteristics for use in specific environments.

The factors that need to be considered for optimum alloy choice are firstly high temperature corrosion phenomena such as oxidation, carburisation, metal dusting, sulphidation and chloride salt attack. Second are thermo-mechanical issues such as creep strength, stress-rupture, distortion, embrittlement, thermal cycling or fatigue, thermal shock and finally life expectancy and cost.

It is important that the correct weld consumable is used to maintain the properties across the weld zone and so it usual for a matching weld consumable to be chosen and each alloy generally has these available either as a filler wire and also an electrode. It is also beneficial that a qualified weld procedure is developed which will give confidence that a weld of optimum quality is achieved. For dissimilar welding, there are several options available and there are separate guidelines for producing various dissimilar welds.

Most heat resisting stainless steel alloys use a higher carbon content for strength, whereas the corrosion resistant grades require a low carbon content to prevent carbide precipitation and maximise chromium content in solution. Therefore, it is not always feasible to utilise corrosion grades in high temperature service or heat resistant grades for corrosion applications. This is true for both base materials and weld consumables.

The Heat resisting alloys can be split into 4 main groups, Stainless steels, Precipitation hardenable Stainless steels, Nickel alloys and finally Precipitation hardenable Nickel alloys/superalloys.

Most of the alloys are austenitic alloys and therefore behave and weld in a similar manner to standard austenitic stainless steels and are readily weldable providing the correct techniques and procedures are used. The remainder are either ferritic or martensitic stainless steels or are precipitation hardenable nickel alloys/superalloys and these require more detailed preheat and post weld heat treatment to prevent cracking and to achieve properties across the weld zone.

The Austenitic alloys they are prone to solidification cracking and each of the alloy groups has a different method to prevent cracking. Further information for each group is detailed in the applicable section later. It is essential to understand and utilise these methods to prevent hot cracking as defects in service can lead to premature failure.

The HSE requirements for welding of these alloys are no different to any welding and are the usual considerations, such as those for the use of electrical equipment, risk of fire and explosion, fume extraction, hot working, care with ultraviolet light, manual handling and slip or trip hazards. These should all be considered and the use of the appropriate PPE and other precautions put in place. Further information is available on the applicable Weld alloy data sheet.