Graphitization cracks in Ck45 steel clips of turbofans

Highlights

• In this research the failure of a DIN Ck45-steelclips of turbofans during the forming was studied.

• The failure was originated from a problem with raw material.

• The problem was related to the slow rate of heating beneath of 850 °C.

• It seems that spheroidize annealing was not done properly and the temperature and time was at the range in which graphitization occurs.

Abstract

In this research the failure of a DIN Ck45-steelclip of a turbofan during the forming process was studied. As the failure was brittle and the forming was done properly, the failure was originated from a problem with raw material. The material and properties like chemical compound, hardness and tensile strength were at the proper ranges. The cold rolled steel passed the heat treatment process like spheroidize annealing and martempering prior using as clips. The problem was related to the slow rate of heating beneath of 850 °C and incomplete microstructure modifying prior to spheroidizing annealing which causes the graphitization in the raw material, and brittle failure during the forming process of parts.

Introduction

The fasteners, clamps and clips are important in different industries. Since these parts work as a connecting component, their performance is critical [1], [2], [3], [4].

Some studies have been done on graphitization of different steels, Ye Jin Kim and et al. [5] investigated the effect of heat treatment temperature and primary grain size on the mechanical properties and graphitization. Zhang and Han [6] surveyed graphitization of free- cutting steels and Guseva et al. [7] investigated the methods of graphitized steels obtaining when there is a need for both steel and gray cast iron properties.

Carbon steels are used due to the variety of mechanical properties provided when suitable heat treatment is used [8]. During the steel production, it is essential that raw material has enough quality, cleanliness, and pass the quality control checks [9].

A small company provided seven coils of DIN Ck45 for production of turbofan clips. When company used new coils of DIN Ck45 faced with the failure problem. It is noticeable that the failure was just observed in one coil out of the seven coils. The failure was observed during the forming of turbofan clips in related stamping die. These products are formed in progressive dies during 8 stages. The progressive die design of the turbofan clips can be seen in Fig. 1. In the 1th, 2nd and 8th stages cracks form in the parts.

As it is shown in Fig. 2, the turbofan clip is a component that holds electromotor. Assembling was done using impact force and the failure of the new specimen was observed during the impact assembling. In order to improve the formability and proper hardness, the microstructure of raw materials was selected to contain alpha ferrite + spheroidized cementite instead of ferrite + pearlite. The products were passed martempering process before assembling. The process of martempering heat treatment was consisted of austenitizing of parts at 860 °C for 15 min and tempering at 330 °C for 1 h with 420 HV. The furnace used in this study was shaker heart type made by SIB (Italian brand).

The production process needed for raw materials was hot rolling until the plate reaches to adequate thickness. Then cold rolling was done for final dimension and finally spheroidizing annealing (or soft annealing) was done using inter critical annealing at 850 °C and 700 °C for total 80 h at hydrogen atmosphere in order to recrystallization and spheroidizing, and to reduce hardness and increase the formability of coils. There was a brittle failure problem in one coil. There are some hypothesizes for this failure and this paper tries to present the reasons of the failure.