Toughness required – composite hardfacing coating
A new composite hardfacing coating for extreme environments has been developed for the energy sector. Dr Andrew Sturgeon, Technical Director at Deloro Stellite in Swindon, UK, explains.
Engineering coatings are a key requirement for many components in the energy sector where exposure to heat, corrosion and wear limit service life.
These coatings are often based on hard metals, cermets or other composite materials deposited principally by weld hardfacing and thermal spraying. The thermal spraying techniques include high velocity oxyfuel, plasma, and flame velocity. Coatings applied by these techniques are characterised by thin deposits, typically under 0.5mm, and are used for temperature-sensitive base materials or where high levels of hardness – greater than 1,000HV – are required. The performance is often limited by a reliance on mechanical bonding to the metal substrate with a risk of coating dis-bond and detachment.
Weld hardfacing techniques, including plasma transferred arc, tungsten inert gas and manual metal arc, are used in applications with extreme demands on wear resistance. These deposits are thicker, over one millimetre, with a high strength metallurgical bond formed between the hardfacing overlay and the underlying component. This ensures that the coating is not compromised under high levels of stress. However, deposit hardness tends to be lower than can be achieved with thermal sprayed coatings and the high heat input during deposition can cause excessive part distortion or degradation of high performance base metals.
To further expand the capability of weld deposited hardfacings, Deloro Stellite, based in Swindon, UK, has developed a proprietary coating technology called DuraStell. This weld deposited coating has high hardness and resistance to abrasive wear in corrosion environments. The coating process allows thin (one-two millimetre), high quality layers to be deposited with minimum base metal heating, while offering a fully fused deposit.
DuraStell is a composite deposit of nickel alloy matrix containing 65% tungsten carbide with the carbide retained in uniform dispersion throughout the deposit. Reaction between the carbide and matrix is minimised so that good ductility is retained by the matrix alloy. Bulk hardness of the deposit has been measured at over 700HV (60HRC), with the carbide particles having a typical hardness of 3,000HV.
The coating was developed to meet the extreme requirements of downhole tooling in the oilfield industry. For this application the coating is used as welded, with control of deposited thickness to +/-0.3mm or following grinding when tighter dimensional control is required. Development of the coating is underway to tailor the carbide type and amount, as well as matrix chemistry, for a range of severe wear applications.