Steel and its Heat Treatment

685 Pages·1984·18.727 MB·685\685

by  Karl-Erik Thelning (Auth.)

Steel and its
heat treatment
Second edition
Karl-Erik Thelning
Head of Research and Development
Smedjebacken-Boxholm Stài AB, Sweden
Butterworth-Heinemann
Linacre House, Jordan Hill, Oxford OX2 8DP
A division of Reed Educational and Professional Publishing Ltd
A member of the Reed Elsevier pic group
^
First published 1975
Second edition 1984
Reprinted 2000
© Jointly owned by Butterworth & Co. and K-E Thelning 1984
All rights reserved. No part of this publication may be reproduced or
transmitted in any form or by aay means, including photocopying and
recording, without the written permission of the copyright holder,
applications for which should be addressed to the Publishers. Such written
permission must also be obtained before any part of this publication is
stored in a retrieval system of any nature.
This book is sold subject to the Standard Conditions of Sale of Net Books and
may not be re-sold in the UK below the net price given by the Publishers in
their current price list.
British Library Cataloguing in Publication Data
Thelning, Karl-Erik
Steel and its heat treatment.—2nd ed.
1. Steel—Heat treatment
I. Title II. Bofors handbook. English
672.3'6 TN751
ISBN 0 408 01424 5
Typeset by Phoenix Photosetting, Chatham
Printed and bound in Replika Press Pvt Ltd, 100% EOU,
Delhi-110 040, India
Preface
Steel and its heat treatment has been thoroughly revised and updated so that
the second edition may incorporate the many developments that have
taken place in the subject since publication of the first edition in 1975. As a
result the coverage has been extended to include the following items.
Chapter 1: The fundamentals of TTT-diagrams are explained in detail. A
description of various hardening mechanisms is given.
Chapter 3: Injection metallurgy and continuous casting are discussed; as is
the influence of sulphur on the properties of steel.
Chapter 4: Existing CCT-diagrams are subjected to a critical review and a
new generation of CCT-diagrams are presented. The mechanisms
controlling hardenability are discussed and this forms the basis for a new
concept of the cooling sequence during hardening. Examples of various
cooling sequences and their effect on the resulting hardening are given.
Chapter 5: Various annealing processes, strain ageing and temper
brittleness are discussed in more detail than in the first edition. Solution
diagrams on heating are explained and discussed. Various methods of
testing cooling media are given along with different interpretations of the
three stages of the cooling curve.
Chapter 6: The various steel grades are arranged in accordance with ISO's
system, with reference to national standards. A recently developed
Swedish hot-work steel is presented. A large section is devoted to boron
constructional steels, micro-alloyed steels and dual-phase steels. A
scientifically interesting case-hardening test series, which is also of practical
use, illustrates the prime importance for a case-hardening steel to have the
right hardenability in the carburized case.
The literature has been critically surveyed and all important references
are listed.
The newly-written sections are based in the main on information and
research released since 1975 which was the year when the author joined
Boxholms AB. The steel division of this company merged in 1982 with
Smedjebackens Valsverk (Smedjebacken Rolling Mill) to form the new
company, Smedjebacken-Boxholm Stài AB (Smedjebacken-Boxholm
Steel Company Limited).
vi Preface
The author is indebted to Dr Allan Hede, the laboratory director of
Bofors AB, Professor Rune Lagneborg of the Swedish Institute for Metals
Research, Professor Torsten Ericson of the Institute of Technology in
Linkoping and Professor Tom Bell of the University of Birmingham, who
have scrutinised the new material and made valuable contributions to the
work.
As in the preceding edition the English translation has been carried out
by Mr Cecil N. Black, BSc (Hons).*
The author wishes to thank all the persons and institutions mentioned as
well as all others who have assisted in and contributed to the publication of
this work. Last, but no means least, my thanks and appreciation to my wife
Iris who has patiently and loyally supported me in all the work connected
with this book.
