Nano structure and transformation mechanism of white layer for AISI1045 steel during impact wear

doi:10.1016/j.wear.2004.06.002

Wear

Volume 258, Issues 1–4, January 2005, Pages 537-544

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Abstract

Impact wear was carried out for annealed AISI1045 steel in this study. With the impact frequency of 600 times per min, an 8.29 kN impact load was applied on the constant contact area of 540.08 mm² at the individual attack angles 60° and 90°. The microstructure of white layer for annealed AISI1045 steel was examined by SEM, TEM and HRTEM. It was found that nano structures with ferrite and cementite have been formed in white layer after impact of AISI1045 steel. Nano crystallization mechanism for ferrite is from moving, interacting and rearranging of dislocations, resulting in production of dislocation walls. With increasing strain, the density of pinned dislocations, whose Burgers vectors are usually vertical to the wall, correspondingly increases along the dislocation wall, i.e., θ/b increases. That, finally, forms subgrain boundaries with small angle θ. In addition, the density of slippery or shear-susceptible dislocations, whose Burgers vectors are often parallel to dislocation walls, also increases at the same time. The slipping of dislocations in many places along the boundary of subgrains will result in the rotation of crystals to form boundaries with large angles. Nano crystallized mechanism for cementite is mainly due to shearing fracture, neck-like shrinkage fracture and carbides dissolving under the impact. The cementite thinning and the formation of slipping stages will increase the surface energy per volume, which causes the dissolution of cementite. The chemical potential energy difference between carbon atoms and cementite is a driver for dissolving cementite. The moving, reacting and proliferating of dislocations promote carbon atoms diffusing and migrating to distant ferrite from the boundary of cementite, which accelerates the dissolution of cementite.

Introduction

Impact wear is frequently met in engineering application, such as piston and rod of rockdrill, liner of jaw crusher, valve and seat of internal combustion engine, liner and balls of ball mill, frog and wheels of railway, padrails of tractor, etc. During service, those parts usually experience strong impact and their impact loads are also two or three magnitude order higher than traditional fatigue loads. This makes impact wear to behave as a dynamic characteristic [1]. Especially, a lot of experimental results will be quite different when impact load increases to two or three magnitude order higher than usual impact load. The reason is that the wear failure mechanism is completely different in the case of large impact load [2].

Under the heavy impact, the “white layer” is easy to form in subsurface of mild steel and shows the bright color in optical microscope. The brittle fracture of “white layer” easily occurs under the impact conditions from the present author's viewpoint. The explanations of “white layer” formation mechanism also vary by now. Some researchers considered that the concentrated frictional heat leads to transformation of martensite, and then is quenched to the secondary martensite [3], [4], [5], [6]. Others thought that the strong plastic deformation of subsurface induces the martensite formation rather than frictional heat [1], [7]. In a word, martensite or austenite–martensite transformation in subsurface becomes a popular viewpoint of “white layer” formation. However, there also exists another viewpoint besides above explanation. It has been reported that oxygen, nitrogen or carbon elements can be diffused, transported to subsurface or chemically reacted with counter pairs or medium to form oxide, nitride or carbide [4], [8], [9], [10]. Dynamic recrystallization is also considered as a mechanism of “white layer” formation [11].

Therefore, it is noticed that the formation mechanism of “white layer” is still unclear at present. In this research, impact wear characteristic of annealed AISI1045 steel was investigated to understand the mechanism of “white layer” formation. Subsurface after impact was examined by transmission electronic microscope (TEM) and high-resolution electron microscope (HRTEM) to analyze the procedure of “white layer” formation.

Section snippets

Experimental procedure

Impact wear was carried out on self-built test rig. Fig. 1 shows the schematic of the rig. The upper specimen was monitored on the upper specimen holder which can move up and down driven by a cam gear system. A dead weight was mounted upon the upper impact holder. The impact load can be changed by choosing different dead weights. In this research, dead weight was 9 kg, adjusting the impact energy was 8.2 J. The impact frequency is fixed to 600 times per min. The bottom specimen was fixed on rigid

Test results

During running-in stage, the evolution procedure of AISI1045 steel impact wear losses showed a conventional characteristic. At the initial impact stage, an incubated period was observed during which the weight loss has not been selected. With increasing impact cycles, the weight losses increased continuously till 18,000 times running-in procedure. After running-in procedure, the wear curve of AISI1045 steel is as shown in Fig. 2. It is shown from Fig. 2 that the wear curves showed uncontinuous

Microstructure of “white layer”

Fig. 7 shows the bright and dark field TEM ferrite microstructure and diffraction pattern in “white layer” individually. Table 1 indicates the measured interplanar spacing D and indices of crystallographic plane of ferrite in “white layer”. It can be concluded that the spots for every diffraction radius become more continuous, which infers that the grain size in “white layer” has been largely decreased after impact. The grain size has stepped in nano scale as shown in Fig. 7(a) and (b). Data in

Conclusions

1.
Impact wear of annealed AISI1045 steel with an impact energy of 8.2 J shows the stage pattern which is related to crack initiating and debris spalling process in brittle “white layer” and interface between “white layer” and plastic deformation zones.
2.
Austenitic or martensitic transformation in “white layer” is not observed by TEM and HRTEM examination. It is found that original ferrite and cementite in “white layer” have been nano crystallized after impact wear.
3.
Moving, interacting and rearranging

Acknowledgements

The present authors would like to thank Dr. Fazhan Wang, Xian University of Architecture and Technology, PR China, for his great help during the impact wear tests. Authors are also grateful to Dr. Zhengxin Lu for his significant suggestions in the microanalysis.

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Nano structure and transformation mechanism of white layer for AISI1045 steel during impact wear

Abstract

Introduction

Section snippets

Experimental procedure

Test results

Microstructure of “white layer”

Conclusions

Acknowledgements

Wear

Tribology Int.

Tribology Int.

Mater. Sci. Eng.

Wear

Wear

Correlation between a laboratory impact-abrasion test and an in-situ marked ball test

Tribology Int.