Role of elevated temperature strength in cold forging
103 pages
English

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103 pages
English
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Description

Properties and service performance
Industrial research and development

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Informations

Publié par
Nombre de lectures 19
Langue English
Poids de l'ouvrage 3 Mo

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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.

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