Cutting performance improvement of MTCVD coated cemented carbide inserts via appropriate heat treatment

doi:10.1016/j.cirp.2020.04.022

CIRP Annals

Volume 69, Issue 1, 2020, Pages 45-48

https://doi.org/10.1016/j.cirp.2020.04.022 Get rights and content

Abstract

The softening of cemented carbide at the elevated temperature of MTCVD processes results in significant reduction of their strength, and thus, high coating deformation during cutting. In this way, at interrupted cutting loads, as during turning cast iron, the cutting performance of MTCVD coated cemented carbide inserts degrades due to premature coating fatigue fracture. To overcome this problem, appropriate heat treatments of MTCVD coated inserts were conducted. The related temperatures and durations for improving the substrate properties were optimized considering obtained strength data of uncoated inserts variously heat treated and the turning performance of coated ones correspondingly heat treated.

Introduction

Moderate Temperature Chemical Vapor Deposition (MTCVD) techniques are often used for producing thin hard coatings on cemented carbide tools employed in various cutting operations. TiCN coatings manufactured by MTCVD possess increased hardness and toughness, and thus, improved wear resistance [1,2]. However, a softening of cemented carbide substrates taking place during the MTCVD process, significantly deteriorates their strength properties [3,4]. In such cases, the coating deforms more and is consequently higher stressed during cutting compared to the same coating deposited on a harder and less deformable substrate. Hence, the capacity of the MTCVD coated cemented carbide tools to withstand cutting loads, especially dynamic ones developed in interrupted cutting processes decreases, leading to pre-mature coating fracture and shorter tool life.

The present paper aims to investigate the potential of improving the wear resistance of MTCVD TiCN coated cemented carbide inserts by increasing the substrate strength properties via convenient heat treatments. The heat treatment temperature and duration for enhancing the substrate strength of coated inserts was optimized considering on one hand the mechanical properties of variously tempered uncoated inserts determined by means of FEM-supported evaluation of related nanoindentation's results [2]. On the other hand, the heat treatment parameters were optimized regarding the tool life of coated and variously tempered inserts in turning cast iron. To help explain the achieved tool life results, XRD-measurements were conducted indicating the crystalline transformations in the substrate material after the conducted heat treatments.

Section snippets

Experimental details

The MTCVD TiCN coating was deposited on HW/K05 cemented carbide inserts. The geometry and properties of the used cutting inserts are presented in Fig. 1. The WC–Co microstructure is classified as fine with WC grain sizes between 1.0 and 1.3 μm. Cast iron GG30 was used in the performed turning investigations to dynamically load the cutting edge as a result of the interrupted chip formation. Main material properties of the employed cast iron are presented in Fig. 1. The MTCVD process took place

Micro-structure and strength properties of the cemented carbide inserts

Nanoindentations were performed at a maximum load of 50 mN, for determining the mechanical properties of the uncoated cemented carbide inserts before and after the heat treatments. The obtained load-displacement diagrams in all investigated insert cases are shown in Fig. 2(a). Each curve represents the average of 40 measurements. The average was stabilized already after 30 indentations. An increase of the maximum indentation depth occurs after annealing at 850 °C for 180 min of an uncoated

Cutting performance of the MTCVD coated inserts

The cutting performance of the untreated and variously heat-treated MTCVD TiCN coated inserts was investigated in turning cast iron GG30. During cutting this material, interrupted chips are formed resulting in high dynamic loads on the coated cutting edge. The courses of the flank wear width versus the cutting time for all examined heat-treatment cases are shown in the diagram of Fig. 6. At the top of this diagram, the developed flank wear on the variously tempered inserts (CTx) after 90 s

Conclusions

Due to the elevated MTCVD process temperature, a softening of the cemented carbide substrates, and thus, a restricted coated tool life especially in interrupted cutting processes occurs. In this paper, we presented a way to overcome this problem and improve the wear behavior of MTCVD TiCN coated cemented carbide inserts via appropriate heat treatment. Tempering at 400 °C up to 60 min in vacuum leads to a significant improvement of the coated tool life. At longer tempering durations the

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Citation Excerpt :
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