0.38.Theimprovedfatiguestrength,asmentionedpreviously,isaresultofironnitrideformedinthediffusion zone,andgenerallyresultsinanimprovement of25%to35%instainlesssteelsz. LSBNProcess Description Thepartsareusuallycleaned firstinanalkaline rinsetoremove anysurface oxides.Theyarepreheated to approximately 400°C(750°F) inordertoreduce thepossibility ofthermal shockandtodrytheparts completely. Thepartsarethenplacedinthesaltbath,generally keptat580°C(1075°F). Thecriticalparameter tocontrol atthisstageisthecyanate concentration, whichmustbekeptat34%-38% forpropercompound layer formation 8.Iftheconcentration fallsbelowthisrange,aproprietary nitrogen-bearing polymer isadded,which reactswiththecarbonate inthebathtoformcyanate. Theresidence timeinthebathdepends onthematerial processed andtherequired compound layerthickness, andisusually 15minutes minimum, 30minutes maximum for13-8PHand17-4PHsteels.Thelastfactortobeconsidered inthebathisaeration, whichis accomplished byusingairspargers onthebottomofthetank.Theairtothespargers isdriedandfiltered, anditsfunction istoimprove surface contact withthepartsandtheliquid. Thepartsarethenremoved fromthesaltbath,andareplacedinaquench tank.Thistankiskeptat400°C (750°F) andcontains oxidizing (nitrate) salts.Theoxidizing saltsconvertanyremaining cyanide andcyanates tocarbonates, thusstopping anyfurthersurface reactions. Inaddition, thelowertemperature ofthequench bathprevents anydistortion ofthepartsduetothermal stress.Thepartsareusuallykeptinthisbathfor5to 20minutes. Afterremoval fromthequench tank,thepartsareaircooledtoroomtemperature, andresidual saltsare rinsedoffwithwater.Atthisstage,thepartsshould haveauniform blackfinish.Onepieceisusually sectioned andtestedforhardness andthickness ofthecompound layer,aswellashardness ofthediffusion zone. Problems Associated withLSBNProcessed Stainless Steels TheLSBNprocess usually provides predictable anduniform resultsforawiderangeofsteelsandevencast iron.However, somedifficulties arisewhenprocessing stainless steels.Thechromium andnickelinstainless steelsarealsonitrideformers; therefore, thecompound layercontains chromium andnickelnitrides in between grainboundaries. Thisresultsinaverybrittlecompound layer,whichincreases thechance of spalling. Themolybdenum in13-8PHsteelalsoscavenges nitrogen, thusaddingtothepossibility ofspalling inthisalloy.Inaddition, thenitriding reactions occurveryquickly, soresidence timeiscriticalinorderto prevent formation ofanexcessively heavycompound layer.Furthermore, thediffusion zoneisnotasdeepas innon-alloyed steelsbecause agreatdealofthefreenitrogen isscavenged bythealloying elements; therefore, thenitrogen cannotdiffusewellintothesteel.Finally,corrosion resistance iscompromised dueto thereactions ofthealloying elements chromium andnickel. CaseHistory The13-8PHpinions thatwereinitiallyprocessed weredoneaccording tothecustomer's specifications, whichwerebasedontheiryearsofexperience usingLSBN.Thepartswererequired tobeheattreatedtoRC 4245priortoLSBNprocessing. Thesaltbathtemperature wasspecified tobelessthantheheattreat temperature inordertoprevent anydegradation inthehardness, andtheresidence timewastobeadjusted toproduce acompound layerbetween 0.005mmand0.025mm(0.0002 inand0.001in).However, when thisspecification wasfollowed fortheAeroflex pinion,achromium nitrideembrittled compound layercloseto 0.025mm(0.001inch)resulted intoomuchbuildup onbothsidesofthefinepitchtooth,andspalling occurred. Inotherwords,thespecification tolerances onthecompound layerthickness forthisparticular part hadtobetightened. 6Easterday, p.9. 7Easterday, p.9. 8Easterday, p.4. 136