Welding dissimilar metals
Introduction
Dissimilar metal welding can be the joining of two different alloys, the joining of one alloy with a different filler metal or could be the overlay of an alloy onto a substrate for improved performance. These different situations all have comparable concerns and require similar care in developing a suitable solution for use.
There are many joining processes including:-
- Bolting
- Adhesives
- Riveting
- Brazing
- Soldering
- Fusion welding
Some of the newer more specialist welding techniques such as laser, electron beam or friction welding are beneficial but are not suitable for all situations and are not routinely available to all. This guide will concentrate on the common fusion welding processes.
The aim of a dissimilar weld is to complete a joint with a filler metal that will produce a continuous ductile weld that has suitable properties to meet the design requirements.
The combination of joints can range from the welding of a single alloy with an over-alloyed filler metal to welding any combination of up to three or more different materials (2 base metals, a weld filler and possibly a buttering layer) so can cover a range of complex metallurgical issues in providing the correct solution for the completed weld.
This guide is intended as an introduction with some guidance to enable users to consider the required information to be able to establish suitable procedures for the dissimilar joining of NeoNickel alloys.
There are several factors that need to be considered when undertaking a dissimilar weld with fusion welding and these are:
Physical Properties, such as melting point, thermal conductivity and coefficient of thermal expansion differences.
Metallurgical properties, formation of Intermetallic phases with limited ductility and the microstructure stability in the short term and long term.
Corrosion and oxidation resistance, fitness for intended service and possible galvanic corrosion of dissimilar joints.
Each of the above factors will affect the completed weld and are discussed in more detail below.
Physical Properties
If there is a marked difference in melting temperature of the materials to be joined then the lower melting point alloy will melt quicker and provide more of the weld pool and more dilution. There are several other factors that can affect the amount of dilution that is obtained, these are welder manipulation giving greater focus on the lower or the higher melting point alloy, split multi-runs and the position of those runs and the welding parameters used. Low heat input processes will be beneficial as they minimise dilution.
Thermal conductivity differences will affect heat flow and allow heat to flow more easily into the side with the higher thermal conductivity. This means that a higher arc energy is required to complete the weld which may not be compatible with all the alloys to be welded.
This will also be affected by geometry (thickness and size) of each alloy, the welding position and by welder manipulation.
Thermal expansion differences between two alloys can be marked and these differences can introduce additional welding stresses that can aggravate weld cracking, HAZ cracking or distortion. If one alloy is susceptible to hot cracking then the additional stresses could make the problem worse. As well as leading to distortion difficulties then thermal expansion differences in a joint has also been known to lead to thermal fatigue in service.
Microstructure
In all 3 dissimilar weld cases the concern is the mixed transition zone between one alloy and another where potential metallurgical effects can take place. There will be a variation in composition across the weld zone from the fusion line on one side through the weld metal and into the fusion zone of the other side of the weld.
Considerations need to be the formation of intermetallic phases which can embrittle the weld and the stability of the weld over the short and long term.
The potential dilution can normally be calculated using the lever rule and depends on the weld process, the melting points of each material and the welding position. Different weld techniques and weld processes can cause differences in the composition obtained in the weld.
There are several diagrams that can help to predict the phases that will be developed in a stainless steel dissimilar weld that are a useful first step to alleviate any potential problems that can arise. These are Schaeffer, for standard Chromium Nickel stainless steels, Delong modified Schaeffer diagram for stainless steels including Nitrogen and the WRC diagram for those stainless steels with additions of Copper. These diagrams will predict the structure that will be obtained in the weld and the predicted amount of ferrite.
For stainless steels care must also be taken with carbon migration when joining carbon steels to stainless steels. This can lead to sensitization both in the short term in fabrication and in the long term during service.
For Nickel Alloys there is a limit of solubility of the major alloying elements Iron and Chromium. All Nickel alloys will tolerate up to 5% iron without complications but above this level is dependent on whether it is pure Nickel, a Nickel Chromium or Nickel Copper alloy. It is a similar situation with Chromium in Nickel alloys with different systems having different limits. The AWS has charts, available in the references at the end of this document, that are based on practical experience and enable users to calculate the dilution and check that it will be within these limits for different alloy systems.
Titanium alloys have limits for Iron, Nickel and Chromium so in general a buffer layer must be used when looking at dissimilar welds between these alloys. Vanadium, Copper and Niobium can provide suitable buffer layers.
Corrosion
As dissimilar welding involves two or three different alloys then it is important that the weld has sufficient corrosion resistance for the intended service. As potentially there are different materials and compositions across the weld zone then Galvanic corrosion needs to be considered between the elements of the weld. If the weld is a very different composition to the other alloys then Galvanic corrosion is a distinct possibility.
There is an unmixed zone on either side of the weld where there is not enough time for the diffusion of alloying elements to take place which will have a markedly different composition and this can cause corrosion issues.
The relative positions of the two dissimilar alloys in the galvanic series and the relative surface areas of the respective alloys need to be considered. A small area ratio of an alloy such as a weld or part of the weld that is lower in the galvanic series could corrode very quickly. The corrosion rate will depend on the driving force dictated by the surface area and the difference in the galvanic series.
Coatings on the less noble material can provide a solution to the galvanic problem and the extent of coating depends on the difference in surface area and the difference in the galvanic series. It should be noted that a coating will protect against corrosion but could if the coating gets damaged, greatly increase local corrosion rates.
Pre- qualification
It is essential that because of the complex nature of dissimilar welding that pre-qualification of the intended joint is carried out to ensure that all the relevant design considerations are met. The completed weld should be tested to ensure that it meets the design requirements, structurally, metallurgical integrity and corrosion resistance.
It is usual that the acceptance criteria of weld strength testing should meet the lower of the base materials but some codes do accept a slight loss of strength of the joint.
It is also important that if one of the components requires post weld heat treatment that the effect of any PWHT is considered in relation to all the other possible compositions in the weld zone and tested in the treated condition.
Improved performance
As well as when joining two different alloys a dissimilar weld is also made if a higher alloy weld metal is deposited as an overlay or is used to join a lower alloy material. Typical instances are weld overlay of carbon steel for improved corrosion resistance. Alloy 625 is widely used for this application.
Alloy 625 is also used to weld the high Alloy Austenitic alloys and the super Austenitic alloys such as the 6% Mo alloys, AL-6XN® (UNS S08367), UNS S31254, UNS S08904 and similar alloys. It is used in this instance to maintain the minimum high molybdenum level across the weld zone because of segregation in the weld metal on cooling.
ZERON® 100X Super duplex weld metal has been successfully used for welding of 22Cr Duplex alloys and 13 Cr Super martensitic alloys.
Alloy 22 nickel, chromium, molybdenum alloy has been successfully used to weld dissimilar Nickel based corrosion resistant alloys.
All the above are still dissimilar welds albeit are common solutions to improve performance and additional information on all these applications is available on request.
NeoNickel has a weld wire wizard available on the website to give possible filler metal solutions for common dissimilar weld metal combinations with most NeoNickel alloys.
References
Dissimilar Metal Welding N F Herbst, Australian Welding Institute.
Guidelines for welding Dissimilar Metals Avery R. E. NIDI Publication 14018.
Schaeffer Diagram
Delong modified Schaeffer diagram
WRC 1992 Diagram
