Duplex stainless steel grades are high strength and extremely corrosion resistant materials. They are an ideal candidate for a wide range of applications. They are chemically balanced to offer a ferrite and austenite structure possessing 40 to 60% ferrite configuration subsequent to solution annealing. Popularly used duplex steel grades are 2205 and 2507.

Stainless steel grade 316 consists of austenitic microstructure with little fraction of ferrite phase. The magnitude of nickel in wrought SS 316 is generally in the range of 10 to 11%.

The chemistry of Duplex stainless steels is adjusted to develop a microstructure that consists of equal proportions of ferrite and austenite phase. It is done in Duplex steel 2205 by decreasing the nickel magnitude up to 5% and modifying manganese and nitrogen inclusions to produce 40 to 50% ferrite. The composition of 2205 steel is balanced to achieve equal corrosion resistance by austenite and ferrite phase. It states the dual austenite and ferrite microstructure.

The increased magnitude of nitrogen and fine grained 2205 duplex steel has higher strength than traditional austenitic grades of SS 304l and SS 316l. In the solution annealed form, grade 2205 has double yield strength of SS 316l. On the base of design code utilized in production of processing capability , this larger strength grade increases the limit of maximum tolerable stress. In several applications, it tends to decrease the material thickness hence becoming more cost and weight savvy material than other steel grades. 

Duplex stainless steels formation

Hardening

The operator should be careful while cold forming of Duplex stainless steel 2205. The larger annealed strength of duplex 2205 grade requires higher power on cold treating apparatus. It has tensile strength 30% more than stainless steel grades 304l and 316l.

Cutting

The processes applied to austenitic stainless steels and carbon steels can be applied in the cutting procedure of duplex stainless steels with some modifications in parameters to compensate the difference in the mechanical and thermal properties.

Sawing

Due to their high strength, large work hardening rate and virtual absence of additions that would act as chip breakers, the duplex steels are tougher to saw as compare to carbon steels. The suitable outcomes are received with strong machines, power blade arrangement systems, coarse toothed blades, slight to medium cutting paces, large feeds and a normal coolant supply, basically a synthetic emulsion that offers lubrication in addition of cooling, provided to enable the blade to carry the coolant into material. The cutting paces and feeds should be identical to those utilized for 316 austenitic stainless steel.

Shearing

The Duplex stainless steels shear on the identical apparatus  that is used to shear steel grades 304 and 316 often without any particular modifications. Although, because of larger shear strength of duplex stainless steels, the power of shear should be larger or sheared thickness should be decreased. The shear strength of steels is up to 58% of the ultimate tensile strength for hot and cold treated plates and sheets. The Duplex steels act in the same manner as anticipated from the thicker piece of SS 316. So, the most feasible thickness for Duplex stainless steel 2205 that can be sliced on a specific shear is 75 percent of that SS 304 or SS 316. The maximum thickness of duplex stainless steels can be cut on a specific shear is 65 % of the common austenitic steel types.

Slitting                                          

Traditional coil slitters are utilized to shear coiled duplex stainless steel sheet or strip. The wounded stainless steel feeds from a payoff reel and by an upper and lower arbor on the slitting line that consists of round slitting knives and a start reel recoils the slit width loops. The location of the slitting knives is changed on the base of needed slit mult width of the coil. As the duplex stainless steels possess higher strength than austenitic steel grades, slitter knife tool wear and slit edge reliability is harder to monitor. For keeping suitable slit edge of duplex steel coils it needs to use tool steel or carbide slitter knives.

Punching

Punching can be considered as a tough form of shearing. The large strength, quick work hardening and wear resistance result in much complicated punching and wear to the tooling for duplex steels. A suitable initial step and instruction is to consider that duplex steel will act identically to austenitic steels double its thickness. The highly alloyed duplex stainless steels with larger levels of nitrogen are excessively tougher.

Plasma and laser cutting

The duplex stainless steels are normally treated with same plasma cutting and laser cutting device as utilized for processing austenitic stainless steels. Normally higher thermal conductivity and smaller sulfur magnitude in duplex stainless steels may influence the suitable factors marginally, however suitable outcomes can be received without particular adjustment.

