7. Low Alloy Steels for Cryogenic Applications

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    Nofrijon

    Sofyan, Ph.D.

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    Introduction

    Steel for cryogenic applications is a steel usvery low temperatures around -200 C.

    At this very low temperature, the materials

    commonly become brittle.

    Because of that, for this low-temperature se

    the materials are required to give a specifi

    strength, ductility, and toughness.

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    Carbon and alloy grades for low-temperatureare required to provide the high strength, duct

    toughness in vehicles, vessels, and structures thaserve at -45C and lower.

    At temperature below ambient, a metals behacharacterized somewhat by crystalline structur

    The yield and tensile strengths of metals that cin the body-centered cubic from iron, molybdevanadium and chromium depend greatly ontemperature.

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    These metals display a loss of ductility in a

    temperature region below room temperatu

    The tensile strength of metals with face-cen

    cubic structures - aluminum, copper, nickel a

    austenitic stainless steel - is more temperatu

    dependent than their yield strength, and thoften increase in ductility as temperature d

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    Cryogenic properties5

    Transformation occurring in compositions that are nstable at room temperature, but metastable at crytemperatures can greatly alter their behavior.

    For example, the combination of gross plastic defoand cryogenic temperatures can cause a normallyand tough stainless steel, such as 301, 302, 304, 3partially transform to bcc structure, resulting in animpairment of ductility and toughness.

    A fully stable stainless steel 310 cannot be transfocryogenic temperatures.

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    The 300 series steels offer a fine combination

    toughness and weldability for service to the lo

    temperatures. In the annealed condition, their strength prope

    adequate for ground-based equipment but ina

    for lightweight structures.

    For aerospace applications, fabricators can ta

    advantage of the alloys strain-hardening char

    and use them in highly cold-worked condition.

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    The principal shortcomings of cold-worked matare: low weld-joint efficiencies caused by anneduring welding and the transformation to martthat occurs during cryogenic exposure.

    Selection of fully stable grade type 310, overtransformation problem.

    Precipitation-hardening A286 stainless has eve

    strength when cold worked before aging. The alloy steel recommended for cryogenic ser

    9% nickel steel.

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    It is satisfactory for service down to -195C

    used for transport and storage of cryogeni

    because of its low cost and ease of fabrica

    Other alloy steels are suitable for service in

    low-temperature range.

    The steels A201 and T-1 can suffice to -45steels with 2.25% Ni can suffice to -59C, a

    nickel steels with 3.5% Ni to -101C.

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    Design for cryogenic applications9

    Designers of cryogenic assemblies base thecalculations on the room-temperature prope

    the material.

    The reason is that it is the highest temperat

    material will encounter.

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    And it stands that if a higher-strength matestands up to super cold conditions were ava

    designers might specify it.At 26C austenitic stainless steel has tensile

    yield strengths that are 172 MPa greater tcorresponding strengths for type 304 stainl

    At -100C its tensile and yield strength excthose of type 304 by 550 MPa and 276 Mrespectively.

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    A grade with following chemical composition shgood mechanical properties at cryogenic tempC - 0.072%, Mn - 16%, P - 0.02%, S - 0.008%0.41%, Ni - 5.85%, Cr - 17.8%, N - 0.36%, FRemainder

    The composition is given for plates with 12.7mthickness

    The material combination of high strength, gootoughness, and weldability should prompt desispecify it for welded pressure vessels for the scryogens.

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    Ductility and temperature12

    Ductility is a critical property for cryogenicapplications.

    In general, BCC metals such as Fe, Carbon

    alloy steels, Molybdenum, and Niobium bec

    brittle at low temperatures.

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    FCC metals such as Cu, Ni, Cu-Ni alloys, Al

    alloys, and austenitic stainless steels remain

    at low temperatures.

    Most plastics and elastomers become brittle

    temperatures.

    Ceramics and glasses are already brittle atemperature and become slightly more so a

    cryogenic temperatures.

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    Brittle materials14

    In brittle materials, the maximum load is the sa

    the yield strength,

    the tensile strength

    the breaking strength

    Yield in brittle materials such as ceramics doesby the motion of dislocations but by planar de

    as cracks.

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    The effect of temperature on the stress-strain curve and

    properties of an aluminum alloy.

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    Brittle materials bend test16

    The bend test is used to measure the properties ofmaterials where a flat specimen is put under load

    The bend test for measuring the strength of brittleand the deflection, d, obtained by bending.

    Brittle Materials

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    Low alloy steel17

    Low-alloy steels constitute a category of fematerials that exhibit mechanical propertiesuperior to plain carbon steels as the result additions of alloying elements such as nickechromium, and molybdenum.

    Total alloy content can range from 2.07% ulevels just below that of stainless steels, whicontain a minimum of 10% Cr.

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    For many low-alloy steels, the primary func

    the alloying elements is to increase hardena

    order to optimize mechanical properties antoughness after heat treatment.

    In some cases, however, alloy additions are

    reduce environmental degradation under cespecified service conditions.

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    Classification of low alloy steels19

    As with steels in general, low-alloy steels can

    classified according to:

    Chemical composition, such as nickel steels,

    chromium steels, molybdenum steels, chromi

    molybdenum steelsHeat treatment, such as quenched and temp

    normalized and tempered, annealed.

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    Because of the wide variety of chemical comp

    possible and the fact that some steels are used

    than one heat-treated, condition, some overlapamong the alloy steel classifications.

    Thus, four major groups of alloy steels can be

    addressed: (1) low-carbon quenched and temp

    (QT) steels, (2) medium-carbon ultrahigh-streng(3) bearing steels, and (4) heat-resistant chrom

    molybdenum steels.

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    Low-carbon quenched and tempered21

    Low-carbon quenched and tempered steels

    high yield strength (from 350 to 1035 MPa

    high tensile strength with good notch toughn

    ductility, corrosion resistance, or weldability

    The various steels have different combinatiothese characteristics based on their intende

    applications.

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    However, a few steels, such as HY-80 and H

    are covered by military specifications.

    The steels listed are used primarily as plate

    Some of these steels, as well as other, simila

    are produced as forgings or castings.

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    Medium-carbon ultrahigh-strength ste23

    Medium-carbon ultrahigh-strength steels ar

    structural steels with yield strengths that can

    1380 MPa.

    Many of these steels are covered by SAE/A

    designations or are proprietary compositio Product forms include billet, bar, rod, forgin

    sheet, tubing, and welding wire.

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    Bearing steels24

    Bearing steels used for ball and roller bear

    applications are comprised of low carbon (

    0.20% C) case-hardened steels and high ca

    1.0% C) through-hardened steels.

    Many of these steels are covered by SAE/Adesignations.

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    Cr-Mo heat-resistant steels25

    Ch

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