What grade of duplex it is? and what process you are using? you want general acceptable values or how to control FN in the weld? what is your process fluid?
Chemical composition, welding and heat treatment are process parameters that determine the properties. To control all these parameters, some specifications are “over-specified”, which sometimes lead to conflicting requirements. One example is when an over-alloyed filler wire with 9% Ni is required for the longitudinal weld, and nitrogen is added to the gas for pitting resistance, in which case the ferrite content after heat treatment may fall below 30% in the weld. Most specifications allow minimum 30% ferrite in the weld, due to an expected risk for SCC, in spite of that the risk for SCC being negligible in the intended process environment.
For duplex steels the ferrite content is largely depending on the chemical composition and the thermal history. For the parent steel it is controlled within a fairly narrow range. In many cases the measuring methods commonly used give a greater variation than the process variation. The influence of variation (+/–15%) of the phase balance on properties such as tensile strength and corrosion resistance is small. The second generation of duplex stainless steels is in general aimed to contain 40 – 50 % ferrite in the parent steel resulting in optimum properties. No ferrite content is prescribed in the international standards. There are, however, other specifications requesting a ferrite range.
The most common specified ranges are 40–60% for the parent metal and 30–60% for the weld area. However, some of the most experienced end users of duplex and superduplex steels are using the Norwegian standard NORSOK that specifies 35–55% for the base metal and 25–60% for the weld area. The general opinion is that a too high ferrite content, i.e. >70%, decreases the toughness and pitting resistance, and a too low ferrite content, i.e. < 25%, decreases the SCC-resistance. What really matters is that the corrosion resistance and mechanical properties fulfill the engineering requirements. The ferrite content is not a property, but a way to check that the welding and heat treatment have been properly done. Therefore the limits should be within a reasonable range and be used for control only, and in case of deviation lead to an extra check of the material properties.
Source: ISSN 1101–0681. Teknisk information/Centrumtryck Avesta 2000
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of pankaj.johri@in.transport.bombardier.com
Sent: Friday, February 26, 2010 10:43 AM
To: materials-welding@googlegroups.com
Subject: [MW:4375] ferrite number of duplex stainless steel weld
To control the parameters of welding, we are measuring ferrite number of weld.
Can anybody tell us how acceptable values of ferrite numbers are decided.
Regards,
Pankaj
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