The heat affected zone of plasma cutting procedure is normally narrow about 0.25mm as the cut is made quickly in one cycle with quick quenching from the plate or sheet. The standard machining of a weld groundwork and melting of neighbor base metal while welding will discard the HAZ of the plasma cutting procedure.

Forming

The duplex stainless steels offer outstanding hot forming property with comparatively small forming stress about at 1230oC or 2250oF. Although if hot forming occurs at very small temperature limit, deformation occurs in the weaker zone however slight ductile ferrite can cause cracking of the ferrite structure in the damaged area.

Moreover, a significant magnitude of sigma phase precipitates upon falling of hot processing temperature limit. Common processes suggest the largest hot processing temperature limit from 1100oC to 1150oC or 2000oF to 2100oF. Such higher limit is recommended as the influence of elevated temperature on the size stability of a component and improved potential to produce scale exceeding 1150oC or 2100oF, at such limits, the duplex steels become soft and produced components like as vessel heads or pipe warp in the furnace without support.

At these temperature limits the steel become extremely soft for specific hot producing operations. Following table shows the recommended temperature limit for hot forming and least soaking temperature limits for steels.

Steel grade	UNS series	EN series	Hot forming temperature		Lowest soaking temperature
	S32101	1.1462	1100 oC to 900 oC	2000 of to 1650 of	950 oC	1750 of
2304	S32304	1.4362	1150 oC to 950 oC	2100 of to 1740 of	980 oC	1800 of
2205	S32205	1.4462	1230 oC to 950 oC	2250 of to 1740 of	1040 oC	1900 of
2507	S32750	1.4410	1230 oC to 1025 oC	2250 of to 1875 of	1050 oC	1920 of
	S32520	1.4507	1230 oC to 1000 oC	2250 of to 1830 of	1080 oC	1975 of
	S32760	1.4501	1230 oC to 1000 oC	2250 of to 1830 of	1100 oC	2010 of
304	S30400	1.4301	1205 oC to 925 oC	2200 of to 1700 of	1040 oC	1900 of
316	S31600	1.4401	1205 oC to 925 oC	2200 of to 1700 of	1040 oC	1900 of

It is not recommended every time to initiate the hot processing at the maximum temperature limit. Although the steel should approach to lowest soaking temperature prior the hot processing. The furnace is charged warm, to prevent sluggish heating by the temperature limit to produce the sigma phase.

The temperature consistency is essential in the efficient hot forming of grades of duplex steel. If the shape of sample is non-compacted, it means edges are considerably cooler than the main area, in such condition cracking may occur in the colder parts. To prevent the deformation, it is essential to heat the material again when such local areas are at the risk of quenching lower than the recommended hot processing temperature. The minimum limit of hot temperature can be increased slightly in case the temperature stability of the material particularly the boundaries or thinner parts is sustained. 

In massive parts, it is suitable to observe the suitability of water cooling to avoid precipitation of intemetallic phases. For plate shapes, the thickness is up to 150 to 200 mm for Duplex stainless steel 2205 plate in wrought form and 75 to 125 mm for wrought super duplex plate, the specific limits alter with the chemistry of the duplex steel and the effectiveness of the cooling device. For a normal cylinder of 2205 steel of dia. 375 mm, if the finished component is to possess a through penetrating in the diameter, the quenching of the component subsequent to eventual annealing is significantly enhanced when the opening is machined before the final heat processing.

Solution annealing

Subsequent to hot forming, it is essential to conduct a complete solution anneal after which the quick quenching is done to recover the mechanical characteristics and corrosion resistance. The material should be kept above the lowest solution annealing temperature and kept sufficiently longer to dissolve the intermetallic precipitates. A beneficial instruction is that the keeping period at the temperature should be equivalent to the complete  period for which the component was kept at the temperature range of 650oC to 980oC or 1200oF to 1800oF after the earlier complete annealing. The component should be water cooled from the solution annealing temperature. Avoid it keeping at 700oC to 1000oC or 1300of to 1830of for a long time while it is transferred to the cooling station subsequent to final annealing.

