Super Ferritic Stainless Steel (UNS #S44660)

Super Ferritic Stainless Steel (UNS #S44660)

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Chemical Composition

Element	Percent
Chromium	25.0-28.0
Molybdenum	3.0-4.0
Nickel	1.0-3.5
Manganese	1.00 max
Silion	1.00 max
Carbon	0.030 max
Nitrogen	0.040 max
Phosphorous	0.040 max
Sulfur	0.030 max
Titanium + Niobium	0.020-1.00
Iron	0.020-1.00

Description

The ferritic structure of UNS #S44660 Stainless steel provides a high strength/low work hardening material with good ductility.

These properties allow high design stress limits with good fabrication characteristics. Because of the nickel addition,UNS #S44660 has a lower ductile-to-brittle transition temperature than similar ferritic steels without nickel additions.

Applications

This alloy is specifically designed for applications where chloride induced pitting, crevice, and stress corrosion cracking may be encountered.

UNS #S44660 stainless steel is used in electric power plant condensers and BOP exchangers, various heat exchangers in chemical, petrochemical, and refining applications, desalination heat exchangers and flue gas handling systems such as the secondary heat exchangers in high efficiency furnaces. The American Gas Association has approved UNS #S44660 for flue gas condensate applications. UNS #S44660 stainless steel has better resistance to general corrosion over a broader range of conditions than the austenitic stainless steels.

Welded Super Ferritic Stainless Steel (UNS #S44660) as per ASTM/ASME A/SA268
O.D/W.T (mm)	12.7	15.9	17.2	19.05	22.0	25.4	26.7	31.8	38.1
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Corrosion Resistance

Resistance to a number of strong acids was evaluated using the Materials Technology Institute of the Chemical Process Industries procedures. Representative data are given below.

Acid Solution	Temperature		Type 304	Type 316
	˚F	˚C	Corrosion Rate—MPY*
0.1% Hydrochloric	212	100 B	17.4	2.08	0.23
1.0% Hydrochloric	210	99 B			0.68
1.0% Hydrochloric + 3% FeCl3	167	75			2.27**
10% Sulfuric	215	102 B			1.05
60% Sulfuric	244	118 B			>1000
93% Sulfuric	171	77		78.0	10.0
50% Phosphoric	228	109 B	2.46	3.87	1.78
10% Nitric	219	104 B	0.37	0.96	0.46
65% Nitric	241	116 B	3.34	3.95	1.20***
60% Nitric + 2% HCL	235	113 B			4.18***
80% Acetic	217	103 B	17.0		0.02
100% Acetic	243	117 B	0.39	0.54	0.44
50% Acetic + 50% Anhyd	164	73	0.40		1.60
50% Formic	221	105 B			0.89
10% Oxalic	216	102 B			1.31
55% NaOH + 8% NaC +3% NaClO3	210	99		6.1	<0.1
50% NaOH	289	143		15.0	1.0

* Corrosion rate in mils per year— evaluated over a 96 hour test period.

** Pitting.

*** Welded UNS #S44660 showed good performance in the nitric tests. However, caution should be exercised in using any titanium stabilized alloy in highly oxidizing environments.

B — Boiling

Acid Condensate Resistance

Heat recovery systems are particularly susceptible to severe corrosion caused by acid condensates in the environment. The process of condensation and evaporation concentrates acids and chlorides, increasing the corrosive attack at the condensate dew point or water boiling point. UNS #S44660 stainless steel has the capability to resist most of these corrodents.

Seawater Corrosion Resistance

UNS #S44660 stainless steel was developed specifically to resist localized pitting and crevice corrosion in aggressive chloride solutions, such as seawater. In comparative accelerated laboratory

and crevice corrosion testing, UNS #S44660 ranks far superior to the common austenitic stainless steels such as Types 304 & 316.

Testing Results

In natural seawater at ambient temperature, several tests have shown no attack in over 10 years. Today, numerous power plant condensers have over 25 years of exposure. Under the same conditions, Type 316 experienced a .039 inch crevice corrosion attack. In areas where organic pollution is present (which can decay to produce hydrogen sulfide), UNS #S44660 stainless steel resulted in significantly higher corrosion resistance than the copper alloys, such as copper-nickel.

Chloride Stress-Corrosion Cracking Resistance

Like most other fully ferritic stainless steels, UNS #S44660 stainless steel has excellent resistance to chloride-induced stress-corrosion cracking. When stressed to 90% of its yield strength and placed in a 212˚F (100˚C) 40% CaC12 solution, UNS #S44660 stainless did not crack even after a 5000 hour exposure. Type 316L stainless cracks within 400 hours under the same conditions. UNS #S44660 U-Bend specimens exposed to 1500 ppm sodium chloride at 212˚ F (100˚C) also did not crack. Like other stainless steels, UNS #S44660 is not resistant to stress corrosion in 40% magnesium chloride solution (boiling) at 284˚F (140˚C).

