Microscopic examination of the fracture surfaces of a cold working die due to premature failure

https://doi.org/10.1016/j.engfailanal.2010.12.007Get rights and content

Abstract

The paper studies the fracture surfaces of a rotary cold working die that broke down during the setup process on the machine it was designed for. The die was manufactured using AISI D2 tool steel and was intended for cutting paper at room temperature. The tool failed during the setup process on the machine. Initially, historical data were collected: operating conditions, lifetime, restitution of sound operation, collection of recorded service history. This was followed by measurement of temperatures, hardness control, optical inspection and selection and preparation of samples. The cutting edges, fracture surfaces and the structure, not affected by the failure, were investigated by optical microscopy. The quality of the nitrated layers were inspected. Additional information concerning the fracture was obtained by examining the fracture surfaces using a scanning electron microscope. Local chemical analysis of the material was made using an EDX spectrometer. Fracture mechanism, the type of the fractures, and the principal causes that led to the premature failure of the die are discussed. Justification of these causes facilitates the prevention of future failures and sometimes contributes to an increased service life of the tools.

Research highlights

► Poor design, improper use and absence of stress removing, led to premature fracture. ► Deficiency of design concerning both shape and mechanical properties of the die. ► Metallographic analysis of the fracture surface has evidenced three distinct areas.

Introduction

Cold work tool steels constitute an important group of steels as they cover a huge field of application. AISI D2 is one of the most characteristic steels for cold working tools and is broadly used for cutting tools and forming dies. It is a high-carbon, high-chromium tool steel alloyed with molybdenum and vanadium and is characterized by high wear resistance, high compressive strength, good through-hardening properties, high stability in hardening and good resistance to tempering-back [1].

The operating capacity and the life of die tools in service are determined by their mechanical properties, hardness, impact toughness, cyclic endurance (resistance to low-cycle fatigue) and wear resistance. Tool steel producers are continuously searching for ways to predict and increase the lifetime of tools. Geiger and Falk [2] presented a methodology for the lifetime calculation of cold forging tools, which are characterized by high load levels and often fail by fatigue crack initiation in surface areas. Okolovich [3] studied the effect of the main alloying elements and carbon on the properties of cold working dies. He proved that the main way for increasing the endurance of such tolls is by raising the fatigue resistance; which depends on mechanical properties of the steel. Xinmin [4] proposed a new heat treating processes to increase service life of nut cold heading dies made of AISI D2 steel. Gagg and Lewis [5] have recently proved that the wear is an important parameter for the tool lifetime or even the tool failure. Yoon et al. [6] studied the effect of Ti-based hard coatings on the performance and endurance of AISI D2 steel.

Since technologies evolve continuously and cold working tools are subjected to severe stresses, the tool failure remains an issue. The objective of failure engineering investigation is to determine and explain the causes leading to premature failure.

The analysis of every single failed piece is important for the prevention and prediction of similar situations. A few articles are available in the literature, presenting analysis of cold working tool steels. Krishnadev and Jain [7] underline the importance of failure control in improving the selection of materials and the application of suitable thermal processing. Mac Cormak and Monaghan [8] studied the failure of cutting tools due to design and showed that the form and operation settings of cutting tools influence their lifetime significantly. Knoerr et al. [9] have studied the causes of failure due to fatigue in cold working hammer tools and have propose a failure analysis sequence for similar tools. Gagg [10], [11] has conducted research on failure in tools made of AISI D2 steel on behalf of a construction company and underlined the importance of the quality of tool manufacturing for the production process.

This paper presents the microscopic examination of the fractured surfaces of an AISI D2 cold working tool, which presented a premature failure.

Section snippets

Experimental

A die was produced for cutting envelope packages in the pharmaceutical industry. The die is part of a cutting machine and is located in an assembly including a roller. A strip of the medical product and its packaging material, which is cut at the required size, pass through the machine. High cutting precision is required, as this process defines the shape and dimensions of the package and also the medical product dose. The tool failed during the setup process on the machine.

Historical data were

Results and discussion

The paper is an exceptionally soft material. It is difficult to cut as it gets stuck very easily and blocks the cutting system. Continuous operation of the machine is ensured if tolerances remain stable. The cutting die is part of this cutting system and is located in an assembly including a roller. A strip of the medical product and its packaging material, which is cut at the required size, pass through them. High cutting precision is required as this process defines the shape and dimensions

Conclusions

The study of historical data revealed significant differences in the performance of cutting tools. The cutting surface of the cutting tools has never broken in the past, but cracks have been observed from time to time. Unsuitable use of cutting tools leads to a drastic reduction (62%) in their functioning time. While not functioning, the cutting tool remains on the machine and is not protected in any way. Between two successive operations, stress is not released in any way.

Dealing with edge

References (12)

There are more references available in the full text version of this article.

Cited by (0)

View full text