The Duplex stainless steels are very soft at the solution annealing temperature and warping and deformation may occur if it is not sufficiently handled. It can be a considerable issue in tubular products, particularly those with big diameters and thin walls. Reforming or straightening warped duplex products are tougher than austenitic stainless steels due to their large strength at the ambient temperature limits. Efforts to reduce this deformation through small annealing periods, slight heating into the annealing temperature limit or smaller than the preferred annealing temperature cannot soften the intermetallic phases or may result in the production of more intermetallic phases. It will decrease the corrosion resistance as well as toughness. The suitable alternative is to use the temporary supports to sustain the shape of the material while annealing of the thin regions.

It is not suitable to follow stress relief processing to decrease the cold work of forming or straightening functions.  Duplex stainless steels intrinsically possess excellent chloride stress corrosion cracking resistance and it can be significantly enhanced by decreasing the level of residual cold work. No suitable temperature limit lower than the solution annealing temperature is available to use the stress relief without the risk of production of damaging phases that will decrease the corrosion resistance as well as toughness.

Warm Forming

 It is significant to somewhat heat a steel specimen for a short time to enhance the forming process. Although extended heating of duplex stainless steels above 315oC or 600of may cause some loss of ambient temperature toughness or corrosion resistance because of embrittlement at 475oC or 885oF. At the elevated temperatures, there is a danger of a quicker and more damaging effect due to precipitation of intermetallic phases. As these phases do not enter the forming procedures, it is feasible to heat the duplex steels while forming. Although when the process temperature is 300oC or 570oF, hot forming is conducted by a complete solution annealing and quick cooling.

Cold forming

The duplex stainless steels offer appreciable formability in the several fabrication processes. Various applications of duplex steels need comparatively straightforward forming like rolling of cylindrical parts, press forming and vessel and tank head forming through pressing or rolling. In several applications, a basic factor is high strength of steel duplex and the potential of the forming apparatus.  A normal idea is that a duplex steel responds to forming identically to austenitic steel 300 series grades at double of thickness. Besides of larger initial deformation forces, duplex steels also powerfully work harden as they are distorted. This quick increase in strength while deformation also should be considered while finding the largest material thickness that is possible to make on a press. When the apparatus has necessary power, permission should be given for the larger springback resulted by the high strength of the duplex steel grades.

The smaller ductility of duplex stainless steels as compare to austenitic stainless steels should be considered. The duplex steels have a lowest needed elongation of 15 to 30% in various specifications, as compare to 40%, least elongation needed for several austenitic steel types. While real elongations may be noticeably larger, the relationship recommended by such least is suitable and there are suitable instructions for cold producing processes. The duplex steels need more munificent bend radius rather austenitic steels or require moderate annealing in intense or complicated forming due to their smaller ductility.

Press forming

The duplex stainless steels are easily press formed, although in several conditions, these are taken as a replacement for a component that has been chosen for austenitic steel grade, carbon steel or ferritic steel .

Initial test is usually conducted without altering the thickness. While the larger strength of the duplex grade would adjust the decreased thickness, expenses of redesign may delay in receiving benefit of the cost and material saving. In various conditions, decreasing the thickness enhances forming. In the starting forming tests of a duplex steel, it is usually considered tough somehow.

While comparing the carbon steel or ferritic steel, the issues are with strength and springback. The duplex steel grades are 30 – 50% larger in yield strength however ferritic steels have only controlled work hardening and the running stress can be significantly small. The duplex steels become stronger significantly then springback is the issue. Unlike, the ductility of duplex steel grades is larger, extensive deformation will adjust the springback. Further comparing with ferritic steel types, the duplex stainless steels are lesser susceptible to the bending route relative to the rolling route. The duplex steels also have anisotropic mechanical characteristics due to the rolling of the duplex structure, however its realistic influence is lower than ferritic steels due to the larger ductility of the duplex grades.

In the duplex grades, deep drawing cannot be successfully implemented. Although a new kind of duplex steel has been made to compensate this limitation. These steels posses an exclusive pair of high strength and significantly enhanced formability through transformation induced plasticity (TRIP). The featured characteristics are better mechanical strength and enhanced formability as compare to other duplex steels by TRIP.

