1、Designation: B 874 96 (Reapproved 2008)Standard Specification forChromium Diffusion Coating Applied by Pack CementationProcess1This standard is issued under the fixed designation B 874; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisio
2、n, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This specification covers the requirements for chromiumdiffusion of metals by the pack cementation meth
3、od. Packdiffusion employs the chemical vapor deposition of a metalwhich is subsequently diffused into the surface of a substrate athigh temperature. The material to be coated (substrate) isimmersed or suspended in a powder containing chromium(source), a halide salt (activator), and an inert diluent
4、such asalumina (filler). When the mixture is heated, the activatorreacts to produce an atmosphere of chromium halides whichtransfers chromium to the substrate for subsequent diffusion.The chromium-rich surface enhances corrosion, thermal stabil-ity, and wear-resistant properties.1.2 This standard do
5、es 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 and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Sta
6、ndards:2B 374 Terminology Relating to ElectroplatingB 487 Test Method for Measurement of Metal and OxideCoating Thickness by Microscopical Examination of CrossSectionB 602 Test Method for Attribute Sampling of Metallic andInorganic CoatingsB 697 Guide for Selection of Sampling Plans for Inspectionof
7、 Electrodeposited Metallic and Inorganic CoatingsB 762 Test Method of Variables Sampling of Metallic andInorganic CoatingsC 664 Test Methods for Thickness of Diffusion CoatingD 3951 Practice for Commercial PackagingE 766 Practice for Calibrating the Magnification of a Scan-ning Electron MicroscopeE
8、1077 Test Methods for Estimating the Depth of Decar-burization of Steel SpecimensF 1330 Guide for Metallic Abrasive Blasting to Descale theInterior of Pipe3. Terminology3.1 Definitions used in this specification are in accordancewith Terminology B 374.3.2 Definitions of Terms Specific to This Standa
9、rd:3.2.1 diffusion coatinga diffusion coating is one producedby causing an element to react with or diffuse into, or both, thesurface of a metallic substrate, thus, chemically altering thesurface of the substrate.3.2.2 retortscontainers in which powder and parts arepacked for processing. They can be
10、 constructed of carbon,stainless, or high alloys and fabricated in all shapes and sizesto accommodate parts being processed.3.2.3 significant surfaceareas that are essential to theserviceability or function of the article. These surfaces must beidentified on a drawing or marked-up sample of product.
11、 Areascan fall into one of three categories as follows:3.2.4 coating requiredthese surfaces must be in accor-dance with all quality requirements of this specification.3.2.5 no coating requiredthese surfaces are areas whereno coating is allowed due to a number of reasons includingdimensional, fabrica
12、tion, and welding, as well as others.Materials used for masking are commercially available.3.2.6 optionalthese surfaces do not require coating, but atthe same time do not require masking.4. Classification4.1 There are four classes of chromium diffusion defined bybase (basis) metal category.4.1.1 Cla
13、ss ICarbon steel.4.1.2 Class IILow-alloy steels.4.1.3 Class IIIStainless steels.1This specification is under the jurisdiction of ASTM Committee B08 onMetallic and Inorganic Coatings and is the direct responsibility of SubcommitteeB08.08.01 on Engineering Coatings.Current edition approved Aug. 1, 200
14、8. Published September 2008. Originallyapproved in 1996. Last previous edition approved in 2003 as B 874 96 (2003).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to t
15、he standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.1.4 Class IVNickel-based alloys.5. Ordering Information5.1 In order to make the application of this specificationcomplete, the pu
16、rchaser shall supply the following informationto the vendor through a purchase order and drawings:5.1.1 Title, ASTM designation number, and date of issue ofthis specification.5.1.2 Deposit by classification (see Section 4).5.1.3 Composition and metallurgical condition of substrateto be coated.5.1.4
17、Location of significant surfaces (see 3.2.3).5.1.5 Samples for destructive metallurgical test (see 8.1).5.1.6 Any post heat treatment required.5.1.7 Acceptance inspection procedure to be used (seeSection 9).5.1.8 Any requirement for certification (see Section 11).6. Processing Requirements6.1 Substr
