11- Superferritic Stainless Steels.pdf
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11 Superferritic Stainless Steels Ferritic stainless alloys are noted for their ability to resist chloride stress corrosion cracking, which is one of their most useful features in terms of corrosion resistance. Consequently development efforts during the 1970s were undertaken to produce ferritic stainlesses that would also possess a high level on general and localized pitting resistance as well. The first significant alloy developed commercially to meet these re- quirements contained 26% chromium and 1% molybdenum. In order to ob- tain the desired corrosion resistance and acceptable fabrication characteris- tics, the material had to have very low interstitial element contents. To achieve these levels the material was electron beam rerefined under a vac- uum. It was known as E-Brite alloy. Carbon plus nitrogen contents were maintained at levels below 0.02%. The E-Brite alloy (S44627) was termed a superferritic because of its high level of corrosion resistance for a ferritic material and partly because it is located so far into the ferritic zone on the Schaeffler diagram. For a period of years the usage of this alloy grew. Finally its benefits for the construction of pressure vessels were overshadowed by the difficult nature of fabrication and a concern over its toughness. Due to the very low level of interstitial elements the alloy had a tendency to absorb these elements during welding processes. Increases in oxygen plus nitrogen to levels much over 100 ppm resulted in poor toughness. Even without these effects, the alloy could exhibit a ductile-to-brittle transition temperature (DBIT) around room temperature. Other superferritic alloys were also developed. The chemical composition of selected superferritic alloys are shown in Table 11.1, while Table 11.2 lists mechanical and physical properties. These alloys exhibit excellent localized corrosion resistance. Although the Copyright © 2003 Marcel Dekker, Inc.
Transcript of 11- Superferritic Stainless Steels.pdf
11Superferritic Stainless Steels
Ferritic stainless alloys are noted for their ability to resist chloride stresscorrosion cracking, which is one of their most useful features in terms ofcorrosion resistance. Consequently development efforts during the 1970swere undertaken to produce ferritic stainlesses that would also possess ahigh level on general and localized pitting resistance as well.
The first significant alloy developed commercially to meet these re-quirements contained 26% chromium and 1% molybdenum. In order to ob-tain the desired corrosion resistance and acceptable fabrication characteris-tics, the material had to have very low interstitial element contents. Toachieve these levels the material was electron beam rerefined under a vac-uum. It was known as E-Brite alloy. Carbon plus nitrogen contents weremaintained at levels below 0.02%.
The E-Brite alloy (S44627) was termed a superferritic because of itshigh level of corrosion resistance for a ferritic material and partly becauseit is located so far into the ferritic zone on the Schaeffler diagram. For aperiod of years the usage of this alloy grew. Finally its benefits for theconstruction of pressure vessels were overshadowed by the difficult natureof fabrication and a concern over its toughness. Due to the very low levelof interstitial elements the alloy had a tendency to absorb these elementsduring welding processes. Increases in oxygen plus nitrogen to levels muchover 100 ppm resulted in poor toughness. Even without these effects, thealloy could exhibit a ductile-to-brittle transition temperature (DBIT) aroundroom temperature. Other superferritic alloys were also developed.
The chemical composition of selected superferritic alloys are shownin Table 11.1, while Table 11.2 lists mechanical and physical properties.These alloys exhibit excellent localized corrosion resistance. Although the
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TABLE 11.1 Chemical Composition of Selected Superferritic StainlessSteels
Alloy C Cr Ni Mo N Other
S44627 0.002 26.0 — 1.0 0.010S44660 0.02 26.0 2.5 3.0 0.025 Ti � Cb 0.5S44800 0.005 29.0 2.2 4.0 0.01
Values are in wt%.
