The present invention relates to a process for producing high quality welding joints
for high strength quench and tempered plates having thickness of upto about 40
mm. More particularly the present invention relates to producing high quality
welding joints using shielded metal arc welding.
The strength of welded joints is a most important criterion for certain applications.
The weld joint should have strength, ductility and toughness properties equivalent
to or superior to the parent metal in order to prevent initiation of defects and
premature failure of the component. Particularly in case of long distance
transportation of crude oils and natural gas where transportation efficiency
depends on the high pressure of transmission. Therefore the need for line pipes
having a higher strength has increased. Also in other applications the high
strength welded steel plates are in demand.
PRIOR ART
US Patent No. 5221374 relates to a process for using agent for improving the
hydrogen cracking resistance of low or intermediate-alloy steels, and pieces
obtained. The agent in question is cobalt added in contents of between 0.05% and
2% to steels containing from 0.05% to 0.6% of carbon and less than 10% of
alloying elements taken from silicon, manganese, nickel, chromium and
molybdenum, to produce optionally normalized blocks, plates, bars or pieces of
large size, with improved hydrogen cracking resistance and with improved weld-
ability and suitability for thermal cutting.
US Patent No. 5430269 relates to a submerged arc welding method for a high
strength (2.25%-3%) Cr-3% Mo-V steel, to obtain a weld metal excellent in
strength at room temperature and high temperature, toughness, and creep
strength, after SR, temper brittleness resistance, cold crack resistance and SR
crack resistance. The Cr-Mo steel contains: 2.00 to 3.25% of Cr, 0.90 to 1.20% of
Mo, and V as essential components, and Nb, Ti, B and Ca as needed. A welding
heat input is in the range of from 20 to 50 kJ/cm. The solid wire contains 0.09 to
0.19% of C, 0.30% or less of Si, 0.50 to 1.40% of Mn, 2.00 to 3.80% of Cr and
0.90 to 1.20% of Mo. The bonded flux contains 5 to 20% of Sio.sub.2, 20 to 40%
of MgO, 2.4 to 12% of a metal fluoride (F-converted value) and 3 to 12% of a
metal carbonate (CO. sub.2 - converted value). A weld metal contains 0.08 to
0.15% of C, 0.05 to 0.30% of Si, 0.50 to 1.20% of Mn, 0.030 to 0.060% of 0, 0.10
to 0.50% of V and 0.005 to 0.035% of Nb. P and Ti in the weld metal are restricted
to 0.010% or less and 0.012% or less, respectively. The components of C, Si, Mn,
0, P. Ti are added from the solid wire and the bonded flux, and the components of
V and Nb are added from at least one of the wire and the bonded flux. Moreover,
the welding is performed such that Ps is 3.50 to 5.50, Ps being expressed by
Ps=10.times.(c).sub.D + 10.times.(Si).sub.D + (Mn).sub.D+ where (X).sub.D is
we% of the component X in weld metal.
US Patent No. 5634988 relates to a high tensile welded steel plate consisting
essentially of, by weight, C:0.03 to 0.20%, Si:0.6 to 2.0%, Mn:0.6 to 2.0%, Al:0.01
to 0.08%, B: not more than 0.0020%, and N: 0.002 to 0.008% and optionally at
least one element selected from Cu, Mo, Ni, Cr, Nb, V, Ti, Ca, and REM with the
balance consisting of Fe and unavoidable impurities, and a process, for producing
a high tensile welded steel plate, usually comprising the steps of : subjecting a
slab comprising the above chemical compositions to hot rolling or alternatively hot
rolling followed by controlled rolling. The present invention enables fatigue
cracking of the as-welded steel, in its heat-affected zone, to be prevented and, at
the same time, the propagation of the crack to be prevented or inhibited. •
Objects of the Invention
It is an object of the present invention to format a process for the safe welding of
high strength quench and tempered plate.
It is further object of the present invention to provide a process for the safe
welding of high strength quench and tempered plate having yield strength of at
least 690 MP a.
