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HIC-resistance steel Estructure and composite effects

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  • Commission of the European Communities

    technical steel research

    Properties and service performance

    HIC-resistant steel - Structure and composite effects

  • Commission of the European Communities

    echnical steel research

    Properties and service performance

    HIC-resistant steel - Structure and composite effects

    R. E Dewsnap British Steel pic

    9, Albert Embankment London SE1 7SN United Kingdom

    Contract No 7210-KE/813 (1.7.1986-30.6.1988)

    Final report

    Directorate-General Science, Research and Development

    1990 Otf\S ^ M < ^ >

    PARL EUROP. Bibiioih.

    N.C./tfCo00S&S<

    CL EUR 12959 EN

  • Published by the COMMISSION OF THE EUROPEAN COMMUNITIES

    Directorate-General Telecommunications, Information Industries and Innovation

    L-2920 Luxembourg

    LEGAL NOTICE Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of

    the following information

    Cataloguing data can be found at the end of this publication

    Luxembourg: Office for Official Publications of the European Communities, 1990

    ISBN 92-826-1903-6 Catalogue number: CD-NA-12959-EN-C

    ECSC-EEC-EAEC, Brussels Luxembourg, 1990

    Printed in Luxembourg

  • HIC RESISTANT STEELS - STRUCTURE AND COMPOSITION EFFECTS

    British Steel pic

    ECSC Agreement No. 7210.KE/813

    SUMMARY

    A laboratory composite mould design has been developed consisting of sand and heavy metal chill plates for the casting of slab ingots with central segregation similar to that in continuously cast slab products. This technique has been used as the basis for a programme involving a study of the HIC resistance of a range of vacuum melted low sulphur calcium treated PRS (Pearlite Reduced Steels) and CMnMoNb and CMnTiB AF (Acicular Ferrite)/bainite steels. All steels including a conventional 0.1% carbon ferrite-pearlite linepipe composition used as control were controlled rolled to 15 mm thick plate and a full standard mechanical properties and HIC assessment was conducted including single sided HIC testing with continuous hydrogen permeation and ultrasonic C scan monitoring followed by backup metallography and microanalysis studies of HIC sensitive and segregated regions. A limited study has also been conducted to determine the effect of phosphorus content and laboratory simulated submerged arc welding on HIC response.

    Tensile properties ranged from Grade X60/X65 in the pearlitic microstructural types to Grade X70/X80 with continuous yielding behaviour and a high strain hardening index in the acicular ferrite steels together with excellent notch toughness. Pronounced centreline HIC was found in the Grade X65 ferrite-pearlite control steel along hard martensite/bainite bands containing manganese contents up to 2.2% with a segregation ratio of 1.8. In contrast the higher strength ultra low carbon high manganese AF/bainite steels and particularly the PRS compositions showed intermediate to high levels of HIC resistance with limited segregation band HIC as a consequence of the lower centreline manganese segregation ratios which were typically less than 1.2. These findings support the claim that a reduction in carbon content has a pronounced beneficial influence in reducing the segregation of solute elements. Typical mean CLR values for the ferrite-pearlite control steel, PRS and AF/bainite steels were 17%, 3% and 4-13% respectively reflecting the generally fewer crack systems and reduced cracking intensity of the low carbon materials. However, surface blistering was more pronounced in the AF/bainite steels and particularly in the Grade X80 C1.9MnTiB alloy. Except for the ferrite-pearlite control steel which showed limited cracking in a 1000 h single sided HIC test in NACE solution with a hydrogen threshold less than 1 ml/100 mg, all other steel types remained crack free. Hydrogen permeation studies on single sided HIC testpieces showed classical behaviour on all except the ferrite-pearlite control steel with a rapid rise in permeation current to a peak value within 100-200 h of start of test followed by a steady decay throughout the remaining test period. Hydrogen breakthrough also occurred in shorter times in the low carbon PRS and AF/bainite steels compared with the higher carbon ferrite-pearlite steel.

    A reduction in phosphorus content from about 0.011% to 0.003-0.005% had a pronounced beneficial effect on the HIC resistance of the ferrite-pearlite control steel and the 1.9% manganese CMnTiB AF/bainite steel, reducing mean CLR values from approximately 17% and 13% respectively to less than 3%, comparable with results for the ~0.011% phosphorus PRS steel. ' This is attributable to a reduction in segregation band hardness. Severe surface blistering in the 1.9% manganese CMnTiB steel was also greatly attenuated with a reduction in phosphorus content. The mean microhardness of cracked and uncracked segregation bands in steels containing approximately 0.011% phosphorus was well below the generally accepted threshold level of 300 HV for substantial HIC and typically of the order of 280 HV for the ferrite-pearlite steel and 230-240 HV for PRS and AF/bainite steels. The low phosphorus variants of the ferrite-pearlite and 1.9% manganese CMnTiB AF/bainite steel showed a mean decrease in segregation band hardness of approximately 60 H V compared with the intermediate and high phosphorus equivalents. All steels exhibited good weldability with excellent weld metal toughness and full HIC resistance in HAZ and weld metal.

