Perspectives on Development of a Methods for Determining Susceptibility to Liquid Metal Assisted Cracking

Transcription

1 Z inc e essent tial fo r life Perspectives on Development of a Methods for Determining Susceptibility to Liquid Metal Assisted Cracking by Frank Goodwin and Doug Rourke International Zinc Assn AGA Tech Forum Columbus, OH October 13, 2011

2 Zinc e essent tial fo r life Liquid metal embrittlement (LME): A phenomenon where certain ductile metals experience loss in tensile ductility or undergo brittle fracture when tested in the presence of specific liquid metals

3 r life Zinc e essent tial fo Liquid Metal Assisted Cracking (LMAC) Caused by a combination of high stress and a temporary decrease of the plasticity of the steel during immersion in liquid metal Stahlbau Symposium 2006 Zeche Zollverein, Essen, Prof. M. Feldmann

4 Zinc e essent tial fo r life Liquid Metal Assisted Cracking (LMAC) requires: -a susceptible steel alloy -a stress/strain concentration site (often already associated with pre-existing surface cracks) -high levels of residual stress -the presence of molten metal T. Kinstler, Intergalva 2006

5 Examples of LMAC r life Zinc e essent tial fo Flame-Cut Cope Cooled Too Rapidly After Cutting Severely Formed and Unrelieved High Strength Structural Section

6 Current guidelines inc e essent tial for life EUROPEAN STANDARD FOR STEEL FABRICATION EN F.7.4 Galvanized components Unless otherwise specified, due to the risk of liquid metal assisted cracking (LMAC), galvanized components shall be subjected to post-galvanizing inspection GERMANY The DASt Guideline 022 applies to hot dip galvanizing of load steel components that are dimensioned and manufactured Zbearing as set out in DIN or DIN EN 1993 and DIN EN It is directed to the constructor, manufacturer and galvanizer and discusses measures to prevent deleterious effects on strength and fitness for use caused by crack formation during the galvanizing process

7 Z inc e essent tial fo r life DAST-022 Guideline Published August 2009 Effective only in Germany Requirements in this guideline 1. Basic requirements for galvanized steel components 2. Delivery conditions for steel components to the galvanizing firm 3. Requirements for pre-treatment before hot dip galvanizing 4. Requirements for the zinc bath 5. Requirements ements for dipping in the zinc bath 6. Inspection after hot dip galvanizing

8 r life inc e essent tial fo The Long Notched Tension (LNT) Test Specimen Used as the basis for the limit-state model underlying DASt-022 Determines the time dependent maximum equivalent strain at the surface of steel parts in the hot zinc bath during stress defined by the test body and the execution of the test, which triggers the crack formation relevant for the detection process Test Parameters: composition of the zinc melt and bath temperature steel type for the LNT test piece Zstructure and surface states of the LNT test piece e.g. after welding or as a result of processes such as drilling, stamping, thermal cutting or milling expansion rate, force rate corresponding to the test plan

9 Loading Arrangement for LNT Z inc e essent tial fo r life specimen in reference Zn bath 1. The test piece is preloaded manually with 1kN (224 lb f ) tensile force 2. After reaching the dipping depth and holding time the test machine is started either with constant rate of load increase or with constant rate of widening 3. The force and the widening of the test piece are measured

10 r life tial fo essent Zinc e Example of LNT Loading Apparatus

11 Evaluation of Test Results inc e essent tial fo r life 1Thefracture 1.The strain at the bottom of the notch in the Zn bath is determined either from the test data or by finite element analysis 2. The equivalent fracture strain is calculated 3. The class of zinc bath associated with specific plastic equivalent strains is identified: Z

12 Other Studies Z inc e essent tial fo r life Ecole Centrale de Lille (France): Characterize the effect of varying combinations of Sn, Bi and Pb on susceptibility to LMAC Aims to deliver a predictive model - based on U-bend testing of cold worked steel samples Approach first reported at Intergalva 2009: D. Balloy et al., Experimentation and Modelling of Thermo Mechanical Phenomena That Appear During Hot Dip Galvanizing

13 r life inc e essent tial fo The 3-point U-Bend Test for determining susceptibility ty to LMAC Moving platen Sample Supports Z The critical deflection D th is the level of deformation imposed on the sample at which cracking appears upon galvanizing of the deformed sample

14 Experimental Determination of inc e essent tial fo r life Critical Deflection ect D th HDG samples that have been bent over a range of deflections D Mount samples for examination by optical microscopy Sum the lengths of all observed ed cracks Below some D level there are no cracks Z The D level where cracks begin to be observed is D th

15 Future Developments inc e essent tial fo r life Further validation of the U-bend test may lead to a model that allows wider range of bath alloy levels deemed acceptable for limiting LMAC occurrence. Meanwhile, the various guidance documents and standards in Europe are likely to continue to be based on the Class 1 alloy defined in DASt-022 in Germany. Z