Summary of the Invention
Thus the present invention relates to a process for producing welded joints for
high strength quench and tempered plates made from steel having the following
composition
the process comprising
providing a suitable electrode ;
rebaking the steel plates at a temperature of between 350-400°C for 2
hours to 1 hour;
preparing the edges to be welded by making complimentary angular edges
on one or both sides of the two plates ;
subjecting the steel plates to a step of pre-heating at a temperature of
between 65°C-120°C and a interpass temperature of 110-170°C ;
placing the plates to be welded side by side with the complimentary angular
edges facing each other, providing a gap of between 0.5 - 4 mm between
the plates and then carrying out welding of the said plates using a heat
input of 12-18 KJ cm"1 for 3.15 to 4 mm dia electrodes ;
if desired, subject the welded plates to a step of heat treatment.'
Detailed Description
The invention will now be described in greater details with respect to the non-
examples :
Example
The quench and temper (Q&T) plates used for the purpose of the present
invention are available commercially and conforming to ASTM-A517 Gr.F.
Specification.
However, for the purpose of better voidability the applicant has modified the
components of steel with the specified grade to meet the quality requirement of
the various end applicators e.g., impellers, penstocks, excavator, etc. The
modification of the composition also makes the product cost efficient.-,
A comparison of the steel specified under ASTM-A517 Gr. F. and the modified
steel is set out in Table III where the modified steel is marked as Proposed.
Best results are obtained adopting the appropriate electrodes, rebaking
conditions, joint shape for butt welding, welding conditions (voltage - current -heat 1
input), preheat and post weld heat -treatment requirements. The above welding
conditions will ensure a tough, defect free weld joint.,..
• Post Weld Heat treatment :
Not normally required. However, it is recommended in cases of higher restraint
levels (K>1200 kg/mm2) or where post weld rolling is involved. Temperature and
time may be arrived at depending on thickness of plates, restraint etc.
To demonstrate the superiority of the process of the invention several tests were
carried out and the results are set out below:
Implant Test
The implant test is carried out to assess the cold cracking susceptibility of the
weld joint for a given steel under different welding conditions. The implant
specimen is a cylindrical test piece of 6mm diameter with a circular notch
positioned at one end of the specimen. The other end is threaded to enable
application of a load through a loading bar. The specimen is inserted with a push
fit into a hole bored in a plate, called "host plate". A weld bead is laid under
conditions of welding. The set up is allowed to cool and when a pre-selected
temperature of 100°C is reached, a predetermined static tensile load is applied to
the implant specimen by a "constant loading system" until failure occurs or the
lapse of 24 hrs., whichever is earlier. As the stress level is increased, the time to
failure decreases. The maximum stress that the material can withstand without
failure for 24 hrs. is determined by testing at different stress level. This critical
stress level for cracking is known as static fatigue limit (SFL).
Results
The implant test was carried out under 3 welding conditions as follows :
Test 1 : Heat input: 9.7 KJ / cm, no preheat, full rebake of 350°C -2 h
Test 2 : Heat input: 14.9 KJ / cm, preheat of 100°C and rebake of 250°C -1 h
Test 3 : Heat input: 14.9 KJ / cm, preheat of 100°C and rebake of 350°C -2 h
The results obtained are given below
Electrode AWS E 11018 M

For adequate cold cracking resistance, the SFL value of the weld joint should be
in excess of the minimum specified yield strength (MSYS) of the steel, which in
this case is 69.0 kg / mm2, I __.
The SFL values obtained (using a preheat of 100°C, partial or full rebake and a
heat input of 14.9 KJ cm-1) were found to be in excess of MSYS (72.5 and 81.0
kg mm"2), indicating excellent cold cracking resistance.,.
SFL Values of other steels
A comparison of SFL values for different grades of steel will not necessarily
estabish the superiority of one grade w.r.t. the other. As indicated earlier, the SFL
values are governed by the yield strength of the parent metal. Thus a steel with
higher strength level is expected under normal conditions to exhibit higher SFL
values, as compared to a lower strength steel.
The SFL values of a few grades of steel is mentioned below:

Comparative ERC test results, lamellar tear test results with varying plate
thickness and WRL in the form of a table and with proper conclusion
Elastic Restriant Cracking (ERC) Test --
In the ERC test, a two part plate specimen is clamped rigidly into the clamping
frame with high strength bolts. The material and geometry of the frame is such
chosen that it behaves elastically at all levels of reaction stress. In the test,
different levels of restraint intensities are generated depending on the restraining
frames and welding conditions. Weld cracking occurs when the restraint intensity
(K) imposed is higher than a critical level, know as critical restraint intensity (Kcr).