Boxholm 1984 Karl-Erik Thelning
* The Swedish edition was edited by Maskinaktiebolaget Karlebo in cooperation with
AB Bofors and Smedjebacken-Boxholm Stài AB
International designations and
symbols
International system of units
On the 1st of January 1971 the metric system was officially introduced into
the UK. For the majority of technicians this involved an adjustment from
inches to millimetres. For several years, work had been in progress to
devise a common standard international system of units. Such a system, SI
(Système International d'Unités), was adopted in 1960.
The following units should be used in ISO standards prepared under the
jurisdiction of ISO/TC 164:
1. Stress —N/mm2
2. Hardness—The hardness designations in current use are retained
but the hardness values are regarded as dimensionless
numbers. The actual testing load shall be specified as N.
Example: H V 5 (Testing load 49.03 N).
3. Impact —J.
During an interim period several European countries have used the
symbol kp/mm2 (kilopond) or kgf/mm2 (kilogram-force) to signify the unit
of stress.
The mechanical strength of steel was previously designated ton/in2 (TS I)
in the UK and kg/mm2 on the Continent.
The first edition of this book was written and published during the period
of transition covering the introduction of SI units both in the U K and other
countries that have adopted it. In several diagrams in this book stress is
designated kp/mm2 and in some tables, kgf/mm2 in accordance with some
editions of ISO's recommendations. (BS 970:1970 gives kgf/mm2 as the
designation for stress.) In older ISO documents the symbols for the units
of the yield and the ultimate tensile strength are given as kgf/mm2 and
tonf/in2. In such documents tonf/in2 has been replaced by N/mm2. Also
when the impact strength is given as kgfm/cm the values have been
supplemented with J.
In order to simplify the transition to SI units several diagrams have been
drawn with double scales, e.g. inches-millimetres, kp/mm2-TSI-N/mm2
and even Celsius (°C)-Fahrenheit (°F).
viii
International steel designations ix
In some tables two systems of units are used. For the conversion of
inches to millimetres the factor 25-0 has often been used since the small
error introduced thereby is of no practical consequence. For other, more
precise applications, such as for Jominy diagrams, the exact conversion
factor has been used.
In other ways, too, an international outlook is favoured, viz. the symbols
of hardness units, e.g. HB and HRC. In line with this principle the symbol
HV is used instead of DPH or VPN. Conversion tables and nomograms
are found in Chapter 8.
In connexion with the change over to SI units, according to the ISO
standard a number of designations for mechanical testing have been
changed. What is characteristic of this transition is that certain designations
in Greek letters have been replaced by Latin ones. The designations
generally used for steel are indicated below, partly old ones, partly
according to the new standard. An example for the use of SI units is given
at the same time.
Old standard
ob-2 OB σ5 ψ HB KV KCU
kp/mm2 kp/mm2 % % kpm kpm/cm2
54 81 19 61 249 7,0 9,2
New standard—SI
Äp0 2 Rm A Z HB KV KU
5
N/mm2 N/mm2 % % J J
530 790 19 61 249 69 45
In oider ISO materials standards cited in this book the proof stress is
designated by i? . The designation Rpo-2 was adopted in 1973. The
e
designation Äpo-2 is used for hardened and tempered steels. The
designations R and /? are used for unhardened steels with a clearly
cL eH
defined yield stress range (see Figure 2.8).
International steel designations
Under the auspices of ISO extensive work has been in progress for several
years on the standardization of steel grades, in particular with respect to
cçmposition and mechanical properties. ISO recommendations covering a
large number of steel grades have already been published. Among them
may be mentioned the group 'heat-treated steels, alloy steels, free-cutting
steels and tool steels'.
The tables covering 'Surveys of various types of steel' contain the
standards as published byAISI,BS,DIN and SS along with such ISO
standards as have been issued.
In the text, tool steels are designated mainly by the type letter and
numeral as used in the US A and the UK for standardized tool steels, e.g.
H 13, O 1. These designations are so well known by steel consumers all
over the world that no qualifying institutional designations are necessary.
Steels fpr which there are no A IS I or BS specifications are designated
according to DIN or SS standards.
x International designations and symbols
Depending on which steel types are being discussed in the text,
constructional steels are designated according to B S standards as well as
AISI,DINorSS standards, respectively. In several instances use is made
of simplified designations, e.g. 42 CrMo 4. Such designations are in
general use on the Continent and indicate in a straightforward manner the
approximate chemical composition of the steel.