    - I l l -

  • CONTENTS

    Page

    SUMMARY III

    LIST OF TABLES VII

    LIST OF APPENDICES VII

    LIST OF FIGURES IX

    1. INTRODUCTION 1

    2. EXPERIMENTAL TECHNIQUES 2

    2.1 Material preparation 2 2.2 Mechanical properties 3 2.3 HIC testing 3

    3. RESULTS AND DISCUSSION 4

    3.1 Ingot mould optimisation trials. 4 3.2 Ferrite-Pearlite steel 5 3.3 Low carbon perarlite reduced steel 7 3.4 Acicular ferrite/bainite steels 9 3.5 Effect of Phosphorus 11 3.6 Single sided HIC tests 12 3.7 Weldability studies 14 3.8 Microanalysis and microhardness 15

    4. CONCLUSIONS 17

    5. REFERENCES 17

    TABLES 20

    FIGURES 49

    APPENDIX 158

  • LIST OF TABLES

    1 Chemical Analysis of Experimental Ingots

    2. Nominal Controlled Rolling Schedule for All Steels

    3. Ingot Casting Variables

    4. Mechanical Properties of Controlled Rolled CMnNb Ferrite-Pearlite Steel - Plate E

    5. HIC Test Results - Ferrite-Pearlite Steel - Plate E

    6. Comparison of HIC Parameters - Ferrite-Pearlite Steel - Plate E

    7. Chemical Composition of Pearlite Reduced Steels

    8. Mechanical Properties of Pearlite Reduced Steels

    9. HIC Test Results - Pearlite Reduced Steels

    10. Chemical Composition of AF/Bainite Steels

    11. Mechanical Properties of AF/Bainite Steels

    12. HIC Test Results - AF/Bainite Steels

    13. Chemical Composition of Phosphorus Variant Steels

    14. Mechanical Properties of Phosphorus Variant Steels

    15. HIC Test Results - Ferrite-Pearlite Steel Plate M 0.005% P

    16. HIC Test Results - Acicular Ferrite Steel Plate N 0.003% P

    17. HIC Test Results - Acicular Ferrite Steel Plate O 0.019% P

    18. Analysis of Crack Data - All Steels

    19. Hydrogen Diffusion Coefficients and Critical Hydrogen Concentrations for Plate E Compared With Commercial Linepipe Steel

    20. Welding Procedure for Triple Wire Submerged Arc Welds on 15 mm Plate

    21. Notch Toughness Properties of Weld Metal

    22. HIC Test Results - Pearlitic Steel Welds

    23. HIC Test Results-AF Steel Welds

    LIST OF APPENDICES

    1. Method of Calculating Critical Hydrogen Concentration for Cracking

    VII

  • LIST OF FIGURES

    1. Ingot Mould Designs

    2. Positioning of Thermocouples in Chill Plate - Ingot A

    3. Test Piece Sampling Procedure

    4. Thermal History in Chill Plate and Advance of Solidification Front During Casting of Ingot A

    5. Photomacrographs of Ingot Sections

    6. Effect of Chill Plate Thickness on Columnar Crystal Growth

    7. Manganese Concentration in Central Segregation Zone of Ingots A and D

    8. Typical Microstructures and Full Plate Thickness Macrostructure Ferrite-Pearlite Steel - Plate E

    9. Typical Non-Metallic Inclusion Distributions Ferrite-Pearlite Steel - Plate E

    10. Standard Charpy V-Notch Transition Curves for Controlled Rolled Ferrite-Pearlite Steel -Plate E

    11. C Scan Traces Showing HIC in Ferrite-Pearlite Steel - Plate E - BP and NACE Solution

    12. HIC Along Pearlite and Bainite/Martensite Bands in Ferrite-Pearlite Steel - Plate E

    13. HIC Defining Variable Segregation Band Width Ferrite-Pearlite Steel - Plate E

    14. Manganese Profiles Across Cracked Pearlite and Bainite/Martensite Bands Ferrite-Pearlite Steel - Plate E

    15. Isometric View Showing Manganese Profile in Cracked Bainite/Martensite Band Ferrite-Pearlite Steel - Plate E

    16. Standard Charpy V-Notch Transition Curves for PRS Plate

    17. Elastic-Plastic Region of Load-Extension Curve - PRS Plate H

    18. Typical Inclusion Distribution in Vacuum Melted Pearlite Reduced Steels - Plate F

    19. Typical Microstructures of PRS Plates

    20. C Scan Traces Showing HIC - PRS Plates

    21. Surface Blisters on PRS Plates

    22. Full Cross Section Photographs of Test Piece Sections Showing HIC in PRS Plate F

    23. Microstructural Aspects of HIC in PRS Plates F and L

    - IX

  • 24. Standard Charpy V-Notch Transition Curves for AF/Bainite Steels

    25. Elastic-Plastic Region of Load-Extens

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