1、Designation: G 41 90 (Reapproved 2006)Standard Practice forDetermining Cracking Susceptibility of Metals ExposedUnder Stress to a Hot Salt Environment1This standard is issued under the fixed designation G 41; the number immediately following the designation indicates the year of originaladoption or,
2、 in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers procedures for testing metals forembrittlement and crackin
3、g susceptibility when exposed understress to a hot salt environment. This practice can be used fortesting all metals for which service conditions dictate the needfor such information. The test procedures described herein aregenerally applicable to all metal alloys; required adjustments inenvironment
4、al variables (temperature, stress) to characterize agiven materials system should be made. This practice describesthe environmental conditions and degree of control required,and suggests means for obtaining this desired control.1.2 This practice can be used both for alloy screening fordetermination
5、of relative susceptibility to embrittlement andcracking, and for the determination of time-temperature-stressthreshold levels for onset of embrittlement and cracking.However, certain specimen types are more suitable for each ofthese two types of characterizations.NOTE 1This practice relates solely t
6、o the performance of the exposuretest. No detailed description concerning preparation and analysis ofspecimen types is offered. However, the optimum sample design may beone that uses the same type of stress encountered in service loadingsituations. Standards describing principal types of stress corr
7、osion speci-mens, their preparation, and analysis, include Practices G30, G38, andG39.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices a
8、nd determine the applica-bility of regulatory limitations prior to use. (For more specificsafety hazard statements see Section 8.)2. Referenced Documents2.1 ASTM Standards:2D 1141 Practice for the Preparation of Substitute OceanWaterD 1193 Specification for Reagent WaterG1 Practice for Preparing, Cl
9、eaning, and Evaluating Cor-rosion Test SpecimensG30 Practice for Making and Using U-Bend Stress-Corrosion Test SpecimensG38 Practice for Making and Using C-Ring Stress-Corrosion Test SpecimensG39 Practice for Preparation and Use of Bent-Beam Stress-Corrosion Test SpecimensG49 Practice for Preparatio
10、n and Use of Direct TensionStress-Corrosion Test Specimens3. Summary of Practice3.1 The hot salt test consists of exposing a stressed, salt-coated test specimen to elevated temperature for variouspredetermined lengths of time, depending on the alloy, stresslevel, temperature, and selected damage cri
11、terion (that is,embrittlement, cracking, or rupture, or a combination thereof).Exposures are normally carried out in laboratory ovens orfurnaces with associated loading equipment for stressing ofspecimens.3.2 The ovens are provided with facilities to circulate air atvarious flow rates and ambient pr
12、essure. However, for certainspecific applications, airflow and pressure may be adjusted toobtain information on material behavior in simulated serviceenvironments. Exposure temperatures and stress levels aregenerally selected on the basis of mechanical property data fora given alloy, or of expected
13、service conditions, or both.4. Significance and Use4.1 The hot salt test as applied to metals is utilized as asecondary design consideration indicator, as cracking has beenshown to occur in laboratory tests simulating possible serviceconditions. Although limited evidence exists linking this phe-nome
14、non to actual service failures, cracking under stress in ahot salt environment should be recognized as a potential designcontrolling factor.1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G01.06 on Environmen-tally
15、 Assisted Cracking.Current edition approved Nov. 1, 2006. Published December 2006. Originallyapproved in 1974. Last previous edition approved in 2000 as G 41 90 (2000).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual
16、 Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.2 The hot salt test is not to be misconstrued as beingrelated to the stress
17、corrosion cracking of materials in otherenvironments. It is considered solely as a test in an environ-ment that might be encountered in service.4.3 Because hot salt cracking under stress is considered asecondary design consideration and service failures have notbeen attributed solely to this phenome
18、non, manufacturingprocesses will be optimized or alloying changes will be madeonly after consideration is given to primary design factors suchas creep resistance of a given high temperature alloy. Theusefulness of the test lies rather in limiting maximum operatingtemperatures and stress levels or ca
