Atlas of continuous cooling transformation (CCT) diagrams applicable to low carbon low alloy weld Metals (matsci

101 Pages·1995·2.75 MB·English

by  Zhuyao Zhang

AN ATLAS OF
CONTINUOUS COOLING
TRANSFORMATION (CCT)
DIAGRAMS APPLICABLE TO
LOW CARBON LOW ALLOY
WELD METALS

AN ATLAS OF
CONTINUOUS COOLING
TRANSFORMATION (CCT)
DIAGRAMS APPLICABLE TO
LOW CARBON LOW ALLOY
WELD METALS
ZHUYAO ZHANG and R.A. FARRAR
Department of Mechanical Engineering
University of Southampton, U.K.,S0171BJ
THE INSTITUTE OF MATERIALS
Book 638
Published 1995 by
The Institute of Materials
1Carlton House Terrace
London SW1Y5DB
© The Institute Materials 1995
ISBN 0901716944
Typeset, printed and bound by
Bourne Press Ltd
Bournemouth, UK
An Atlas ofCCT Diagrams Applicable to Low Carbon Low Alloy Weld Metals 1
I. Introduction
Sincethe pioneering studies on continuous cooling transformation (CCT)diagrams
carried outby Christenson etal:'were published almost 50years ago,many hundreds
ofCCT diagrams have been constructed throughout the world to describe the y-a
transformation kinetics of most grades of commercial steels. Because most of the
metallurgical processes occurring in steels involve continuous cooling before the
final microstructure is obtained, the use of CCT diagrams to present the "I-a
transformation reactions has obvious practical advantaget!smpared with other
methods such asthe well-known time temperature transfor tion (TTT)diagrams.
The initial CCT diagrams were constructed for wrought steels and these cannot
usually be directly applied to the cooling cycles experienced in welding situations.
Byemploying modified reaustenitising procedures, the method was applied to the
weldability ofsteels and consequently, several CCTdiagrams applicable tothe coarse
grained region ofthe weld heat affected zones (HAZ) were published.r" However,
since mid-1970s, increasing demands for weld metals of high toughness at low
temperatures with the appropriate microstructures has produced the requirement
for amore systematic and detailed study oftransformation kinetics and mechanical
properties oflow alloy weld deposits. This resulted in anumber of CCT diagrams
which were directly applicable toweld metals and these have significantly improved
our understanding of weld metal microstructural development and the effects of
different factors, such aschemical composition, oxygen content (thus sizedistribution
and population of inclusions), welding parameters (e.g. cooling rate) and prior
austenite grain size, on the "I-atransformation behaviour ofweld metals."?'
Itistherefore ofboth practical aswell as academic importance to draw together
an atlas ofCCTdiagrams applicable tolow carbon low alloy weld metals. Itishoped
that these diagrams willbeofassistance towelding engineers, welding metallurgists,
welding-consumables designers in industry. At the same time, they will also prove
useful tothose in academia who are involved into investigations ofsteel weld metal
phase transformation kinetics.
2.Microstructural terminology for low carbon low alloy weld metals
The microstructural constituents commonly found in low carbon low alloy weld
deposits can be classified as follows, arranged in the order of decreasing
transformation temperature-A"
(1) Primary ferrite (orpolygonal ferrite);
(2) Ferrite side-plates (or Widmanstatten ferrite);
(3) Fine grained acicular ferrite;
(4) Lath structure (lath ferrite or bainite, or lath martensite).
Within the large number ofinvestigations, however, there has been considerable
inconsistency among various classification schemes used to define the different
2 An Atlas ofCCT Diagrams Applicable to Low Carbon Low Alloy Weld Metals
transformation phases. It is therefore necessary to briefly compare these different
schemes. Table 1summarises some earlier schemes used for low carbon low alloy
weld metals.
Table 1.Review ofmicrostructural terminology used for low carbon low
alloy steel weld metals, after The Japan Welding Society" and others.
CA. Dube28
H.I.Aarronson29 Japanese
R.C Cochrane30 T.G.Davey31 D.J.Abson32 Others15, 25,26,33-41 researchersta 42-49
Allotriomorphic Proeutectoid ferrite; Grain boundary Proeutectoid ferrite; Proeutectoid ferrite;
(polygonal) ferrite; ferrite; Grain boundary Grain boundary
ferrite; ferrite.
Polygonal ferrite; Polygonal ferrite;
Blocky ferrite;
True grain boundary
Polygonal ferrite Ferrite islands. ferrite;
Polygonal ferrite.
Primary and Lamellar component Ferrite with aligned Ferrite sideplates; (Widmannstatten)
secondary ferrite (product). MAC; Widmannstatten Ferrite sideplates;
sideplates. Upper bainite. ferrite sideplates; Lath like ferrite.
Lath ferrite
Sidegrain boundary
ferrite.
Intragranular ferrite Acicular ferrite. Acicular ferrite; Acicular ferrite; Acicular ferrite.
plates. Fine bainite ferrite. Needle-like ferrite;
Fine grained ferrite;
Labelled intregranular
ferrite;
Intragranular ferrite.
Massive ferrite; Granular ferrite.
Microphases
Pearlite; Ferrite-carbide Pearlite;
aggregate;
Lath martensite; Martensite. Martensite; Martensite; Martensite;
Twinned martensite; M-A constituent M-A constituent; M-A constituent;