TABLE 11.2 Mechanical and Physical Properties of SelectedSuperferritic Alloys
Property
Alloy
S44627 S44660 S44800
Modulus of elasticity � 106 (psi) 31.5Tensile strength � 103 (psi) 75 95 90Yield strength 0.2% offset � 103 (psi) 60 80 75Elongation (%) 25 30 25Density (lb/in.3) 0.280 0.280 0.277Specific gravity 7.7 7.7 7.63Thermal conductivity (Btu/ft hr �F) at
70�F/20�C9.2
Rockwell hardness B-84 B-95
superferritic materials alloyed with some nickel have improved mechanicaltoughness and are less sensitive to contamination from interstitial elements,their availability is still limited to thicknesses below approximately 0.200in. This is related to the formation of embrittling phases during cooling fromannealing temperatures. Section thicknesses over these levels cannot becooled quickly enough to avoid a loss of toughness.
Toughness is determined primarily by means of high impact CharpyV-notch tests which measure the absorption of energy of a given thicknessof material at a given temperature. The values at different temperatures arethen delineated in a nil ductility transition temperature diagram (NDTT)(Fig. 11.1). Other types of tests have also been devised to study thisphenomenon.
Nil ductility is the phenomenon in which an otherwise ductile metalor alloy becomes brittle as the temperature decreases, as shown in Fig. 11.1.
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FIGURE 11.1 Nil ductility transition temperature diagram.
Steels are particularly susceptible to this phenomenon, but so are such cor-rosion resistant alloys as the low-interstitial ferritic stainless steels. The exactnil ductility transition temperature will vary with the thickness of the ma-terial, but is also influenced by compositional variables and heat treatment.Special impact tests have been devised to determine the NDTT for a specificset of conditions, including flaw size (which can be very critical). Note thatthis problem is distinct from the behavior of such brittle materials as castiron or 14.5% silicon iron.
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I. TYPE XM-27 (S44627)
This alloy is also manufactured under the trade name of E-Brite by Alle-gheny Ludlum Industries Inc. It is a high chromium specialty alloy. Referto Table 11.1 for the chemical composition. Compared to the 300 series ofstainless steel, alloy S44627 has a high thermal conductivity and a lowcoefficient of thermal expansion. Physical and mechanical properties will befound in Table 11.2. Table 10.11 indicates the maximum stress allowance.
In general, E-Brite has good general corrosion resistance in most ox-idizing acids, organic acids, and caustics. It is resistant to pitting and crevicecorrosion and free from chloride stress corrosion cracking. Refer to Table11.3 for the compatibility of alloy S44627 with selected corrodents.
This alloy also resists intergranular corrosion and is approved for usein contact with foods. Applications include heat exchanger tubing, overheadcondensers, reboilers, and feed heaters (petroleum refining), pulp and paperliquid heaters, organic acid heaters and condensers, and nitric coolercondensers.
II. ALLOY S44660 (SEA-CURE)
Sea-Cure is a trademark of Trent Tube. It is a chromium-nickel-molybdenumsuperferritic alloy. The chemical composition is shown in Table 11.1.
This alloy exhibits high strength and good ductility. Table 11.2 liststhe mechanical and physical properties.
Because of its chromium, nickel, and molybdenum content it possessesexcellent resistance to chloride-induced pitting, crevice corrosion, and stresscorrosion cracking. It has better resistance than austenitic stainless steels togeneral corrosion in diverse conditions. Good to excellent resistance isshown to organic acids, alkalies, salts, and seawater, with good resistanceshown to sulfuric acid, phosphoric acid, and nitric acid.
Sea-Cure is used in electric power plant condensers and feedwaterheaters and heat exchangers in chemical, petrochemical, and refiningapplications.
III. ALLOY S44735 (29-4C)
The chemical composition of alloy 29-4C is shown in Table 11.4. This alloyhas improved general corrosion resistance and improved resistance to chlo-ride pitting and stress corrosion cracking in some environments. The absenceof nickel reduces the cost.
Applications are found in the utility industry, chemical processingequipment, household condensing furnaces, and vent pipes.
