Effect of Boride Layer on PM HSS AISI M2 on the Mechanisms Acting in the Transverse Rupture Strength Test

Abstract

Herein, it was conducted a study in order to examine the main fracture characteristics in the transverse rupture strength test (TRS) of a high speed steel (HSS) AISI M2 produced by powder metallurgy (PM) and with boriding treatment. Four conditions have been studied: as-sintered, borided, normalized and borided + TiN. The TRS tests were conducted in accordance with ASTM B528-12 standard. Density, Vickers hardness and transverse rupture strength properties were evaluated. Fractography was observed by scanning electron microscope (SEM). TRS test in the as-sintered condition had the best performance. The fractography of samples showed very irregular appearance, with interconnected porosity for the matrix of the samples and the boride layer had a smooth surface with low porosity. It was observed on the boride layer, the growth of boride layers inside the pores, filling them, which is a positive factor on the TRS. However, cracks were observed in the FeB/Fe 2 B interface of boride layer, which embrittle the material and reduce the TRS. In fractography of borided samples, it is observed that two fracture modes occur, mode I and mode II. This should be explained owing to the difference between the elasticity modulus of FeB and Fe 2 B interface.

Effects of Sodium Octaborate on AISI 4140 Steel Machined by Die-sinking EDM

“…The insertion of boron, which resulted in iron borides, can explain the increase in surface hardness 3 , as reported by 4 , an increase of up to 5 times the surface hardness as a result of the boriding process at 1000 °C for 4 h. The increase in hardness values shown in Table 3 can be attributed to the borided layer produced, as also noted by 43 . A borided layer thickness of approximately 50.6 µm can also be seen in Figure 4, corroborating the data from 35 , which obtained a borided layer of approximately 51 µm on PM HSS AISI M2 steel. While 44 obtained experimentally a Fe 2 B boriding layer thickness from 20 to 160 μm when both the process temperature and treatment time varied from 850 to 1000 ºC and from 2 to 8 hours, respectively.…”

“…The recast and intermediate layers hardness, as well as the substrate hardness, were measured by the Vickers method in a Shimadzu HMV-2T E microhardness tester with a load of 25 gf 30,35 and dwell time of 15 seconds. The hardness indentations were performed in the sample's cross-sections following a minimum indent spacing of 20 μm and were repeated three times.…”