Spin forming

The strength and corrosion resistance of duplex steels particularly because of resistance to chloride induced stress corrosion cracking, these are a reputed material for rotating parts operations for example, centrifuges. Spinforming is a cost-effective and commonly employed technique to produce such components.

It is a sophisticated process with considerable reliance on apparatus and operator knowledge. Austenitic steels are usually spinformed however they need several moderate annealing works to recover ductility while the forming series. The controlled use in sprinforming of duplex grades shows that the forming stresses are very large, particularly when the component thickness is not lesser as compared to austenitic stainless steel grade. With required power and strength in the device, the duplex steels sufficiently spinform however their smaller ductility may need more immediate moderate anneals than those of austenitic steels. Few duplex steel components are spinformd at temperatures exceeding 650oC or 1200oF prior the complete solution annealing.

Machining

The duplex steel grades possess double yield strength than austenitic steel grades and their first work hardening rate is almost equivalent to the austenitic grades. The chip produced while machining of duplex steel is strong and rough to tools, particularly for the highly alloyed duplex types. Since such steel grades are made using smaller magnitude of sulfur, it is sufficient to improve chip damage.

Therefore duplex steel grades are usually tougher to machine as compare to austenitic steel 300 series grades however both have similar resistance to corrosion properties. Larger cutting potential is needed and quicker tool wear is common in machining of duplex steel grades. This harder machining tendency becomes more perceptible with carbide apparatus.

Instructions for duplex steel grades machining

1. Strong and sturdy devices with very strong and sturdy mounting of the tools and specimen.
2. Decrease vibration by minimizing the tool extension.
3. Use nose radius on the apparatus only when required
4. Use edge shape for carbide apparatus that offers a sharp edge in addition of necessary strength.
5. Design machining series offers cut depth beneath the work hardened surface received from earlier cycles
6. Using necessary but not over speed prevents construction of edge and quick damage,
7. Alter the tooling inserts at scheduled intervals, make sure that pointed cutting edges are received.
8. Use normal coolant flows through cutting oils or emulsions along extreme pressure inclusions.
9. Utilize finished carbide inserts with positive chip breaker shape.