19、tegorizing different alloysas to susceptibility, or both, if it is found that hot salt damagemay accelerate failure by creep, fatigue, or rupture.4.4 Finally, the test does not lend itself to the utilization ofpre-cracked specimens because cracking reinitiates at anysalt-metal-air interface, resulti
20、ng generally in many smallcracks which extend independently. For this reason, specimensthat are recommended for utilization in routine testing are ofthe smooth specimen category.5. Interferences5.1 Hot salt cracking under stress is often considered ahydrogen-related phenomenon, and the source of hyd
21、rogen is acorrosion reaction involving moisture, available either from thehydrated salt, trapped as fluid inclusions in nonhydrated salt, orfrom humidity in the test atmosphere if absent in the saltcrystals. Because of this fact, considerable variation in testresults can be obtained, simply from the
22、 method of saltdeposition on the test specimen, even when effective controlson other test variables are realized. Efforts should be made tostandardize the salt deposition techniques and to control ormonitor humidity in order to achieve desired test validity.5.2 The effects of cycling time at tempera
23、ture to achieve agiven total cumulative exposure have been shown to have asignificant effect on test results, with shorter cycle duration andgreater cycle frequency generally resulting in less damage forthe same cumulative exposure time. For this reason, selectionbetween continuous and cyclic exposu
24、re, duration, and fre-quency of cycling, and heating and cooling rates must be madewith the end purpose of the test in mind.5.3 Variations in heat to heat or product forms, or both, havebeen shown to have a significant effect on damage thresholdsdetermined from experimental testing. This effect may
25、be morepronounced than is observed in more conventional stresscorrosion testing of the aqueous type. For this reason, it isimportant to obtain and document to the fullest extent possibleall certified analyses and tests associated with the material to betested and associated fabrication and treatment
26、 histories. Inter-stitial concentration levels, chemical contaminants, and ther-momechanical processing should be included in the documen-tation (see Section 12).5.4 Details regarding general surface preparation and use ofbent-beam stress-corrosion specimens are outlined in PracticeG39. Procedures f
27、or making and using direct tension stress-corrosion specimens is described in Practice G49. However,because of the highly localized nature of onset of attack at thesurface in hot salt exposure testing, it is desirable to charac-terize as fully as possible the surface condition of the material.If an
28、as-received surface condition is to be investigated, effortsshould be made to ascertain the state of residual stress asregards the material surface. Both magnitude and algebraicsign (tension or compression) of residual stress should bedetermined and reported if possible. Chemical milling can beemplo
29、yed in final surface preparation in order to avoid extra-neous surface effects. However, care should be taken to ensurethat proper chemical milling techniques are employed, and thathydrogen uptake does not occur during the surface preparation.6. Apparatus6.1 Apparatus for Salt CoatingA conventional
30、air brushshould be used for spraying the specimens to accomplish thesalt-coating procedure. This will generally provide a thinuniform salt deposition of the desired density.6.2 Apparatus for Conducting Exposure Test:6.2.1 Apparatus required for conducting the exposure testdepends on the selection of
31、 the specimen type to be used. If aconstant-deflection type specimen is utilized for which noexternal loading requirement exists, conventional laboratoryovens are suitable for conducting the exposure test. Provisionfor controlling or monitoring inlet air humidity is recom-mended.6.2.1.1 Specimen Hol
32、ders, suitable for applying stress toconstant-deflection type specimens should be made of the sameor a similar alloy as the material to be tested in order to avoidgalvanic effects. The requirement for the use of a fixture toapply stress can be avoided when testing sheet materials byutilizing a self-
33、stressed specimen design.36.2.1.2 Racks, suitable for supporting specimens in the ovenand for transferring specimens should be made of the same ora similar alloy as the material to be tested. Open circuitconditions should be maintained, although galvanic effects areconsidered to be highly localized
34、on the surface.6.2.2 If a constant-deflection type specimen is utilized, caremust be taken to either avoid or take into account differencesin thermal expansion between test specimen and test fixture.Thermal expansion differences can substantially change thestress level applied at ambient temperature
