The following questions reflect the experience of fabricators in the questions most typically asked during fabrication of duplex stainless stell. Answers are suggested but in these practical matters, there is a wide range of possibly “correct” answers. The answer given may not be applicable to all possible situations. For further reading on welding guidelines please visit the Fabrication and Welding page.

Kiriman dari Moderator KBK Welding – Bapak Darmayadi.

Duplex Stainless Steels – FAQ’s during fabrication

The following questions reflect the experience of fabricators in the questions most typically asked during fabrication of duplex stainless steel. Answers are suggested but in these practical matters, there is a wide range of possibly ‘correct’ answers. The answer given may not be applicable to all possible situations. For further reading on welding guidelines please visit the Fabrication and Welding page. Feel free to send us your own question to duplex.ssw@kci-world.com.

1. Although it is recommended to use plasma torches for back gouging of the root and defect removal, can a conventional carbon arc be used? What is a minimum grinding that should follow arc gouging in order to remove heat-affected layer?

Carbon arc back gouging has been successfully used in the construction of 2205 duplex stainless steel vessels, but care must be taken to minimise the heating and the potential for carbon contamination. When care is taken in the back gouging procedure, the minimum grinding is not burdensome. It would be appropriate to perform a weld procedure qualification in which the typically applied back gouging has been included as it will be used in the practical construction.

2. What is the maximum allowed thickness reduction resulting from cold forming before solution anneal/ water quenching treatment would be required?

A precise answer to this question has not been developed. However, it has been common in construction of 2205 duplex stainless steel vessels to apply the same limits that are applied to carbon steels by the ASME Code. This limitation, while possibly overly conservative, has not led to any problems in service. For vessels not being constructed to ASME Code, significantly more aggressive deformation has been permitted, with no reports of problems attributed to this cold worked condition.

3. What is a proper method of repairing small defects and metal tears on the process side (for example, caused by knocking off of the pre-cut ladder supports and lifting lugs – usual method of removal)?

The repair procedure most typically satisfactory is to open the defect by grinding, if necessary, and then to repair by GTAW with the typical matching filler. Because of the size of the weld involved, it is unlikely that small defects or tears will lead to excessive time at temperature for these repair welds. However, care must be taken to avoid too rapid cooling of the weld (with resulting excessive ferrite). Slight warming of the metal under shielded conditions using the weld torch before the filler is introduced will typically.

Prevent too rapid cooling. Autogenous repairs are not recommended because of the likelihood of forming excessive ferrite in the weld.

4. Excessive heat input may result from weld repair of the defect discovered by post-weld NDT. Should such procedure be pre-qualified, and how?

It is appropriate to demonstrate that the weld repair has not damaged the material, i.e., to qualify the repair in much the same way as the procedure was qualified. So fabricators have qualified reasonably anticipated repair procedures in advance. Alternatively, the repair practice can be documented and simulated on a production runout tab, with the usual production test plate procedures then being applied to the repaired weld.

5. What is in fact the upper limit for weld heat input, provided base metal fully passed A-923 criteria?

Because the goal is to limit total time at temperature, it is generally better to complete a weld in fewer passes with relatively high heat input than many passes of lower heat input. The duplex stainless steels can tolerate relatively high heat inputs. It is not impossible to hot crack a duplex stainless steel during welding, but it is rare. The duplex stainless steels have relatively low thermal expansion and high thermal conductivity. The solidification of the duplex filler metals is not prone to hot cracking as is a fully austenitic solidification. Maximum heat input values as high as 65-100 kJ/mm have been found to be satisfactory, depending on the welding process.