18、ate PreparationThe metal to be chromizedshall be free of flaws and defects that will be detrimental to thecoating. Thorough cleaning is essential to ensure satisfactorydiffusion. Materials used for cleaning should not damage thebase metal. Oils, dirt, grease, and stains must be removed.When blasting
19、 is also required, use aluminum oxide (90 to 220mesh) at 60 to 80 psi from 4 to 6 in. standoff. (Refer to GuideF 1330.)6.2 Materials:6.2.1 Masteralloy with 50 to 100 % Pure ChromiumSoldunder a number of trade names, in various mesh sizes.Percentage in mix depends on material being processed.6.2.2 Ac
20、tivatorMost commonly used is ammonium chlo-ride, but many others are available. Percentage in mix dependson material and type of activity.6.2.3 Inert FillerAluminum oxide or calcined alumina isused and commercially available in numerous mesh sizes.Percentage in mix depends on material being processe
21、d andmesh size being used.NOTE 1It is important that both proper equipment is used and timeestablished to guarantee uniform blend of pack mix.NOTE 2It is common practice to rejuvenate used mixes by addition ofcoating elements, alloys, and activators. Chemical analysis of used mixesfacilitates contro
22、l of critical constituents. Disposal of used mixes must beperformed in compliance with environmental regulations.6.3 LoadingPlace pack mix and parts in retorts. Retortsize is limited by dimensions of furnace used for processing.6.4 Furnace CycleParts are heated to a temperature be-tween 1900 and 240
23、0F depending on base material and packmix and held for amount of time needed to produce desireddepth of diffusion.NOTE 3Due to the high temperature of the process, a positive pressureof an inert gas, usually argon, must be maintained in retorts throughout theentire furnace cycle.6.5 Post-CleaningRet
24、orts should be allowed to cool suf-ficiently before opening. When parts are removed from retorts,residual pack mix is removed from surfaces by a supplier-approved method.6.6 Post-StraighteningLong tubes may have distortiondue to the high temperature of process. Use rotary straighteneror hydraulic pr
25、ess to restore straightness and ovality.6.7 Visual InspectionInspect in accordance with 7.3.6.8 Marking and PackagingTo be defined by the pur-chaser in the purchase order. Parts processed for the U.S.government and military, including subcontract, shall be pack-aged in accordance with Practice D 395
26、1.7. Coating Requirements7.1 Diffusion ThicknessThe following are minimums forthe different classes of materials based on standard processparameters. Thicker coatings require special processing andmust be called out for in the purchase order if required.Class Minimum, in.Class I (carbon) 0.003Class
27、II (low alloys) 0.003Class III (stainless steels) 0.002Class IV (nickel base alloys) 0.0017.2 DecarburizationFor certain Class I and Class II ma-terials, the maximum depth of decarburization to be defined bythe purchaser.7.3 Chromium ContentThe outer 15 % of coating shallcontain a minimum of 20 % (b
28、y weight) chromium.7.4 AppearanceThe diffusion zone shall be nonporousand adherent to the base metal and shall have a uniform surfacefree from objectionable imperfections. Minor variations incolor and surface appearance shall be considered acceptable,providing the requirements of 7.1 and 7.2 are met
29、 (see 8.7).8. Test Methods8.1 Special Test Specimens:8.1.1 The permission or the requirements to use special testspecimens, the number to be used, the material from whichthey are to be made, and their shape and size shall be stated bythe purchaser.NOTE 4Test specimens are often used to represent coa
30、ted articles in atest if these articles are of a size, shape, or material that is not suitable forthe test, or if it is preferred not to submit articles to a destructive testbecause, for example, the articles are expensive or few in number. Thespecimen should duplicate the characteristics of the art
31、icle that influencethe property being tested.8.1.2 Special test specimens used to represent articles in anadhesion, weldability, corrosion resistance, or appearance testshall be made of the same material, shall be in the samemetallurgical condition, and shall have the same surfacecondition as the ar
32、ticles they represent, and they shall be placedand processed in the production lot along with the articles theyrepresent.8.2 ThicknessPerform the destructive test on samples inaccordance with Test Methods C 664. Read and record thethickness of samples in accordance with Test Method B 487.8.3 Decarbu
