A BASIC UNDERSTANDING OF THE MECHANICS OF ROLLING MILL ROLLS Dr. Karl Heinrich Schröder K. H. Schröder: A BASIC UNDERSTANDING OF THE MECHANICS OF ROLLING MILL ROLLS File: SCHROEDER_ROLLS_010703.DOC PREFACE TO THE 1st REVISION I published this e-book in March 2003, for free downloads from ESW homepage http://www.esw.co.at. Immediately some friends and also myself found some spelling - of course - and even worse, real errors, sorry. Some colleagues claimed, “my material toughness” was not precise enough, and they made proposals for improvement. So I tried to eliminate (reduce) errors and to do definitions better. In April 20003, I joined the conference ROLLS 2003 in Birmingham, GB, and some presentations stimulated me to re-think and re-write chapter 4.6. “Roll Damage Caused by Fatigue”. The description of the impact of "particle cleanliness" on sup-surface starting, particularly "crumply" (high cycle) fatigue spalling was so far missing in this chapter totally. After the first release of this book, I was hoping for more stimulations and proposals for improvements, corrections, supplementations and additions by friends, customers, competitors or … Indeed, I am grateful for any comment - this helps to make it better - but in fact, there have been only very few messages. This leaves me waiting for more feedback. I expect to edit some more re-written chapters later. Especially I am waiting for results from "roll cooling simulation" to give my theory of fire-cracks more background. Secondary scale on hot strip and the impact on wear is another issue. Stresses from Hertzian pressure in 4-high cold mill back up rolls were measured years ago, the results were never published, but of high interest - also, maybe later. K. H. Schröder July 2003 K. H. Schröder: A BASIC UNDERSTANDING OF THE MECHANICS OF ROLLING MILL ROLLS File: SCHROEDER_ROLLS_010703.DOC CONTENTS 1. PREFACE / INTRODUCTION ............................................................................................................ 1 2. WHERE DO WE COME FROM AND WHERE WE ARE GOING? .................................................... 4 2.1. GENERAL OVERVIEW .................................................................................................................. 4 2.2. HISTORICAL DEVELOPMENT OF ROLLING MILLS.................................................................... 5 2.3. HISTORICAL DEVELOPMENT OF MATERIALS USED IN ROLLING MILLS............................... 8 3. BASIC BUT IMPORTANT KNOWLEDGE FROM MATERIAL SCIENCE ........................................ 11 3.1. HARDNESS .................................................................................................................................. 11 3.1.1. INTRODUCTION ................................................................................................................... 11 3.1.2. PRINCIPLES OF HARDNESS READINGS .......................................................................... 12 3.1.3. HARDNESS CONVERSION.................................................................................................. 14 3.1.4. FUNDAMENTAL PROBLEMS OF HARDNESS READINGS................................................ 14 3.2. MATERIAL PROPERTIES FROM STATIC TESTING.................................................................. 16 3.2.1. ELASTIC DEFORMATIONS.................................................................................................. 16 3.2.2. TENSILE TEST...................................................................................................................... 19 3.2.2.1. TENSILE TEST WITH STRAIGHT TEST BAR............................................................. 19 3.2.2.2. TENSILE TEST WITH NOTCHED TEST BAR ............................................................. 22 3.2.3. TENSILE TESTS USING GREY IRON WITH LAMELLAR GRAPHITE SPECIMENS ......... 24 3.2.4. TENSILE STRENGTH ........................................................................................................... 28 3.2.5. WORK-HARDENING............................................................................................................. 30 3.2.6. COMPRESSION STRENGTH............................................................................................... 34 3.2.7. TORSIONAL STRENGTH ..................................................................................................... 36 3.3. FATIGUE....................................................................................................................................... 37 3.3.1. HISTORICAL BACKGROUND .............................................................................................. 37 3.3.2. FATIGUE STRENGTH .......................................................................................................... 39 3.3.3. THE IMPACT OF PRE-STRESS/RESIDUAL STRESS ON FATIGUE STRENGTH ............ 41 3.4. FRACTURE MECHANICS............................................................................................................ 43 3.5. WEAR ........................................................................................................................................... 47 3.6. FRICTION ..................................................................................................................................... 51 3.7. PHYSICAL PROPERTIES ............................................................................................................ 53 3.8. REMARKS ON MATERIALS USED FOR ROLLS........................................................................ 54 4. ROLLS .............................................................................................................................................. 57 4.1. DEMAND FOR ROLLING-MILL ROLLS....................................................................................... 57 4.2. REMARKS ON ROLL MAKING .................................................................................................... 59 4.3. HARDNESS OF ROLLS ............................................................................................................... 63 4.4. RESIDUAL STRESS..................................................................................................................... 66 4.5. ROLL DAMAGE CAUSED BY ONE SINGLE LOAD .................................................................... 71 4.5.1. THERMAL BREAKAGE......................................................................................................... 71 4.5.2. TORSIONAL BREAKAGE OF DRIVEN ROLL NECKS......................................................... 73 4.5.3. "FIRE-CRACKS".................................................................................................................... 74 4.5.4. LOCAL OVERLOAD .............................................................................................................. 76 4.6. ROLL DAMAGE CAUSED BY FATIGUE ..................................................................................... 77 4.6.1. FATIGUE BREAKAGE UNDER THE INFLUENCE OF DESIGNED NOTCHES .................. 78 4.6.1.1. FATIGUE BREAKAGE OF SECTION MILL ROLLS..................................................... 78 4.6.1.2. FILLETS OF ROLLS ..................................................................................................... 79 4.6.2. FATIGUE ON ROLLS WITHOUT INFLUENCE OF DESIGNED NOTCHES........................ 81 4.6.2.1. SADDLE SPALLS IN WORK ROLLS ........................................................................... 81 4.6.2.2. EDGE SPALLING ......................................................................................................... 81 4.6.2.3. SPALLS, SPALLING IN GENERAL .............................................................................. 83 K. H. Schröder: A BASIC UNDERSTANDING OF THE MECHANICS OF ROLLING MILL ROLLS File: SCHROEDER_ROLLS_010703.DOC 4.7. DAMAGE OF STEEL ROLLS DUE TO HYDROGEN................................................................... 85 4.8. WEAR AND FRICTION OF ROLLS.............................................................................................. 87 4.8.1. WEAR AND ROLL PERFORMANCE.................................................................................... 87 4.8.2. WEAR ON ROLL NECKS...................................................................................................... 90 4.8.3. BITE ANGLE AND COEFFICIENT OF FRICTION................................................................ 92 4.9. ROLL REPAIR .............................................................................................................................. 94 4.9.1. ROLL INSPECTION AND REDRESSING OF BARREL SURFACE ..................................... 94 4.9.2. SPALLS ................................................................................................................................. 96 4.9.2.1. SPALLS IN WORK ROLLS........................................................................................... 96 4.9.2.2. SPALLS IN BACK-UP ROLLS ...................................................................................... 96 4.9.3. WEAR ON NECKS ................................................................................................................ 97 4.9.3.1. WEAR ON NECKS AT THE BEARING AREA ............................................................. 97 4.9.3.2. WEAR ON THE DRIVING ELEMENT........................................................................... 97 4.9.4. CRACKS IN NECKS.............................................................................................................