Welding-duplex

Stainless Steels Successfully.

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Duplex Stainless Welding

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Welding-duplex stainless should be performed only after rigorous study.

Special requirements imposed by this family of materials must be understood.

Practical welding parameters must be completely determined.

Thorough validation of the properties obtained must be accomplished.

Duplex Stainless Steels contain about equal amounts of ferrite and austenite.

A short account was given in my page on Stainless Steel Welding.

Duplex Stainless Steels are a group of materials offering good corrosion resistance, improved mechanical properties with good ductility and toughness.

As compared to austenitic grades, they show relative ease of fabrication, including weldability, when due precautions are taking place.

Modern duplex stainless steels can be grouped according to similarity of composition to four main Grades identified for ease of reference:

  • (UNS No.S32304) - Lean duplex 2304 with low or no Mo
  • (UNS No.S32205) - Duplex 2205, the most used of these steels
  • (UNS No.S32550) - 25 Cr like 255 and DP-3
  • (UNS No.S32750) - Superduplex like 2507

The balanced microstructures must be maintained also when fusion welding.

Edges of base metals must be carefully machined to facilitate full penetration without burn through.

Filler metal must always be used for Welding-duplex

Cleaning of base metal and consumables before welding should follow the highest standards.

For most of the Welding-duplex stainless, a matching composition of filler metal (similar to base metal composition) is used.

A post weld heat treatment (PWHT) at high solutioning temperature (1050-1100 °C = 1920-2010 °F) should be performed, followed by water quenching.

The usual low temperature (600-650 °C = 1100-1200 °F) stress relief treatment as used for carbon steel is harmful and must be avoided.

In common practice even the matching filler metals have a slight excess of nickel relative to the wrought base metal products, mainly to avoid high ferrite content of the weld metal.

If filler metal with higher (8-10%) Nickel is preferred (called overmatching), weldments can be put into service as welded.

Among the favorable properties of Duplex Stainless Steels, are chloride pitting and crevice corrosion resistance.

These are obtained by carefully balancing the amounts and the proportions of the different microstructures developed during primary material making but also during fabrication.

Proper control required to reach a stable duplex structure, depends on specific chemical composition, due to complex interactions between chromium, molybdenum, nitrogen and nickel.

Furthermore the thermal history of the material must avoid the formation, at elevated temperatures, of detrimental intermetallic structures, called generally sigma and chi phases.

The presence of nitrogen in suitable proportions effectively contrasts their appearance.

Welding-duplex stainless steel is made easier by the current commercial grades that are low in carbon (less than 0.03 %C).

These are free from the risk of sensitization and of intergranular corrosion due to carbide precipitation.

Solidification hot cracking risk is reduced by the adequate presence of ferrite.

Also by the fact that sulfur and phosphorus levels are kept low both in base material and filler metals.

Therefore hot cracking is seldom a concern when Welding-duplex stainless steel.

Hydrogen cold cracking resistance is satisfactory due to high hydrogen solubility in austenite that is present in the matrix in high percentage.

Not all the problems likely to occur when processing these materials may be readily apparent in the shop.

Problems cannot be detectable by non destructive inspection.

Those responsible for Welding-duplex stainless steel should be aware of this fact.

The weld quality, as demonstrated by achieved toughness and corrosion resistance in service, depends on strict observance of proven and qualified welding procedures.

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Any unauthorized deviation involves unacceptable risks of failure.

Preheating is generally not required, except for eliminating moisture in cold weather, and could be harmful.

The concerns associated with Welding-duplex stainless steel refer to the Heat Affected Zone, (HAZ) not generally to the weld metal.

Primary among these are loss of corrosion resistance and of toughness or post weld cracking.

In order to avoid these problems, Welding-duplex stainless procedures should try to minimize the total time at high temperature.

That should be preferred to limiting the heat input for any single pass.

In fact duplex steels have good tolerance for relatively high heat inputs.

Best practice allows for rapid cooling from weld temperature, by keeping the weldment mass relatively cool.

The requirement for maximum interpass temperature, is usually established at 150 °C (300 °F).

It must be enforced with suitable instrumentation both for welding procedure qualification and for production.

Contrary to common practice for different kinds of steel, limiting the heat input to very low values can have deleterious effects as loss of toughness and corrosion resistance.

However very high heat input increases the risk of intermetallic phase forming.

For Welding-duplex stainless standard welding processes are used.

Except that flux cored arc welding (FCAW) is not recommended, especially for the super duplex grades.

An Article on Spot Welding Duplex Stainless Steels was published (11) in Issue 81 of Practical Welding Letters for May 2010.
Click on PWL#081 to read it.

An Article on Precautions when welding Superferritic was published (2) in Issue 140 of Practical Welding Letters for April 2015.
Click on PWL#140 to see it.

An Article on Duplex Stainless Filler Metals and Procedure Qualification Problems was published (4) in Issue 152 of Practical Welding Letters for April 2016.
Click on PWL#152.

An Introduction to Bulletin_119 on Stainless Steels was published (8) in Issue 154 of Practical Welding Letters for June 2016.
Click on PWL#154.

An Article on Filler Metal for Stainless Steels was published (4) in Issue 156 of Practical Welding Letters for August 2016.
Click on PWL#156.

An Article on Comparing measured and calculated Ferrite content was published (4) in Issue 158 of Practical Welding Letters for October 2016.
Click on PWL#158.

An Article on FILLER METAL for welding super duplex stainless was published (4) in Issue 160 of Practical Welding Letters for December 2016.
Click on PWL#160.

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Watch the following Video

Research on Duplex Stainless Steels

http://www.youtube.com/watch?v=bvA6l9EVMzM

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