Top Ten Ordering Tips

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You are a business, expert. Your design needs stainless steel tubing, so you call up your favorite manufacturer. You speak to a tubing expert who is friendly, helpful, and knowledgeable, but you still feel bombarded with questions. This guide walks you through the top 10 things to know when ordering stainless steel tubing and related products.

• WHICH ALLOY? To start your order, your supplier will want to know what kind of material you need. Saying, “stainless steel,” is too broad since there are a series of alloys that fall under this designation. These include 300 series austenitic, 400 series martensitic, 400 series ferritic, precipitation hardening, and duplex grades. The higher alloys in 300s and precipitation hardening series resist chemical corrosion and are the most popular and readily available alloys. The most common alloys are T304, T304L, T316, and T316L, which are stocked in most standard fractional and hypodermic sizes.

• WHAT TEMPER OR HARDNESS? You will also have to specify the temper of your stainless steel. Hardness is often used as a surrogate measure for strength because it is easier to measure. Temper is specified by description, Annealed to Full Hard, or by mechanical properties such as hardness, tensile strength, yield strength, and/or elongation. For example, full hard #3 temper is common for hypodermic grade tubing made from T304 and T316 stainless steels.

• WELDED, WELDED AND DRAWN, OR SEAMLESS? Welded tubes start as long coils of cold-rolled stainless steel strip that are rolled into tube form. A welding head melts the strip edges together to form a strong weld without flux or filler metal. Cold-working operations are followed by annealing to recrystallize the weld area and make it as strong as the parent metal. The seamless tube is made from tube hollows that are then drawn down to desired diameters.

• WHAT SPECIFICATIONS DO YOU NEED TO MEET? Tubing can be made to a variety of specifications; ASTM, AMS, ASME, and ISO designations are the most popular. For example, Eagle-welded tubes with outer diameters up to 5 in. are certified to ASTM A-249/A-269, ensuring tensile, yield, elongation, flaring, flattening, flanging, hardness, and hydrostatic or nondestructive electrical tests have been performed. Seamless tubes are typically made to ASTM-A213.

• WHAT CROSS-SECTIONAL DIMENSIONS? Which cross-sectional dimensions are critical to your tubing order? Most standard tubing is produced to meet exact outside diameter and wall thickness dimensions. If your application is more sensitive to inside diameter, you can special order tubing using that dimension plus wall thickness. Some applications need tight tolerances on both inside and outside diameters, in which case the wall thickness is taken as theoretical.

• HOW LONG? Stainless steel tubing is typically made in either 10-ft or 20-ft random lengths; however, tubing may be cut to shorter lengths for delivery purposes. Furthermore, Eagle Stainless can cut and deburr tubing to 0.008 in. and longer. Depending upon the tube OD and length, tolerance such as +/-0.010 in. or +/-0.005 in. can be held on small diameter tubing applications. If you require a smaller tolerance please ask.

• WHAT SURFACE FINISH? Stainless steel is often chosen for its aesthetic appearance, so make sure to specify the surface finish you need. The most common is the shiny, silvery finish manufacturers refer to as “bright”; alternatively, you can request a “pickled” or dull finish. Polishing and centerless grinding operations can be performed as secondary processes to produce desired RMS finishes as needed.

• HOW STRAIGHT? Stainless steel is typically produced in “reasonable” straight conditions; however, where straightness is critical, then additional straightening operations can be performed to ensure desired requirements.

• DO YOU NEED OVALITY, CONCENTRICITY, OR ECCENTRICITY? Ovality is the roundness or the difference between the max and min OD, obtained by measurement for high and low points at any one cross section of the tube. Also, the concentricity or eccentricity is the difference between the center of the OD of the tube when compared to the center of the ID of the tube.

• HOW CLEAN DOES IT NEED TO BE? Commercially cleaned tubes are defined as tubes that are free from drawing compounds, oils, dirt, grease, or other compounds and are visually inspected with an unaided eye. Precision cleaned parts can be specified where the parts are visually inspected under magnification and/or have a lint-free cloth or tissue passed through the ID. The cloth should be free from compounds, dirt, and oils; however, a light-gray discoloration is not cause for rejection.

