Properties and service performance

Directorate-General For Research And Innovation European Commission

Description

Basic constitution of 9 to 11 % Cr steels for elevated temperature service
Industrial research and development

Sujets

• Sciences et techniques

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 4 Mo

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ISSN 1018-5593
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European Commission
technical steel research
Properties and service performance
Basic constitution of 9 to 11% Cr steels
for elevated temperature service
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 92-827-7205-5
© ECSC-EC-EAEC, Brussels · Luxembourg, 1996
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Luxembourg A STUDY OF THE BASIC CONSTITUTION OF 9-11% Cr STEELS FOR ELEVATED
TEMPERATURE SERVICE
British Steel pic
ECSC Agreement No. 7210.KF/804
SUMMARY
The 9% Cr 1% Mo NbVN steel type known colloquially as 'Steel 91' has become established as a candidate
material for many high temperature applications.
Following a brief review of the history of the development of high strength 9 CrMo type steels, this project
describes the metallurgical stability, tempering characteristics and elevated temperature strength of
compositions related to Steel 91. Seventeen compositions were studied.
The metallurgical stability of this steel type is high in terms of prior austenite grain sizes with the grain
coarsening temperature at ~1125°C and little risk of forming delta ferrite S1150°C. The presence of fine
Nb(CN) particles is considered to be mainly responsible for the control of austenite grain size at
normalising temperatures in the range 1000-1200°C.
The tempering resistance is significantly higher than that of 9% Cr 1% Mo steel due to the precipitation of
vanadium and niobium rich carbides and nitrides. There is a secondary hardening peak in the tempering
curve after ~1 h at 700°C. Tempering has to be carried out at ^800°C due to the risk of reforming
martensite at 810°C and higher. The useful tempering range is therefore 730-800°C with optimum
performance over the range 750-800°C. The as-tempered hardness is increased by increasing the
normalising temperature over the range 1050-1200°C and/or adding additional solid solution
strengthening by chromium or tungsten. Furthermore, the adoption of a V:N ratio of ~3.5 i.e.
stoichiometric, maximises the precipitation hardening contribution.
The strength at ambient and elevated temperatures namely tensile and stress rupture respectively, are
direct reflections of the tempering characteristics. The test data include results from stress rupture tests
completed to durations of 27 165 h at 600 and 650°C.
Analysis of the data collected in this project has identified routes for increasing the strength of Steel 91
type material through heat treatment and composition control.
It is recommended that the optimum normalising temperature is 1100°C which gives high strength with a
fine prior austenite grain size, without the risk of intergranular cracking and therefore reduced rupture
ductility values as found for material normalised from 1200°C.
The recommended tempering temperature is considered to be 750CC. By normalising from 1100°C rather
than the 'standard' 1050°C, the material has a sensitivity to tempering temperature. This may be a useful
control parameter in relation to the need for subsequent heat treatments of welded components.
Three composition factors relate to the strength of Steel 91 type material. The base strength arises from
the 9% Cr 1% Mo basic alloy content and the precipitation of VN and Nb(CN). By careful control of the
vanadium and nitrogen contents within typical ranges of 0.15-0.2% and 0.050-0.065% respectively,
precipitation strengthening by VN can be maximised. Further solid solution strengthening can be
achieved either by increasing the chromium content or adding 1.5% W. These two solid solution factors
were found not to be additive. Precipitation of Laves phase occurring in these types of steel appears to
reduce the solid solution strength component which may become an important consideration in long
service durations. Thus from this work the optimised conditions for Steel 91 are considered to be; normalise from 1100°C and
temper at 750°C, with a composition similar to those in ASTM/ASME specifications except for 10.5% Cr or
1.5% W + 0.15/0.20% V + 0.050/0.065% N to maximise the precipitation strength and solid solution
