The Development of Comprehensive High Density Hardness Survey Mapping Techniques BAE Systems Maritime , Barrow-in-Furness, Cumbria, UK

Mr. M.W.Wilson – Consultant Metallurgist Dr. S. J. Hambling– Principal Metallurgist

She is 97 metres long and weighs 7,400 tonnes. The vessel has a system with the processing power of 2,000 laptop computers which is so sensitive it can hear other vessels 3,000 miles away and can carry a payload of 38 cruise missiles capable of hitting targets 1,200 miles away. She is clad in high- density rubber tiles utilising noise and vibration suppression techniques to maintain the ’s stealth.

Figure 1 HMS Astute

Built by BAE Systems Submarines at its yard in Barrow-in-Furness, HMS Astute (Figure 1), one of the ’s newest nuclear submarines, is one of the most sophisticated and powerful vessels ever built routinely described as being of comparable complexity to the space shuttle. The nuclear reactor, which drives the propulsion system, will last the 30 year life of the vessel without the need for refuelling and enables it to travel over 500 miles in a day, allowing deployment anywhere in the world within two weeks.

The pressure hull containment structure of the submarine is fabricated from high strength steel known as Naval Quality 1 (NQ1) to the highest standards. The choice of welding consummable for the hull circumferential butt welds is therefore critical and carefully controlled to ensure that the required levels of quality are always maintained. To achieve this, representative test welds are produced from any proposed new weld metal consumables and subjected to a comprehensive volumetric non destructive and a full destructive examination. The destructive tests include macro-examination, tensile, charpy impact, hardness surveys, flawed bulge explosion and crack tip opening displacement. The QC hardness surveys of the double sided full penetration butt welds comprise of a series of three parallel runs of hardness tests. These indents cover the weld caps at 2mm below the surface on side 1, side 2 and through the root of the joint and are set to incorporate the parent metal, heat affected zones (HAZ) and weld. A hardness force of 10Kgf is applied to an 800 grit finish transverse-section-macro of the full weld section. These had been traditionally carried out manually and were both time consuming and costly to conduct.

When the time came to procure a new hardness tester it was decided to try and automate the process as much as possible. The magnitude of the welds associated with submarine construction was beyond the capability of most of the available equipment. The limiting aspects were either the image capture capacity and/or the physical x-y movement of the mechanised stage. It was decided that an integrated micro hardness capability would additionally be beneficial. Indentec was selected as the supplier of a sophisticated Vickers micro/macro, 0.2 through to 30Kgf, hardness testing system which incorporated automatic indentation, motorised XYZ stage, multiple lenses and sample scanning software (Figure 2). The installed system also utilises sophisticated software to facilitate stitching of multiple images, accommodating the microscopic assessments of complete welds.

Figure 2 Indentec ZHV 30 Micro-Macro Hardness Tester

The survey process was duly automated utilising templates specially created from the supplied Zwick Roell software. As familiarisation with the hardware and the software developed it was recognised that far more comprehensive hardness surveys could be generated. The use of micro hardness loads over larger and larger areas uncovered teething issues associated with the sample preparation techniques. Because of the lower loads a micro surface finish was necessary to facilitate the automatic indentation recognition system requiring that the sample was completely flat and parallel over the entire area of interest.

Having developed the necessary sample preparation techniques the hardness surveys gradually evolved in size from several tens of points; incorporating small areas of HAZ etc, until eventually the complete weld could be covered involving in excess of 25,000 individual points to date. This was achieved using an indentation spacing of just 250 micrometres in conjunction with a 1Kgf hardness force. The loading and spacing were carefully chosen to maximise the density of the testing whilst still retaining sufficient resolution of the hardness results to differentiate subtle microstructural differences within the fabricated joint. By manipulating commercially available software we were then able to produce visual maps of hardness values from the acquired data to readily distinguish the marginal variations in hardness, and corresponding mechanical properties, across the full welded section.

At BAE System Submarines Materials Technology Centre the ease of post hardness testing analysis, involving the development of standardised colour palettes, combined with the level of resolution detail and extent of area coverage is most certainly a world first. In order to support the development of pressure hull steels for the next generation of submarines one of the studies compared submerged arc tee-butt-weld joints manufactured using various welding parameters. The specimens were manufactured from NQ1 and similar high strength steel plates with the aim of establishing any potential effects of parent plate dilution into the weld metal arising due to the weld preparation geometry of the double sided Tees. In particular, focus on the root run and the proceeding weld runs in their order of sequence were compared.

In some cases the parent abutting plates were shown to clearly exhibit minor levels of through thickness plate segregation aligned parallel with the centreline. Of particular interest the weld metal appears to have no discernable localised regions of increased hardness associated with dilution of the parent plate. However, the micro-hardness survey clearly exhibited hardness banding attributable to individual weld runs, Figures 3 and 4. At the set level of resolution the details observed within the root of the weld were sufficient to highlight hardness bands directly associated within successive columnar and recrystallized equiaxed regions within of the weld metal. The peak hardness associated with each weld run was clearly shown to associate with the highest hardness regions within the HAZ (Figures 3 and 4). It was also possible to note small differences in HAZ width and hardness range associated with varied welding parameters from automated welding practices. Additionally, the survey analysis was able to highlight elevated hardness regions commonly associated within the cap weld bead and to a lesser extent, the toe weld bead runs. Further, the post-hardness-testing analysis comes into its own when the surveys were viewed in combinations of 2D / 3D plan and side elevations allowing for complete understanding of the hardness contour visualisations, Figure 4. Additionally, when coupled with gridded-map overlays individual areas are easily identified for SEM element mapping analysis using the hardness indent co-ordinates as a location guide.

Optical Macro Image 2D Hardness Survey Map

Figure 3 Hardness Indenter Array and Resultant Hardness Profiles

The generation of this fuller more comprehensive picture of the variations associated with the complete welded joint can prove invaluable for current and future submarine design. It will, for example, enable more precise targeting for the fatigue cracks associated with the CTOD test piece to locations of interest within welds with respect to both the local microstructure and the mechanical properties. This is of particular importance where changes in weld technique, joint design and welding consumables are proposed. A much more comprehensive database of the fracture toughness characteristics of the weld can then be generated which will enhance the safety case for any new design.

Figure 4 3D Generated Elevation of Hardness Survey Map