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TABLE 11.3 Compatibility of E-Brite Alloy S44627 with Selected Corrodents
Chemical �F/�C Chemical �F/�C
Acetic acid, 10%Acetic acid, 20%Acetic acid, 50%Acetic acid, 80%Acetic acid, glacialAcetic anhydride*Ammonium chloride, 10%*Aqua regia, 3:1BeerBeet sugar liquorsBenzaldehyde*Bromine water, 1%Calcium hydroxide, 50%*Chromic acid, 10%Chromic acid, 30%Chromic acid, 40%Chromic acid, 50%Citric acid, 10%Citric acid, 25%Copper chloride, 5%Ethylene chloride*Ferric chlorideFluosilicic acidFormic acid, 80%Hydrochloric acidLactic acid, 80%
200/93200/93200/93130/54140/60300/149200/93
X160/71120/49210/9980/27
210/99130/5490/3280/27
X200/93210/99100/38210/9980/27
X210/99
X200/93
Methylene chlorideNitric acid, 5%*Nitric acid, 10%*Nitric acid, 20%*Nitric acid, 30%*Nitric acid, 40%*Nitric acid, 50%*Nitric acid, 70%*Oxalic acid, 10%Phosphoric acid, 25–50%*Sodium chloriteSodium hydroxide, 10%Sodium hydroxide, 15%Sodium hydroxide, 30%Sodium hydroxide, 50%Sodium hypochlorite, 30%*Stearic acidSulfamic acidSulfur dioxide, wetSulfuric acid, 10%Sulfuric acid, 30–90%Sulfuric acid, 95%Sulfuric acid, 98%Sulfurous acid, 5%*Tartaric acid, 50%Toluene
X310/154310/154320/160320/160200/93200/93200/93
X210/9990/32
200/93200/93200/93180/8290/32
210/99100/38550/293
XX
150/66280/138210/99210/99210/99
The chemicals listed are in the pure state or in a saturated solution unless otherwiseindicated. Compatibility is shown to the maximum allowable temperature for which dataare available. Incompatibility is shown by an X. When compatible, corrosion rate is <2mpy except for those marked with an *, whose corrosion rate is <20 mpy.Source: Ref. 1.
IV. ALLOY S44800 (29-4-2)
The chemical composition of alloy 29-4-2 is shown in Table 11.1, and themechanical properties will be found in Table 11.2. Applications are foundin chemical processing equipment and the utility industry for use in corrosiveenvironments.
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TABLE 11.4 Chemical Composition of Alloy S44735(29-4C)
Chemical Weight percent
Carbon 0.03 max.Manganese 0.30 max.Silicon 1.0 max.Chromium 28.0–30.0Nickel 1.0Phosphorus 0.03Molybdenum 3.60–4.20Titanium � niobium 6 (%C � %N): 0.045 N min.Iron Balance
TABLE 11.5 Chemical Composition ofAlloy S44700 (29-4)
Chemical Weight percent
Carbon 0.010 max.Manganese 0.30 max.Chromium 28.0–30.0Nickel 0.15Molybdenum 3.50–4.20Silicon 0.02 max.Copper 0.15Nitrogen 0.02Iron Balance
This alloy has improved resistance to chloride pitting and stress cor-rosion cracking and improved general corrosion resistance in someenvironments.
V. ALLOY S44700 (29-4)
This is a chromium-nickel-molybdenum alloy, with its composition shownin Table 11.5. It has excellent resistance to chloride pitting and stress cor-rosion cracking environments. It finds use in the chemical processing andutility industries.
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REFERENCES
1. PA Schweitzer. Corrosion Resistance Tables, 4th ed., Vols. 1–3, New York:Marcel Dekker, 1995.
2. PK Whitcraft. Corrosion of stainless steels. In: PA Schweitzer, ed. CorrosionEngineering Handbook. New York: Marcel Dekker, 1996.
3. CP Dillon. Corrosion Control in the Chemical Process Industries, 2nd ed. St.Louis, Mo, Materials Technology Institute of the Chemical Process Industries,1994.
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