·Table of Contents
·Materials Characterization and testing
Local Thermal Processing Effects in the Morphological Transformations Compression
Z. CHERROUF, S. OUALLAM, F. SELLIDJ, A. HAMMOUDA, G. KAMEL
Laboratory of metallurgy
SCIENTIFIC AND TECHNICAL RESEARCH CENTER IN WELDING AND NDT:
C.S.C. BP 64 CHERAGA
The different welding parameters influencing the steel weld ability
complexity determination study, brings us to assimilate this assembly process
to a black box which have several inputs such as: the welding energy
(temperature) , the assemblies geometry, the filing metal and the work
environment , and as for output, materials weld ability aptitude.
Our work consists in establishing an identification temperature effect methodology on the heat affected zone (H.A.Z.) apparition, proceeding by a local heat
treatment and using characterisations means : metallography, mechanical (micro
hardness) and non destructive testing (ultrasonic technique).
The importance that assigns the industry to assemblies, holds these
different assemblies types the convenient possibilities and advantages
However, the soldering realisation permit to get some
mechanical properties can resist the applied solicitations to the welded
sides, depends, strongly, on a metallurgic study.
This study puts in evidence
the thermal treatment structural and constitutional transformations exits
associated to the operation of weldering.
The goal of this work consists in
identifying zones transformed by a prompt source that constitutes the simplest
welding thermal cycle simulation .
This specific thermal treatment is
considered like a following successive thermal treatments adjoining the
treated locally zone.
This approach misleads a structure gradient
characterised by a micro hardness gradient creation. The mechanical holding
structures welded assessment by tests no destructive and the rupture mechanics
in the HAZ presents a considerable interest nowadays. Well that progresses
have been accomplished in welding and physical metallurgy, scientific research
various ways are again in development on the thermal simulation phenomena and
the microstructures forecasting so the HAZ tenacity .
In order to be able to
deduct all logical morphological transformation according to the local thermal
treatment, the reference to samples stallions is indispensable.
objective, these stallions must have thick grains and all internal constraint
II - LOCAL THERMAL TREATMENT
Because of the relations between numerous variable ignorance (qualitative
and quantitative) composing the soldering, the welding sequence complexity
obliges us to orient our research on the welding cycle thermal simulation by a
local treatment on the type Z30C13 rustproof steel.
The thermal source metallurgic effects on a steel are assimilated to the welding operation cycle.
This study rests on two essential parameters that are the temperature reach
and its maintenance length. Being given the influential factors number
intervening in the physic-chemical transformations process, the induced
morphological variation interpretation is not simple.
Following a samples homogenisation , the every point thermal history knowledge is got thanks to thermocouples implanted at different distances and different depths away from
the source impact point.
The chosen steel is the Z30C13 type greatly allied
belonging to the rustproof martensitic family considering contents in carbon
This steel, whose utilisation concerns the thermal power station
overheater tubes present weldability problems in the strong thickness case.
The chromium encourages the phase «a » and widens the
With a superior chromium percentage to 13%, the alloy
undergoes only little or not transformation and remain in phase «a » while holding carbon, nitrogen and nickel rates amount,
and knowing, in particular, that these elements widen the phase«c » austenitic existence domain .
The pseudo-binary Fe-C
diagram (Fig1) is a cut to constant chromium (13%) ternary Fe-Cr-C gives the
different phases and carbides constituting steel to the balance diagram state.
Curves Fe-Cr-C isotherms (Fig2 a and b) give cuts to 20°C and 700°C and
facilitate the phases identification and the existing carbides.
Fig 1: pseudo-binary Fe-C Diagram|
Fig 2: isotherm Cuts Fe-Cr-C
Kc : (Fe,Cr)3C ; K1 :(Fe,Cr)23C6 ;K2 : ( Fe,Cr)7C|
The TTT(time temperature transformation ) curves and CCT(continuous cooling transformation)  give a more precise idea on transformations dynamics during the cooling.
Besides, three Z30C13 conventional HT(heat treatment) types are given by the
documentation: hardening, tempering and annealing treatments [1,2].
III- TENTATIVE study
The samples chemical analysis by X fluorescence gave the mass contents:
On the six half-cylindrical samples, three have been used for the thermocouples
implantation . Other samples cut up on the same log served to reference tests,
analyses and to the preliminary study for the most suitable treatment choice.
Steel no treaty gives a martensitic structure (Fig3-a) to acicular structure
corresponding probably to a austenitisation follow-up a steeping.
Micro-hardness Tests give 296 HV mean .
Fig 3: Micrographes sample 2 Austenitisation 1200°C; Maintenance 3h30mn; Cooling into the oven and tempering at 600°C during 2h
The big disparity between
micro-hardness achieved values (Vmax=335 ,Vmin=267 HV) can be explained by the
splitting of the martensite at the cooling time as suggested by the
documentation , which consider as a carbides precipitation before the Ms
point reach or as a carbon atoms grouping in the privileged zones.
1-Homogenisation thermal treatment :
treatment choice is an austenitisation at 1200°C, a maintenance of 3h30mm with
a cooling to the oven.
Fig 4: LTT principle diagram
The micrograph (fig.3) gives a structure to thick
grains corresponding to an homogeneous matrix and less acicular with regard to
the basis martensitic material .
