ISSN 1018-5593 European Commission technical steel research Properties and service performance Role of elevated temperature strength in cold forging STEEL RESEARCH LEGAL NOTICE Neither the European Commission 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, 1996 ISBN 92-827-7199-7 © ECSC-EC-EAEC, Brussels · Luxembourg, 1996 Reproduction is authorized, except for commercial purposes, provided the source is acknowledged Printed in Luxembourg THE ROLE OF ELEVATED TEMPERATURE STRENGTH IN COLD FORGING STEELS British Steel pic ECSC Agreement No. 7210.KC/810 SUMMARY In order to develop steels for cold forging applications, it has been necessary to develop an understanding of the mechanisms which limit the ductility in commercial cold heading applications. A model which defines the thermally aided instability limit has been utilised. A series of experimental steels has been made to determine the effects of steel composition and material condition on the thermally aided critical instability strain and thereby on cold forgeability. The steels have been drawn to wire to give five levels of cold work and they have also been assessed in the as-rolled, fully annealed and spheroidise annealed conditions. The stress-strain-temperature characteristics of the steels in each of the processed conditions have been determined using compression testing techniques. The results have been used to calculate the empirical constants in a constitutive equation, which is cubic in temperature, from which the critical instability strains have been determined for each of the steels. The spacer size to give a 50% failure rate has been adopted as the parameter for assessing the cold heading performance in a commercial cold heading operation. This parameter has been found to provide a more precise correlation between the critical instability strain, derived from compression testing, and the cold forgeability than the direct comparison with total failure rate for a constant spacer size. This improved precision has revealed differences in cold heading performance due to changes in composition and processing conditions. The validity and importance of the thermally aided instability criteria has thus been reaffirmed. Statistical analysis of the steel compositions has indicated that increasing the carbon, silicon, phosphorus, sulphur and nitrogen contents increased the failure rate experienced in cold forging. A positive beneficial effect of cold work up to levels of 36% on the cold forgeability has been observed. Variations in the cold forging performance around the waps of commercial coils were discerned. No conclusive explanations could be given to reliably account for these observations. Statistical analysis has shown that aluminium has a significant effect on the critical instability strain. However, no other elements were found to be significant. In addition, the critical instability strain, unlike the failure rates in cold heading, showed no significant dependence on prior cold work. On this basis, the existing experimental techniques to determine the thermally aided instability strain may be limited and may not be of sufficient precision for revealing small differences in steel composition or processing. A number of modifications have been made to the existing compression testing facilities. The development and utilisation of a small preheated subpress having a relatively high thermal capacity has been found effective in minimising heat losses during compression testing, thereby leading to a more homogeneous temperature distribution throughout the specimens. Modelling of the heat losses during the upsetting of cylinders revealed a difference in critical instability strains of approximately 12% from those determined under ideal, adiabatic conditions. Using the present compression testing facilities, improvements to the presently used curve fitting technique by use of numerical analysis techniques proved unsuccessful. For such a technique to be beneficial would necessitaten testing at 10°C intervals. This was considered impractical for the number of materials under test. Hopkinson split bar testing facilities have been used to generate stress-strain data in torsion at strain rates of up to 10*/s. The critical instability strains determined in torsion were found to compare III favourably with those determined in compression. This adds credence to the applicability of a model of a thermally aided instability criterion to high strain rate compressive forming techniques. The strain ageing properties