The Kcr value is determined under different welding conditions.
Thus for a weld joint to be safe in actual service condition, the Kcr value should be
higher than the restraint level (K) imposed on the weld structure for a given end-
application. The restraint intensities experienced for different structural
applications are shown below. It may be noted that K varies between a range of
200 -1800 kg/mm2. Thus for a weld joint to be safe for most structural
applications, its Kcr should be in excess of 1800 kg / mm2.

The Kcr values for weld joints subjected to preheat of 65 / 100°C, rebake of 350°C
-2h and heat input of 15.4 KJ cm"1 was found to be well in excess (3540 kg / mm2)
of the normal restraint levels experienced in practice (200 -1800 kg / mm2),
indicating excellent resistance to cold cracking and adequate safety in practice.
Lamellar Tear (L T) Test ^
Lamellar tears are separations lying beneath the weld and parallel to the plane of
the plate. These separations are caused by stresses generated in the through
thickness direction resulting from weld contraction, high surface area of planar
inclusions, high hydrogen level and faulty weld joint design. Lamellar tear test was
conducted using different plate thicknesses simulating the normal conditions of
fabrication involving fillet welds. A multirun weld was applied, starting from the
same side for all the passes. After each pass was deposited and the bead
temperature dropped to around 100°, the weld throat thickness was measured
and the load corresponding to the required WRL was applied. After the test,
visual, ultrasonic and macro examinations were conducted to detect the presence
of lamellar tears.

No evidence of lamellar tear was observed in 16 of the 17 samples tested, upon
visual, ultrasonic and macro examination. This includes steel plates machined
down to different thickness (13 to 20 mm), so as to study the effect of weld
thermal cycle on the lamellar tear susceptibility of different sections of the plate
thickness. Only one sample (SI. No. 14) showed some evidence of micro-tears,
which appears to be an aberration.
Details of test such as LPI, MPI, radiography etc.
Liquid Penetrant Inspection (LPI)
This is a highly sensitive method used for detecting minute flaws such as cracks,
pores and porosity which are open to the surface of the material being inspected.
The kit consist of 3 aerosol can containing a cleaner, a red dye penetrant and a
developer. The area to be inspected is cleaned with a solvent cleaner to remove
any dirt, grease or film. After cleaning, the dye penetrant is sprayed on the weld
joint and allowed to settle for 5-10 minutes. Subsequently, the red dye is removed
from the surface with a cloth. Next a white developer is sprayed which dries and
forms a white powdery coating. The result is a blotting action which draws the
penetrant from within the cracks. A thin red line forms on the white background
and thus delineates the nature, size and shape of the defect.
Magnetic Particle Inspection (MPI)
The method consists of establishing a magnetic field and applying a reddish
coloured iron powder to the surface of the test object. If there is any surface
defects, the iron powder forms a fine line along the contour of the defect and can
be visually observed.
Radiography
This test method utilizes invisible X-ray or gamma radiation to examine the
internal defects within materials. It is used to detect porosity, voids, inclusions,
cracks and other internal defects in welds.
An X-ray film is kept behind a weld joint and exposed to a X-ray source. The
amount of energy absorbed by the material depends on its thickness, duration of
exposure and intensity of X-ray beam. The remaining (not absorbed by the
material) energy will cause exposure of the film. Any defects or discontinuities in
the material will lead to lower absorption of energy, higher exposure of the film
and will appear darker. The X-ray film is subsequently developed and inspected
against an illuminated glass. Defects can be easily observed using this technique.
Ultrasonic Test
Ultrasonic testing is a versatile technique used to find surface and internal defects.
A beam of ultrasonic energy is directed into the specimen to be tested. The beam
travels through the material with only a small loss, except when it is intercepted
and reflected by a defect / discontinuity within the material. This is known as the
ultrasonic contact pulse technique.