Machining table for 2205 duplex stainless steel grade

Machining	Metallurgical condition	Cutting depth or width (inches)	Cam functioned –coated carbide			CNC –coated carbide
			Cutting speed SFPM	Feed (ipr)	Tool type	Cutting speed SFPM	Feed (ipr)	Tool type
Turning	Annealed/ cold drawn	.04 inch	325 to 445	.004 ipr to .010 ipr	C5 / C6	296 to 395	.004 ipr to .008 ipr	C6 / C7
		.08 inch	295 to 430	.004 ipr to .012 ipr		263 to 360	.006 ipr to .010 ipr	C6
		.012 inch	230 to 360	.005 ipr to .014 ipr		230 to 330	.008 ipr to .014 ipr	C5 / C6
	Annealed	.04 inch	360 to 525	.004 ipr to .010 ipr		490 to 660	.003 ipr to .008 ipr	C6 / C7
		.08 inch	295 to 395	.008 ipr to .014 ipr		295 to 495	.008 ipr to .016 ipr	C6
		.25 inch	195 to 330	.010 ipr to .020 ipr		230 to 395	.012 ipr to .024 ipr	C5 / C6
Milling	Annealed and cold drawn	.04 inch	325 to 445	.004 ipr to .010 ipr	C5 / C6	296 to 395	.004 ipr to .008 ipr	C6 / C7
		.08 inch	295 to 430	.004 ipr to .012 ipr		260 to 360	.006 ipr to .016 ipr	C6
		.012 inch	230 to 360	.005 ipr to .014 ipr		230 to 330	.012 ipr to .024 ipr	C5 / C6
	Annealed	.04 inch	360 to 525	.004 ipr to .010 ipr		490 to 660	.004 ipr to .008 ipr	C6 / C7
		.08 inch	295 to 395	.008 ipr to .014 ipr		295 to 495	.006 ipr to .010 ipr	C6
		.25 inch	195 to 330	.010 ipr to .020 ipr		230 to 395	.012 ipr to .024 ipr	C5 / C6
Forming / grooving	Annealed and cold drawn	.06 inch	330 to 445	.0012 ipr to .003 ipr	C5 / C6	130 to 200	.001 ipr to .003 ipr	C6 / C7
		.25 inch	265 to 430	.002 ipr to .006 ipr		165 to 265	.002 ipr to .006 ipr	C6
		.50 inch	230 to 360	.003 ipr to .008 ipr		195 to 365	.003 ipr to .008 ipr	C5 / C6
	Annealed	.06 inch	295 to 510	.0012 ipr to .003 ipr		180 to 215	.001 ipr to .003 ipr	C6 / C7
		.25 inch	165 to 430	.002 ipr to .006 ipr		195 to 300	.002 ipr to .006 ipr	C6
		.50 inch	165 to 330	.003 ipr to .008 ipr		230 to 395	.003 ipr to .009 ipr	C5 / C6
Cut off / part off	Annealed and cold drawn	.04 inch	360 to 525	.0015 ipr to .003 ipr	C5 / C6	115 to 185	.001 ipr to .002 ipr	C6
		.08 inch	295 to 395	.002 ipr to .006 ipr		130 to 230	.001 ipr to .002 ipr	C6
		.12 inch	195 to 330	.003 ipr to .009 ipr		160 to 265	.001 ipr to .004 ipr	C6
	Annealed	.08 inch	330 to 490	.001 ipr to .002 ipr	C5 / C6	130 to 200	.001 ipr to .002 ipr	C6
		.12 inch	195 to 360	.0015 ipr to .002 ipr		145 to 265	.001 ipr to .002 ipr	C6
		.25 inch	165 to 330	.0015 ipr to .005 ipr		160 to 396	.001 ipr to .005 ipr	C6
Threading	Whole	< 20 threads	15 to 30	–	C2	45 to 110	–	C2 / C5
		>= 20 threads	20 to 40			60 to 130
Drilling	Whole	.0625 inch	115 to 195	.002 ipr to .004 ipr	C5	95 to 200	.002 ipr to .004 ipr	C5 – C6
		.125 inch	115 to 215	.002 ipr to .006 ipr		160 to 395	.002 ipr to .006 ipr
		.25 inch	130 to 230	.003 ipr to .008 ipr		195 to 495	.003 ipr to .008 ipr
		.5 inch	95 to 180	.004 ipr to .009 ipr		230 to 530	.004 ipr to .009 ipr
		.75 inch	95 to 195	.005 ipr to .010 ipr		260 to 530	.005 ipr to .010 ipr
Boring	Annealed and cold drawn	.04 inch	–	–		296 to 395	.004 ipr to .008 ipr	C7
		.08 inch				263 to 360	.006 ipr to .010 ipr	C6
		.12 inch	–			230 to 330	.008 ipr to .014 ipr	C5
	Annealed	.04 inch	–	–		490 to 660	.003 ipr to .008 ipr	C7
		.08 inch				295 to 495	.008 ipr to .016 ipr	C6
		.12 inch				230 to 395	.012 ipr to .024 ipr	C7
Reaming	Whole	.0625 inch	95 to 215	.002 ipr to .005 ipr	C5 / C6	80 to 165	.002 ipr to .005 ipr	C5 to C6
		.125 inch	115 to 215	.001 ipr to .002 ipr		160 to 200	.003 ipr to .006 ipr
		.25 inch	150 to 260	.0015 ipr to .002 ipr		195 to 495	.003 ipr to .007 ipr
		.5 inch	195 to 295	.0015 ipr to .005 ipr		230 to 465	.004 ipr to .010 ipr
		.75 inch	115 to 180	.002 ipr to .004 ipr		260 to 530	.006 ipr to .012 ipr
Taping	Whole	Whole	30 to 50	–	T 15	–	–	–

Welding            

Choosing an initial material

The behavior of duplex stainless steels to welding can be significantly changed by differences in composition or treatment. The significance of the base metals comprising of necessary nitrogen has been frequently underlined. If the initial material is quenched slightly from 700oC to 1000oC or 1300oF to 1800oF or if it is permitted to air cool in this limit for a few seconds or before water cooling, these steps are followed for some time for welding to accomplish the weld without occurrence of damaging precipitation reactions. It is essential that the metallurgical condition of the material utilized in fabrication is of the same type related to the chemistry and production method as the material utilized should be eligible for the welding process.