Duplex Stainless Steel: A Technical Guide to Grades, Properties, and Applications

by AMC

Posted on April 10, at 05:01 PM

Navigating material selection for demanding environments often leads engineers to consider advanced options. Duplex stainless steel, with its unique microstructure and balanced properties, frequently emerges as a prime candidate. However, understanding the nuances between grades like and , interpreting PREN values, and assessing suitability for specific corrosive conditions requires detailed knowledge.

This guide provides engineers, designers, and procurement specialists with a comprehensive technical overview of duplex stainless steels. We will dissect the microstructure, analyze key properties of mechanical and corrosion, compare common grades including super duplex, examine chemical compositions, and detail relevant industrial applications, enabling informed material selection for critical projects.

Defining Duplex Stainless Steel: Microstructure and Key Advantages

At its core, duplex stainless steel possesses a two phase microstructure containing approximately equal proportions of austenite and ferrite. This deliberate metallurgical balance, achieved through controlled alloying and processing, is the foundation of its distinct advantages over purely austenitic or ferritic grades. This structure inherently provides higher strength and enhanced resistance to certain forms of corrosion, particularly stress corrosion cracking SCC.

Core Properties of Duplex Stainless Steel: Strength, Corrosion Resistance, and Weldability

The practical benefits derived from the duplex structure translate into specific, measurable properties critical for engineering design:

1. Elevated Mechanical Strength: Duplex grades exhibit significantly higher yield and tensile strengths compared to common austenitic steels for example 304L, 316L, often double the yield strength. This allows for designs with reduced section thickness, leading to weight savings and potential cost reductions.
2. Enhanced Corrosion Resistance:
• General and Localized Corrosion: High chromium, molybdenum, and nitrogen content provide excellent resistance to general corrosion and critically superior resistance to localized attack mechanisms like pitting corrosion, high PREN values and crevice corrosion, especially in chloride rich environments.
• Stress Corrosion Cracking SCC: Duplex steels offer vastly improved resistance to chloride induced SCC compared to standard austenitic grades, a crucial factor in warm, saline, or chloride containing process environments.
3. Good Weldability: While requiring specific procedures controlled heat input, appropriate filler metals to maintain the phase balance in the heat affected zone HAZ, duplex steels are readily weldable using common arc welding processes.

Duplex vs. Super Duplex Stainless Steel: Composition and Performance Differences

• Composition: Super Duplex Grades e.g., UNS S / : Contain higher percentages of chromium Cr, molybdenum Mo, and nitrogen N.
• Performance Impact: This enriched composition translates to:
• Higher PREN Pitting Resistance Equivalent Number: Typically ≥ 40, indicating superior resistance to pitting initiation in highly aggressive chloride environments.
• Increased Strength: Usually exhibit even higher yield and tensile strengths than standard duplex grades.
• Enhanced Crevice Corrosion Resistance: Better performance in tight geometries where corrosives can concentrate.
• Selection Criteria: Standard duplex e.g., UNS S / is suitable for many demanding applications, while super duplex is reserved for the most severe service conditions where maximum corrosion resistance is paramount e.g., high temperature seawater, aggressive chemical processing, deep offshore.

Common Duplex Stainless Steel Grades: S, S, and Lean Variants

Several specific grades cater to different performance and cost requirements:

For more duplex steel tubesinformation, please contact us. We will provide professional answers.