Retained austenite; Lath ferrite; High carbon
Upper (occasionally Upper bainite; martensite;
lower) bainite Lower bainite & Upper bainite.
Martensite
Efforts have been made by TheInternational Institute ofWelding (IIW)todevelop
a standard scheme for the identification of ferritic weld metal microstructures.Y"
Harrison and Farrar14,16,17 used aterminology similar tothat ofthe IIWproposal, but
also considered the morphologies ofvarious types offerrite present in low carbon
low alloy welds. This allowed them to describe satisfactorily the microstructures in
C-Mn and C-Mn-Ni weld metals. More recently, Zhang and Farrar
employed a modified terminology which extended the Harrison and Farrar
21-24
scheme. Table2lists this terminology and the description for each constituent
14,16,17
along with the equivalent terminology 'proposed by the IIW.Some examples ofthe
different microstructures are illustrated in Fig.I.
An Atlas ofCCTDiagramsApplicabletoLowCarbonLowAlloy WeldMetals 3
Table 2.Definition ofmicrostructural terms used bythe current
authors and the equivalent terminology under the IIWscheme.
22-32
Transformation product General description Equivalent terminology inIIW
(Z.Zhang and scheme
R.A. Farrar22-24)
Polygonal fenite(PF) Polygonal orequiaxed atlow cooling Primary ferrite (PF)or (PF(G»
rates;
Grain boundary allotriomorph athigher
cooling rates.
Pearlite (P) Pearlite orpearlitic carbides. Ferrite-carbide aggregate
(FC(P»
Ferrite with non-aligned Ferrite completely surrounding either Ferrite with non-aligned
second phase (FS(NA» (i)microphases which are second phase (FS(NA»
approximately equiaxed and randomly
distributed or(ii)isolated laths of
acicular ferrite.
Ferrite sideplates (FSP) Sideplate structures growing directly Ferrite with second phase
from polygonal ferrite orgrain boundary (FS(SP»
allotriomorphs, i.e.Widmannstatten
secondary sideplates.
Acicular ferrite (AF) Intragranular product offine Acicular ferrite (AF)
interlocking ferrite grains separated by
high angle boundaries, and aspect ratio
from ,..,3:1-10:1.
Coarse acicular ferrite Refers tothe intragranular product
(CAF) formed atslower cooling rates than
acicular ferrite with larger grain size and Acicular ferrite (AF)
may be associated with carbides.
Lath ferrite (LF) Refers toapredominantly intragranular Ferrite with second phase
product resembling bainite which (FS(B»
sometimes forms amongst acicular
ferrite orsideplate structures.
Carbides mayor may not bepresent.
Martensite (M) Lath martensite Martensite (M(L»
In this monograph, the terminology of most of the CCT diagrams will be
essentially in line with the scheme of Table 2. However, the microstructural
descriptions employed by some other authors, which are not clearly defined by
those authors, such as Homma et al." are respected and retained in their CCT
diagrams, and the equivalent terminology tothesemaybefound either fromTable
lor Table2.
4 An Atlas ofCCT Diagrams Applicable toLow Carbon Low Alloy Weld Metals
a.PF andCAF b.FSP
c.AF d.AF with PF an FSP
e.FS(NA) f.M
Fig. 1Definitions ofweld metal microstructural constituents used in CCT diagrams:
(a)PFand CAF;(b)FSP;(c)AF;(d)AFwith PFand FSP;(e)FS(NA); (f) M.
An Atlas ofCCT DiagramsApplicable toLow CarbonLowAlloy WeldMetals 5
3.Construction of CCT diagrams for low carbon low alloy weld metals
Continuous cooling dilatometry technology is by far the most commonly used
method ofproducing CCT diagrams applicable towelding. Inthe case ofsteels, the
transformation temperatures for corresponding microstructural products can often
be obtained by locating the temperature at which the dilation versus temperature
curves start to deviate from linearity. The CCT diagram can then be constructed by
plotting temperature versus time. This procedure isshown schematically in Fig.
2.54
(a)
(b)
ee)
(d)
LDGTlME -
P,PsFs
TEMPERATURE ---+
Fig. 2The normal procedure ofproducing a CCT diagram for steel."
(a) Schematic length versus temperature plots for four different cooling
rates; (b)schematic CCT diagram produced from data in (a).
Although for low carbon low alloy weld metals, especially at cooling rates
experienced under welding conditions (typically 1-30 Ks-l, ~T 800-500 °C), the
transformed microstructure from theparent austenite (A)usually consists ofdifferent
forms of ferrite phase, i.e. polygonal ferrite (PF),ferrite side-plates (FSP),acicular
ferrite (AF) and sometimes lath ferrite (LF). These do not lead to a very clear
dilatometric resolution (deviation from linearity) unless some martensite (M)forms.
Inthese cases, quantitative metallography isused tolocate the temperature atwhich
each ferrite phase transforms. The transformation order ofthese ferritic structures
are known.P" and assuming that the contribution ofeach amount oftransformation
to the volume change ofthe sample isthe same, it ispossible to calculate the micro
structural constituent start temperatures as shown in Fig. 3.54 The corresponding
CCTdiagram can then be constructed accordingly. This dilatometry-metallography
method has recently been completely verified byFarrar and Zhang using systematic
55
step-quenching and detailed metallographic examination.