Erosion-Corrosion Resistance

UNS #S44660 exhibits excellent resistance to all types of erosion. It is not affected by high water velocities, which may result from either tube blockage or mechanical design, nor by steam or impingement erosion. In a wear-erosion test using silica sand and water impinging on various stainless steels, it shows only 25% of the weight loss of Type 316.

Galvanic Corrosion

Whenever the tubes and the tubesheet of a heat exchanger or condenser are of dissimilar materials and in contact with conductive water (usually more than 1000ppm dissolved solids), there is a possibility of galvanic corrosion of the other alloy.

UNS #S44660 has a high electrode potential in seawater, making it very noble or cathodic. It is slightly below titanium, gold and platinum, and is more noble than the copper alloys, copper-nickel or carbon steel in the galvanic series. Therefore, there is a possibility of galvanic attack to the material that is lower in the galvanic series. Thus, if UNS #S44660 tubes are used with a Muntz metal tubesheet in seawater, the Muntz metal tubesheet can pit in the ligament section between the tubes. Covering the tubesheet with an epoxy-type coating or using an impressed voltage cathodic protection system usually protects the tubesheet. If a cathodic protection system is used, the voltage should be maintained more positive than -0.800 volts as measured against a standard calomel electrode to prevent generation of hydrogen, which can cause hydrogen embrittlement.

Marine Fouling

All metals foul in seawater over time. Because most stainless steels do not contain copper, which dissolves and forms copper ions that are poisonous to marine growth, fouling may occur earlier.

The tendency for marine fouling of all materials can be minimized by chlorination, mechanical cleaning or high water velocity.

UNS #S44660 stainless, by virtue of its erosion resistance, is ideally suited to either mechanical cleaning or high water velocity. In softer copper alloys, these methods can cause severe wear.

Sulfide Pitting Attack

Pitting corrosion in the presence of sulfur compounds and certain bacteria in polluted seawater may occur with copper-nickel, aluminum-brass and other alloys high in copper. UNS #S44660 is not attacked by these sulfur compounds and the associated bacteria.

Manganese Bacteria Attack

Manganese can be extracted from certain waters by certain types of bacteria and deposited on heat exchanger surfaces as hydrated manganous oxide. In the presence of chlorine, this compound can be oxidized to the permanganate and the chlorine reduced to the chlorine ion. This reaction can cause pitting in the 300 Series stainless steels and Admiralty brass. UNS #S44660 is essentially immune to this reaction because of its very high resistance to pitting.

Ammonia Attack

Copper-base alloys are very susceptible to ammonia attack resulting in accelerated general corrosion, pitting attack, or ammonia-induced stress corrosion crack. UNS #S44660 like other stainless steels is essentially immune to ammonia attack.

Physical Properties

 UNS #S44660 stainless steel has a number of attractive physical properties, including low thermal expansion, good thermal conductivity, and a high elastic modulus which provides high stiffness. High stiffness allows less vibration than with other engineering materials. The thermal expansion coefficients are similar to those of carbon steel and lower than those of the austenitic stainless steel or copper alloys. The thermal conductivity is similar to titanium and higher than the austenitic stainless steels of high nickel alloys. The passive corrosion resistant film is extremely thin, which allows good heat transfer performance.

Comparative Properties of Various Alloys
	Ti Gr.2	90-10 Cu/Ni	UNS 44660
Yield Strength* (ksi)	40	15	65
Tensile Strength* (ksi)	50	40	85
Elongation* (%)	20	25	20
Elastic Modulus (PSI x 106)	15.5**	18	31.5
Density (lb/in3)	0.16	0.32	0.278
Expansion Coefficient (in/in-˚Fx106)	4.7	9.5	5.38
Thermal Conductivity (Btu/hr-ft2-˚F/ft)	12.6	26	10.1
Specific Heat(Btu/lb-˚F)	0.124	0.092	0.12
Fatigue Endurance(ksi)	16	25	35

* Minimum ASTM Value

** Maximum ASTM Value

Vibration Resistance

Because of its very high modulus of elasticity,  UNS #S44660 stainless steel is very resistant to vibrational fatigue damage. For the purpose of comparison, the following minimum tube wall would be required to prevent vibration damage under the same conditions of turbine exhaust steam velocity, steam density, tube support spacing, and tube diameter:

Mechanical Properties

The ambient temperature strength of UNS #S44660 stainless steel is retained over the temperature range encountered by most heat exchanger applications.  UNS #S44660 is approved for ASME Boiler and Pressure Vessel Code construction Section VIII, Division I. The allowable stresses for both sheet and tube are substantially higher than those of lower alloy ferritic and austenitic stainless steels. This factor can produce substantial savings through reduced section thickness or higher operating pressures. An upper temperature limit of 500˚F is imposed to avoid danger from 885˚F embrittlement which is a characteristic of all ferritic steels that contain more than 12% chromium.