have been found to be enhanced by a spheroidise annealing treatment. The results in the present work indicated that, in spite of the improved cold forgeability by prior cold work, it was doubtful whether materials which had not undergone a spheroidise annealing treatment could attain the required properties, including cold forgeability, as those of the spheroidise annealed product. Even so, for fastener components involving a deformation process of relatively low severity in their manufacture, wire in the as-drawn condition may possess more than adequate ductility. The work has shown that to improve the cold forging performance of low carbon steels it is necessary to control both the steel composition and processing of the steels to produce a material having a high critical instability strain. IV CONTENTS PAGE 1. INTRODUCTION 1 2. MATERIALS FOR INVESTIGATION 2 2.1 Experimental Steels 2.2 Commercial Coil Product 3. MATERIAL PROCESSING 3.1 Experimental Steels 3.2 Cold Heading of Commercial Coil Product 2 4. DEVELOPMENTS IN EXPERIMENTAL TECHNIQUES 3 4.1 Temperature Control 4 4.2 Variables Related to the Stress-Strain-Temperature Surface 5 4.3 Torsion Testing 6 5. EXPERIMENTAL RESULTS 5.1 Chemical Analyses 5.2 Room Temperature Tensile Properties 7 5.3 Characterisation of the Experimental Steels 5.4 Failure Rate Determination and Critical Instability Strain 8 Assessment of the Commercial Coil Material 5.5 Comparison of Critical Instability Strains Derived from 9 Testing in Compression and Torsion 6. DISCUSSION 9 6.1 Factors Affecting the Correlation Between the Critical Instability Strain and the Failure Rates in Cold Heading 6.2s Affecting the Critical Instability Strain 10 6.3 Metallurgical Factors Affecting the Critical Instability Strain1 6.4 Microstructural Features 12 6.5 Interaction of Cold Forging with the Metallurgy of Low Carbon Steels 13 7. CONCLUSIONS3 REFERENCES4 TABLES 16 FIGURES 27 APPENDICES 51 V LIST OF TABLES 1. Specified Composition of Experimental Casts 2. Chemical Composition, Critical Strain and Cold Heading Failure Rates 3.l Composition and Critical Instability Strains of As-Normalised Experimental Steels 4. Chemical Composition, Critical Strain and Cold Heading Failure Rates of Commercial Low Carbon Cold Heading Steels 5.l Composition, Cold Work and Critical Instability Strains of Other Commercial Coil Materials 6. Tensile Properties of Experimental Steel Compositions 7.e Properties of Commercial Coil Material 8. Tensile Properties of the Experimental Steels in the As-Drawn, Fully Annealed and Spheroidise Annealed Conditions 9. Critical Instability Strains of Experimental Steels in the As-Drawn, Drawn and Fully Annealed and Drawn and Spheroidise Annealed 10. Effect of Cold Work on the Critical Instability Strain 11. Grain Size Measurements of Selected Experimental Casts 12. Details of Cold Heading Failure Rates 13. Comparative Critical Instability Strains Determined by Compression and Torsion Techniques LIST OF APPENDICES 1. Aspects of the Thermally Aided Instability Criteria 2. Temperature Measurement During Compression Using the AGEMA Thermovision 450 System 3. Heat Loss in Compression Testing 4. Strain Behaviour and Fracture at the Surface of Upset Cylinders VI LIST OF FIGURES 1. Processing Route for the Experimental Steels 2. Upsetting Sequence in Final Heading Die 3. Redesigned Subpress Assembly 4. Experimental Data for Samples Tested at Room Temperature and 600°C Under the Three Test Conditions 5.l Data for Samples Tested at 100°C Intervals from Room Temperature to 600°C Using Test Condition (C) 6. Experimental Stress-Strain-Temperature Surface of Cast No. 07113 Generated Using Compression Data from Tests at (a) 100°C Intervals and (b) 50°C Intervals 7.le Surface of Cast No. 09380 Generated Using Compression Data from Tests at (a) 100°C Intervals and (b) 50°C Intervals 8. Fitting of the Experimental Data to the Constitutive Equation a = Κ εη exp (ΑχΤ + A2T2 + A3T3) 9. Hopkinson Split Bar Facility (Schematic) 10. Stress-Temperature Data for Selected Experimental Steels 11 Variation of Flow Strength with Temperature at a Strain of 0.5 (a) CastNo.VS662A (b) Cast No. VS663B (c) CastNo.VS719A (d) Cast No. VS719B 12. Optical Micrographs (Transverse) (a) CastVS718A (b) CastVS664B (c) CastVS622A (d) CastVS663B 13. Comparison of Component Failure with Spacer Size for Various Cold Heading Quality Steels 14. Critical Instability Strains ν Spacer Sizes for 50% Failure Rate 15. Examples of the Distribution of Failed Components in Cold Heading Around