We Claim :
1. A process for producing welded joints for high strength quench and tempered
plates made from steel conforming to ASTM 517 Gr. F and having the following
composition
Carbon - 0.10-0.20% by wt.
Manganese - 0.60 to 1.0% by wt.
Silicon - 0.15 to 0.35% by wt.
Sulpher & Phosphorous - , 0.040 Max.
Nickel - 0.7 to 1.0% by wt.
Chromium - 0.4 to 0.65% by wt.
Molydenum - 0.4 to 0.6% by wt.
Vanadium - 0.03 to 0.08% by wt.
Copper - 0.15 to 0.50% by wt.
Boron - 5 to 60% ppm,
the process comprising
providing a suitable electrode ;, ^?
,< " rebaking the steel plates at a temperature of between 350-400°C for 2
hours to 1 hour;
preparing the edges to be welded by making complimentary angular edges
on one or both sides of the two plates ;
subjecting the steel plates to a step of pre-heating at a temperature of
between 65°C-120°C and a interpass temperature of 110-170°C ;
placing the plates to be wejded side by side with the complimentary angular
edges facing each other, providing a gap of between 0.5 - 4 mm between
the plates and then carrying out welding of the said plates using a heat
input of 12-18 KJ cm"1 for 3.15 to 4 mm dia electrodes ;
2. A process as claimed in claim 1, wherein the composition of the steel is as
follows :
Carbon - 0.12-0.16% by wt.
Manganese - 0.80 to 1.0% by wt.
Silicon - 0.2 to 0.35% by wt.
Sulpher & Phosphorous - 0.02 Max.(P) & 0.015 max (S)
Nickel - 0.7 to 0.8% by wt.
Cromium - 0.0 to 0.65% by wt
Molydenum - ., 0.4 to 0.5% by wt.
Vanadium - 0.05 to 0.08% by wt.
Copper - 0.25 to 0.35% by wt.
Boron - 5 to 20% ppm,
3. A process as claimed in claim 1, wherein, if desired, the welded plates are \ msubjected to a step of heat treatment. ' *
4. A process as claimed in claim 3, wherein the heat treatment after the step of ,
welding is carried out where the welded plate is subjected to rolling o rhidjier y^
restraint levels (k> 1200 Kg/mm2).
5. A process as claimed in Claim 1, wherein the thickness of the steel plate is
upto 40 mm.
6. A process as claimed in Claim 1, wherein the step of rebaking is carried out at
a temperature of between 350-400/°C for a time of between 2 hrs. to 1 hour.
7. A process as claimed in Claim 1, wherein the electrode used is a low hydrogen
| type electrode conforming to AWS A 5.5 E 11018 M type.
8. A process as claimed in Claim 1, wherein the angle at which the edges of the
plates to be welded is in the region of 70°.
9. A process as claimed in claim 1, wherein the heat input is maintained at 12-14
KJ cm 1 for 3.15 mm dia and 14-18 KJ cm"1 for 4 mm dia electrodes.
10. A process as claimed in Claim 1, wherein the space between the edges of the
plates to be welded is in the region of 2.5 mm.

Process for producing welded joints for high strength quench and tempered plates made from steel conforming to ASTM 517 Gr. F and having the composition Carbon - 0.10 - 0.20% by wt., Manganese - 0.60 to 1.0% by wt., Silicon - 0.15 to 0.35% by wt., Sulpher & Phosphorous -0.040 Max., Nickel - 0.7 to 1.0% by wt., Chromium - 0.4 to 0.65% by wt, Molydenum - 0.4 to 0.6% by wt., Vanadium -0.03 to 0.08% by wt., Copper - 0.15 to 0.50% by wt., Boron - 5 to 60% ppm. A suitable electrode is provided and the steel plates are rebaked at a temperature of between 350-400°C for 2 hours to 1 hour. The edges of the plates to be welded are prepared by making complimentary angular edges on one or both sides of the two plates and the steel plates are subjected to a step of pre-heating at a temperature of between 65°C-120°C and interpass temperature of 110-170°C. The plates are placed side by side with the complimentary angular edges facing each other, providing a gap of between 0.5 - 4 mm between the plates and then carrying out welding of the said plates using a heat input of 12-18 KJ cm-1 for 3.15 to 4 mm dia electrodes.