Abstract

The market today have four main types of duplex steels, lean duplex, standard duplex, super duplex and hyper duplex. The differences between them are the chemical analysis and their mechanical and corrosion properties. Their similarities are that they all have a ferritic solidification and that the austenite formation starts at app. 1.420 deg.C, at app. 800 deg.C the austenite formation is finalised. The fact that the steels have two crystal structures results in a combination of desirable properties that is beneficial to the user of the steel. The ferrite basically gives high strength and resistance to SCC (Stress Corrosion Cracking), the austenite gives ductility and general corrosion resistance. 

In choosing duplex in a construction it is often possible to reduce plate thickness due to the higher strength and thereby reduce weight of the construction. In addition to this the world market prices for alloying elements as Ni has increased a lot and therefore the lean duplex grades have increased in popularity. One of the first grades were 2304 (1.4362) and the intention with this material was to compete with 316 grades as the strength was higher and the corrosion properties at least as good. The latest grade 2101 (1.4162) has gained a large market share in a relatively short time.

One issue that often occurs is the weldability with the “leaner duplex grades”. Welding standard duplex is not a problem and with all processes there are consumables well suited for the applications. To weld the 2101 (1.4162) is from a metallurgical point of view no problem at all, actually it is even easier to weld than a standard grade as it is possible to perform autogenous welding, and this shall always be avoided for standard duplex material. The practical problem in welding 2101 is that the viscosity of the molten pool is different, and thereby the wettability is a bit worse. This forces the operator to be more active with the arc during welding, and this is a problem. It can be counteracted by choosing an over alloyed consumable, but we often want a matching consumable.

In the 2101 we also have an interaction in HAZ between the microstructures in LTHAZ and HTHAZ that is more favourable than with 2304 or 2205 respectively 2507. In the trials with 2101 it has also been found that due to the lower Ni-content we have a different type of “heat tint” containing more N and Mn and this affects the corrosion properties. This element loss that occurs in the arc and weld pool due to evaporation and deposition of N and Mn is new to the duplex grades and is more described in the lecture.

Introduction

During the recent years prices for alloying elements and Ni in particular has been very high, this has been changed by the finance crisis when prices dropped but prices are now on the way up again. The Ni-price has been one driving factor for the development of the new lean duplex grades, the steel producers tried to find a chemical composition that gave the same high strength properties and a bit reduced corrosion properties, with a much lower level of Ni. This has been achieved in the new composition according to EN 1.4162 and ASTM S32101. The actual word duplex stems from the Latin language and means “two parts” in this case relating to the ferrite and the austenite. As the steel starts to solidify it does so in a ferritic structure, at 1.420 deg. C the austenite starts to form and this continues down to 800 deg. C when the phase balance is reached. How the balance looks at room temperature is to a large extent depending on the temperature history between 1.420 an 800 deg. C. As can be seen in the table 1. the Ni-level is considerable lower compared to standard duplex steels.

                       
Material properties

Lean duplex combines high strength with good fatigue resistance, very good weldability (when using the right technique), good corrosion resistance and high energy absorption. The applications are many and are increasing all the time, to mention some it is used for storage tanks, water piping, reinforcement bars, building and construction. The table 2. shows the mechanical properties.

                 
In many cases lean duplex materials are selected to replace austenitic materials (EN 1.4301, UNS S304). The benefits in the lean duplex steel are better resistance to stress corrosion, better corrosion resistance in Cl-containing environments, twice the yield strength and thereby savings in material consumption as thinner material can be selected. When general weight is reduced for road, rail and water moving transport containers the fuel efficiency is improved and has a positive effect on environment.

The graph shows corrosion comparison between some stainless steels in sulphuric acid.

          
The actual composition of lean duplex with C = 0,03; Mn = 5,1; Cr = 21,4; Ni = 1,6; Mo = 0,3 and     N = 0,23, has created a very well balanced composition. The interaction between Nitrogen and Manganese contributes to a very rapid austenite formation in the weld metal and in the heat affected zone, even faster than for other duplex grades. This has proven to give a change compared with other grades and that is that it now is possible to do autogenous welding, i.e. to weld without any filler metal. If this is done in standard duplex grades the formation of austenite will be to low because the thermal cycle during heating and cooling is too fast to allow enough austenite to be formed, therefore a filler metal with higher level of Ni than that of the base material is needed to promote the austenite formation.

Standards, approvals and products

On the market today a number of different products of lean duplex are available in different dimensions. To mention some; hot rolled plate, cold rolled plate and coil, cold rolled coil and sheet, billet, wire rod and bar, welded tube and pipe, rectangular hollow section, welding consumables. Standards available are; 

