A steely controversy

Materials World magazine
27 May 2015

Harry Bhadeshia’s conclusions on the formation of white etching matter, seen as one of the primary issues in large wind turbine failure, may seem straightforward but will they be adopted by the metallurgy community? Khai Trung Le spoke with the Cambridge professor.

‘Imagine you’re being punched in the face 6,000 times a minute at a stress of two gigapascals. That would be unbearable.’ That’s how Professor Harshad Kumar Dharamshi Hansraj ‘Harry’ Bhadeshia, director at SKF Steel Technology Centre, University of Cambridge, describes the degree of pressure applied to steel bearing raceways in large wind turbines. One product of this extreme pressure is white etching matter – highly visible microstructure markings that intitate around defects and are often found in instances of wind turbine malfunction.

The impairment of contacting components such as steel bearings are among the most frequent and costly types of failure for large wind turbines, and as the number of breakdowns occuring far short of their intended service lives continues to swell, engineers, scientists and metallurgists are focusing on white matter as the cause of premature bearing failure.

However, Bhadeshia’s latest paper Critical Assessment 13: elimination of white etching matter in bearing steels attempts to persuade people of a different approach – ‘A key issue is whether the regions are a symptom or a cause of damage […] This confusion hinders the development of clear solutions to the problem.’

Proposed speculation for white matter formation includes severe localised deformation, adiabatic shear, carbide dissolution and low temperature crystallisation. However, the paper asserts that these theories ‘miss the obvious observation that white etching matter is associated with pre-existing defects’.

Critical Assessment 13 seeks to establish that white matter associates with pre-existing defects, and while it exacerbates existing faults, it should not be the primary focus on resolving large wind turbine failure rates.

Bhadeshia commented, ‘Our conclusion is that the white etching matter itself is not the issue. The cause of the initial defects where the white etching matter forms, to make the white etching material more tolerant - that is the essence of what we are saying in this critical assessment so that we can direct where the effort is needed.’

The paper also seeks to challenge routine observations on the influence of hydrogen in white matter formation. Hydrogen encourages the problem by reducing the fracture toughness of steel and causing cracking. However, conventional theory attributes enhanced local plasticity as the cause without supporting evidence or a theory for the localisation of white matter itself.

The report notes, ‘The role of hydrogen, therefore, appears simply to be to embrittle the steel and hence provide the parted surfaces that initiate localised plastic deformation’, establishing that large cracks are more frequently formed in hydrogen-charged specimens compared with uncharged samples.

Despite what is seemingly an uncomplicated conclusion, Bhadeshia anticipates a degree of dissention, noting, ‘There are hundreds of papers written on this subject and many, many different interpretations. It is far from straightforward. The interpretation that we’ve put forward might be a source of controversy because there is a lot of data that exists to a lot of different resolutions.’

Instead, Bhadeshia looks to prioritise an increase in resistance to cracking, i.e. making the steel tougher. Potential initiatives include carbide-free structures – heat treatments designed so that carbides need not be present in the final microstructure are already employed in specialist aerospace applications – and increasing the stability of carbide by enhancing the chromium concentration, which has previously been established in Fujita and Mitamura’s Research of new factors affecting rolling contact fatigue life, published in 2005.

It is this area in which Bhadeshia foresees the most challenge, ‘This is where you will need to do the research, to see that none of the other properties are compromised in the process.’

Harry Bhadeshia’s Critical Assessment 13 is due for publication in Materials, Science and Technology volume 31, issue nine. IOM3 members can read the journal free of charge from our journals page at tinyurl.com/iom3journals.