Cleaning

Caution to clean all the areas that need to be heated before welding is applicable not only on the duplex grades even also on the all stainless steel grades. The compositions of the base metal and filler metal have been made considering that there are no more causes of contamination. Cleaning of dirt, grease, oil, paint and other type of humidity is important to prevent any harmful result while welding as it will otherwise affect the corrosion resistance and mechanical characteristics of the weldment. Welding cannot be done successfully if any extent of dirt is present on the weld surface.

Joint design

For welding of duplex steel grades, the weld joint pattern should provide complete pierce and prevent unmixed base metal in the solidification of the weld metal. It is great to machine instead grind the weld boundary groundwork to offer evenness of the land thickness and gap. While grinding is conducted, more care should be paid to the evenness of the weld make and suitability. If grinding burr is present, it should be discarded to sustain the complete mixture and pierce. A professional welder can control some shortages in joint make by changing the torch.

For the duplex steel grades few of the methods like weaving may result in more than required exposure in the deleterious temperature limit that causes the results beyond the professional process.

Heat supply and interpass temperature  

 The grades of duplex steel can withstand very high heat supplies. The solidification shape of the weld metal prevents hot fractures more than austenitic weld materials. The duplex grades have superior thermal conductivity as well as smaller coefficient of thermal expansion and different high intensity of local thermal loads at the welds as compare to austenitic grades.

The identical welding procedures used for Steel grade 316l can be implemented for joining duplex stainless steel 2205. Although heat supply and interpass temperature should be controlled to retain the required austenitic + ferritic structures and prevent precipitation of unwanted intermetallic phases. A nominal magnitude of nitrogen in welding gas can be advantageous in preventing such issues. For a suitable duplex stainless welding , it is a common practice to assess the austenite – ferrite ratio through ferrite gauge or metallographic test. The ASTM A 923 test procedures are followed to confirm the absence of unwanted intermetallic phases.

The suitable weld filler metal is ER2209 and welds without filler are only preferred if the solution annealing of weld is possible subsequent to welding to recover the corrosion resistance properties. The solution annealing is done through heating up to least temperature limit of 1900oF or 1040oC then quick quenching.

Duplex steel grades demonstrate smaller penetration and fluidity as compared to steel 316l that can cause to decrease the welding speeds. The lowered penetration of grade 2205 may need enhancement of joint geometry. To attain complete penetration, grade 2205 may need larger joint angles, bigger root gaps and minor root lands as compare to SS grade 316l.

For those welding apparatus that can attain using a filler wire, orbital welds for joining 2205 tubing can be constructed with 2209 filler wire. Otherwise overalloyed consumable insert can also be utilized as a replacement to filler wire.

The welding methods utilized are Gas tungsten arc welding (GTAW), Tungsten inert gas welding (TIG), shielded metal arc welding (SMAW), flux core wire arc welding (FCW), submerged arc welding (SAW), electro – slag welding (ESW), electron beam and laser welding and resistance welding.

Electropolishing of Duplex steel grades

For several pharmaceutical and biotechnology applications, electropolishing of product contact surfaces is required. The potential to receive an excellent electropolished surface is hence an essential material attribute. Electropolishing of duplex stainless steel grade 2205 is done to softness of 15 micro-inches or softer that meets the needs of surface finish for electropolished surfaces as per ASME BPE standard.

However 2205 duplex stainless steel can immediately meet the surface finish criteria for pharmaceutical and biotechnology industries, the finished 2205 surface is less brighter and shining than finished steel grade 316l. This variation lies because of potential for nominally larger metal dissolution rates in the ferrite phase as compared to the austenite phase while electropolishing.

Specification and quality Standards

Duplex stainless steel 2205 is made under different American and European standards. Presently two main differences exist in grade 2205 specified in ASTM A240 are UNS S31803 and UNS S32205. The UNS grade S32205 needs nominally larger magnitudes of chromium, molybdenum and nitrogen however it lies within the chemistry range of S31803. It is made to meet the needs of recovery of corrosion and hardness properties in the heat affected regions of few welds of S31803 grade. Therefore use of S32205 is a commonly recommended grade. Following table lists the essential ASTM specifications and control standards for duplex steel grade 2205.

