Here is some more food for thought (exerpt) that comes from an article of this month's AWS Homepage:
The nuclear Submarines
In 1959, the QT28 electrode was replaced by the QT35 electrode, which had a 35-ton-per-square-in. 0.2 percent proof stress. That was attained by stringent heat treatment from an open hearth steel making process. This steel was intended to match the properties required for Dreadnought, Britain's first nuclear submarine that was based on an American design that used HY80, also a quenched and tempered steel.
Because the properties of these steels were obtained by heat treatment, the very process of welding degraded the properties of the heat affected zone, so that very strict controls of pre-heat, heat input (run out length) and temper bead sequences were developed, to minimise the size, hardness and brittleness of the HAZ.
The as-deposited weld metal could not be heat-treated, and therefore had to have a higher alloy content to match the strength of the plate. That, in turn, increased the risks of weld cracking. Therefore the weld metal analysis/ strength was increased even more, to over-match the plate strength, to try to compensate for the lower toughness of the weld metal.
Whole joint testing was carried out by the MOD(N) Research Establishment using a specially devised "Pellini Bulge Explosion" test, in which whole test weld plates were subjected to severe mechanical deformation by explosions close to their surface.
Automatic hull butt welding still was carried out with the development of the Fusarc process, where alloying elements were added to the continuous flux coating to improve the deposited weld strength. This reduced the welding characteristics of the flux coating, but was offset by the use of an extra granulated arc flux, as in then-common submerged arc process.
This hybrid process was called "Fusemelt," and provided by BOC who had taken over Quasi-Arc. Low basicity powdered fluxes had not been developed, so that the standard submerged arc process was not capable of giving the required joint properties
Ultrasonic testing
In 1959, for the first time anywhere in large scale, heavy fabrication, the quality of (pressure hull) welding was examined not only by gamma radiography, but also ultrasonic testing. At first, normal probes scans were used.
As with the introduction of X-radiography in the 1940s, there were very few objective acceptance standards at first, and no truly standardised ultrasonic testing procedure. No significant problems were identified for some time, until the absence of a properly standardized ultrasonic testing calibration system lead to considerable confusion and widely differing defect reports by various ultrasonic testing teams in the shipyard and subsequent in-service monitoring inspections. Sequential in-service surveys appeared to show defects in the frame to pressure hull welds growing at alarming rates, causing consternation.
Eventually, a section of hull was actually cut out and tested by all the teams who each found different defect lengths. The sample then was cut up to prove that the "indications" were merely surface profile effects that had been found during the post welding inspection in the yard, but not recorded.
A more significant problem resulted from the change to the connection of the stiffener frame web to the pressure hull from simple fillet welds to full penetration tee-butts. This may have been introduced to improve and simplify the inspection and interpretation by the ultrasonic testing process, and perhaps to improve resistance to the possibility of fatigue toe cracking in subsequent service.
These welds were deposited by a semi-automatic MIG process using a 1 percent argon/oxygen shield gas and 1.6-mm-diameter Airco A632 low alloy bare wire (at 380-420 amps).
The tensile strength of this deposit was 25 percent greater than the plate strength (overmatched), to compensate for the lower toughness of as-deposited weld metal compared with the plate toughness.
It is interesting to note that, even today, submarine hulls designed with a maximum working stress close to the plate yield stress, and subjected to external pressures with extreme modes of failure, are not, and cannot be subjected to post weld stress relief. Compare this with almost universal code requirements for post weld treatments for commercial pressure vessels designed with much greater factors of safety.
Lamellar Tearing
The resulting very high tensile residual stresses induced in the tee butt welds, from the over-matched weld metals and the pre-heating regime used, exceeded the "through the thickness" or short-transverse strength and ductility of the QT35. Soon, the incidence of lamellar tearing (cracking) in the pressure hull plating below the frame webs was revealed by the ultrasonic testing. Many strange theories for the cause of the mainly sub-surface cracking were postulated, together with procedures to both avoid its incidence and repair it when found.
In the end, it was the shipyard welding engineers who deduced that the problem lay with the previously unrecognised poor through-the-thickness properties of the QT35. It was clear that the residual stresses of the tee-butt connections had to be reduced. Lower strength electrodes, with pro-active control of preheating and deposition techniques were developed that effectively overcame the problem for the frame welds.
New repair techniques were devised involving management of the application of preheat and buttering using under-matched electrodes. More research showed that it was the open hearth steel making process for QT35 which gave rise to distribution of lamellar, brittle, manganese-silicates which caused the poor throughthickness, which now is known as "Z" quality, properties.
It was realised that these were not a feature of the comparable HY80 steel, which was produced by electric furnace and vacuum de-gassed. Thus, by 1969, NQ1 steel, a modified version of HY80 steel, was introduced. This steel still is used in current submarine fabrication, and the experience gained has lead to the development of "Z" quality steels for susceptible heavy joints in commercial use in the United Kingdom.
Welding process developments
In 1967, the first commercial lowhydrogen basic flux was tested and approved using a low alloy wire, as a conventional submerged arc process. The old Fusemelt process was abandoned, leading to significantly improved applications of automatic welding for hulls.
The older MMA electrodes were replaced by slightly higher strength rods, but with improved weld metal toughness and reduced hydrogen levels. Unfortunately, careful and extensive developments of pulsed arc positional processes in the late 1960's failed to produce a viable semi-automatic process to replace positional MMA. During the early 1970's, ultrasonic testing began to be carried out using both shear and normal probes, giving much better defect location and representation. In the mid 1980's, a flux-cored wire, with acceptable mechanical properties, was introduced, which was much more productive than MMA for positional work.
Current Processes
Current welding practice for submarine manufacture involves twin tandem submerged arc for rotated sub-unit circumferential butts, and for frame to hull and web-to-table tee butts. Pressure hull static circumferential butts and sub-unit vertical seams are welded by a mechanised (positional) FCAW process, and semi-auto FCAW is used for all other welding. The use of MMA is currently limited to very few applications, mainly where access is very restricted. Current NDT practice promotes the use of digitised ultrasonics (time-of-flight diffraction), replacing radiography for butt welds wherever practical.
Respectfully,
Henry
Forum