Previously, the Swedish Standard was designated as SIS. All standards
that have been revised or issued since 1978-01-01 are designateci as SS.
For the sake of uniformity all Swedish Standards are designated as SS in
this book.
It is the author's aim and hope that this book will help in promoting the
introduction of the SI units.
1
Fundamental
metallographìc concepts
Metallography reveals the structure of metals and leads to a better
understanding of the relationship between the structure and properties of
steel. With the aid of modern developments such as the electron
microscope and the scanning electron microscope it is now possible to
obtain a much deeper insight into the structure of steel than was possible
only some twenty years ago.
In order to understand the process occurring during the heat treatment
of steel, it is necessary to have some knowledge of the phase equilibriae
and phase transformations which occur in steel as well as of its
microstructure. Therefore, a brief summary of these topics is given in this
chapter which forms the groundwork for subsequent discussion.
1.1 The transformations and crystal structures of iron
On heating a piece of pure iron from room temperature to its melting point
it undergoes a number of crystalline transformations and exhibits two
different allotropie modifications. When iron changes from one
modification to another heat is involved. This is called the latent heat of
transformation. If the sample is heated at a steady rate the rise in
temperature will be interrupted when the transformation starts and the
temperature will remain constant until the transformation is completed.
On cooling molten iron to room temperature the transformations take
place in reverse order and at approximately the same temperatures as on
heating. During these transformations heat is liberated which results in an
arrest in the rate of cooling, the arrest lasting as long as the transformation
is taking place.
The two alloptropic modifications are termed ferrite and austenite and
their ranges of stability and transformation temperatures on heating and
cooling are shown in Figure 1.1. The letter A is from the French arrêter,
meaning to delay, c from chauffer, meaning to heat, and r from refroidir,
meaning to cool. Ferrite is stable below 911 °C as well as between 1392 °C
and its melting point, under the names a-iron and d-iron respectively.
Austenite, designated y-iron, is stable between 911 °C and 1392 °C. Iron is
1
2 Fundamental metallographic concepts
Solidifying point -
Curie
Time
Figure 1.1 Heating and cooling curve for pure iron
ferromagnetic at room temperature; its magnetism decreases with
increasing temperature and vanishes completely at 769 °C, the Curie point.
The atoms in metals are arranged in a regular three-dimensional pattern
called a crystal structure. In the case of iron it may be pictured as cubes
stacked side by side and on top of one another. The corners of the cubes
are the atoms and each corner atom is shared by eight cubes or unit cells.
Besides the corner atoms the iron unit cell contains additional atoms, the
number of positions of which depend on the modification being studied.
Ferrite, besides having an atom at each corner of the unit cell, has
another atom at the intersection of the cube body diagonals, i.e. a
body-centred cubic lattice (BCC). The length of the unit cube edge or
lattice parameter is 2-87 Â at 20 °C (Â = Angstrom = 10"™ m).
Austenite has a face-centred cubic lattice (FCC), the parameter of which is
3*57 Â (extrapolated to 20 °C). the structure of the unit cells of a-iron and
y-iron respectively may be envisaged as shown in Figure 1.2. The y-iron
unit cell has a larger lattice parameter than the a-iron cell but the former
contains more atoms and has a greater density, being a 8-22 g/cm3 for
y-iron at 20 °C and 7-93 g/cm3 for a-iron.
1.2 The iron-carbon equilibrium diagram
The most important alloying element in steel is carbon. Its presence is
largely responsible for the wide range of properties that can be obtained
Ferrite Austenite
Figure 1.2 The crystal structure of ferrite and austenite
Figure 1.3 Microstructure of carbon steels with varying carbon content.
(a) Ferrite 0-0% C. 500x ; (b) Ferrite + pearlite 0-40% C. 500 x ;
(c) Pearlite 0-80% C. 1000 X ; (d) Pearlite -I- grain boundary cementite
1-4% C. 500 x