High strength steel wire for composites

Tom Shelley reports on an old idea brought up to date

Designers of blast barriers and cars are beginning to use a composite that is reinforced with high strength steel wire. The wire is made by Birmingham company Webster & Horsfall, using a process invented in the 1850s. It is passed through tubes in a furnace at about 970°C. This produces uniform austenite of large grain size. The subsequent fast cooling in air or molten lead ensures that the final structure has alternating plates of ferrite (typically 60nm thick) and (10nm thick plates of iron carbide or cementite – with no separation of primary ferrite. The process, known as ‘patenting’, produces what is effectively piano wire. Now US company Hardwire is looking to exploit its properties – including strengths up to 3,780N/mm2 – in composite materials. Hardwire makes tapes, mats and single end rovings of wire. The tapes are 12in (305mm) wide and are sold in 15, 152 and 762m lengths, with 4,12 or 21-23 wires per inch width. The 4 wires per inch tapes are used with high viscosity resins such as cement-based mixtures and thermoset putties. The mat is a 3 ply fabric available in a 14.4oz/yd2 basis (0.5kg/m2) with fibres in the outer two plies running in the warp direction while the centre ply is an isotropic non-woven layer. Hardwire claims that these composites are up to 70% thinner and 25% lighter than those made with glass fibres. Because patented steel wire has elasticity and a certain amount of ductility compared with glass and carbon fibre, it is good for absorbing blast. It is also of interest to automotive engineers. The steel wire is less expensive than carbon fibres and conducts electricity well, so panels made of steel-reinforced composites may be used as a return path for vehicle electrics. It also works with matrix materials, giving reinforcement to multi-layer structures made of wood and concrete. It can be laminated into thermoplastics that are too viscous to be reinforced with other fibres. Explosion tests have been undertaken by the US Navy on Hardwire-reinforced polyurethane and polyureas with good results. Hardwire cords are supplied with standard or adhesion optimised brass coatings. The latter uses a titanium based primer which increases bonding to polyesters, vinyl esters and epoxies by as much as 500%. The brass, only 6 microns thick, lubricates the wire drawing dies instead of oil. Some automotive designers are also interested in using brass-coated steel reinforcement mats because of its effect on vehicle panel appearance. Hardwire LLC Webster and Horsfall Pointers * Composites can be made with reinforcements made of steel wire, manufactured by a process that has been in use since the 1850s * Hardwire claims that its composites can be 70% thinner and 25% lighter than those made with glass fibres * Steel reinforcements show greater elasticity and ductility than glass fibres, hence the interest in using them for blast resistance