Individual Coil Waps Taken from Two Commercial Coils 16. Stress-Strain Curves Derived from Hopkinson Split Bar Tests (Cast No. 12826) 17. Variation in Strain at xmax ν Log Strain Rate 18. Failure Rate ν Spacer Size Curves 19. Features Observed in Failed Headed Component VII LIST OF APPENDICES FIGURES Al. 1 Schematic Diagram of Subpress and Connections for Compression Tests Al .2 Maximum Surface Strain Axes A2.1 Room Temperature Test Frame Showing Maximum Temperatures in °C A2.2 400°C Test Specimen Prior to Compression A3.1 Cooling Constant ν Length of Sample A3.2 Strain Path and Instability Locus ν Temperature During a 3 Second Compression A4.1 Change in Hoop Strain and Axial Strain with Overall Height Strain During Compression of Cylindrical Specimens VIII LE ROLE DE LA RESISTANCE AUX TEMPERATURES ELEVEES DANS LE FORGEAGE A FROID British Steel pic Accord ECSC No. 7210.KC/810 RESUME Pour pouvoir élaborer des aciers convenant au forgeage à froid, il a été nécessaire d'acquérir une bonne compréhension des mécanismes qui limitent la ductilité dans les applications commerciales de refoulement à froid. Pour ce faire, on a utilisé un modèle qui définit la limite de déformation critique due à un critère d'instabilité thermiquement influencé. Une série d'aciers expérimentaux a été élaborée pourr les effets de la composition des acier et de l'état des matières sur la déformation critique due à un critère d'instabilité thermiquement influencé et, dès lors, sur la forgeabilité à froid. Les aciers ont été tréfilés pour produire cinq niveaux de travail à froid et ont par ailleurs été évalués dans les états bruts de laminage, recuits nettement au- dessus de Ac3 et après recuit d'adoucissement. Les caractéristiques de contrainte-allongement-température des aciers dans chacun des états de traitement ont été déterminées à l'aide de techniques d'essais par compression. Les résultats ont été utilisés pour calculer les constantes empiriques dans une équation constitutive, dans laquelle la température est élevée au cube et à partir de laquelle on a déterminé la déformation critique due à un critère d'instabilité pour chacun des aciers. On a adopté le dimensionnement des pièces d'écartement produisant un taux de défaillances de 50% comme paramètre pour l'évaluation des performances de refoulement à froid dans une exploitation commerciale de refoulement à froid. Ce paramètre s'est avéré fournir une corrélation plus précise que la comparaison directe avec le taux de défaillances total en fonction d'une dimension constante des pièces d'écartement entre la déformation critique due à un critère d'instabilité obtenu à partir d'essais de compression d'une part, et la forgeabilité à froid d'autre part. Ce surcroît de précision a révêlé des différences dans les performances de refoulement à froid, différences dues à des changements de compositions et de conditions de traitement. La validité et l'importance du critère d'instabilité thermiquement influencé ont ainsi été confirmées. L'analyse statistique de la composition des aciers a indiqué qu'en augmentant la teneur en carbone, en silicium, en phosphore, en soufre et en azote, on augmentait les taux de défaillances survenues durant le forgeage à froid. On a par ailleurs pu observer un effet positif du travail à froid sur la forgeabilité à froid, atteignant des niveaux de 36%. On a distingué des fluctuations des performances de forgeage à froid au niveau des spires de bobines de qualité marchande. Il a été impossible de formuler des explications conclusives et fiables de ces observations. L'analyse statistique a révélé que l'aluminium exerce un effet significatif sur la déformation critique due à un critère d'instabilité. En revanche, aucun autre élément ne s'est avéré important. Par ailleurs, au contraire des taux de défaillances durant le refoulement à froid, la déformation critique due à un critère d'instabilité n'a pas présenté de variations dues à des travaux à froid précédents. Sur la base de ces constatations, il est possible que les techniques expérimentales existantes utilisées pour définir la déformation critique due à un critèreé thermiquement influencé soient limitées, et n'offrent pas une précision suffisante pour révéler de petits écarts éventuels de composition ou de traitement des aciers. Plusieurs modifications ont été apportées aux installations existantes d'essais par compression. La création et l'utilisation d'une petite presse à colonnes de guidage possédant une capacité thermique relativement élevée a permis de minimiser IX