Effect of boron concentration on two-body abrasive wear behavior of high boron high speed steel

Xiangyi Ren¹, Hanguang Fu ², Jiandong Xing¹, Shuli Tang¹

¹State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, ²School of Materials Science and Engineering, Beijing University of Technology, Beijing

INTRODUCTION:  China is a country rich of boron. In precent years, plenty of researches focus on investigations of various alloys with boron addition.¹ As a new-type alloying element, boron can improve the microstructures and properties of materials, such as reducing the cell size, producing finer hard phases, decreasing the cost of production, simplifying the production process, as well as obtaining excellent properties like hardenability, tensile strength, bending strength and wear-resistance.² However, generally, Fe–B–C alloy has awful toughness due to its brittle and continuous net-like distributed Fe₂B borides.

For the purpose of improving the toughness of borocarbides, various alloying elements such as molybdenum, chromium, vanadium, cobalt, niobium, tungsten, etc. are tried.³ Among those elements, chromium had been systematically investigated by many researchers and they all found that chromium addition in Fe–B–C alloy could change the morphology of borocarbides and obviously improve their fracture toughness.⁴ As a kind of common alloying element, molybdenum could also be added to Fe–B–C alloy, so that the fracture toughness and heat treatment properties of alloy were further improved.⁵ High boron high speed steel is the product of M2Al high speed steel with boron addition whose microstructure consists of metal matrix and eutectic borocarbides with high hardness and excellent wear resistance. Though chromium, molybdenum and other elements were necessary in high boron high speed steel,⁶ boron was the most important factor to influence the wear resistance of alloy. For obtaining the composition of high boron high speed steel with best wear resistance. It is necessary to explore the suitable combination of boron and other elements. This work investigated the effect of various boron concentrations on two-body wear behavior of high boron high speed steel with Fe–x wt% B–0.4 wt% C–6.0 wt% Cr–4.0 wt% Mo–1.0 wt% Al–1.0 wt% Si–1.0 wt% V–0.5 wt% Mn alloy (x=1.0, 2.0, 3.0).

METHODS:  The microstructure and two-body abrasive wear behavior of samples were also studied. Samples were austenized at 1050 °C for 2 h, quenched by water and then tempered at 500°C for 1h. The ML-100 pin-on-disk abrasion tester was used to investigate the two-body wear behavior of the alloys. SiC abrasive paper with 240 mesh was fixed on the disk with a rotation rate of 60 r/min, where samples slid on from the center of the disk along radical direction at 4 mm/r under contact loads of 15 and 35 N. Samples were worn down under the load of 10 N before wear tests. Weight loss of samples measured on electronic balance with the accuracy of 0.1 mg was defined as the average value of three experimental results under the same load. Worn surfaces were observed by SEM, and VK-9700 3D laser scanning microscopy was utilized to the three-dimensional morphology observing and roughness measuring of worn surface. Wear mechanism of alloys was analyzed through observing worn surface and oblique section.

RESULTS:  The two-body abrasive wearing test results, worn surface SEM morphologies and oblique sections through worn surfaces of tempered high boron high speed steel with different boron contents are presented in Fig. 1. It is observed that with the increase of boron content, the weight loss of alloys decreases and the larger the load, the higher weight loss alloys possess. Furthermore, by comparing with alloy with 1.0wt% B, wear resistance of alloys with 2.0wt% and 3.0wt% B is significantly improved. Variation trend of wear loss is exactly opposite to the macrohardness results, which indicates that macrohardness is the 

main influencing factor of wear resistance (not shown). Fig. 1B shows the oblique section through worn surface under 35N load of high boron high speed steel with 3.0wt% B. It is seen that there are only ploughings on the worn surface. Besides, no cracks or other defects are observed on the oblique section.

Figure 1 - A) Weight loss of high boron high speed steel with different boron contents and loads.  B) Oblique section through worn surface under 35N load of high boron high speed steel with 3.0wt% B.

DISCUSSION:  As is known to us, borocarbide acts as hard phase to resist abrasive particles, protecting matrix from being damaged directly, while matrix supports borocarbides. The synergistic reaction has a positive effect on wear resistance of high boron high speed steel.⁷ According to the theory of Ref. 48, the ratio of hardness of abrasive SiC (HV2600) and eutectic borocarbide (HV1400-1800)⁸ is below 0.8, which means that SiC can be regarded as hard abrasive. In alloy with 1.0wt% B, only small-size borocarbides exist, which are easy to be fractured by moving abrasives. Consequently, ploughings, pits and scratches caused by abrasive and broken borocarbides appear on its worn surface. When boron content increases to 2 or 3 wt%, only ploughings are observed because the size of borocarbide becomes larger, it is difficult to be broken by abrasives.

By observing the oblique sections together with worn surface morphologies, it is proved that the wear mechanism of alloy with 1.0wt% B is micro cutting accompanied with spalling of borocarbides. Rest of the alloys are single micro cutting From Ref.⁷, borocarbides with Cr, Mo, and V addition have excellent toughness, so no cracks are observed on borocarbides.

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