Steel form	ASTM specification
Forged pipe flanges and fittings	A 182 for forged or rolled alloy, stainless steel pipe flanges, forged fittings as well as valves and components for applications in the elevated temperatures.
Plate, sheet and strip	A240 for chromium, chromium-nickel steel plate, sheet, strip for pressure tubes for standard uses
Seamless and welded hygienic tubes	A 270 for seamless and welded austenitic and ferritic/ austenitic steel sanitary tubes
Bars and shapes	A 276 for stainless steel bars and shapes
Seamless and welded tubes	A 789 for seamless and welded ferritic and austenitic stainless steel tubes for general service
Seamless and welded pipe	A 790 for seamless and welded ferritic and austenitic stainless steel pipes
Pipe fittings	A 815 for wrought ferritic, ferrtic and austenitic, martensitic stainless steel pipes fittings
Welded pipe	A 928 for ferritic and austenitic (duplex) stainless steel pipe electric fusion welded with inclusion or filler metal
Castings	A 890 for standard size for castings, Fe-Cr-Ni-Mo corrosion resistant steel duplex for standard usesA 995 for normal size for casting, austenite and ferrite duplex steel for pressure components
Quality control	A 923 for finding damaging intermetallic phase in duplex stainless steel
Corrosion analysis	G 48 for standard test method for pitting and crevice corrosion resistance of stainless steels and alloys for use of ferric chloride solution

European standards

EN 10028 – 7	Flat materials for pressure applications
EN 10088 – 2	Stainless steels for corrosion resistance sheets, plates, strips for normal and construction applications
EN 10088 – 3	Stainless steels for corrosion resistance partial finished materials such as bars, rods, wires and components for general and manufacturing applications
EN 10217 – 7	Welded steel tubes for pressure applications made of stainless steel
EN 10272	Stainless steel rods and bars for pressure applications
EN 10296 – 2	Welded circular steel tubes for mechanical and engineering applications as steel tuves
VdTUV WB 418	Ferritisch austenitic walz and schmiedestahl, 1.4462
Norsok M-CR 630, MDS D45	Piping components made of duplex stainless steel

Corrosion Resistance

Duplex stainless steels containing austenitic and ferritic structure are recognized as outstanding corrosion resistant materials in the several aqueous conditions. Containing large content of chromium in addition of large magnitudes of molybdenum and nitrogen prevents pitting attack in the chloride conditions. The availability of about 50% ferrite offers better strength than austenitic steel grades. It also has greater resistance to stress corrosion cracking in chloride induced conditions. Molybdenum increases firmness of stainless steel to prevent the localized attack such as pitting and crevice, specifically in conditions of chloride. The presence of nickel alloying agent has a significant effect on the corrosion resistance of ferritic steels.

The common use of duplex steels is in the production of chemicals like sulfuric and phosphoric acids. The general corrosion resistance is featured by uniform corrosion on the flat surface that is kept in contact with a corrosive agent. The duplex grades offer superior corrosion resistance to SS 304 and SS316 even the general resistance to corrosion is ten times better than 316l and 18 times better than 304l in a sulfuric acid solution.

Pitting corrosion

It is the most commonly occurring corrosion type in stainless steels that are used in the pharmaceutical and biotechnology applications. The increased magnitudes of chromium, molybdenum, and nitrogen in Duplex 2205 steel offers potentially better resistance to pitting and crevice corrosion as compared to 316l grade. The comparative pitting resistance of steel grade can be assessed by measuring the temperature needed to create pitting in a standardized solution like 6 percent ferric chloride. Duplex steel grade 2205 has critical pitting temperature higher than 316l grade however lower than super austenitic stainless steel 6% Mo.

Pitting corrosion resistance improves with increasing chromium, molybdenum and nitrogen contents. The duplex steels offer larger PREN values than standard austenitic grades. Pitting attack is the most common type of attack in steels. The deleterious effects of chloride ions produce small spots in the metal’s surface.

Duplex steels have double pitting corrosion resistance than SS 316l. Their critical pitting temperature is double of 316l and double yield strength in 6% ferric chloride solution.