33、rizationDetermined by metallographic ex-amination in accordance with Test Method E 1077.8.4 Chromium ContentPerform by scanning electron mi-croscope (refer to Practice E 766).8.5 Mechanical Properties (if required)Tensile testing onsamples from each process batch.B 874 96 (2008)28.6 Hardness Testing
34、 (if required)Macrohardness and mi-crohardness testing are both acceptable means.8.7 AppearanceThe coating can be examined at up to 53magnification for conformance to appearance requirements.9. Sampling9.1 The sampling plan used for inspection of a quantity ofcoated articles shall be agreed upon by
35、the purchaser and thesupplier.NOTE 5Usually, when a collection of coated articles, the inspectionlot (9.2), is examined for compliance with the requirements placed on thecoating, a relatively small number of the articles, the sample, is selectedat random and is inspected. The inspection lot is then
36、classified ascomplying or not complying with the requirements based on the results ofthe inspection of the sample. The size of the sample and the criteria ofcompliance are determined by the application of statistics. This procedureis known as sampling inspection. Three standards, Test Method B 602,G
37、uide B 697, and Method B 762, contain sampling plans that are designedfor the sampling inspection of coatings.Test Method B 602 contains four sampling plans, three for use with teststhat are nondestructive and one for tests that are destructive. The buyerand seller may agree on the plan or plans to
38、be used. If they do not, TestMethod B 602 designates the plan to be used.Guide B 697 provides a large number of plans and also gives guidancein the selection of a plan. When Guide B 697 is specified, the buyer andthe seller need to agree on the plan to be used.Method B 762 can be used only for coati
39、ng requirements that have anumerical limit, such as a coating thickness. The test must yield anumerical value, and certain statistical requirements must be met. MethodB 762 contains several plans and also gives instructions for designatingplans to meet special needs. The buyer and the seller may agr
40、ee on theplan or plans to be used. If they do not, Method B 762 designates the planto be used.NOTE 6When both destructive and nondestructive tests exist for themeasurement of a characteristic, the purchaser needs to state which one isto be used so that the proper sampling plan is selected. A test ma
41、y destroythe coating, but in a noncritical area; or, although it may destroy thecoating, a tested part may be reclaimed by stripping and recoating. Thepurchaser needs to state whether the test is to be considered destructive ornondestructive.9.2 An inspection lot shall be defined as a collection ofc
42、oated articles of the same kind that have been produced to thesame specifications, coated by a single supplier at one time orapproximately the same time under essentially identical con-ditions, and that are submitted for acceptance or rejection as agroup.9.3 If special test specimens are used to rep
43、resent the coatedarticles in a test, the number used shall be that required in 8.1.1.10. Rejection and Rehearing10.1 Parts that fail to conform to the requirements of thisspecification may be rejected. Rejection shall be reported to theproducer promptly in writing. In the case of dissatisfactionwith
44、 the results of a test, the producer may make a claim for arehearing. Coatings that show imperfections may be rejected.11. Certification11.1 When specified in the purchase order or contract, thepurchaser shall be furnished with certifications that the samplesrepresenting each lot have been either te
45、sted or inspected asdirected by this specification and the requirements have beenmet. When specified in the purchase order or contract, a reportof the test results must be furnished.12. Record Retention12.1 The seller is to maintain all job records and samples fora period of two years unless the pur
46、chaser requires a longerperiod.13. Keywords13.1 chromizing; corrosion-resistant coatings; diffusioncoating; pack cementationASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are exp
47、ressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revis
48、ed, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may
49、 attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).B 874
