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Cold Work Embrittlement

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  Project Fact Sheet C OLD  W ORK  E MBRITTLEMENT  (CWE) OF I NTERSTITIAL -F REE  (IF) S TEELS D EVELOPMENT   OF   A   RELIABLE   METHOD   TO   PREDICT  CWE FAILURE   IN  IF STEELS Steel and parts manufacturers need to determine the sensitivity of IF steels toCWE in order to reduce the occurrence of fracture during secondary deforma-tion and to ensure the in-service reliability of parts. Ultra-low carbon, interstitialfree steel (ULC-IF) sheets are widely used in the automotive industry. Theabsence of solute interstitial elements in these steels leads to sensitivity to coldwork embrittlement. This may lead to brittle fracture during secondary formingof deep-drawn parts or in-service failure. CWE may also affect fatigue perfor-mance.A standard, reliable methodology for CWE testing needs to be developed. To berelevant to steel users, CWE measurements must be conducted using conditionsthat simulate those in-service. They should be representative of the fracturecondition observed in formed parts and should establish the relationship betweenlaboratory data and failures within real parts. To date, the most commonapproach appears to be measuring the ductile-to-brittle transition temperature(DBTT) by fracture of a deep-drawn cup using a plunger to open it. The transi-tion temperature obtained with this test varies with the test conditions. A betterunderstanding of the factors that affect the DBTT will help to determine moreaccurately the temperature at which brittle fracture may occur in more complexparts. STEELSTEELSTEELSTEELSTEEL B ENEFITS ã Enable steel producers to increasesupply of medium-strength IF steel ã Enable parts manufacturers toreduce the occurrence of fractureduring secondary deformation ã Establish ability for vehiclemanufacturers to produce morereliable cars ã Enable positive environmentalimpact through weight reductionresulting from a better usage of medium-strength IF steel in theautomotive industry A PPLICATIONS  The research will provide informationto improve the performance of IF steelgrades in the production of steel parts. OFFICE OF INDUSTRIAL TECHNOLOGIESENERGY EFFICIENCY AND RENEWABLE ENERGY ã  U.S. DEPARTMENT OF ENERGY 86LD S323 115KN S train distribution in a drawn cup with 2.3 draw ratio.      C   u   p   W   a   l   l   S   t   r   a   i   n   (   %   ) 150100500-50 Trimming Linefor the DR=2.3 Cups Forming CWE CracksDistance from Bottom of the Cup (cm)20151050  C HART   OF  S TRAIN  D ISTRIBUTION  Project Description Goals: To develop a correlation between CWE laboratory test results and fracture in realparts used by industry. This project also aims to determine the relationship between steelCWE sensitivity, primary strain conditions and fatigue resistance.A survey was carried out to select suitable candidate materials for this study and to docu-ment parts failure (part geometry, steel chemistry, strain level, and deformation mode). Thecup draw expansion and bend tests were evaluated. Validation of the laboratory CWE testswas made with CWE failure in real parts. Seven commercial steels were selected toevaluate the effect of the steel chemistry and phosphorus segregation on CWE, and to studythe influence of several test conditions on CWE. One steel was selected for fatigue tests todetermine if a relationship can be established between fatigue in drawn parts and intergranu-lar fracture by CWE in deformed parts and undeformed materials. Project Start: February, 1998 Project Completion: First Quarter, 2001 Progress and Milestones ãCompletion of two reports on a company and literature survey on CWE test methods(September, 1998).ãDevelopment of a test procedure for the 150-mm cup/expansion test (September, 1998).ãProduction and delivery of seven steels for the parametric study (April, 1999).ãProduction and delivery of five steels for the fatigue study and for the fabrication of realparts (June, 1999).ãFabrication of real parts using low and high sensitivity steels for impact testing in labora-tory (July, 1999).ãStamping of deep-drawn parts for production of pre-strained specimens for the fatiguestudy (August, 1999).ãCompletion of the parametric study on the effect of strain, cup edge condition, specimensize, impact speed, steel chemistry, and sheet thickness (February, 2000).ãCompletion of the impact tests on real parts (April, 2000).ãDissemination of preliminary results at the Mechanical Working and Steel ProcessingConference in Toronto (October, 2000).ãDraft report on correlation between laboratory test results and fracture in real parts(October, 2000).ãFinal report on CWE study (March, 2001).ãCompletion of fatigue tests on prestrained specimens (January, 2001).ãFinal report on fatigue (March, 2001).ãProject status meetings held with participating companies (eight). P ROJECT  P ARTNERS MTL/CANMETOttawa, Ontario, Canada(Principal Investigator)American Iron and Steel InstituteWashington, DC(Project Manager)AK Steel CorporationMiddleton, OHDofasco, IncorporatedHamilton, Ontario, CanadaLTV Steel CompanyIndependence, OHNational Steel Corporation Trenton, MIRouge Steel CompanyDearborn, MIStelco, IncorporatedHamilton, Ontario, CanadaU.S. Steel ResearchMonroeville, PAWeirton Steel CorporationWeirton, WV F OR   ADDITIONAL   INFORMATION , PLEASE   CONTACT : Gobind JagtianiOffice of Industrial TechnologiesPhone: (202) 586-1826Fax: (202) 586-3237gobind.jagtiani@ee.doe.govhttp://www.oit.doe.gov/steelPlease send any comments,questions, or suggestions towebmaster.oit@ee.doe.gov.Office of Industrial TechnologiesEnergy Efficiencyand Renewable EnergyU.S. Department of EnergyWashington, D.C. 20585 February 2001
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