Microstructure and fracture mechanism of a flash butt welded 380CL steel

Highlights

• The failure modes and fracture mechanism of the failed wheel rim was investigated.

• The microstructure and mechanical properties of the flash butt welded joints cut from the failed and survived wheel rim were examined.

• The continuous banded structure and the serious Widmanstatten ferrite of the joint were the major reasons for the rim failure.

• The failure did occur at the weld location, then the cracking propagated along the weld metal.

Abstract

High strength low alloy (HSLA) steels have been widely used in the manufacture of wheel rim on account of the higher strength and larger elongation compared to the traditional low carbon steels. This experimental investigation was aimed to evaluate the microstructure and fracture mechanism of a flash butt welded 380CL steel by comparing the failed wheel rim and the survived wheel rim. The results showed that the continuous banded structure in the HAZ and the serious Widmanstatten ferrite in the weld were the major reasons for the failure of the wheel rim. The micro-hardness of two joints was similar in the overall profile. For the joint of the failed rim, the micro-hardness at the weld was the highest at 254 HV and the lowest micro-hardness value was in the HAZ. The high yield ratio of the failed wheel rim resulted in poor formability in the flaring process. The fracture mechanism of the failed rim was the mixture of ductile and brittle fracture modes. The crack initiation was the fusion zone of the weld having highest micro-hardness, then the cracking propagated along the weld metal to the middle of the wheel rim, a limited deviation was found in the terminal of the crack. The large ferrite grain in the HAZ was the main reason for the brittle–ductile transition.

Introduction

With the continuous development of production technology and research of materials, especially in the automobile industry, the expectations for improving materials properties, energy efficiency and cost-effectiveness have been the widespread goal in the auto industry [1], [2]. In recent decades, reducing the weight of a vehicle has become the focus of attention of whole auto industry. Automobile lightening could save energy and decrease environmental pollution, as well as ensure the safety performance of the car [3], [4]. The application of lightweight materials such as aluminum alloy and high strength steel plate was an effective way to achieve automobile lightening. However, the high strength low alloy (HSLA) steels remained the most widely-used material in the forming of rim of trucks. In order to replace the HY steels, the high strength low alloy (HSLA) steels were firstly put forward and studied in the early 1980s by the US navy [5], [6]. The high strength steels were widespread used in the production of automobile components due to its excellent combined mechanical properties. The welding technologies as the efficient and mostly used connection operation played a crucial role in the construction of vehicles [7].

In the aforetime design of automobile body structure, the resistance spot welding (RSW) can basically meet the demands of automobile manufacturers. But with the development of advanced materials especially the high strength low alloy (HSLA), a multitude of welding methods such as flash butt welding (FBW), friction stir welding (FSW), laser welding, and gas metal arc welding (GMAW) are becoming increasingly popular in the automobile industry [8], [9]. Flash butt welding, a solid state welding method, belongs to resistance welding. Due to the advantages of high efficiency and speed, high strength in welded joints, and flash butt welding has broadly applied in the production of automobile rim [10], [11]. A sea of researches can be found on the weldability of high strength low alloy (HSLA) steel and effects of welding on the mechanical properties, yet the limited studies on the microstructures and fracture mechanisms of flash butt welded joints have been reported, in the forming process of rim, the flash butt welded joints would be inclined to failure. Therefore, it is necessary to investigate the microstructures and fracture mechanism of flash butt welded HSLA steel joints in failed and survived wheel rims.

The cracking often occurs at the flaring procedure because of the huge forming force. Sandip Bhattacharyya and M. Adhikary [12] have reported the thinning and cracking were observed in the flash butt welded joint, it was also pointed out that inherent material properties such as high yield ratio, low strain hardening exponent of steel plate, and coarse Widmanstatten ferrite in the weld seam were the main factors which caused the failure of wheel rim. In the flash butt welding of 16MnCr5 steel, Cemil Cetinkaya and Ugur Arabaci [13] harbored the idea that the mechanical properties and microstructures were affected by welding parameters, for example, increasing build up pressure can increase the hardness and reduce the inclusions of weld. In addition, the short flashing time and high welding heat-input may lead to the softening in the heat affect zone (HAZ), which was unfavorable for the mechanical properties of welded joint [4], [14]. The present study was aimed at evaluating the microstructures and mechanical properties of flash butt welded joints in failed and survived wheel rims. On the basis of the studied above, the failure mechanism of joints in the flaring process was identified, highlighting causes that contributed to the fracture of the wheel rim. Finally, the related recommendations were proposed to avoid the cracking of welded joints in the flaring procedure.