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NDT.net Issue - 2008-08 - NEWS

Effects of Heat Treatment Procedures on the Cold Cracking Behaviour of High Strength Steel Welds

BAM Federal Institute for Materials Research and Testing1256, Berlin [Germany]
NDT.net Journal
Issue: 2008-08
Most of the research on Hydrogen Assisted Cold Cracking (HACC) in high strength steel welds conducted over the last several decades has focused on single-pass welds, especially considering materials with yield strengths of about 700 MPa. Most of the weld procedure specifications, guidelines and standards targeted at HACC avoidance recommend preheating procedures. Application of such regulations to multi-pass welds of modern high strength structural steels with yield strengths of up to 1300 MPa is very limited. Actually there is no decent knowledge and only an empirical experience how to weld such joints in real components subjected to a respective shrinkage restraint. Consequently, an increasing number of failure cases, partly of catastrophic dimensions, have been reported in the present decade.

The present contribution is targeted to close this knowledge gap by elucidating the principal effects of various inhomogeneous Hydrogen Removal Heat Treatment (HRHT) procedures on the HACC avoidance in high strength structural steel welds. As a typical representative in the upper yield strength range of this category of materials, a S 1100 QL weld using UNION X96 filler wire has been chosen. The results were achieved by indirectly coupled thermal, structural and hydrogen diffusion finite element modeling of HACC in single-layer and five-layer welded V-bevelled butt joints with plate thicknesses of 20.0 mm and 12.0 mm, respectively, at realistic restraint conditions and have been partly been confirmed by respective Instrumented Restraint Cracking (IRC) Tests. The numerical simulations are based on the interacting three local effects on HACC, i.e. local microstructure, local mechanical load and local hydrogen concentration. HACC has thus been regarded as a cracking phenomenon occurring, if the local mechanical load in a specific microstructure exceeds the limit for the respective hydrogen concentration. The various heat treatments proposed in literature, guidelines, specifications and standards, i.e. sole preheating, controlled interpass temperature, combined preheating and controlled interpass temperature application as well as postheating have been investigated with respect to their effects on the mechanical loading of the butt joints in terms of stresses and strains as well as on the hydrogen removal capabilities. As a particular item, a numerical model for Hydrogen Assisted Stress Corrosion Cracking (HASCC) has been developed further that it can be applied to HACC, in order to study, how such heat treatments influence crack initiation and propagation.

By such modeling procedures as the most important results have been achieved:
    1. Further development and adaptation of a model for hydrogen assisted cracking to HACC and usage validation of the model for this material.
    2. Evaluation of the effects of pre- and postheating as well as interpass temperature on the stress-strain distribution in multi-pass welds.
    3. Clarification of the difference between single- and multi-pass welding with respect to stress-strain and hydrogen distribution as well as to HACC initiation and propagation.
    4. Establishment of practical hydrogen removal heat treatment diagrams.
    5. Assessment of the effects of the amount of hydrogen picked up during welding on crack location and propagation.

(Volltext, PDF, 8.9 MB)

BAM-Dissertationsreihe Band 36
M. Eng. Pornwasa Wongpanya
Effects of Heat Treatment Procedures on the Cold Cracking Behaviour of High Strength Steel Welds
2008, ISBN 978-3-9812072-7-9

Prof. Dr.-Ing. Michael Rethmeier
Fachgruppe V.5 Sicherheit gefügter Bauteile
Telefon: +49 30 8104-1550, E-Mail: michael.rethmeier@bam.de

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