Welding of ferritic/martensitic stainless steels

Job Knowledge 101

The ferritic stainless steels contain up to some 27% chromium and are used in applications where good corrosion/oxidation resistance is required but in service loads are not excessive, e.g. flue gas ducting, vehicle exhausts, road and rail vehicles.

The martensitic grades contain up to 18% chromium and have better weldability and higher strengths than the ferritic grades. They are often found in creep service and in the oil and gas industries where they have good erosion and corrosion resistance.

Now for a little metallurgy! Chromium is an alloying element that promotes the formation of ferrite in steel; in the case of the ferritic stainless steels, this ferrite is the high temperature form known as delta-ferrite. Unlike the low alloy steels, therefore, this type of steel undergoes no phase changes as it cools from melting point down to room temperature; they cannot therefore be hardened by heat treatment and this has implications with respect to the properties of welded joints.

Carbon and nitrogen, however, are two elements that promote the formation of austenite so, as the percentage of carbon and/or nitrogen increases, the ferritic steel can be designed to transform, wholly or partially, to austenite before transforming back to ferrite. This series of phase changes are similar to those in a low alloy steel, enabling the steel to be hardened by producing martensite - the martensitic stainless steels. Compositions and typical properties of some of the alloys are given in Table 1.

Table 1 Typical properties of ferritic and martensitic steels

There are a number of welding problems with the ferritic steels. Although they are not regarded as hardenable, small amounts of martensite can form, resulting in a loss of ductility. In addition, if the steel is heated to a sufficiently high temperature, very rapid grain growth can occur, also resulting in a loss of ductility and toughness.

Although the ferritic steels contain only small amounts of carbon, on rapid cooling carbide precipitation at the grain boundaries can 'sensitise' the steel making it susceptible to inter-crystalline corrosion. When this is associated with a weld it is often known as weld decay. Developments in recent years of extra low carbon, titanium or niobium containing grades have, however, improved this situation.

The ferritic stainless steels are generally welded in thin sections. Most are less than 6mm in thickness where any loss of toughness is less significant. Most of the common arc welding processes are used although it is regarded as good practice to limit heat input with these steels to minimise grain growth (1kj/mm heat input and a maximum interpass temperature of 100-120°C is recommended) implying that the high deposition rate processes are inadvisable. Preheat is not required although it may be helpful when welding sections over, say, 10mm thick, where grain growth and welding restraint may result in cracking of the joint.

Welding consumables for the ferritic steels are generally of the austenitic type; type 309L (low carbon grade) is the most commonly used. This is to ensure that any dilution that occurs does not result in a low ductility austenitic/ferritic/martensitic weld metal micro-structure. However, provided care is taken to control dilution, types 308 and 316 may be used. Nickel based consumables may also be used and will result in better service performance where the component is thermally cycled. A matching filler metal is available for welding of Grade 409 steel, often used in vehicle exhaust systems.

Post weld heat treatment (PWHT) at around 620°C is rarely carried out although a reduction in residual stress will give an improved fatigue performance: nickel based fillers are a better choice in this context than the Cr/Ni austenitic consumables.

The martensitic grades are used in more challenging environments and, as the name suggests, present rather more problems than the ferritic steels. Both the higher carbon (>0.1%) and low carbon (<0.1%) versions, with a few exceptions, require preheat and PWHT to avoid weldment cracking problems and to provide a sufficiently tough and ductile joint.

Matching welding consumables are available for most grades so that corrosion resistance and mechanical properties can be matched to those of the parent metal. To reduce the risk of hydrogen induced cracking, low hydrogen welding processes are essential and preheat temperatures of 200 to 300°C are recommended. A weld that has been completely transformed to untempered martensite by allowing the joint to cool to room temperature can be extremely brittle and great care is needed in handling to prevent brittle failure. In addition, such joints are sensitive to stress corrosion cracking even in a normal fabrication shop environment. It is highly advisable therefore to PWHT as soon as possible on completion of welding.

A conventional heat treatment cycle would be to cool the joint to below 100°C to ensure full transformation of the weld and HAZ to martensite, closely controlled heating to minimise stresses from temperature variations, PWHT at around 700°C for one to four hours and controlled cool to ambient.

A hydrogen release treatment from the preheat temperature, say 350°C for four hours, is unlikely to reduce the risk of cold cracking. If the steel is not allowed to cool to a sufficiently low temperature so that full transformation to martensite takes place then there will be austenite present during the hydrogen release treatment.

This austenite will retain hydrogen and may generate cracks when it transforms to martensite as the joint is cooled to ambient. If cold cracking is a real issue, even with good hydrogen control, then it may be necessary to PWHT directly from the preheat temperature, cool to ambient and repeat the PWHT to temper any martensite that was formed following the first cycle of PWHT.

Welding consumables matching the base metal composition are available for most of the martensitic stainless steels, often with small additions of nickel to ensure that no ferrite is formed in the weld. Nickel lowers the temperature at which martensite transforms to austenite so it is important with such filler metals that the PWHT temperature is not allowed to exceed about 750°C otherwise untempered martensite will form in the weld as the item cools to ambient.

Conventionally, when welding dissimilar metal joints the filler metal is selected to match the composition of the lower alloyed steel. Experience has shown that this can cause cold cracking problems so filler metals matching the martensitic steel should be used. An alternative is to weld with austenitic stainless steel fillers, type 309 for example, but the weld may then not match the tensile strength of the ferritic steel and this must be recognised in the design of the weld. Nickel based alloys may also be used; alloy 625 for instance, has a 0.2% proof strength of around 450MPa; and will give a better match on coefficient of thermal expansion.

The metallurgy of these types of steels is complex and they are frequently used in challenging and safety related environments. An article such as this can only give a partial picture so if there are any doubts surrounding their fabrication it is recommended that advice is sought from suitable specialists.

This article was written by Gene Mathers.