• Grade UNS S / S: The most widely utilized duplex grade. Offers an excellent balance of strength, resistance to SCC, pitting, and crevice corrosion. The workhorse for chemical processing, marine applications, oil and gas, and pulp and paper. S offers tighter nitrogen control than the older S. PREN typically 34-36.
• Grade UNS S: The leading super duplex grade. Engineered for maximum corrosion resistance PREN ≥ 42.5 and high strength. Specified for highly corrosive offshore, desalination, and chemical processing environments.
• Lean Duplex Grades e.g., UNS S, S: Developed for cost effectiveness with lower nickel and sometimes molybdenum content. They provide better strength and SCC resistance than 304L/316L but lower pitting resistance than . Suitable for structural applications, storage tanks, and less aggressive environments.

Chemical Composition Analysis: Key Alloying Elements in Duplex Steels Cr, Ni, Mo, N

Understanding the role of key alloying elements is crucial for predicting performance :

The precise ratio of ferrite stabilizers Cr, Mo, Si to austenite stabilizers Ni, N, Mn, C, Cu determines the final phase balance.

Mechanical Properties Deep Dive: Yield Strength, Tensile Strength, and Hardness

The mechanical properties underscore the design advantages of duplex steels:

The high yield strength is often the most significant factor, allowing for substantial material savings in pressure containing equipment and load bearing structures. Good ductility elongation ensures adequate toughness.

Industrial Applications: Where Duplex & Super Duplex Excel Oil & Gas, Chemical, Marine

The unique property set of duplex steels makes them ideal solutions in specific industries facing challenging conditions:

• Oil and Gas: Subsea flowlines, umbilicals, manifolds, separators, heat exchangers, structural components on offshore platforms, high strength to weight, resistance to H₂S and chlorides. Super duplex is prevalent.
• Chemical Processing Industry CPI: Pressure vessels, reactors, tanks, piping, heat exchangers handling chlorides, organic acids, caustic solutions where SCC or pitting are risks for standard austenitics.
• Marine: Propeller shafts, rudder stocks, pumps, valves, fasteners, seawater cooling systems, deck hardware resistance to seawater corrosion and high strength.
• Desalination: Evaporator tubing, high pressure piping, pumps resistance to hot brine and high chloride concentrations. Both duplex and super duplex are used depending on temperature/concentration.
• Pulp and Paper: Digesters, bleach plant equipment, liquor tanks resistance to specific chemical liquors and abrasion.
• Power Generation: Flue Gas Desulfurization FGD scrubbers, absorbers, ducting resistance to acidic, chloride containing condensates, some feedwater heaters.
• Infrastructure and Architecture: Bridges especially in coastal/deicing salt areas, structural supports, swimming pool components, building facades long life, low maintenance, corrosion resistance.

For a detailed look at the uses and advantages of duplex steel, read our article on Applications & Uses of Duplex Stainless Steel

Advantages Summary: Performance Benefits and Cost Effectiveness

Choosing duplex or super duplex steel offers tangible engineering and economic advantages:

• High Strength to Weight Ratio: Enables lighter designs, reducing material volume, fabrication effort, and installation loads.
• Excellent Corrosion Resistance: Provides reliable performance in environments where standard stainless grades would fail prematurely due to SCC, pitting, or crevice corrosion.
• Improved Durability and Service Life: Reduces maintenance cycles and replacement frequency, leading to lower lifetime costs.
• Potential Overall Cost Savings: While initial material cost per unit weight might be higher than 316L, the ability to use thinner sections and avoid costly failures often results in lower total installed and operational costs.

Conclusion: Selecting the Optimal Duplex or Super Duplex Grade

Duplex and super duplex stainless steels represent highly engineered materials offering a superior combination of mechanical strength and corrosion resistance compared to conventional stainless steels. Their selection is warranted in applications where standard grades are inadequate due to high stress, aggressive chemical environments particularly chlorides, or susceptibility to stress corrosion cracking.

A thorough analysis of the service environment temperature, chemical concentrations, chloride levels, pH and mechanical design requirements pressure, load is essential for choosing the most appropriate and cost effective grade Lean Duplex, , , or others. Consulting material specialists and leveraging resources from reputable suppliers like Amardeep Steel, who offer certified materials and technical support, is crucial for successful implementation.

For more information, please visit super duplex pipe.