Floating metal - steel in ships
Improving steel strength is enhancing the security of warships and affecting manufacturing routes. Norman McPherson from BVT Surface Fleet, and Norman Cooper from BAE Systems, describe developments.
For new naval surface ships the main steel type used is Lloyds Grade DH36. Up to 80% of the plate weight in the Type 45 destroyer (shown in the image below, left) is DH36, and the equivalent amount for the Carrier Vessel Future (CVF) project – a two-ship class of aircraft carrier being developed for the Royal Navy - is 74%.
In the CVF, 19% of the plate weight is Lloyds Grade EH46. It can mainly be found in the flight and hanger decks. Typical chemical analysis and mechanical properties are shown in the table below.
The drive to increase the use of DH36 on the Type 45 was based on weight reduction by using its higher strength to displace thicker D grade steel. A similar situation can be seen on the CVF flight deck where Lloyds Grade EH46 displaced Lloyds Grade EH36.
DH36 is a widely available steel, except in four- and five-millimetre thickness, where potential plate supply options are limited. To counteract this, consideration is being given to the use of wide high-strength hot-rolled strip steels. Also, the overall geometry of thin plate (below eight millimetres) is important. Flatness is a key issue as deviation can result in slower rates of work, and can also act as a site for thin plate distortion. For the Type 45, thin plate gauge control was a major factor to control weight and a limit of +0/-0.3mm was required. However, for CVF, the main concern is the mechanical properties of the steel.
There are a number of potential process routes for EH46, and in this specific case there was a sound techno-economic case to use thermomechanically processed (TMCP) plate. The possibility of using accelerated cooled TMCP plate with higher toughness was considered but found to create no obvious added value.
On both contracts the main steel bar product used was the offset bulb bar shown in the image right.
This design offers the best strength-to-weight ratio of any section, and the shape gives added benefits when it is painted due to the lack of corners, such as those found in T sections. However, corners will have to be catered for in CVF as a considerable amount of welded T sections will be used. The corresponding steel grades are DH36 and EH46. In all of the build yards in the CVF project this will be a bought in item.
Military submarine plate steels differ significantly from those used for surface ships. A military submarine is a pressure vessel, but with significant differences. The pressure is on the outside of the structure. While under pressure, the submarine has to withstand hostile explosive shock, and, in the instance of nuclear submarines, it contains a reactor.
One of the current submarine builds at BAE Systems Submarine Solutions in Barrow, UK, is shown in the main image. For a submarine pressure hull application, the main properties required of a steel are strength, which, in association with geometric constraints, defines the safe diving depth, high dynamic toughness to withstand explosive shocks, and at the same time the ability to be welded, albeit with tight process controls.
For current UK submarine construction, a three per cent NiCrMo steel is used for the pressure hull, designated Q1 (Navy) Quality known as Q1N. This steel is based on HY80, a material that was developed in the USA in the late 1940s from armour plate by reducing the carbon and nickel content. Lamellar tearing problems occurred with HY80 in the 1950s and early 1960s, with the UK introducing the Q1N specification in the mid-1960s to take advantage of the improvements in steelmaking practices related to steel cleanliness. In the following years, the HY80 and Q1N specifications advanced, and they continue to aid improvements in steelmaking technology.
Q1N is a quench and tempered steel made using an approved basic electric and vacuum treatment and fully killed using aluminium grain refining practice. Together with tight controls on chemistry, this route produces a clean plate product with uniform mechanical properties up to 130mm in section. Thickness testing has shown that the material exhibits a high degree of isotropy. Although the material is a 550MPa proof stress steel, the Q1N plate specification requires a minimum average Charpy toughness of 100J at -84ºC for material up to 60mm thick, and 70J at -84ºC for material of higher thickness. In practice, Charpy values at the plate centreline are typically in the range of 150-250J at -84ºC for plates up to 130mm thick.
The importance of the Q1N application means that the specific manufacturing route for the material has to be approved for each supplier, with any changes to the route, such as different melting, rolling or heat treatment facilities, requiring evaluation. In addition to assessing a potential supplier’s process controls and quality system, mechanical testing is undertaken on trial material to ensure uniformity of properties throughout the product. The mechanical testing not only involves standard tensile, Charpy and hardness testing, but static fracture toughness tests, and finally, explosive tests to demonstrate the material possesses dynamic toughness.
Chemical analysis (wt%) and mechanical properties of DH36 and EH46 rolled steel plate (X = -20 for DH36 and X = -40 for EH46)
C 0.16 0.13
Si 0.34 0.49
S 0.010 0.004
P 0.017 0.018
Mn 1.25 1.49
Ni 0.030 0.006
Cu 0.08 0.12
Cr 0.07 0.061
V 0.002 0.071
Nb 0.032 0.035
Al 0.039 0.032
N 0.0045 0.006
UTS MPa 555 627
YS MPa 375 527
Toughness at XoC (J) 204 120