ASTM A240 – UNS S32101 – Plate, sheet and strip

ASTM A276 – Bars and shapes

ASTM A479 – Bars for boilers and other pressure vessels

ASME Code Case 2418 – Plate, sheet and strip

ASTM 789/A790 – Seamless and welded tube and pipe

PED97/23/EG: PMA 1.4162 – TUV hot rolled plate, cold rolled sheet/plate

Preparation of a harmonised EN application is in progress

Welding methods and ferrite content

This material is suitable for welding with all normal welding methods such as GTAW, GMAW, PAW, SMAW, FCAW, SAW, Laser, HybridLaser, resistance welding. For GTAW it is common to use a filler wire composition of the same content as the lean duplex plate but for other processes a slightly higher content of Ni is used. The reason for this is to further promote the formation of austenite in the weld metal. If we look at the ferrite content in the weld metal we can identify some important factors that will influence, they are; the chemical composition, dilution with the base material, heat input, material thickness and the interpass temperature. When welding with gas shielded processes the composition of the shielding gas is very important as N will be lost from the weld metal, especially sensitive is the root side. Here a purging gas with additions of N is needed to counteract the diffusion of N from the weld metal to the gas. All systems containing gas will try to reach equilibrium if they can and therefore the need of N in the root gas. If the level of ferrite becomes too low we will have risk of decreased strength and risk of decreased resistance to stress corrosion cracking. On the other hand if the ferrite content is too high we have risk of decreased resistance to pitting corrosion, risk of decreased toughness and increased risk of hydrogen cracking.

     
SMAW weld metal

FCAW weld metal

PAW no filler

Above photos shows three different weld metals from different methods with different phase balances but they are all within the normal allowance of ferrite levels stipulated by different classification societies. Those limits are often from 30% up to 70% a bit depending on the application and project requirements and specifications. The microstructure in the duplex weld metal is characterised by almost no risk of solidification cracking, the growth of intermetallic phases such as sigma phase is very slow, and the risk of hydrogen cracking is also very low. In the HAZ the grain growth rate is slow due to the composition of the steel. Special care needs to be taken in consideration for the joint preparation in lean duplex, first of all joint angles shall be slightly larger than for normal austenitic steels, the reason is that the viscosity of the lean duplex is lower and this means that the wettability properties are not so good. Root opening shall also be larger than for normal stainless steels, 3 – 4 mm opening is preferable and no problem to weld in. Tack welds shall be closer than normal. If plasma cutting is used for joint preparation it is necessary with a slight grinding of the surface before welding.

Weld defects

It is relatively problem free to weld in lean duplex but it is important to be aware of the different behaviours of the weld metal compared to an austenitic stainless steel, 304 or 316. Due to the fluidity of the weld pool and the slag control it is a risk to get inclusions in a joint especially if the joint angles are too small. Due to the material properties of lean duplex, the penetration into the base material is less than for 1.4301 or 1.4401 material. Hence the importance to make the right joint design especially in thicker sections and when welding from two sides, X-joint, here the root face and opening is of the highest importance to avoid defects.

Typical defect of inclusion

Typical defect due to less penetration

Due to the high degree of N alloying in both the plate and the filler material there is a potential risk of getting porosity in the weld due to either too thick or too thin weld beads, wrong joint preparation and also if the arc is not properly protected.

 
Porosity formed and included           

Porosity formed and surface breaking

Welding

As stated before it is relatively easy to weld if proper procedures are followed. Joining lean duplex to other materials is easy and commonly done and in most cases the best solution for the joining is a matching consumable. As it comes to the question of post weld heat treatment that is normally not needed but if it should be necessary the material is annealed at 1020 – 1080 deg.C. Rapid cooling is recommended after the annealing. Under normal conditions preheating is not necessary, welding outdoors in humid or cold weather condition requires a slight preheating (max 50 deg.C.) to avoid moist in the joint. Interpass temperature shall never exceed 150 deg.C. To keep microstructure as it shall be, large fusion of parent material shall be avoided, for SAW welding a basic flux shall be used and no more than 2,5 mm wire diameter shall be used.

Weld oxides

As on all stainless steels the lean duplex steel is covered by an oxide layer of Cr-oxide, this layer forms naturally in all oxidising environments and are to an extent self-healing. The thickness is 2 – 5 nm. One of the most detrimental forms of corrosion that can attack the steel is, pitting corrosion, this corrosion can make deep and fast penetrations into the material. When welding in the steel the oxide layer increases in thickness rapidly from the heat and the presence of oxygen and Cr, this leads to a weld oxide that is often more than 200 nm thick and leaves a Cr-depleted layer below, this must be removed after welding in order to prevent future corrosion attack. This phenomena takes place on both the root side and the front side, this is the reason why root-gas systems are used as well as proper gas shield on the front side. The removal of the weld oxides is really important for the performance and the life span of the welded object. This can be done by grinding, sandblasting, pickling, passivation. Lean duplex objects that have been welded with an over alloyed consumable such as standard 2205 and after this has to be pickled will be difficult to get the proper surface as the weld will be more difficult to get clean and this leads to that the surface of the HAZ will be over pickled. So, it is important to use the right composition that is as close as possible to the composition of the steel.

Conclusion

The lean duplex steels that are a result of producing a more price stable duplex steel than before have a lot of good properties that many fabricators will take advantage of, many years to come. There are many new applications still waiting to be explored. The use of the lean duplex steels instead of normal austenitic steels will contribute to a better environment through reduced weights of constructions. The welding in lean duplex is relatively problem free if precautions are taken and the operators have the proper education and training. The post surface treatment is important for the life of the construction and here the right choice of consumable is important.