strength parameters. In view of the Laves phase precipitation it may be possible to reduce the tungsten
content to below the 1.5% used in this project and still retain a significant solid solution strength
contribution. This could be adopted in further projects in this area as required.
The optimised conditions indicated above will probably give a long term strength increase of ~ 20% over that
of the material as specified and used currently.
IV CONTENTS PAGE
1. INTRODUCTION 3
2. PROGRAMME OF WORK AND MATERIAL USED 3
3. EXPERIMENTAL RESULTS 4
3.1 Microstructural Characterisation
3.2 Normalising Temperature and Hardness 5
3.3 Tempering Characteristics 6
3.4 Tensile Properties 7
3.5 Stress Rupture Properties
3.6 Hardness after Ageing 8
3.7 Metallography
4. DISCUSSION 10
4.1 Microstructural Stability
4.2 Tempering Resistance1
4.3 Mechanical Properties2
4.4 Stress Ruptures
4.5 Optimisation Considerations4
5. CONCLUSIONS5
REFERENCES6
TABLES 18
FIGURES 33
APPENDIX DEVELOPMENT OF HIGH STRENGTH 77
9Cr Mo STEELS
V LIST OF TABLES
1. Steel Compositions Studied
2. Grain Coarsening Data
3. Prior Austenite Grain Size Assessments
4.r Austenite Grain Size (pm) for Various Austenitising Conditions
5. Hardness Data for Steels 1-17 Normalised 1050°C 1 h AC
6.s Values (HV30) for Different Normalising and/or Tempering Treatments for
Selected Experimental Casts
7. Tensile Properties of Experimental Casts
8. Effect of Normalising and Tempering Temperatures on Ambient Temperature Mechanical
Properties of Steel 15
9.t of Heat Treatment Temperatures on Rupture Properties of Steel 15
10. Stress Rupture Results on Experimental Casts
11. The Effect on Normalising Temperature on the Stress Rupture Properties of Selected
Experimental Casts
12. Hardness Values after Ageing at 700°C Following Different Normalising Treatments
13. Main Precipitate Types and Compositions in Base Composition Steels
14. Precipitate Type and Heat Treatment Variables
15.e Compositions in Base Composition Casts
16. Precipitates in 10.5Cr Steel (Steel 14)
17.e Compositions in Base Composition Casts
18. Stress Values for 100 000 h at 600°C
LIST OF APPENDICES
1. Development of High Strength 9Cr Mo Steels
VI LIST OF FIGURES
1. Section of Schneider Diagram to Predict Microstructures
2. Prior Austenite Grain Sizes of Steel 1
3.r Austenite Grain Sizes
4. Grain Coarsening Characteristics of Base Composition Casts
5.ngs of Experimental Steels
6. Grain Coarsening Characteristics ofl Steels
7. Hardness after Normalising from 1050°C
8. Effect of Change in Normalising Conditions on Hardness
9.t of Compositions on Hardness (with Respect to Base Composition Cast - Steel 1)
10. Tempering Curves for Base Compositions
11.g Curves - Effects of Carbon and Nitrogen
12. Temper Resistance - Effects of Carbon and Nitrogen
13.r Resistance - Effects of Vanadium and Niobium
14. Temper Resistance - Effect of Titanium, Phosphorus, Boron, Chromium and Tungsten
15.r Resistance of Steels 16 and 17
16. Temper Resistance Curves for Steels 1, 2,5 and 6
17.r Resistance Curves for Steels 8,9,13 and 14
18. Hardness of Steel 91 Compositions after Tempering for 1 h at 750°C - The Effect of
Normalising Temperature
19. Influence of Heat Treatment on Tensile Properties
20. Variation of Strength with Vanadium:Nitrogen Ratio
21. Effect of Heat Treatment on Stress Rupture Strength
22.t of Tempering on Stress Rupture Strength of Material Normalised from 1100°C
23. Rupture Ductility Values for Material Normalised + T.750°C
24. Stress Rupture Data (N.1050°C+T.750°C)
25.s Rupture Data (N. 1050°C+T.750°C)
26. Stress Rupture Data)
27.s Rupture Data - Effect of Normalising Temperature
VII 28. Stress Rupture Data - Effect of Normalising Temperature
29. Hardness Data after Ageing (Material Normalised 1050°C/1 h)
30. Change in Hardness after Ageing at 700CC
31. Hardness Changes Related to Initial Normalising Temperature
32. M23C6 Precipitation after Various Heat Treatments
33. NbC and V4C3 Precipitates
34. (V, Nb) C Precipitates
35. Thin Foil Micrographs of Steel 15 in N.1050 + T.750°C Condition
36. Vanadium and Niobium Rich Precipitates in Steels 6 and 13
37.m and Chromium Based Precipitation in Steels 14 and 17
38. Compositions of M23C$ Type Carbides
39. Laves Type Phase in Steels 13 and 16
40. Creep Failure in Base Steel
41(a) Solubility Curves for VN in Austenite
41(b) Hardness - Vanadium Nitride Correlation
42.s Vanadium:Nitrogen Ratio Correlation
43. Stress for 10 0O0 h at 600°C as a Functi