Intergranular corrosion:  The duplex configuration and low carbon magnitude of duplex steel grades offer outstanding intergranular corrosion resistance.

Stress corrosion: 

For temperature limits more than 150oF or 60oC, the pair of tensile stress and chlorides can immediate corrode steel 316l grade. Such type of terrible attack is called chloride induced stress corrosion cracking that should be taken into account while selecting materials for applications in hot process streams. Steel 316l grade is not recommended for use in conditions of chlorides and temperatures of 150oF or 60oC and above. 2205 duplex steel resists such kind of corrosion attack in salt solutions minimum up to 250oF or 120oC.

Duplex steels offer good stress corrosion resistance properties. When subjected to NACR standard medium of hydrogen sulfide sour gas for 720 hours, these grades offer greater performance than SS 904l.

Rouging

Stainless steel grades are subjected to high purity water conditions where they receive surface stain called as rouge. This staining layer constitutes of ferric oxide or hydroxide particles that can be of different shades such as red, grey and brown. In pharmaceutical and biotechnology sectors, clean steam conditions and high purity water conditions occurred in water for injection equipments usually get rouging. The equipments for example storage containers, process vessels, pumps, valves and pipes may be attacked.

Due to the risk of extensive equipment material contamination, severely rouged surfaces should be cleaned that can be a costly and time taking process. Therefore the equipments should be made of corrosion resistant materials such as steel 316l and duplex grade 2205.

Applications of Duplex stainless steel

Pulp and Paper Industry                

Leading applications of Duplex stainless steels started in 1930 in sulfite paper industry. The Duplex steels are utilized in the pulp and paper units in bleaching apparatus, digester, chip storage tank, liquor storage containers and suction roll shells. The Duplex steels have substituted austenitic stainless steels and carbon steel by their higher strength as well as greater corrosion resistance for which thinner plates can be utilized for the same pressure level. Production with thinner plate decreases cost of material and lowers welding time as well as shipping and utility cost.

Desalination

This application sector includes intense testing of materials in the high chloride and elevated temperature corrosive conditions. The background of desalination is widely a background of materials production because desalination equipment traders and customers look for options to meet the needs for corrosion resistance while controlling the investment costs. Earlier desalination plants include evaporators of both multi-stage flash MSF and multi-effect desalination MED plants were constructed with mild steel. Later, MSF evaporators were clad with austenitic stainless steel 316l.

The advantages of duplex stainless steel for this applications sector are large strength almost twice of that offered by traditional austenitic steels with excellent corrosion resistance. Hence the evaporators made of Duplex steel grades can be constructed using thinner sheets, decreasing the cost and they do not need extensive welding.

Oil and Gas

Duplex stainless steel has a significant role in operations that are conducted in the oil and gas plants. It is because of its higher strength and greater pitting and crevice corrosion resistance than traditional austenitic stainless steels.

The key uses for duplex steels are in flow lines, process pipes, devices and apparatus such as separators, scrubbers and pumps. Subsea materials are utilized in the downhole production tubes, pipes, flowlines and pipelines for transit of corrosive oil & gas. Superduplex stainless steel containing 25% chromium is utilized for their large strength and hence they are suitable as bars, sheet, plate, tube etc. They also offer outstanding fatigue strength and are galvanically well – suited with other high alloyed steel grades.

Duplex stainless steels are also most commonly used material as umbilicals for monitoring the well top functions through hydraulic lines as well as chemical injection. The duplex umbilicals are also used in analyzing the deep sea surfaces because of their excellent corrosion resistance as well as high strength.

Food Industry

Duplex stainless steel has worthy applications in the food and drink hubs. The steel grades 2205 and 2507 are used in food storage and liquor storage units. They are higher performance and affordable alternative to steel types 304 and 304l.

Architecture

Duplex stainless steel has significant role in manufacturing bridges that need excellent resistance to corrosion combined with saline conditions and large load bearing strength. Duplex stainless steel 2205 pipe and plate are used. These steels are widely used in the bridge constructions in the various countries particularly in sheet and pipe forms. These are also used in the construction of airport roofs. They provide necessary strength and are cost-effective as compared to other steel types.