Improvement in the service life of cutting tools

Short term industrial goal!

Elodie Sorlier, doctor of MINES ParisTech since 5 juin 2009

Mechanical properties of  the graduated cutters

Oil and gas wells are drilled deeper and deeper nowadays, in hard to reach formations. In addition, geothermal heat extraction is booming. Thermomechanical stresses are very high under these conditions and are responsible for heavy demands on tools, in terms of shock and abrasion. The service life of the active part of these drilling tools is therefore strongly decreased. As a result, the change of tools, which is very expensive and risky, becomes more frequent.

Cutters are mainly made up of cemented carbide WC-Co. Properties of these composites are governed by carbide grain size and metallic binder phase content. Toughness, responsible for shock resistance, and hardness, responsible for resistance to abrasion, remain two antagonistic properties. The most frequent solution to circumvent this opposition is the generation of property gradients through the cemented carbides. The gradation processes developed until now depend on various phenomena, but have two points in common, the small distance concerned by the gradient and the discontinuous character of this gradient. The goal of this study is thus to develop a new gradation process for the WC-Co, generating continuous gradients on several millimetres in order to enhance the service life of drilling cutters. This process will have to be amenable to industrialisation.

In the study of Elodie Sorlier, the mechanical properties of the graduated cutters have been tested in terms of hardness, shock resistance, resistance to abrasion and compression. Furthermore, residual stresses have been measured on commercial and graduated cutters. These mechanical tests show that the gradation process developed in this work makes it possible to improve the shock resistance of the cutters without decreasing their abrasion resistance. These results can be the beginning of the development of a new type of cemented carbide WC-Co, with applications to fields broader than drilling.

Background

The background of this study is the search for technical solutions aimed at cost-cutting in drilling. The contribution to this strategic axis concerns the improvement in the service life of cutting tools in order to minimize downtime. The idea is to improve the hardness of the tool surface, in contact with rocks, for better efficiency, and, at the same time, to improve the toughness of the tool core in order to enhance shock resistance.

Based on precursory work by AF. Lisovsky, a gradation process has been developed and improved, according to the specifications. In particular, a boron-rich coating is used in association with the imbibition technique. This process, which is the subject of two patents, consists in enriching a commercial WC-Co fully-dense cutter in the metallic phase via a short thermal treatment around 1400 °C under a controlled atmosphere. Gradients superior to an 800 HV amplitude were measured in 10 mm-high samples. The influence of all the process parameters on the shape and amplitude of the gradients were studied. This parametric study, in addition to a better understanding of the mechanisms implied in the process, makes it possible to consider envision an important process flexibility concerning the geometry of the pieces and of the gradients. A kinetic study has also been carried out.

The process developed in the study is getting several advantages. It applies to commercial parts, it is flexible and adapts to the tool geometries and/or to gradient types. Its implementation is quite simple. The gradients obtained are continuous and of big amplitude. Further studies could enable it to be applied at the same time as the sintering process or during the pressing of the diamond layer.


This study has been carried out in partnership with two laboratories of MINES ParisTech Centre des Matériaux in Evry and Centre de Géosciences in Fontainebleau, for Varel Europe, a drilling tool manufacturer based at Ibos (64).

 


Elodie Sorlier decided to do a PhD after one year at the Material Research Centre in Evry where she was completing her “advanced master”. She was interested in the practical aspect of the subject. Her motivation comes from the existence of a short term industrial goal. That’s why she decided to continue to work in industry and not in the academic field.

Now, she's working for a bearing manufacturer, SNR, in the R&D department. She's very satisfied with this position because she's in contact with the products and interact with everyone in relation with these products. In addition, bearings are typically a transversal product: you find bearings everywhere, in the aeronautic field, in automotive applications or for the industry at large. She continue to learn something every day and the perspectives are huge.