There is place to note that the maintenance
permits a growth and even a grains coalescence . Micro-hardness tests give the
703 Hv average value on the grain.
These elevated micro-hardness value is
bound to a shearing phenomenon in the matrix stimulated by an interns
constraints concentration and a carbides formation type ((Fe-Cr)23C6)
dispersed in the matrix. The tempering treatment at 600°C during 2 hours with
a cooling to the oven, justify the constraint concentration reduction
corresponding to a 334 Hv micro-hardness grain average value.
the got homogenisation, even though it remains partial, constitute a
reference that could permit the induced transformations registration at the
local thermal treatments.
2-Local thermal treatment (LTT):
The local and prompt
thermal treatment bases itself in the act that the heat source section is very
small in the one treated face front correspondent to the two half-cylinder
To every LTT one uses:
- a half-cylinder treated (homogenised) and,
- a half-cylinder no treated (no homogenised), on which are implanted
a. First test:
The applied heat source for the treatment is
produced by the bow welding process (micro -plasma). The applied cycle
consists on a heating until the zone fusion where the source is applied
,with a fusion 3 mn maintenance and a 20mn cooling to air period.
implanted thermocouples, to the identical depths and different distances,
are joined to an tension indicating equipped electronic system displaying
every three seconds tension delivered by thermocouples one after the other.
The maximal temperatures don't pass 800°C as shown by curves of the figure
(Fig.5) that represent the thermal cycle measured by four of the implanted
Fig 5: Curve heating - cooling (1st test)
These curves show, also, an important heating speed with
regard to the cooling one without a considerable maintenance length to
The structures observation to the microscope begotten
by the heat source effect is given by figures 6,7 and 8.
Fig 6: no homogenised sample transformed zone
Fig 7: homogenised sample transformed zone
The grain size , to
acicular structure, decreases while moving away the source impact to be only
a basis metal structure in the half-cylindrical no homogenised case.
homogenised half-cylinder case, the smaller grains than those of the basis
metal, are refined to a substantial manner.
Micro hardness in the samples
transverse and longitudinal senses are given by the figure 9.
thermal treatment effect creates a structure gradient confirmed by a micro
Fig 8: homogenised sample transformed zone
Fig 9: Micro hardness HV
no homogenised sample, homogenised sample
Transformed zone measurements and temperatures reaches
values constituted a handicap in an interrelationship temperature -
structural topography research .
For it, another test of local thermal
treatment has been considered.
b. Second test:
The second test has been achieved with the following modifications:
- another source utilisation permitting a more important maintenance
- a better eight thermocouples distribution to surround the best
thermally affected zone:HAZ.
With a blowtorch oxyacetylenic, the
fusion temperature has been reached and has been maintained during 9mn,
follow-up by a cooling to air.
Fig 10: Curve heating -cooling (2nd test)
Curves (Fig10) give four thermocouples
implanted temperatures measurement.
The microscope observation (Fig11) gives
the different zones transformation.
The zones transformed widening is more
important than the first test , which can be explained by the more important
maintenance length fusion temperature.
On the other hand, the three under -
zones apparition explanation (Fig11) in each half-cylinder is the following:
- the first under strip shape holds the fact that it is nearer the
fusion place, constituent thus, a zone skin as those observed on a cast
- the second under granular structure shape is owed to the reached
maximal temperatures (950°C) to the regions level to thick grains and
(850°C) to the regions level to thin grains,
- The third under zone shows a slightly modified structure with regard
to the initial structure, which is owed to transformation beginning
temperatures (750°C and 850°C).
The corresponding micro hardness measures
are represented by the figure12 curves.
modifications achieved on the two tests
experimentation techniques permit:
- to establish a better approach concerning interrelationships
- to note the structural gradient within the zone transformed, described
by the three zones, which is appreciable to the source energy used and in
the maintenance length fusion temperature.
Comparative two half-cylinders treaties different zone microstructures transformed micrographes
Fig 12: Micro hardness HV measures no homogenised sample,homogenised sample.a)In the same way to the half-cylinder main axis.b)Perpendicularly to the half-cylinder axis to:
0.05 (1) , 0.55 (2) , 3.80 mm (3) of the treated face.
IV - ULTRASONIC CHARACTERIZATION :
In a no destructive zone transformed characterisation tentative goal ,the
ultrasonic measures has been done, in immersion to the normal and oblique
impacts, and concern longitudinal and transverse waves velocities.
have been achieved with a 2Mhz ultrasonic focalised probe .
The curves got
(Fig.13) are not revealing the transformed zones minute extent .
Fig 13: speeds ultrasonic measure : longitudinal(Vl) and transversals(Vt) according to the heat source application point distance
Metallographic Analysis permitted us to determine the different grains
morphologies owed to the thermal effect and to value the affected thermally
zone extent degree and fragility zones by micro hardness curves.
the got results oblige us to foresee other steels for a deepened analysis, in
particular more according to the equivalent carbon and the weldability in
- the engineer techniques M2I - 1996
- the engineer techniques D3I - 1996
- Revue science of materials -dec.1996
- H. GRAJON: Basis of Metallurgy of Soudage-1989
Other references :
- TESTARD: Rev. Weldering and Related Techniques. (July / August 1994).
- J. DEFOUNI: Rev. Weldering and Related Techniques. (July / August 1994).
- D. SEFERIAN: Metallurgy of the Soldering - 1965.
- L. DEVILLERS, D. KAPLAN, J. P. JANSEN,: Rev. Weldering and Related
Techniques (mars/April 1987).