35、 when specimens areheated to the test temperature.6.2.3 If a constant-load type specimen is to be utilized,provision must be made to combine both heating and loadingequipment. Vertical-tube resistance-wound furnaces can beutilized with dead-weight loading or conventional creep frameequipment for low
36、 and high loading conditions, respectively(Note 2). Direct induction or resistance heating of the specimenitself is not recommended.NOTE 2When using vertical-tube furnaces care must be taken toavoid a chimney effect through the furnace, which could result inexcessive airflow and uneven temperature d
37、istribution along the specimenlength. Sealing at both ends will allow control of air flow and improvetemperature distribution within the furnace.7. Reagents and Materials7.1 Reagent grade salts shall be used when preparingsolutions from which the salt coating is derived. Sodium3See “A Stress Corrosi
38、on Test for Structural Sheet Materials,” MaterialsResearch and Standards, Vol 5, No. 1, January 1965, pp. 1822.G 41 90 (2006)2chloride (NaCl) should be used for routine testing. Other saltsthat may be encountered in service can be used for specializedapplications. Synthetic sea water (Note 3), shoul
39、d be used forcharacterizing alloys for use in marine environments.NOTE 3If tests are to be conducted on specimens with salt depositsderived from substitute ocean water, solutions should be prepared inaccordance with Specification D 1141.7.2 Purity of WaterUnless otherwise indicated, referencesto wat
40、er shall be understood to mean Type IV water preparedin accordance with Specification D 1193.8. Hazards8.1 Shatterproof glasses with side shields should be wornwhen handling and examining stressed samples. Generally therequired safety equipment is similar to that used for conductingroutine mechanica
41、l tests.8.2 Appropriate heat-resistant equipment, for example,gloves, may be required when exposing test samples to hightemperatures.9. Calibration and Standardization9.1 When conducting elevated temperature exposure tests,determination of the temperature profile within the oven orfurnace should be
42、made, including temperature sampling alongthe width, depth, and height of the hot zone to ensure thattemperatures within all locations of specimen exposure arewithin prescribed limits. Deviation from the desired testtemperature should not be more than 62 % of the absolutetemperature.9.1.1 Temperatur
43、e control of the exposure test shall beaccomplished by determining true specimen temperature. Thiscan be done by means of affixing a thermocouple of appropri-ate sensitivity for the temperature range to be investigated ontoa control specimen either by spotwelding or mechanicalfastening. In either in
44、stance it must be determined that thetechnique of thermocouple fastening does not introduce anyinterference effects.9.2 The degree of control required on the applied stress of atest specimen depends on the nature and purpose of the test.When determining threshold values of time-temperature-stressfor
45、 onset of embrittlement or cracking for a given alloy as asecondary design consideration, control should be more strin-gent than that for indication of trends or determination ofrelative material susceptibilities.9.2.1 In determining threshold values for onset of embrittle-ment or cracking, constant
46、-load type specimens, for which thelevel of applied stress can be more tightly controlled, arerecommended. Deviation from the desired target stress levelshould not be more than 62%.9.2.2 When utilizing constant-deflection type specimens forthe determination of behavior trends or relative materialsus
47、ceptibilities, specimen geometry should be limited such thatcontrol of the applied stress level can be maintained within610 % of the desired stress level.9.3 Humidity ControlFor routine testing, active control ofhumidity is not considered mandatory. Most testing is accom-plished using ambient labora
48、tory air. However, daily monitor-ing and recording of humidity should be made and humidityconsidered as a potential cause of data scatter. In tests forascertaining the effects of humidity on cracking behavior,moisture levels can be adjusted by mixing various ratios ofsaturated and dry air to oven or
49、 furnace air inlet. Sampling ofdew point at oven or furnace inlet will allow determination ofhumidity of the air at ambient conditions.9.4 AirflowCare must be taken to prevent airflow veloci-ties beyond that achieved in recirculating ovens (30 to 120m/min (100 to 400 ft/min). Variations in this factor have beenshown to produce differences in test results. If airflow is anexperimental variable to be investigated, it should be con-trolled and monitored.10. Procedure10.1 Cleaning of SpecimensBefore salt coating, thor-oughly clean the specimens to remove all identification mark
